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A reinterpretation of stomatal response to humidity
Abstract
The stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ∂g/∂E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance gm obtained by extrapolation to zero transpiration, and a maximum rate of transpiration Em obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO2 concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.
... This approach uses the Penman-Monteith formulation for plant transpiration based on an assumed constant minimum stomatal resistance (TSEB-PM Rc,min ). We also examine an alternative minimum stomatal resistance formulation, dependent on a VPD formulation and based on the studies by Monteith (1995) and Leuning (1995), within a modified TSEB framework that explicitly accounts for the partial canopy condition through the use of the Shuttleworth-Wallace model (TSEB-SW Rc,VPD ). These different TSEB formulations are applied to local tower-based hemispherical LST observations from a longwave radiometer over a vineyard in a strongly advective environment and compared to eddy covariance tower measurements of ET to identify the optimal formulation. ...
... where ρ is the air density (kg m −3 ), C P is the specific heat of air (assumed constant at 1013 J kg −1 K −1 ), γ* = γ(1 + r C /r A ), r C is the bulk canopy resistance (s m −1 ), r A is the aerodynamic resistance between the canopy and the air above the canopy (s m −1 ), e S and e A are the saturation and actual vapor pressures of the air (kPa), respectively, and all other terms are as defined previously. An increase in vapor pressure deficit (VPD = e S − e A ) may be offset by an increase in r C (and, thus, γ*) due to stomatal response to environmental factors (Jarvis, 1976;Lohammar et al. 1980;Monteith, 1995); however, Allen et al. (2006) concluded that r C is generally constant and recommended values of 50 s m −1 during the day ...
... In those cases, it is key to parameterize the stomatal closure with increasing VPD, whose behavior might indeed differ between species and even between varieties (Grossiord et al. 2020). Accounting for the negative feedback observed between transpiration (T) rates and stomatal closure based on a wide variety of plant level measurements, Monteith (1995) proposed a method to parameterize the relationship between leaf stomatal conductance (g s ) and VPD, based on measurements of transpiration as follows in Eq. (3): ...
Water conservation efforts for California’s agricultural industry are critical to its sustainability through severe droughts like the current one and others experienced over the last two decades. This is most critical for perennial crops, such as vineyards and orchards, which are costly to plant and maintain and constitute a significant fraction of the regional water use. It is no longer feasible to access groundwater for irrigation to replace deficit surface water resources during drought due to a significant overdraft of aquifers and new regulation limiting its use. To achieve significant water savings, the actual crop water use or evapotranspiration (ET) needs to be mapped from field to regional scales on a daily basis. This can only be achieved using remote sensing-based models, particularly thermal-based energy balance models that are sensitive to deficit irrigation conditions. The two-source energy balance (TSEB) model has been successfully applied over vineyards in California, but challenges still remain. In particular, much of the irrigated cropland in the California Central Valley is affected by advection of hot dry air masses from surrounding non-irrigated areas and the TSEB model appears to need modifications to adequately estimate ET under such conditions, as well as the partitioning between evaporation and transpiration. This study investigates the application of the TSEB model, using local observations in a vineyard having significant advection. Four versions of the transpiration algorithm in TSEB are applied and evaluated with tower eddy covariance measurements spanning 4 growing seasons. The results suggest the performance of the original transpiration algorithm based on Priestley–Taylor used in TSEB is satisfactory in all but the most extreme advective conditions, while a transpiration algorithm based on Shuttleworth–Wallace with a canopy resistance formula, which relates maximum stomata conductance to vapor pressure deficit (VPD), performs well in all cases. These modifications have potential for improving regional applications of the TSEB model in support of water management in the Central Valley.
... This plateau occurred when VPD was high (>2.5 kPa), in July 2021, indicating a partial closure of stomata. Numerous studies have reported stomatal regulation in response to increasing VPD [60][61][62]. As VPD continues to rise, a plateau is reached, and eventually, sap flow declines at high VPD values [61,63,64], as observed in our results (Figure 2), suggesting stomatal closure. ...
... Numerous studies have reported stomatal regulation in response to increasing VPD [60][61][62]. As VPD continues to rise, a plateau is reached, and eventually, sap flow declines at high VPD values [61,63,64], as observed in our results (Figure 2), suggesting stomatal closure. Stomatal closure in trees serves to reduce transpiration and prevent embolism in the xylem conduits of conifers [65]. ...
In the face of ongoing climatic changes, understanding the species’ sap flow responses is of crucial importance for adaptation and resilience of ecosystems. This study investigated diurnal variability and radial sap flux density (Js) in a natural Juniperus drupacea forest on Mt Parnon and determined the climatic factors affecting its total sap flow (Qs). Between July 2021 and March 2022, Granier-type sensors and automatic weather stations monitored Js of J. drupacea trees and environmental factors. Utilizing a multi-point sensor for Js radial profile variability, correction factors were applied to calculate (Qs), ranging from 4.78 to 16.18 L day−1. In drier months of the study period (July–September), Qs progressively increased with increasing PAR and soil temperature, reaching a plateau at maximum values (app. 600 µmol m−2 s−1 and 26 °C respectively) indicating partial stomatal closure. Whereas, during the wetter period (October–March), when water was no longer a limiting factor, VPD and PAR emerged as significant controllers of stand transpiration. In this period, Qs responded positively to increasing soil water content (θ) only on days with high VPD (>0.5 kPa). The studied J. drupacea stand demonstrated adaptability to varying environmental conditions, crucial for the species’ survival, considering anticipated climate change scenarios.
... Responses of leaf conductance to increasing D generally follow a hyperbolic or exponential decrease, while the magnitude of the decrease has been used to describe the stomatal sensitivity (Leuning, 1995;Oren et al., 1999;Medlyn et al., 2011). The mechanistic basis of this reaction has been subject to a number of theories regarding the stimulus factor (RH or D), the existence and site of a 'humidity sensor', the reaction type (feedback, feedforward), and the involvement of hormonal and genetic factors (Farquhar, 1978;Ball et al., 1987;Grantz, 1990;Monteith, 1995;Xie et al., 2006;Bauer et al., 2013;Cardoso et al., 2020). The actual site of evaporation, i.e., the end of the plant´s hydraulic system, might also play an important role. ...
... The decrease of G with increasing D has often been reported to be exponential or hyperbolic, including a considerable intra-and interspecific variability (Aphalo and Jarvis, 1991;Leuning, 1995;Monteith, 1995;Oren et al., 1999). A frequent assumption is the proportionality between stomatal conductance gs at low D and the sensitivity of the closure response, where sensitivity refers to the magnitude of the reduction in gs with increasing D (Oren et al., 1999). ...
Introduction
Many atmospheric aerosols are hygroscopic and play an important role in cloud formation. Similarly, aerosols become sites of micro-condensation when they deposit to the upper and lower surfaces of leaves. Deposited salts, in particular can trigger condensation at humidities considerably below atmospheric saturation, according to their hygroscopicity and the relative humidity within the leaf boundary layer. Salt induced water potential gradients and the resulting dynamics of concentrated salt solutions can be expected to affect plant water relations.
Methods
Hydroponic sunflowers were grown in filtered (FA) and unfiltered, ambient air (AA). Sap flow was measured for 18 days and several indicators of incipient drought stress were studied.
Results
At 2% difference in mean vapor pressure deficit (D), AA sunflowers had 49% higher mean transpiration rates, lower osmotic potential, higher proline concentrations, and different tracer transport patterns in the leaf compared to FA sunflowers. Aerosols increased plant conductance particularly at low D.
Discussion
The proposed mechanism is that thin aqueous films of salt solutions from deliquescent deposited aerosols enter into stomata and cause an extension of the hydraulic system. This hydraulic connection leads – parallel to stomatal water vapor transpiration – to wick-like stomatal loss of liquid water and to a higher impact of D on plant water loss. Due to ample water supply by hydroponic cultivation, AA plants thrived as well as FA plants, but under more challenging conditions, aerosol deposits may make plants more susceptible to drought stress.
... The most significant environmental variable that affects stomata is the VPD (Medlyn et al., 2011). Stomatal closure occurs as a feedback response to the leaf and whole plant water status to limit transpiration in response to increasing VPD (Domec et al., 2009;Monteith, 1995). However, despite complete decoupling from the atmosphere, equilibrium transpiration still exists, mainly controlled by incoming solar radiation (Boese et al., 2017). ...
... However, despite complete decoupling from the atmosphere, equilibrium transpiration still exists, mainly controlled by incoming solar radiation (Boese et al., 2017). If plants prioritize primary production over water conservation, transpiration may decrease with rising VPD (Monteith, 1995). This is why carbon and water tended to decouple under transitional and energy-limited conditions. ...
Water use efficiency (WUE) is a crucial parameter for describing the relationship between carbon and water cycles in plants and ecosystems. Previous studies suggest that an additional intercept is necessary for the empirical WUE model. However, the impact of soil moisture on model performance remains uncertain. In this study, 71 sites of FLUXNET2015 were categorized into energy‐limited, transitional, and water‐limited climates based on the Budyko dryness index (DI). Modified WUE models were utilized to forecast daytime transpiration based on GPP × VPD0.5 with an intercept term, which evaluated whether soil moisture content regulates the intercept term with increasing dryness levels. Our results demonstrated that the WUE model with an additional net radiation (RN) term effectively predicts transpiration in energy‐limited and transitional regions. On the other hand, the WUE model with an additional soil water content (SWC) term performs well in water‐limited regions with annual precipitation of less than 400 mm and energy‐limited regions with annual mean temperature of less than 5°C. The significance of SWC on Nash‐Sutcliffe efficiency increased from energy‐limited to water‐limited regions. The coupling of the RN and SWC terms as intercept terms in the empirical WUE model can enhance the prediction of Et in both energy‐limited and transitional regions. With global warming leading to more extreme climates in both humid and arid regions, incorporating the SWC term in the empirical WUE model can enhance the accuracy of modeled daytime transpiration globally, providing an easy‐to‐use method to simulate global transpiration.
... There is no consensus on the exact mechanism by which increases in VPD induce stomatal closure; physiological and metabolic changes might be induced by the perception of changes in VPD by cells in leaves, and hormone signals such as abscisic acid (ABA) might also play an important role [43−45] . The transpiration rate is the product of G s and VPD under the same boundary layer conductance of the leaves, and the magnitude of the increase in VPD is far greater than the magnitude of the decrease in G s , which results in an increase in the transpiration rate under high VPD [46] . A higher transpiration rate results in lower turgor of guard cells and higher water loss. ...
... Additionally, the allocation of nutrients to the stems and roots increases and that to the leaves decreases under low VPD (0.63 kPa) under adequate irrigation. This is because a lower transpiration rate increases the retention of nutrients in the roots and stems [46,106] . Additionally, the nutrient content in substrate decreases, and the nutrient absorption in vegetables increases under low VPD (0.90 kPa) under adequate irrigation [26,105] . ...
In previous decades, the global temperature has risen, and the saturation vapor pressure deficit (VPD) has increased. VPD is an important environmental factor affecting crops, especially their yields. However, the effects of various VPD conditions on water transport dynamics, anatomical structure, stomatal morphology, photosynthetic physiology, nutrient absorption, yield, and quality remain unclear. Many studies have shown that atmospheric transpiration is enhanced, water transport dynamics in the soil-plant-atmosphere continuum and water potential gradient are increased, and crop water potential is reduced under high VPD. Crops have undergone a series of changes that have enhanced their adaptation to high-VPD environments. Mesophyll thickness and conductance and stomatal size and conductance have decreased, and this has led to reductions in the photosynthetic rate and nutrient accumulation. High VPD seriously reduces the yield and water use efficiency of protected vegetables but improves fruit color and flavor quality. Reductions in VPD can improve water and nutrient transport in protected vegetables, alter the anatomical structure of crops, promote crop photosynthesis, and increase fruit yield, nutritional quality, and water use efficiency. Comprehensive analysis of the effect of VPD on the physiology and productivity of protected vegetables will provide insights that will aid the cultivation of protected vegetables with high quality and yield.
... Rising VPD affects tree transpiration in two ways: first, it comes with an increase in the evaporative demand and thus a higher force driving the water loss through stomata, and second, it induces stomatal closure through its effect on leaf water status (Cochard et al., 2002;Monteith, 1995). Therefore, transpiration responses to changes in VPD take a non-linear shape, with an initial increase until a certain threshold where the maximum is reached, after which stomatal closure may result in decreased transpiration (Monteith, 1995). ...
... Rising VPD affects tree transpiration in two ways: first, it comes with an increase in the evaporative demand and thus a higher force driving the water loss through stomata, and second, it induces stomatal closure through its effect on leaf water status (Cochard et al., 2002;Monteith, 1995). Therefore, transpiration responses to changes in VPD take a non-linear shape, with an initial increase until a certain threshold where the maximum is reached, after which stomatal closure may result in decreased transpiration (Monteith, 1995). Transpiratio n regulation in response to changes in VPD is highly species-specific due to differences in hydraulic architecture Grossiord et al., 2017;Hassler et al., 2018;Peters et al., 2019). ...
Temperature rise and more severe and frequent droughts will alter forest transpiration, thereby affecting the global water cycle. Yet, tree responses to increased atmospheric vapour pressure deficit (VPD) and reduced soil water content (SWC) are not fully understood due to long‐term tree adjustments to local environmental conditions that modify transpiration responses to short‐term VPD and SWC changes.
We analysed sap flux density (SFD) of Fagus sylvatica , Picea abies , Pinus sylvestris and Quercus ilex from 25 sites across Europe to understand how daily variation in SWC affects the sensitivity of SFD to VPD (β VPD ) and the maximum SFD (S 95 ). Furthermore, we tested whether long‐term adjustments to site climatic conditions and stand characteristics affect β VPD and S 95 .
The studied species showed contrasting β VPD and S 95 with the largest values in F. sylvatica , followed by Q. ilex , which surpassed the two conifers that showed low β VPD and low S 95 . We observed that β VPD and S 95 dropped during days of low SWC in F. sylvatica , P. sylvestris and Q. ilex , but not in P. abies . Both β VPD and S 95 were driven by tree height, and the temperature and precipitation at the sites. However, stand basal area was the most important driver of β VPD and S 95 , explaining 30% of their total variance.
Synthesis and applications : A future warmer and drier climate will restrict tree transpiration and thereby heavily affect the soil–plant‐atmosphere coupling. However, the effect of basal area, being the largest driver of tree transpiration sensitivity to vapour pressure deficit across a broad range of conditions, provides the opportunity to pre‐adapt European forests to future climate conditions. While stand thinning can increase the soil water availability for remaining trees, it also increases transpiration sensitivity to high air temperatures and may thereby amplify tree vulnerability to heat and drought.
... Canopy conductance determined from sap flux tends to show similar trends to those shown by leaf area-based conductance (Oren et al., 1999;Addington et al., 2004). It can be argued that the G si is autocorrelated with VPD because VPD is also used in the calculation of G si (Monteith, 1995). However, using an alternative method to calculate G si did not change the shape of the relationship between VPD and G si (Monteith, 1995;Oren et al., 1999). ...
... It can be argued that the G si is autocorrelated with VPD because VPD is also used in the calculation of G si (Monteith, 1995). However, using an alternative method to calculate G si did not change the shape of the relationship between VPD and G si (Monteith, 1995;Oren et al., 1999). In fact, b is the stomatal conductance at 1 kPa VPD which often occurs in field measurements. ...
Although the effects of nitrogen deposition on tree water relations are studied extensively, its impact on the relative sensitivities of stomatal and xylem hydraulic conductance to vapor pressure deficit and water potential is still poorly understood. This study investigated the effects of a 7-year N deposition treatment on the responses of leaf water relations and sensitivity of canopy stomatal conductance to vapor pressure deficit (VPD) and water potential, as well as the sensitivity of branch hydraulic conductance to water potential in a dominant tree species (Quercus wutaishanica) and an associated tree species (Acer mono) in a temperate forest. It was found that the N deposition increased stomatal sensitivity to VPD, decreased stomatal sensitivity to water potential, and increased the vulnerability of the hydraulic system to cavitation in both species. The standardized stomatal sensitivity to VPD, however, was not affected by the N deposition, indicating that the stomata maintained the ability to regulate the water balance under nitrogen deposition condition. Although the increased stomatal sensitivity to VPD could compensate the decreased stomatal sensitivity to water potential to some extent, the combined response would increase the percentage loss of hydraulic conductivity (PLC) when 50 % loss in stomatal conductance occurred, particularly in the dominant species Q. wutaishanica. The result indicates that N deposition would increase the risk of hydraulic failure in those species if the soil and/or air becomes drier under future climate change scenarios. The results of the study can have significant implications on the modelling of ecosystem vulnerability to drought under the scenario of atmospheric nitrogen deposition.
... This model has been found to reflect differences in drought stress conditions between plants, and the slope factor g 1 is inversely related to both WUE and carbon isotope composition during carbon assimilation . The attraction of aerosols to water vapor might affect modeling outputs, mainly because the HAS mechanism creates a parallel transpiration pathway of liquid water, while the model relies on equivalent pathways of water vapor and CO 2 (Aphalo and Jarvis, 1993;Monteith, 1995;Burkhardt, 2010). The original objective of this first study on aerosol-and HAS-caused effects under field conditions was the identification of physiological responses to aerosols on C. camphora in two field sites with different aerosol regimes, and their confirmation and explanation under greenhouse conditions with seedlings of the same species in filtered versus unfiltered air. ...
... The lower g 1 value of AA camphor leaves compared to FA can thus possibly be interpreted as aerosol induced drought stress. The reason for the involvement of H s for plant transpiration in the original, semi-empirical Ball-Berry model has remained elusive and its relevance was questioned, compared to VPD which seems to be physiologically more meaningful (e.g., Monteith, 1995). The successful H s use, however, might well be due to the direct interaction of hygroscopic, deposited aerosols with water vapor on the leaf surface. ...
Major parts of anthropogenic and natural aerosols are hygroscopic and deliquesce at high humidity, particularly when depositing to leaf surfaces close to transpiring stomata. Deliquescence and subsequent salt creep may establish thin, extraordinary pathways into the stomata, which foster stomatal uptake of nutrients and water but may also cause stomatal liquid water loss by wicking. Such additional water loss is not accompanied by a wider stomatal aperture with a larger CO2 influx and hypothetically reduces water use efficiency (WUE). Here, the possible direct impacts of aerosols on physical and physiological parameters of camphor (Cinnamomum camphora) were studied (i) in a greenhouse experiment using aerosol exclusion and (ii) in a field study in Taiwan, comparing trees at two sites with different aerosol regimes. Scanning electron microscopy (SEM) images showed that leaves grown under aerosol exclusion in filtered air (FA) were lacking the amorphous, flat areas that were abundant on leaves grown in ambient air (AA), suggesting salt crusts formed from deliquescent aerosols. Increasing vapor pressure deficit (VPD) resulted in half the Ball-Berry slope and double WUE for AA compared to FA leaves. This apparent contradiction to the wicking hypothesis may be due to the independent, overcompensating effect of stomatal closure in response to VPD, which affects AA more than FA stomata. Compared to leaves in a more polluted region in the Taiwanese Southwest, NaCl aerosols dominated the leaf surface conditions on mature camphor trees in Eastern Taiwan, while the considerably lower contact angles and the 2.5 times higher minimum epidermal conductances might have come from organic surfactants. Interpretations of SEM images from leaf surface microstructures should consider amorphous areas as possible indicators of aerosol deposition and other hygroscopic material. The amount and type of the material determine the resulting impacts on plant water relations, together with the surrounding atmosphere and ecophysiological traits.
... The transpiration rate is well known as a function of stomatal conductance [19], and the leaf temperature is also well known as a function of stomatal conductance [20]. Leaf temperatures measured with the infrared thermometry method are often used to estimate stomatal conductance [21][22][23][24][25]. Thermography, often used in conjunction with other image sensors and data mining techniques, is critical for enabling more automated, accurate, and sustainable agriculture. ...
Environmental control in greenhouse horticulture is essential for providing optimal conditions for plant growth and achieving greater productivity and quality. To develop appropriate environmental management practices for greenhouse horticulture through sensing technologies for monitoring the environmental stress responses of plants in real time, we evaluated the relative value of the stomatal opening to develop a technology that continuously monitors stomatal aperture to determine the moisture status of plants. When plants suffer from water stress, the stomatal conductance of leaves decreases, and transpiration and photosynthesis are suppressed. Therefore, monitoring stomatal behavior is important for controlling plant growth. In this study, a method for simply monitoring stomatal conductance was developed based on the heat balance method. The stomatal opening index (SOI) was derived from heat balance equations on intact tomato leaves, wet reference leaves, and dry reference leaves by measuring their temperatures in a growth chamber and a greenhouse. The SOI can be approximated as the ratio of the conductance of the intact leaf to the conductance of the wet reference leaf, which varies from 0 to 1. Leaf temperatures were measured with infrared thermometry. The theoretically and experimentally established SOI was verified with tomato plants grown hydroponically in a greenhouse. The SOI derived by this method was consistent with the leaf conductance measured via the porometer method, which is a standard method for evaluating actual leaf conductance that mainly consists of stomatal conductance. In conclusion, the SOI for the continuous monitoring of stomatal behavior will be useful not only for studies on interactions between plants and the environment but also for environmental management, such as watering at plant production sites.
... As plantas do morfotipo de folíolo pequeno tiveram maior acúmulo de massa seca total no ambiente de alto DPVar e alta temperatura (A3) incremento na temperatura do ar causa a abertura estomática (Kudoyarova et al., 2011;Sadras et al., 2012;Way et al., 2012, Maenpaa et al., 2011. Contudo, em valores elevados desta variável ambiental, a redução na condutância estomática ocorre em maior intensidade devido ao aumento no DPVar, (Monteith, 1995;Oren et al., 1999). À medida que as plantas são submetidas por longos períodos as condições climáticas com variação tanto de temperatura quanto de DPVar, pode-se haver influência de processos associados à aclimatação, e tais processos modificam as dinâmicas estomáticas, o que pode tornar complexa a compreensão das respostas fisiológicas das plantas às variações climáticas e ambientais (Marchin, 2016). ...
As condições climáticas em especial as variações de demanda atmosférica, condicionam as respostas fisiológicas e o crescimento de espécies florestais. Neste sentido, o presente estudo objetivou investigar a influência dos efeitos conjugados da variação do déficit de pressão de vapor (DPVar) e da temperatura do ar na ecofisiologia de dois morfotipos de Paubrasilia echinata (folíolos pequeno e médio). Buscou-se compreender os mecanismos de aclimatação fisiológica e mensurar o crescimento de ambos os morfotipos em distintas combinações de DPVar e temperatura. Para isso, foram realizados experimentos em casas de vegetação climatizadas, em delineamento inteiramente casualizado. Durante 125 dias, mudas dos dois morfotipos cresceram sob quatro condições atmosféricas de DPVar e temperatura: Baixo déficit de pressão de vapor e baixa temperatura (DPVar↓ + T↓), Baixo déficit de pressão de vapor com alta temperatura (DPVar↓ + T↑), Alto déficit de pressão de vapor e alta temperatura (DPVar↑ + T↑), Alto déficit de pressão de vapor com baixa temperatura (DPVar↑ + T↓). Foram avaliadas as características fisiológicas, relacionadas às trocas gasosas foliares, e o crescimento. Como resultado, as variações climáticas de DPVar e temperatura ocasionaram diferentes mecanismos de aclimatação fisiológica e crescimento nos morfotipos de Paubrasilia echinata. Sob alta demanda e alta temperatura o morfotipo de folíolo pequeno realizou um ajustamento estomático que favoreceu a homeostase das suas relações de carbono e obteve maior crescimento frente as demais condições climáticas, enquanto o morfotipo folíolo médio apresentou um menor ajuste estomático, aparentemente priorizando a manutenção do seu status hídrico e assim apresentou baixa eficiência no uso da água e menor crescimento. Já quando submetidos aos ambientes de baixa temperatura independentemente do DPVar, a estratégia fisiológica do morfotipo médio propiciou o seu maior crescimento. Os morfotipos pequeno e médio apresentam respostas fisiologias e estratégias de aclimatação contrastantes quando submetidos as variações de demanda atmosférica.
... (9) (see (Colaizzi et al., 2012b(Colaizzi et al., , 2014(Colaizzi et al., , 2016a, or light-use efficiency (LUE) parameterization that enables estimation of coupled transpiration and carbon assimilation fluxes (Anderson et al., 2008;Houborg et al., 2011;Schull et al., 2015). More recently, Kustas et al. (2022) examined an alternative minimum stomatal resistance formulation, dependent on a VPD formulation derived from the studies of Monteith (1995) and Leuning (1995), and applicable to partial canopy conditions following the Shuttleworth-Wallace sparse canopy model. Results from each of these studies suggest there is potential improvement in ET estimation, but that the most significant impact is a potential improvement in the partitioning between LE S and LE C (e.g., Colaizzi et al. 2016a;Knipper et al., 2023). ...
Thermal infrared (TIR) remote sensing of the land-surface temperature (LST) provides an invaluable diagnostic of surface fluxes and vegetation state, from plant and sub-field scales up to regional and global coverage. However, without proper consideration of the nuances of the remotely sensed LST signal, TIR imaging can give poor results for estimating sensible and latent heating. For example, sensor view angle, atmospheric impacts, and differential coupling of soil and canopy sub-pixel elements with the overlying atmosphere can affect the use of satellite-based LST retrievals in land-surface modeling systems. A concerted effort to address the value and perceived shortcomings of TIR-based modeling culminated in the Workshop on Thermal Remote Sensing of the Energy and Water Balance, held in La Londe les Maures, France in September of 1993. One of the outcomes of this workshop was the Two-Source Energy Balance (TSEB) model, which has fueled research and applications over a range of spatial scales.
In this paper we provide some historical context for the development of TSEB and TSEB-based multi-scale modeling systems (ALEXI/DisALEXI) aimed at providing physically based, diagnostic estimates of latent heating (evapotranspiration, or ET, in mass units) and other surface energy fluxes. Applications for TSEB-based ET retrievals are discussed: in drought monitoring and yield estimation, water and forest management, and data assimilation into – and assessment of – prognostic modeling systems. New research focuses on augmenting temporal sampling afforded in the thermal bands by integrating cloud-tolerant, microwave-based LST information, as well as evaluating the capabilities of TSEB for separating ET estimates into evaporation and transpiration components. While the TSEB has demonstrated promise in supplying water use and water stress information down to sub-field scales, improved operational capabilities may be best realized in conjunction with ensemble modeling systems such as OpenET, which can effectively combine strengths of multiple ET retrieval approaches.
... Q was the strongest determinant of T, which explained 34% of the average variance across all sites in our energy-limited ecosystems (Figure 10). High VPD typically causes plants to close their stomata to minimize water loss (Monteith, 1995), but high VPD conditions tend to co-occur with high radiation therefore making it difficult to separate their effects (Grossiord et al., 2020), hence our use of dominance analysis to separate the effects of covarying drivers. T tends increase as VPD increases to a certain threshold depending on the ecosystem (Ficklin & Novick, 2017;Franks et al., 1997;Marchin et al., 2016;Sulman et al., 2016;Will et al., 2013). ...
Climate change is intensifying the hydrologic cycle and altering ecosystem function, including water flux to the atmosphere through evapotranspiration (ET). ET is made up of evaporation (E) via non‐stomatal surfaces, and transpiration (T) through plant stomata which are impacted by global changes in different ways. E and T are difficult to measure independently at the ecosystem scale, especially across multiple sites that represent different land use and land management strategies. To address this gap in understanding, we applied flux variance similarity (FVS) to quantify how E and T differ across 13 different ecosystems measured using eddy covariance in a 10 × 10 km area from the CHEESEHEAD19 experiment in northern Wisconsin, USA. The study sites included eight forests with a large deciduous broadleaf component, three evergreen needleleaf forests, and two wetlands. Average T/ET for the study period averaged nearly 52% in forested sites and 45% in wetlands, with larger values after excluding periods following rain events when evaporation from canopy interception may be expected. A dominance analysis revealed that environmental variables explained on average 69% of the variance of half‐hourly T, which decreased from summer to autumn. Deciduous and evergreen forests showed similar E trajectories over time despite differences in vegetation phenology, and vapor pressure deficit explained some 13% of the variance E in wetlands but only 5% or less in forests. Retrieval of E and T within a dense network of flux towers lends confidence that FVS is a promising approach for comparing ecosystem hydrology across multiple sites to improve our process‐based understanding of ecosystem water fluxes.
... Atmospheric water stress, characterized by atmospheric VPD, is a major driver of transpiration (Monteith, 1995). In addition to soil water stress, atmospheric water stress plays a significant role in photosynthetic performance and stomatal regulation (Kimm et al., 2020;Zhang et al., 2021). ...
Water stress can severely decrease crop productivity by restricting photosynthesis, while the use of plastic film mulching can mitigate these water stress effects. However, the intricacies of photosynthetic and stomatal responses to soil water stress under plastic film mulching, particularly when combined with atmospheric water stress, have not been well studied, especially in arid irrigation areas. Limited research has investigated photo-synthetic chlorophyll fluorescence parameters, stomatal responses, grain filling process and crop productivity to soil and atmospheric water stress under plastic film mulching. Our study addresses this knowledge gap through a comprehensive field experiment in an arid irrigation area involving maize (Zea mays L.). Well-watered and water deficit conditions with and without plastic film mulching treatments, alone or combined with atmospheric water stress (different vapor pressure deficits) were conducted. Our findings revealed that soil water stress significantly increased stomatal limitations (by 6.4-12.4 %) and may cause non-stomatal limitations. Plastic film mulching significantly improved plant photosynthetic performance (increased net photosynthesis rate by 12.2-39.8 %), chlorophyll fluorescence parameters, and stomatal regulation. Under mulched conditions, soil water stress primarily affected photosynthetic performance through stomatal limitations. Moreover, plastic film mulching significantly improved grain filling process (increased grain-filling rate by 6.3-78.5 %) and productivity (increased grain yield by 12.1-45.8 %) in spring maize subjected to soil water stress. Atmospheric water stress, alone or combined with soil water stress, influenced plant photosynthetic performance, decreasing the net photosynthesis rate and stomatal conductance. Mulching enhanced photosynthetic performance under atmospheric water stress. Overall, the positive effect of mulching on spring maize photosynthetic performance and productivity under soil and atmospheric water stresses holds promise for alleviating water resource shortages and addressing global climate warming issues in arid irrigation areas.
... Interestingly, the impact mechanisms of temperature and VPD on crop growth and development are different. In detail, temperature influences plants primarily through the temperature dependence of biochemical and developmental processes, such as photosynthesis and respiration [51], whereas a VPD influences plants mainly by increasing atmospheric water demand and plant water loss [77]. Because of the intimate connection between temperature and VPD, the effect of VPD on crop growth and development is often neglected or attributed to temperature. ...
As a key indicator of agricultural production capacity, crop production potential is critical to evaluate the impacts of climate variability on agriculture. However, less attention has been paid to the pattern of biomass accumulation in response to crop climatic production potential and its relation to grain yield formation at an intra-seasonal time scale, especially under evolving soil water limitation. In this study, we combined a mechanism-based empirical model with field experiments conducted at different growth stages of maize on the Northern China Plain (NCP) to assess the dynamic response of biomass accumulation to climatic production potential and its relation to grain yield. The results showed that the ability of climatic production potential to estimate biomass was significantly improved when a vapor pressure deficit (VPD) was incorporated, with the root mean square error (RMSE) reduced by 33.3~41.7% and 45.6~47.2% under rainfed and evolving soil water limitation conditions, respectively. Drought significantly decreased biomass accumulation mainly by decreasing the intrinsic growth rate and delaying the occurrence timing of maximum growth. Moreover, grain yield shared a nonlinear and saturating relationship with biomass across rainfed and water deficit conditions. The results imply that evolving soil water limitation changes the process of biomass accumulation but not its relationship with grain yield. These findings provide useful information to estimate crop production potential under abiotic stresses and improve the accuracy of crop yield prediction.
... For a given stomatal opening, transpiration would increase linearly with VPD leaf , without any gain in carbon uptake. Stomatal conductance (g s ) however decreases with increasing VPD leaf , avoiding excessive water loss, but restricting carbon uptake (Dai et al., 1992;Monteith, 1995;Oren et al., 1999). In angiosperms the reduction of g s in response to an increase in VPD leaf is believed to be abscisic acid (ABA) mediated (Xie et al., 2006;Bauer et al., 2013;McAdam and Brodribb, 2015). ...
The rise in global temperature is not only affecting plant functioning directly, but is also increasing air vapour pressure deficit (VPD). The yield of banana is heavily affected by water deficit but so far breeding programs have never addressed the issue of water deficit caused by high VPD. A reduction in transpiration at high VPD has been suggested as a key drought tolerance breeding trait to avoid excessive water loss, hydraulic failure and to increase water use efficiency. In this study, stomatal and transpiration responses under increasing VPD at the leaf and whole-plant level of 8 wild banana (sub)species were evaluated, displaying significant differences in stomatal reactivity. Three different phenotypic groups were identified under increasing VPD. While (sub)species of group III maintained high transpiration rates under increasing VPD, M. acuminata ssp. e rrans (group I), M. acuminata ssp. zebrina (group II) and M. balbisiana (group II) showed the highest transpiration rate limitations to increasing VPD. In contrast to group I, group II only showed strong reductions at high VPD levels, limiting the cost of reduced photosynthesis and strongly increasing their water use efficiency. M. acuminata ssp. zebrina and M. balbisiana thus show the most favourable responses. This study provides a basis for the identification of potential parent material in gene banks for breeding future-proof bananas that cope better with lack of water.
... When VPD is low and the stomata are fully open, the leaf guard cells sense the increased rate of transpiration through the stomatal pores and induce stomatal opening (35). As a result of this "feedback" response (36), with an increase of VPD, the transpiration rate increases and the nutrient (such as nitrogen, phosphorus, and potassium) uptake from the soil is promoted (37)(38)(39)(40), which is beneficial for vegetation growth. Moreover, some field experiments performed in northern Europe where the positive effect of VPD on vegetation productivity was detected (Fig. 1A) revealed adverse effects of an increase in atmospheric humidity (or decrease in VPD) on photosynthetic capacity and growth rate in vegetation. ...
The impact of atmospheric vapor pressure deficit (VPD) on plant photosynthesis has long been acknowledged, but large interactions with air temperature (T) and soil moisture (SM) still hinder a complete understanding of the influence of VPD on vegetation production across various climate zones. Here, we found a diverging response of productivity to VPD in the Northern Hemisphere by excluding interactive effects of VPD with T and SM. The interactions between VPD and T/SM not only offset the potential positive impact of warming on vegetation productivity but also amplifies the negative effect of soil drying. Notably, for high-latitude ecosystems, there occurs a pronounced shift in vegetation productivity's response to VPD during the growing season when VPD surpasses a threshold of 3.5 to 4.0 hectopascals. These results yield previously unknown insights into the role of VPD in terrestrial ecosystems and enhance our comprehension of the terrestrial carbon cycle's response to global warming.
... Isohydric behavior involves maintaining constant leaf water potential, even under water deficit conditions, as in water abundance, by limiting transpiration. Similarly, during severe drought, when the water vapor gradient between the substomatal cavity and the atmosphere above the leaf is high, transpiration is suppressed [5,6] to prevent water loss from the plant. Under such conditions, xylem cavitation can also drive stomatal closure [7]. ...
... However, a decrease in SWC leads to a decrease in the available water taken in by plant roots [3]. Insufficient soil moisture can have negative impacts on vegetation productivity, limiting photosynthesis and biomass accumulation, leading to decreased GPP and NPP [4,5], while an increase in VPD will induce plants to close stomata to minimize leafscale water loss and further inhibit plant photosynthesis in the ecosystem [6,7], and lower photosynthetic efficiency usually indicates reduced utilization of light energy for photosynthesis and lower production of organic matter. This is because plants need to adjust stomatal conductance to maximize carbon gains and reduce water loss as much as possible under the condition of high VPD. ...
The soil water supply and atmospheric humidity conditions are crucial in controlling plants’ stomatal behavior and water use efficiency. When there is water stress caused by an increase in saturated water vapor pressure (VPD) and a decrease in soil water content (SWC), plants tend to close stomata to reduce water loss. This affects the gross primary productivity (GPP) and evapotranspiration (ET), subsequently leading to changes in water use efficiency (WUE) and carbon use efficiency (CUE) in plants. However, land–atmosphere interactions mean that water vapor in the atmosphere and soil moisture content causing water stress for plants are closely related. This study aims to compare and estimate the effects of VPD and SWC on the carbon cycle and water cycle for different plant functional types. Based on the fluxnet2015 dataset from around the world, the WUE and CUE of five plant functional types (PFTs) were estimated under varying levels of VPD and SWC. The results showed that high VPD and low SWC limit the stomatal conductance (Gs) and gross primary productivity (GPP) of plants. However, certain types of vegetation (crops, broad-leaved forests) could partially offset the negative effects of high VPD with higher SWC. Notably, higher SWC could even alleviate limitations and partially promote the increase in GPP and net primary production (NPP) with increasing VPD. WUE and CUE were directly affected by Gs and productivity. In general, the increase in VPD in the five PFTs was the dominant factor in changing WUE and CUE. The impact of SWC limitations on CUE was minimal, with an overall impact of only −0.05μmol/μmol on the four PFTs. However, the CUE of savanna plants changed differently from the other four PFTs. The rise in VPD dominated the changes in CUE, and there was an upward trend as SWC declined, indicating that the increase in VPD and decrease in SWC promote the increase in the CUE of savanna plants to some extent.
... 2. An indirect response of stomata to changing VPD through changes in leaf water potential caused by changes in transpiration is known as a 'feedback mechanism' (Mott and Parkhurst 1991;Monteith 1995;Matzner and Comstock 2001). ...
... 2. An indirect response of stomata to changing VPD through changes in leaf water potential caused by changes in transpiration is known as a 'feedback mechanism' (Mott and Parkhurst 1991;Monteith 1995;Matzner and Comstock 2001). ...
Global warming and consequent changes in climate will put intense pressure on crop production, undermining global food security. Adaptation strategies are needed to minimise the adverse impact of high temperature on crops, and physiological trait-based breeding is considered as a promising strategy in this regard. Identifying and assessing physiological traits associated with improved crop performance under warmer climatic conditions will assist physiological breeding programmes. In this context, it is vital to assess the role of transpiration in ameliorating leaf temperature as a potential heat avoidance strategy as the literature is limited and controversial. The complex relationship between transpiration, the physical environment, and plant anatomical and morpho-physiological attributes could explain the contrasting views on the role of transpiration in ameliorating leaf temperature. A comprehensive examination of transpiration and leaf cooling in relation to its controlling factors will assist in unravelling this complex relationship. This chapter discusses the different heat-dissipating mechanisms, the contribution of transpirational cooling as a heat avoidance strategy evidenced by previous research findings on different crops, the complex relationship of transpiration and leaf temperature with morpho-physiological attributes and environmental parameters, and the application of transpirational cooling in physiological breeding.KeywordsHeat avoidanceTranspirationCanopy temperatureMorpho-physiological attributesPhysiological breeding
... The transpiration rate is influenced by atmospheric conditions and, over a short time-scale, is regulated by the function of stomatal apparatus [32]. It has been argued that a declining stomatal conductance (g s ) concomitantly with increasing VPD would rather occur as a feedback response to E and water loss from the leaf, than as a direct response to humidity [33][34][35]. ...
... With increasing VPD, leaf stomata close gradually (Jarvis & McNaughton 1986;Monteith 1995). ...
Temperature (T) and vapor pressure deficit (VPD) are important drivers of plant hydraulic conductivity, growth, mortality, and ecosystem productivity, independently of soil water availability. Our goal was to disentangle the effects of T and VPD on plant hydraulic responses. Young trees of Fagus sylvatica L., Quercus pubescens Willd. and Quercus ilex L. were exposed to a cross-combination of a T and VPD manipulation under unlimited soil water availability. Stem hydraulic conductivity and leaf-level hydraulic traits (e.g., gas exchange and osmotic adjustment) were tracked over a full growing season. Significant loss of xylem conductive area (PLA) was found in F. sylvatica and Q. pubescens due to rising VPD and T, but not in Q. ilex. Increasing T aggravated the effects of high VPD in F. sylvatica only. PLA was driven by maximum hydraulic conductivity and minimum leaf conductance, suggesting that high transpiration and water loss after stomatal closure contributed to plant hydraulic stress. This study shows for the first time that rising VPD and T lead to losses of stem conductivity even when soil water is not limiting, highlighting their rising importance in plant mortality mechanisms in the future.
... Through the land- atmosphere interactions, higher VPD is mostly determined by lower SWC and higher mean Ta during droughts (Chen et al., 2021;Kimm et al., 2020). Under high VPD conditions, plants tend to close their stomata to minimize water loss at the leaf scale, thus restricting photosynthetic activities (Monteith, 1995). Although VPD has a weaker impact on GPP than SWC during droughts on the TP (Chen et al., 2021;Zhang et al., 2018) and global scale (Liu et al., 2020), higher VPD also plays a vital role in restraining GPP and LAI during droughts (Chen et al., 2021;Kimm et al., 2020;Xu et al., 2021). ...
Under global climate change, climate warming and changing precipitation patterns would greatly affect plant phenology and photosynthetic capacity and hence affect the gross primary productivity (GPP) of the sensitive alpine meadow. To deeply understand the variations in GPP, it is important to clarify the joint effects of plant phenology and photosynthetic capacity. In this study, 10-year continuous flux and microclimatic data and in situ phenology period observation data were used to explore the roles of growing season length (GSL) and the seasonal maximal capacity of carbon uptake (GPPmax) on annual cumulative gross primary productivity (GPPann). The results indicated that the temperature and water conditions, especially water conditions, dominated GSL and GPPmax and hence determined GPPann. Both GSL and GPPmax were dominated by annual precipitation, and the soil water content in August also showed a strong influence over them. GSL and GPPmax together could explain 87% of the variation in GPPann. Compared with GSL, GPPmax had stronger effects on the interannual variability (IAV) in GPPann. This study provides a new perspective on clarifying the proximate causes of IAV in GPPann. The IAV in GPPann of alpine ecosystems could be determined by these two key underlying processes, the length of the carbon uptake period and amplitude. These results are of critical importance for improving our ability to predict future changes in alpine meadow ecosystems.
... For a given stomatal opening, transpiration 73 would increase linearly with VPDleaf, without any gain in carbon uptake. Stomatal conductance (gs) 74 however decreases with increasing VPDleaf, avoiding excessive water loss, but restricting carbon 75 uptake (Dai, Edwards and Ku, 1992;Monteith, 1995;Oren et al., 1999). In angiosperms the reduction 76 of gs in response to an increase in VPDleaf is believed to be abscisic acid (ABA) mediated (Xie et al., 77 2006;Bauer et al., 2013;McAdam and Brodribb, 2015). ...
The predicted rise in global temperature is not only affecting plant functioning directly, but is also increasing air vapour pressure deficit (VPD). The yield of banana is heavily affected by water deficit but so far breeding programs have never addressed the issue. A reduction in transpiration at high VPD has been suggested as a key drought tolerance breeding trait to avoid excessive water loss, hydraulic failure and to increase water use efficiency. In this study, stomatal and transpiration responses under increasing VPD at the leaf and whole-plant level of 8 wild banana (sub)species were evaluated, displaying significant differences in stomatal reactivity. Three different groups were identified under increasing VPD. M. acuminata spp. e rrans (group I), M. acuminata spp. zebrina (group II) and M. balbisiana (group II) showed the highest transpiration rate limitations to increasing VPD. In contrast to group I, group II only showed strong reductions at high VPD levels, limiting the cost of reduced photosynthesis and strongly increasing their water use efficiency. Group II genotypes thus show favourable responses for high water use efficiency in regions with high VPDs. This provides a basis for the identification of potential parent material within their wild populations for drought tolerance breeding.
Highlight
Wild banana species respond significantly different to water deficit caused by VPD increases and differ in the rate of stomatal reduction, revealing opportunities for drought tolerance breeding.
... For instance, favorable temperatures and strong radiation generally accelerate photosynthesis (Choudhury, 1987;Yamori et al., 2014), which leads to stomatal opening and greater transpiration. Furthermore, air humidity affects stomatal aperture and transpiration rates (Monteith, 1995;Morison & Gifford, 1983). Thus, the activity of canopy gas exchange affects the canopy surface temperature and CTD, and it can be assumed that CTD can be estimated from micrometeorological data. ...
Canopy photosynthesis is an important component of biomass production in field-grown rice (Oryza sativa L.). Although canopy temperature differences (CTD) provide important information for evaluating canopy photosynthesis, the measurement of CTD is still a labor-intensive task. Therefore, we designed this study to establish a model for predicting CTD under different field conditions using meteorological data and evaluated the environmental response of CTD using the established model. Our study collected 2,056,264 CTD data points from two rice cultivars having different photosynthetic capacities, ‘Koshihikari’ and ‘Takanari’, and then used these data to create a novel model using a neural network (NN). The input variables were limited to meteorological data, and the output variable was set to CTD. The established NN model produced a prediction accuracy of R² = 0.792 and RMSE = 0.605°C. We then used this NN model to simulate the CTD response of the Koshihikari and Takanari cultivars in response to various environmental changes. These predictions revealed that Takanari had a lower CTD than Koshihikari when exposed to high relative humidity (RH) or low to moderate solar radiation (Rs). In contrast, the CTD of Koshihikari tended to be lower than that of Takanari under lower RH or higher Rs. This result implies that the advantages of the single-leaf gas exchange system in Takanari can be mitigated under extremely high-VPD conditions. Thus, our new method may provide a powerful tool to gain a better understanding of gas exchange, growth processes, and varietal differences in rice cultivated under field conditions.
... Kustas et al. (2022) showed the advantages of accounting for the sensitivity of stomatal closure at higher VPD in canopies highly coupled with the atmosphere (Jarvis and McNaughton 1986). Based on the method proposed by Monteith (1995), Kustas et al. (2022) derived the stomatal parameters for the Leuning (1995) stomatal conductance model of Eq. 2: with g m = 0.58 mol m −2 s −1 and D 0 = 15.85 mb. ...
Precision irrigation management requires operational monitoring of crop water status. However, there is still some controversy on how to account for crop water stress. To address this question, several physiological, several physiological metrics have been proposed, such as the leaf/stem water potentials, stomatal conductance, or sap flow. On the other hand, thermal remote sensing has been shown to be a promising tool for efficiently evaluating crop stress at adequate spatial and temporal scales, via the Crop Water Stress Index (CWSI), one of the most common indices used for assessing plant stress. CWSI relates the actual crop evapotranspiration ET (related to the canopy radiometric temperature) to the potential ET (or minimum crop temperature). However, remotely sensed surface temperature from satellite sensors includes a mixture of plant canopy and soil/substrate temperatures, while what is required for accurate crop stress detection is more related to canopy metrics, such as transpiration, as the latter one avoids the influence of soil/substrate in determining crop water status or stress. The Two-Source Energy Balance (TSEB) model is one of the most widely used and robust evapotranspiration model for remote sensing. It has the capability of partitioning ET into the crop transpiration and soil evaporation components, which is required for accurate crop water stress estimates. This study aims at evaluating different TSEB metrics related to its retrievals of actual ET, transpiration and stomatal conductance, to track crop water stress in a vineyard in California, part of the GRAPEX experiment. Four eddy covariance towers were deployed in a Variable Rate Irrigation system in a Merlot vineyard that was subject to different stress periods. In addition, root-zone soil moisture, stomatal conductance and leaf/stem water potential were collected as proxy for in situ crop water stress. Results showed that the most robust variable for tracking water stress was the TSEB derived leaf stomatal conductance, with the strongest correlation with both the measured root-zone soil moisture and stomatal conductance gas exchange measurements. In addition, these metrics showed a better ability in tracking stress when the observations are taken early after noon.
... For instance, sensible and latent heat fluxes from vegetation affect the dynamics and thermodynamics of the atmospheric boundary layer and, at the same time, vegetation responds to changes in air temperature and humidity (Monteith and Unsworth, 2013). Vegetation closes its stomata in absence of light or water in the soil so that both radiation and soil moisture are variables directly related to transpiration (Monteith, 1995). ...
We present an extension of the stochastic ecohydrological model for soil moisture dynamics at a point of RodríguezIturbe et al. (1999) and Laio et al. (2001). In the original model, evapotranspiration is a function of soil moisture and vegetation parameters, which makes the model suitable for water–limited environments. Based on the Leuning’s stomatal conductance approach, the C3 photosynthesis model of Farquhar et al. and the Penman–Monteith equation, we model daily transpiration as a negative exponential function of available photosynthetically active radiation. This function allowed us to broaden the Rodríguez-Iturbe et al. (1999) and Laio et al. (2001) model to encompass both water– and energy–limited ecosystems by introducing the dependence of maximum evapotranspiration on available photosynthetically active radiation. We illustrate the extended model with two study cases from the FLUXNET database, DE–Hai in Germany and GF–Guy in French Guiana, and analyze the sensibility of soil moisture dynamics and the long-term water balance to available radiation. Our results show that the analytical solution presented by Rodríguez-Iturbe et al. (1999) continues to be valid as the maximum evapotranspiration rate is calculated in terms of available energy and assuming stationary in the radiation regime.
... Through the land- atmosphere interactions, higher VPD is mostly determined by lower SWC and higher mean Ta during droughts (Chen et al., 2021;Kimm et al., 2020). Under high VPD conditions, plants tend to close their stomata to minimize water loss at the leaf scale, thus restricting photosynthetic activities (Monteith, 1995). Although VPD has a weaker impact on GPP than SWC during droughts on the TP (Chen et al., 2021;Zhang et al., 2018) and global scale (Liu et al., 2020), higher VPD also plays a vital role in restraining GPP and LAI during droughts (Chen et al., 2021;Kimm et al., 2020;Xu et al., 2021). ...
The knock-on effects between earlier vegetation activities and summer droughts may have important consequences for broad ecological processes. To date, little is known about how the chained effects drive the carbon and water cycles on the Tibetan Plateau (TP). Using the naturally occurring above-mentioned sequential events in spring and summer in 2015 and 2017, we applied the observations at the site, landscape, and regional scales to evaluate the chained effects on the TP. Our findings indicated that higher spring vegetation productivity is caused by early vegetation activities, partially compensated for summer drought-induced loss. Concurrently, increased spring evapotranspiration induced by earlier spring may drain soil water resources earlier, exacerbating summer water restrictions caused mainly by sparse precipitation. This lagged effect of early spring, accompanied by summer drought, significantly increased summer sensible heat flux by 23.2%. Remarkably, the mean air temperature (Ta) was lower than the baseline during drought. This decrease was contributed mainly by lower nighttime Ta, indicating that the region-specific characteristics of the TP could offset the heating effects as mentioned above. The characteristics of high altitude, low air pressure, and thin air could strongly weaken the cloud insulations. More substantial decreases in cloud amount during drought further decreased atmospheric counter radiations, leading to lower mean/nighttime Ta. The simulation results showed that lower mean Ta alleviated the decreases in gross primary productivity by 4.3% through reducing vapor pressure deficit by 5.1%. In conclusion, the present study highlighted the need to comprehensively consider the buffering effects of lower temperature during summer drought to precisely assess the chained effects on the TP.
... where a and b are two fitted parameters. In general, a ranged from 1.1 to 1.4 regardless of surface wetness (Monteith, 2010). ...
Evapotranspiration (ET) is an important component in water budgets in forest ecosystems and the associated latent heat flux has important implications for regional climate. However, biophysical control mechanisms on variations in ecosystem ET on intra‐annual scales may have year‐to‐year variations in different years with contrasting precipitation and soil water contents. Related research is lacking in natural oak forests in Central China. Based on eddy‐covariance technique, we investigated variations of ET and controlling factors in a natural oak forest in central China over three years (2017‐2019) with contrasting soil water contents (SWC) and precipitation. The ET of the oak forest was mainly affected by surface conductance (gs) in growing seasons, and the restriction from stoma on ET was aggravated in dry year. High vapor pressure deficit (VPD) promoted gs and ET in wet year, but decreased gs and ET in dry year. There was a positive relationship between shallow soil water contents and gs in dry year, but no relationship between the two variables in wet year. The ET were 738.1, 750.6 and 513.1 mm, respectively for 2017‐2019, with a coefficient of variation (CV) of 20%, while the corresponding precipitation (P) were 1239.4, 855.8 and 645.6 mm, with a CV of 33%. Annual ET was the lowest in the year with the lowest P, SWC and gs. Short‐term droughts (periods with relative extractable water content less than 0.1 for less than 15 days) only constrained gs and ET in short timescales (e.g., hours). Long‐term droughts (periods with relative extractable water content less than 0.1 for more than 15 days) evidently decreased the gs, thus reducing the ET and affecting energy partitioning in the corresponding period. ET/P were 0.6, 0.88 and 0.79 in 2017‐2019, respectively, indicating that ET consumed most of the rain. The ratios of latent heat flux (LE) to net radiation (Rn) were 0.86, 0.88 and 0.6 during growing seasons in 2017‐2019, respectively. Our study suggested that the natural oak forest has high ET rate and soil moisture affects the variations of ET.
... The transpiration rate is influenced by atmospheric conditions and, over a short time-scale, is regulated by the function of stomatal apparatus [32]. It has been argued that a declining stomatal conductance (g s ) concomitantly with increasing VPD would rather occur as a feedback response to E and water loss from the leaf, than as a direct response to humidity [33][34][35]. ...
In this study, leaf hydraulic functionality of co-occurring evergreen and deciduous shrubs, grown on Olympus Mountain, has been compared. Four evergreen species (Arbutus andrachne, Arbutus unedo, Quercus ilex and Quercus coccifera) and four deciduous species (Carpinus betulus, Cercis siliquastrum, Coronilla emeroides and Pistacia terebinthus) were selected for this study. Predawn and midday leaf water potential, transpiration, stomatal conductance, leaf temperature and leaf hydraulic conductance were estimated during the summer period. The results demonstrate different hydraulic tactics between the deciduous and evergreen shrubs. Higher hydraulic conductance and lower stomatal conductance were obtained in deciduous plants compared to the evergreens. Additionally, positive correlations were detected between water potential and transpiration in the deciduous shrubs. The seasonal leaf hydraulic conductance declined in both deciduous and evergreens under conditions of elevated vapor pressure deficit during the summer; however, at midday, leaf water potential reached comparable low values, but the deciduous shrubs exhibited higher hydraulic conductance compared to the evergreens. It seems likely that hydraulic traits of the coexisting evergreen and deciduous plants indicate water spending and saving tactics, respectively; this may also represent a limit to drought tolerance of these species grown in a natural environment, which is expected to be affected by global warming.
... It was considered a feedforward signal to avoid increased transpiration rates in dry conditions, and with it the rate of photosynthesis lowers due to reduced intake of CO2 from the air. Contrastingly, it has been argued that the change in stomatal conductance might rather be a negative feedback in response to higher transpiration rates that optimizes photosynthesis accordingly (Friend, 1991;Monteith, 1995). More so, as shown by Tardieu and Simonneau (1998), in many cases the stomatal openings of well-watered plants do not experience any change over a range of leaf-to-air vapour pressure deficits (VPD) 16 . ...
Current farming practices in Flanders, Belgium use large amounts of inorganic fertilizers to attain high yield and quality. Especially in open field vegetable production the amount of applied nitrogen fertilizer exceeds the crop demand all too often. This practice in addition to the mineralization of soil organic matter and the use of organic fertilizer results in nitrate concentrations in ground and surface water that are frequently above the thresholds set by the European Union. In order to understand the impact of these legislative norms for farmers and in search of solutions for the growers, this work proposes a crop-soil-climate interaction model that enables studying the impact and interaction of weather variability and different nitrogen fertilization schemes on yield and environment for open field cauliflower and leek production systems.
The different fertilizer scenario simulations gave a clear indication of production limits and the model allowed the estimation of plausible production outcomes under variable weather. These outcomes were used to generate on-the-go information for pre-season decision support and in-season managerial recommendations, and included a real-time estimation of the likeliness of not complying with the environmental threshold under present weather conditions. Finally, the presented model allowed determining optimal fertilizer strategies and defining best-bet solutions to the growers depending on the production priorities.
... Moderate thermal stress inhibits net photosynthesis and stomatal conductance in many plants and thus reduces the ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO) activation (Crafts-Brander & Salvucci 2002;Morales et al., 2003). Monteith (1995) also found that stomata had closed irregularly in all studied species as a result of higher respiration rates to reduce the loss of water. In several studies, various genotypes for heat tolerance were identified by evaluating cotton and wheat germplasm based on stomatal conductance and photosynthesis (Lu et al., 1998;Cornish et al., 1991;Ulloa et al., 2000;Rahman, 2005). ...
Nowadays, the climate is changing globally. High temperature affects cellular membrane stability, chlorophyll fluorescence, photosynthesis, and antioxidant enzymes activities in plants. Cotton is susceptible to high temperature during the reproductive growth stage, particularly during the flowering and boll formation periods. Reduced yields associated with heat stress are common in upland cotton areas and have been partially attributed to reproductive dysplasia, including increased fruit shedding, reduced pollen viability, fertilization efficiency, and boll size and seed number. Hence, it is imperative to explore the cotton germplasm to identify heat-tolerant genotypes. The rapid developments in modern biotechnology, including molecular marker-assisted selection and next-generation sequencing technologies, have facilitated cotton breeding. This chapter begins with an introduction to the cause of heat stress in cotton and the underlying theory of heat tolerance in cotton. Then, breeding methods based on marker-assisted selection and even genomic selection, together with how to explore elite alleles of heat tolerance from wild species or mutated materials are summarized. Finally, new prospects of cotton breeding based on the above-mentioned methods are presented and discussed.
... Some potential improvements of the SW-S2 model include modifications to substrate and canopy resistances parametrizations for vineyards, better treatment of canopy height and surface roughness, and improvement in the automated derivation of wet/dry lines in W calculation. For instance, the maximum and minimum stomatal resistances (conductances) are considered to be highly sensitive to VPD as well as other short-term environmental factors including solar irradiance and temperature (Collatz et al. 1991;Jarvis 1976;Monteith 1995;Noilhan and Planton 1989). Hence, the thresholds used for r leaf (100-400 s m −1 ) may not be well representative of actual conditions and future studies could use more adaptive thresholds based on actual VPD and local conditions. ...
Sustainable use of available water resources in viticulture can be aided by frequent high-resolution information on vineyard water status. Recently, a new Shuttleworth–Wallace evapotranspiration (ET) model, which uses a contextual framework to determine dry and wet extremes from the Sentinel-2 surface reflectance data (SW-S2), showed promising results when tested over a GRAPEX (Grape Remote-sensing Atmospheric Profile and ET eXperiment) site in California. However, current knowledge on its applicability across the climate gradient in California and how the selections of modeling domain and meteorological data influence model outputs are limited. This study expands the evaluation of the SW-S2 model across multiple domains and meteorological inputs covering all three GRAPEX sites over the 2018–2020 growing seasons. In comparison with flux tower observations, the size of the modeling domain did not have a strong influence on model performance, although the model performed marginally better under a larger domain (yielding root mean square error within 1.03–1.11 mm d−1 and mean biases within 2%). The source and quality of meteorological forcing data, in particular vapor pressure deficit (VPD) and wind speed (u), were found to have a strong influence on model output as indicated by the poor performance of the model with less accurate regional and coarse-scale gridded meteorological inputs. Results suggest that simple regression for local bias correction of VPD and u significantly improved model performance. Overall, this study supports future research aiming to merge outputs from more frequent spectral and less frequent thermal-based ET models and reduce latency in ET monitoring of California vineyards.
... Hence, the reported changes in r s are solely due to stress factors driven by air temperature and vapor pressure deficit. Monteith (1995) described that ET increases as D increases up to an optimal D, after which it stabilizes and eventually decreases in very dry air because of the patchy closure of stomata. One interpretation of our results is that, on average, the summertime vapor pressure deficit in California is at or close to the optimal value and any further increase would result in minimal ET reaction due to regulation of stomata resistance. ...
Climate modeling studies and observations do not fully agree on the implications of anthropogenic warming for evapotranspiration (ET), a major component of the water cycle and driver of irrigation water demand. Here, we use California as a testbed to assess the ET impacts of changing atmospheric conditions induced by climate change on irrigated systems. Our analysis of irrigated agricultural and urban regions shows that warmer atmospheric temperatures have minimal implications for ET rates and irrigation water demands—about one percent change per degree Celsius warming (∼1% °C⁻¹). By explicitly modeling irrigation, we control for the confounding effect of climate‐driven soil moisture changes and directly estimate water demand implications. Our attribution analysis of the drivers of ET response to global anthropogenic warming shows that as the atmospheric temperature and vapor pressure deficit depart from the ideal conditions for transpiration, regulation of stomata resistance by stressed vegetation almost completely offsets the expected increase in ET rates that would otherwise result from abiotic processes alone. We further show that anthropogenic warming of the atmosphere has minimal implications for mean relative humidity (<1.7% °C⁻¹) and the surface available energy (<0.2% °C⁻¹), which are critical drivers of ET. This study corroborates the growing evidence that plant physiological changes moderate the degree to which changes in potential ET are realized as actual ET.
... Moderate thermal stress inhibits net photosynthesis and stomatal conductance in many plants and thus reduces the ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO) activation (Crafts-Brander & Salvucci 2002;Morales et al., 2003). Monteith (1995) also found that stomata had closed irregularly in all studied species as a result of higher respiration rates to reduce the loss of water. In several studies, various genotypes for heat tolerance were identified by evaluating cotton and wheat germplasm based on stomatal conductance and photosynthesis (Lu et al., 1998;Cornish et al., 1991;Ulloa et al., 2000;Rahman, 2005). ...
Nowadays, the climate is changing globally. High temperature affects cellular membrane stability, chlorophyll fluorescence, photosynthesis, and antioxidant enzymes activities in plants. Cotton is susceptible to high temperature during the reproductive growth stage, particularly during the flowering and boll formation periods. Reduced yields associated with heat stress are common in upland cotton areas and have been partially attributed to reproductive dysplasia, including increased fruit shedding, reduced pollen viability, fertilization efficiency, and boll size and seed number. Hence, it is imperative to explore the cotton germplasm to identify heat-tolerant genotypes. The rapid developments in modern biotechnology, including molecular marker-assisted selection and next-generation sequencing technologies, have facilitated cotton breeding. This chapter begins with an introduction to the cause of heat stress in cotton and the underlying theory of heat tolerance in cotton. Then, breeding methods based on marker-assisted selection and even genomic selection, together with how to explore elite alleles of heat tolerance from wild species or mutated materials are summarized. Finally, new prospects of cotton breeding based on the above-mentioned methods are presented and discussed.
... The complex array of aboveground and belowground tissue interactions is strongly related to atmospheric diurnal variations. One of the main driving forces of transpiration rate is the VPD (Monteith, 1995;Lobell et al., 2014). In response to water stress, high VPD, or their combination, the plant will increase or limit its water use. ...
Drought intensity as experienced by plants depends upon soil moisture status and atmospheric variables such as temperature, radiation, and air vapour pressure deficit. Although the role of shoot architecture with these edaphic and atmospheric factors is well characterized, the extent to which shoot and root dynamic interactions as a continuum are controlled by genotypic variation is less well known. Here, we targeted these interactions using a wild emmer wheat introgression line (IL20) with a distinct drought-induced shift in the shoot-to-root ratio and its drought-sensitive recurrent parent Svevo. Using a gravimetric platform, we show that IL20 maintained higher root water influx and gas exchange under drought stress, which supported a greater growth. Interestingly, the advantage of IL20 in root water influx and transpiration was expressed earlier during the daily diurnal cycle under lower vapour pressure deficit and therefore supported higher transpiration efficiency. Application of a structural equation model indicates that under drought, vapour pressure deficit and radiation are antagonistic to transpiration rate, whereas the root water influx operates as a feedback for the higher atmospheric responsiveness of leaves. Collectively, our results suggest that a drought-induced shift in root-to-shoot ratio can improve plant water uptake potential in a short preferable time window during early morning when vapour pressure deficit is low and the light intensity is not a limiting factor for assimilation.
... As the hydraulic regime can change spatially (e.g., soil properties) and temporally (e.g., VPD), stomatal responses should be adaptive to changing conditions. While stomatal responses to humidity have been studied to a considerable extent (e.g., Knauer et al., 2015;Monteith, 1995;Peak & Mott, 2011), it is unclear how stomatal closure during drought changes with soil properties . Moreover, ...
Understanding stomatal regulation during drought is essential to correctly predict vegetation‐atmosphere fluxes. Stomatal optimization models posit that stomata maximize the carbon gain relative to a penalty caused by water loss, such as xylem cavitation. However, a mechanism that allows the stomata to behave optimally is unknown. Here, we introduce a model of stomatal regulation that results in similar stomatal behaviour without presupposing an optimality principle. By contrast, the proposed model explains stomatal closure based on a well‐known component of stomatal regulation: abscisic acid (ABA). The ABA level depends on its production rate, which is assumed to increase with declining leaf water potential, and on its degradation rate, which is assumed to increase with assimilation rate. Our model predicts that stomata open until the ratio of leaf water potential to assimilation rate, proportional to ABA level, is at a minimum. As a prerequisite, the model simulates soil‐plant hydraulics and leaf photosynthesis under varying environmental conditions. The model predicts that in wet soils and at low vapour pressure deficit (VPD), when there is no water limitation, stomatal closure is controlled by the relationship between photosynthesis and stomatal conductance. In dry soils or at high VPD, when the soil hydraulic conductivity limits the water supply, stomatal closure is triggered by the sharp decline in leaf water potential as transpiration rate increases. Being adaptive to changing soil and atmospheric conditions, the proposed model can explain how plants are enabled to avoid critical water potentials during drought for varying soil properties and atmospheric conditions.
Flash drought events (FDEs) are projected to increase frequently in a warming world, significantly impacting ecosystem productivity and the global carbon cycle. The development of FDEs, induced by anomalies in different environmental variables, may cause different responses to the ecosystem’s gross primary productivity (GPP). However, the GPP variations and underlying mechanisms during the FDEs have rarely been quantified. This study collected long-term (>10 years) high-quality flux observations from the FLUXNET 2015 dataset to investigate GPP variations and their driving mechanisms during FDEs. Results showed that all vegetation types have two contrasting GPP variations during FDEs. One variation is a decreasing then increasing standardized GPP anomaly (V-shape response). The other shows an increase followed by decreasing standardized GPP anomaly (inverted V-shape response). The V-shape GPP response to FDEs was induced by increased soil water content deficit at the onset stage of FDEs. In contrast, the inverted V-shape GPP response to FDEs was induced by increased net radiation at the onset of FDEs. Such results indicated competing moisture supply and atmospheric moisture demand at the onset of FDEs, controlling the two contrasting ecosystem’s carbon responses with its development. Moreover, the contribution of water use efficiency to the magnitude of the V-shape GPP response (64.5% ± 22.4%) is greater than that to the inverted V-shape GPP response (47.6% ± 18.7%). This study identified the two contrasting types of GPP variations during FDEs and their driving mechanisms across multiple ecosystem types which can improve our ability to predict the future effects of more frequent FDEs on ecosystem productivity.
Convection‐Permitting Model (CPM) simulations of the Central United States climate for the summer of 2011 are studied to understand the causes of warm biases in 2‐m air temperature (T2m) and related underestimates of precipitation including that from mesoscale convective systems (MCSs). Based on 10 CPM simulations and 9 coarser‐resolution model simulations, we quantify contributions from evaporative fraction (EF) and radiation to the T2m bias with both types of models overestimating T2m largely because they underestimate EF. The performance of CPMs in capturing MCS characteristics (frequency, rainfall, propagation) varies. The pre‐summer precipitation bias has large correlation with mean summertime T2m bias but the relationship between summertime MCS mean rainfall bias and T2m bias is non‐monotonic. Analysis of lifting condensation level deficit and convective available potential energy suggests that models with T2m warm biases and low EF have too dry and stable boundary layers, inhibiting the formation of clouds, precipitation and MCSs. Among the CPMs with differing model formulations (e.g., transpiration, infiltration, cloud macrophysics and microphysics), evidence suggests that altering the land‐surface model is more effective than altering the atmospheric model in reducing T2m biases. These results demonstrate that land‐atmosphere interactions play a very important role in determining the summertime climate of the Central United States.
Purpose of Review
Harsher abiotic conditions are projected for many woodland areas, especially in already arid and semi-arid climates such as the Southwestern USA. Stomatal regulation of their aperture is one of the ways plants cope with drought. Interestingly, the dominant species in the Southwest USA, like in many other ecosystems, have different stomatal behaviors to regulate water loss ranging from isohydric (e.g., piñon pine) to anisohydric (e.g., juniper) conditions suggesting a possible niche separation or different but comparable strategies of coping with stress. The relatively isohydric piñon pine is usually presumed to be more sensitive to drought or less desiccation tolerant compared to the anisohydric juniper although both species close their stomata under drought to avoid hydraulic failure, and the mortality of one species (mostly piñon) over the other in the recent droughts can be attributed to insect outbreaks rather than drought sensitivity alone. Furthermore, no clear evidence exists demonstrating that iso- or anisohydric strategy increases water use efficiency over the other consistently. How these different stomatal regulatory tactics enable woody species to withstand harsh abiotic conditions remains a subject of inquiry to be covered in this review.
Recent Findings
This contribution reviews and explores the use of simplified stomatal optimization theories to assess how photosynthesis and transpiration respond to warming (H), drought (D), and combined warming and drought (H+D) for isohydric and anisohydric woody plants experiencing the same abiotic stressors. It sheds light on how simplified stomatal optimization theories can separate between photosynthetic and hydraulic acclimation due to abiotic stressors and how the interactive effects of H+D versus H or D alone can be incorporated into future climate models.
Summary
The work here demonstrates how field data can be bridged to simplified optimality principles so as to explore the effect of future changes in temperature and in soil water content on the acclimation of tree species with distinct water use strategies. The results show that the deviations between measurements and predictions from the simplified optimality principle can explain different species’ acclimation behaviors.
Vapor pressure difference between the leaf and atmosphere (VPD) is the most important regulator of daytime transpiration, yet the mechanism driving stomatal responses to an increase in VPD in angiosperms remains unresolved. Here, we sought to characterize the mechanism driving stomatal closure at high VPD in an angiosperm species, particularly testing whether abscisic acid (ABA) biosynthesis could explain the observation of a trigger point for stomatal sensitivity to an increase in VPD. We tracked leaf gas exchange and modelled leaf water potential (Ψl) in leaves exposed to a range of step-increases in VPD in the herbaceous species Senecio minimus Poir. (Asteraceae). We found that mild increases in VPD in this species did not induce stomatal closure because modelled Ψl did not decline below a threshold close to turgor loss point (Ψtlp), but when leaves were exposed to a large increase in VPD, stomata closed while modelled Ψl declined below Ψtlp. Leaf ABA levels were higher in leaves exposed to a step-increase in VPD that caused Ψl to transiently decline below Ψtlp and in which stomata closed compared to leaves in which stomata did not close. We conclude that the stomata of S. minimus are insensitive to VPD until Ψl declines to a threshold that triggers the biosynthesis of ABA and that this mechanism might be common to angiosperms.
Climate change‐associated rise in VPD (atmospheric vapor pressure deficit) results in increased plant transpiration and reduced stomatal conductance, photosynthesis, biomass, and yield. High VPD‐induced stomatal closure of Arabidopsis is an active process regulated via the kinase SnRK2.6 (OPEN STOMATA 1, OST1). Here, we performed gas exchange, leaf water potential and rosette growth measurements to study, whether (1) high VPD‐induced stomatal closure is detected in plants carrying loss‐of‐function mutations in OST1 ( ost1‐3 ) when they are grown at reduced soil water content or measured at increased air temperature; (2) ost1‐3 plants expressing OST1 construct with no ABA‐activation domain, but intact ABA‐independent activation, show stronger stomatal VPD response compared with ost1‐3 plants; and (3) rosette area and biomass of ost1‐3 are more affected by growth at high VPD compared with Col‐0. The stomata of well‐watered ost1‐3 plants were insensitive to high VPD regardless of air temperature, but in deficit‐irrigated ost1‐3 , leaf water potential decreased the most and stomata closed at high VPD. Differences between VPD‐induced stomatal closures of ost1‐3 plants and ost1‐3 plants expressing OST1 with no ABA‐activation domain point at gradual VPD‐induced ABA‐independent activation of OST1. High VPD conditions led to similar reductions in rosette area and specific leaf area of well‐watered Col‐0 and ost1‐3 plants. Rosette dry mass was unaffected by high VPD. Our results show that OST1 loss‐of‐function plants display conditional stomatal closure and no extra sensitivity of rosette area growth compared with Col‐0 wildtype under high VPD conditions.
High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests worldwide. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (Anet ) and stomatal conductance (gs ). However, evidence for decoupling of gs from Anet under extreme heat has been found. Such a response both enhances survival of leaves during heat events but also quickly depletes available water. To understand the prevalence and significance of this decoupling, we measured leaf gas exchange in 26 tree and shrub species growing in the glasshouse or at an urban site in Sydney, Australia on hot days (maximum Tair > 40°C). We hypothesized that on hot days plants with ample water access would exhibit reduced Anet and use transpirational cooling leading to stomatal decoupling, whereas plants with limited water access would rely on other mechanisms to avoid lethal temperatures. Instead, evidence for stomatal decoupling was found regardless of plant water access. Transpiration of well-watered plants was 23% higher than model predictions during heatwaves, which effectively cooled leaves below air temperature. For hotter, droughted plants, the increase in transpiration during heatwaves was even more pronounced-gs was 77% higher than model predictions. Stomatal decoupling was found for most broadleaf evergreen and broadleaf deciduous species at the urban site, including some wilted trees with limited water access. Decoupling may simply be a passive consequence of the physical effects of high temperature on plant leaves through increased cuticular conductance of water vapor, or stomatal decoupling may be an adaptive response that is actively regulated by stomatal opening under high temperatures. This temperature response is not yet included in any land surface model, suggesting that model predictions of evapotranspiration may be underpredicted at high temperature and high VPD.
Through the Stomata of plants, water (H2O) and carbon dioxide (CO2) are transferred between leaves and the atmosphere. The intakes of CO2 during photosynthesis and water loss through transpiration are facilitated by stomata. To effectively model plant transpiration, and study the mass, energy transfer between plants, and the atmosphere, stomatal conductance of plant leaves requires accurate modelling. Abnormal changes in soil moisture result from frequent droughts in water-strapped environmental regions in a warming context. This has an impact on the stomatal conductance models's applicability and, in turn, the precision of the carbon and water cycles in agro-ecosystems. Four commonly used stomatal conductance models-Jarvis, Ball-Woodrow-Berry (BWB), Ball-Berry-Leuning (BBL) and unified stomatal optimization (OS) were investigated in the simulation of spring maize during persistent water stress to determine the impact of introducing a soil moisture response function on the simulation effect of the stomatal conductance model. The results showed that the BWB model was the best model for spring maize during persistent water stress, followed by the OS and BBL models, and the Jarvis model was the worst model. The OS and BBL models' simulation effects were improved by the addition of the soil moisture response function, while the Jarvis and BWB models' simulation effects were diminished. The OS-θ model was the best, followed by the BBL-θ and BWB-θ models, and the Jarvis-θ model was the worst, according to the model simulation effect. The 95 % confidence intervals of BWB-θ and OS-θ models were simulated with the addition of the soil moisture response function. The addition of soil moisture function improved the model's applicability, allowing it to be used for a variety of relative soil moisture contents, including 13∼68 %, 13∼89 %, 13∼78 % and 13∼89 %forthe Jarvis, BWB, BBL and USO models. With or without the addition of a moisture response function, the OS model performs optimally and is appropriate for various soil moisture conditions. The study's finding may serve as a foundation for choosing an appropriate stomatal conductance model for effective simulation of carbon and water cycles of agricultural ecosystems under drought conditions in water-limited environmental regions. This may support effective use and evaluation of agricultural water resources.
Agricultural intensification is often seen as an appropriate approach to meet the growing demand for agricultural products. In regions affected by seasonal or chronic water scarcity, closing the yield gap depends on irrigation. Brazil is the second-largest wheat importer of the world and irrigated wheat has high yield potential in Brazil, especially in tropical and sub-tropical regions of the country. The crop coefficient (Kc) is still a practical and simple way of quantifying crop irrigation requirements. In this paper, we provided Kc values based on a robust experimental dataset across different wheat-producing regions in Brazil. Four experiments were conducted in three sites [season 2017 in Piracicaba, State of São Paulo (Southeast region); season 2012 Maringá, State of Paraná (Southern region); and seasons 2016 and 2017 Rondonópolis, State of Mato Grosso) (Midwest region)] using lysimeters and the Bowen ratio energy balance method for crop evapotranspiration measurements. Our data showed that Kc values varied among the regions analyzed. For the Midwest region, Kc values ranged from 0.88 to 1.36; the Southeastern region from 0.81 to 1.15, and the Southern region from 0.67 to 1.01. Considering values suggested by FAO, those for arid climates should be used in the Midwest region, sub-humid values for the Southeast, and humid values in the Southern. Differently, that was observed for other crops, our database showed wheat Kc values being stable disregarding the ETo range, which might be related to relatively lower ETo during the winter.
Daytime transpiration is driven by evaporative demand (vapor pressure deficit, VPD) and radiative energy, dominantly accounting for daily transpiration (T). Recent modeling approaches recommend weighing VPD for diurnal cycle of T using radiation to predict T, as opposed to using daily mean VPD. This proposition requires field evaluation so that diurnal patterns of water use are represented fairly. In this research, hourly T was measured concurrently for irrigated maize, sorghum, and soybean in a dry sub-humid climate. Various VPD conditioning approaches available to the user that differed in measurement frequency, spectral sampling, and weighing strategies were evaluated to explain variance in T. The locally derived radiation-based weighing coefficient (Frac) showed substantial variability with mean estimates of 0.61 (annual) and 0.58 (growing season), which were lower than the classically recommended Tanner-Sinclair coefficient (0.75). The use of a constant Frac value for weighing VPD was suboptimal to dynamic weighing of VPD for each day, accounting for day-to-day variability. This is primarily because of considerable nighttime water use: Tn demonstrated values that were 6–16% of T across the three crops. Tn was driven by wind speed and VPD during nighttime hours. These field observations of Tn hold implications for (un)suitability of radiation-based weighing procedures, which assume no Tn. Diurnally weighted VPD was suboptimal to using daily mean VPD, as the former does not account for dark period-VPD. Additionally, reference evapotranspiration (ETo), the standardized metric of crop water use was either negative or zero for nocturnal periods when Tn ≠ 0, and thus, failed to appropriately represent Tn. A variable of transpiration, based on daily mean VPD and total daily radiation, was the most effective in explaining T variance in all crops. With increasing asymmetry between daytime and nighttime warming and aridity, it becomes increasingly important to appropriately precondition VPD data to predict T. Caution should be exercised while using radiation-weighted VPD in crops, environments and seasons with non-negligible Tn.
Cotton Breeding and Biotechnology presents information on one of the most economically important crops of the world, cotton. This book contains chapters on the history of cotton; breeding approaches; technologies for increasing germination, crop growth and yield; and fiber quality issues. It emphasizes sustainable development in the cotton industry analysing the progress of breeding technologies under environmental adversity. The book explores the national and global status of cotton crop, including cotton production, possible impacts of climate change, and the vulnerability of cotton to pest infestations and disease attacks.
Features
Focuses on cotton breeding and biotechnology
Proposes ideas, data, and strategies to mount breeding programs for enhancing cotton production
Details strategies for cotton quality improvement against abiotic and biotic stresses
Emphasizes the revival of cotton in Pakistan and South Asian region
This book is useful to researchers, cotton breeders and growers, farmers, and the agriculture industry.
Stomatal conductance (gs), the process that governs plant carbon uptake and water loss, is fundamental to most Land Surface Models (LSMs). With global change accelerating, more attention should be paid to investigating stomatal behavior, especially in extremely arid areas. In this study, gas exchange measurements and environmental/biological variables observations during growing seasons in 2016 and 2017 were combined to investigate diurnal and seasonal characteristics of gs and the applicability of the optimal stomatal conductance model in a desert oasis vineyard. The results showed that the responses of gs to environmental factors (photosynthesis active radiation, PAR; vapor pressure deficit, VPD; and temperature, T) formed hysteresis loops in the daytime. The stomatal conductance slope, g1, a parameter in the unified stomatal optimal model, varied in different growing seasons and correlated with the soil-to-leaf hydraulic conductance (KL). These results indicated the potential bias when using a constant g1 value to simulate gs and highlighted that the water-use strategy of oasis plants might not be consistent throughout the entire growing season. Our findings further help to achieve a better understanding of stomata behavior in responding to climate change and encourage future efforts toward a more accurate parameterization of gs to improve the modeling of LSMs.
The responses of leaf water potential, leaf conductance, transpiration rate and net photosynthetic rate to vapour pressure deficits varying from 10 to 30 Pa kPa-1 were followed in Helianthus annuus as the extractable soil water decreased. With a vapour pressure deficit of 25 Pa kPa-1 around the entire plant as the soil water content decreased, the leaf conductance and transpiration rate showed a strong closing response to leaf water potential at a value of-0.65 MPa, whereas with a vapour pressure deficit of 10 Pa kPa-1 around the entire plant, the rate of transpiration and leaf conductance decreased almost linearly as the leaf water potential decreased from-0.4 to-1.0 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps decreased the leaf conductance by a similar proportion at all extractable soil water contents. At high soil water contents, the decrease in conductance with leaf water potential was greater when the vapour pressure deficit was increased than when it was not, indicating a direct influence of vapour pressure deficit on the stomata. The rate of net photosynthesis decreased to a smaller degree than the leaf conductance when the vapour pressure deficit around the leaf was varied. Overall, the net photosynthetic rate decreased almost linearly from 20 to 25 μmol m-2 s-1 at-0.3 MPa to 5 μmol m-2 s-1 at-1.2 MPa. As the soil water decreased, the internal carbon dioxide partial pressure was maintained between 14 and 25 Pa. No unique relationship between leaf conductance, transpiration rate or photosynthetic rate and leaf water potential was observed, but in all experiments leaf conductance and the rate of net photosynthesis decreased when about two-thirds of the extractable water in the solid had been utilized irrespective of the leaf water potential. We conclude that soil water status, not leaf water status, affects the stomatal behaviour and photosynthesis of H. annuus.
The literature on the micrometeorology of temperate and tropical forests is reviewed to determine whether structural or species difference between these biomes alters their interaction with the atmosphere. Considerable consistency is found in the value of those whole-canopy features of most importance to this interaction, namely solarreflection coefficient, through-canopy radiation absorption, aerodynamic roughness, the symptoms of near-surface K-theory failure, the canopy store for rainfall interception and the magnitude and environmental response of their bulk stomatal (surface) resistance. Typical values of these parameters and functions are given with a view to their potential use in climate simulation models. Attention is drawn to the fact that this similar micrometeorological response can generate different timeaverage surface-energy partitions when interacting with different climates and, in particular, alters between the edge and the middle of continents. This is of considerable significance, implying tropical deforestation is likely to have most effect on river flow (though not climate) at continental edge and island locations. The similar micrometeorological response of forests is interpreted as the necessary consequence of energy and mass (water) conservation acting as an area average on vegetation that is, by definition, dense and extensive, to reconcile a characteristically tall growth habit with a perennial nature.
The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.
This paper presents a system of models for the simulation of gas and energy exchange of a leaf of a C3 plant in free air. The physiological processes are simulated by sub-models that: (a) give net photosynthesis (An) as a function of environmental and leaf parameters and stomatal conductance (gs); (b) give g, as a function of the concentration of CO2 and H2O in air at the leaf surface and the current rate of photosynthesis of the leaf. An energy balance and mass transport sub-model is used to couple the physiological processes through a variable boundary layer to the ambient environment.
In the past, stomatal responses have generally been considered in relation to single environmental variables in part because the interactions between factors have appeared difficult to quantify in a simple way. A linear correlation between stomatal conductance (g) and CO2 assimilation rate (A) has been reported when photon fluence was varied and when the photosynthetic capacity of leaves was altered by growth conditions, provided CO2, air humidity and leaf temperature were constant (1). Temperature and humidity are, however, not consistent in nature. Lack of a concise description of stomatal responses to combinations of environmental factors has limited attempts to integrate these responses into quantitative models of leaf energy balance, photosynthesis, and transpiration. Moreover, this lack has hindered progress toward understanding the stomatal mechanism. We have taken a multi-variant approach to the study of stomatal conductance and we show that under many conditions the responses of stornata can be described by a set of linear relationships. This model can be linked to models of leaf carbon metabolism and the environment to predict fluxes of CO2, H2O and energy. In this paper, we show how the model of conductance can be linked to a description of CO2 assimilation as a function of intercellular CO2 (whether empirical or the output of a model) to predict the distribution of flux control between the stornata and leaf “biochemistry” under conditions in a gas-exchange cuvette.
Thesis (Ph. D.)--Stanford University, 1988. Includes bibliographical references. Photocopy.
Leaf resistance of Sesamum indicum L. increased when the humidity gradient between leaf and air was increased, at moderate temperatures, even though calculated carbon dioxide concentrations within the leaf decreased slightly. Mesophyll resistance remained relatively constant when humidity gradients were changed, indicating that the increases in leaf resistance were mainly caused by reductions in stomatal aperture and that nonstomatal aspects of photosynthesis and respiration were not affected. Low carbon dioxide concentrations inside the leaf decreased but did not eliminate resistance response to the humidity gradient. Internal carbon dioxide concentrations had little effect on resistance in humid air but had moderate effects on resistance with large humidity gradients between leaf and air. Stomatal response to humidity was not present at high leaf temperatures. Effects of humidity gradients on photosynthetic and stomatal responses to temperature suggested that large humidity gradients may contribute to mid-day closure of stomata and depressions in photosynthesis.
The sensitivity of stomatal conductance to changes of CO(2) concentration and leaf-air vapor pressure difference (VPD) was compared between two C(3) and two C(4) grass species. There was no evidence that stomata of the C(4) species were more sensitive to CO(2) than stomata of the C(3) species. The sensitivity of stomatal conductance to CO(2) change was linearly proportional to the magnitude of stomatal conductance, as determined by the VPD, the same slope fitting the data for all four species. Similarly, the sensitivity of stomatal conductance to VPD was linearly proportional to the magnitude of stomatal conductance. At small VPD, the ratio of intercellular to ambient CO(2) concentration, C(i)/C(a), was similar in all species (0.8-0.9) but declined with increasing VPD, so that, at large VPD, C(i)/C(a) was 0.7 and 0.5 (approximately) in C(3) and C(4) species, respectively. Transpiration efficiency (net CO(2) assimilation rate/transpiration rate) was larger in the C(4) species than in the C(3) species at current atmospheric CO(2) concentrations, but the relative increase due to high CO(2) was larger in the C(3) than in the C(4) species.
The literature on the micrometeorology of temperate and tropical forests is reviewed to determine whether structural or species difference between these biomes alters their interaction with the atmosphere. Considerable consistency is found in the value of those whole-canopy features of most importance to this interaction, namely solar-reflection coefficient, through-canopy radiation absorption, aerodynamic roughness, the symptoms of near-surface K-theory failure, the canopy store for rainfall interception and the magnitude and environmental response of their bulk stomatal (surface) resistance. Typical values of these parameters and functions are given with a view to their potential use in climate simulation models. Attention is drawn to the fact that this similar micrometeorological response can generate different time-average surface energy partitions when interacting with different climates and, in particular, alters between the edge and the middle of continents. -from Author
Models for the responses of stomatal conductance to changes in humidity difference between leaf and air are developed. Feedforward of the stomatal response to humidity difference is present when stomata respond to a portion of the rate of transpiration of the leaf that is unaffected by changes in stomatal aperture (cuticular transpiration). A simple linear model of the responses of conductance is sufficient to predict many phenomena reported in the literature. These include the presence, under specified conditions, of a peak rate of transpiration, and an increase in the conductance and humidity difference at which this peak occurs, with increased general levels of stomatal opening.
Stomatal apertures are the major pathway for the movement of CO2 from the atmosphere into the mesophyll of leaves. The presence of this pathway for the movement of gases also results in water loss from the hydrated surfaces within leaves to the atmosphere. Stomatal aperture appears to be controlled by complex mechanisms which operate to maintain a variable balance between allowing CO2 uptake to proceed, while restricting the loss of water vapor, and preventing leaf desiccation. Recent reviews have examined the physiological bases of stomatal function (Raschke 1979; Allaway and Milthorpe 1976) and stomatal responses to environment (Sheriff 1979; Burrows and Milthorpe 1976; Hall et al. 1976). Analyses which integrated stomatal effects on CO2 exchange, transpiration, and energy balance were developed based upon theory (Cowan 1977), which have led to hypotheses concerning optimal stomatal function (see Chap. 17, this Vol.; Cowan and Farquhar 1977). However, information concerning the simultaneous responses of stomata, water loss, and CO2 assimilation rates has not been reviewed for plants in natural environments.
Gas-exchange measurements on Eucalyptus grandis leaves and data extracted from the literature were used to test a semi-empirical model of stomatal conductance for CO2
gSc=go+a1A/(cs-I) (1+Ds/Do)]
where A is the assimilation rate; Ds and cs are the humidity deficit and the CO2 concentration at the leaf surface, respectively; g0 is the conductance as A → 0 when leaf irradiance → 0; and D0 and a1 are empirical coefficients. This model is a modified version of gsc=a1A hs/cs first proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus Mijhoff, Publ., pp. 221–224), in which hs is relative humidity. Inclusion of the CO2 compensation point, τ, improved the behaviour of the model at low values of cs, while a hyperbolic function of Ds for humidity response correctly accounted for the observed hyperbolic and linear variation of gsc and ci/cs as a function of Ds, where Ci is the intercellular CO2 concentration. In contrast, use of relative humidity as the humidity variable led to predictions of a linear decrease in gsc and a hyperbolic variation in ci/cs as a function of Ds, contrary to data from E. grandis leaves. The revised model also successfully described the response of stomata to variations in A, Ds and cs for published responses of the leaves of several other species. Coupling of the revised stomatal model with a biochemical model for photosynthesis of C3 plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO2 concentration. Best results are obtained for well-watered plants.
An empirical model for stomatal conductance (g), proposed by Leuning (1995, this issue) as a modification of Ball, Woodrow & Berry's (1987) model, is interpreted in terms of a simple, steady-state model of guard cell function. In this model, stomatal aperture is a function of the relative turgor between guard cells and epidermal cells. The correlation between g and leaf surface vapour pressure deficit in Leuning's model is interpreted in terms of stomatal sensing of the transpiration rate, via changes in the gradient of total water potential between guard cells and epidermal cells. The correlation between g, CO2 assimilation rate and leaf surface CO2 concentration in Leuning's model is interpreted as a relationship between the corresponding osmotic gradient, irradiance, temperature, intercellular CO2 concentration and stomatal aperture itself. The explicit relationship between osmotic gradient and stomatal aperture (possibly describing the effect of changes in guard cell volume on the membrane permeability for ion transport) results in a decrease in the transpiration rate in sufficiently dry air. Possible extension of the guard cell model to include stomatal responses to soil water status is discussed.
Leaf conductance responses to leaf to air water vapour partial pressure difference (VPD) have been measured at air speeds of 0.5 and 3.0 ms−1 in single attached leaves of three species in order to test the hypothesis that leaf conductance response to VPD is controlled by evaporation from the outer surface of the epidermis, rather than by evaporation through stomata. Total conductance decreased linearly with increassing VPD at both air speeds, but was decreased 1.6 3.0 times as much as by a given incrase in VPD at high than at low air speed. depending on species. In all species the relationship between leaf conductance and the gradient for evaporation from the epidermis was the same at both values of boundary layer conductance, supporting the hypothesis that direct epidermal evaporation controls stomatal guard cell behaviour in responses of stomata to VPD in these species.
A model of photosynthesis (PGEN) is presented. The model assumes that optimal use is made of the leaf nitrogen available for partitioning between the carboxylase and thylakoid components. This results in predictions of Rubisco and chlorophyll concentrations very similar to those measured elsewhere. A function is incorporated which represents the detrimental effects of negative leaf water potentials on the Calvin cycle, producing a quantitative and mechanistic trade-off between CO2 entering, and H2O leaving, the leaf. Thus, an optimal stomatal conductance and associated internal partial pressure of CO2 exists for any given set of environmental conditions. The model calculates this optimal state for the leaf, which is its output. The model was subjected to changes in the following parameters: soil water potential, irradiance, ambient CO2 partial pressure, leaf temperature, leaf-to-air vapour pressure deficit, wind speed, atmospheric pressure, leaf nitrogen content, root dry weight and leaf width. These perturbations resulted in changes in predicted optimal conductance which were very similar to what has been observed. In general, as the capacity of the leaf to fix CO2 increased, so did the predicted optimal conductance, with the internal partial pressure of CO2 being maintained close to 22Pa.
The objective of the work reported was to answer the following questions: (1) Do stomata respond to both humidity and temperature? (2) Do these responses interact in such a way that relative humidity at the leaf surface is a more appropriate variable than water vapour saturation deficit at the leaf surface and yields a simpler description of the compound response? To answer these questions, we measured the response of leaf conductance to humidity under constant leaf temperature, and the response to increasing leaf temperature under constant relative humidity and under constant water vapour saturation deficit. We found that, in Hedera helix subsp. canariensis (Willd.) Coutinho, there was a reversible response to humidity under constant temperature, and that there was also a response to temperature under constant relative humidity. The relationship between leaf conductance and relative humidity was different when measured at the same temperature rather than at different temperatures. An inversely proportional response was consistently obtained when stomatal conductance was expressed in relation to water vapour saturation deficit. The interaction between the effects of leaf temperature and water vapour saturation deficit was not compatible with a mechanism of response to humidity and temperature based solely on relative humidity. From these data, we conclude that water vapour saturation deficit is a more appropriate variable for describing stomatal responses to humidity.
Plants growing in environments differing in prevailing humidity exhibit variations in traits associated with regulation of water loss, particularly cuticular and stomatal properties. Expansive growth is also typically reduced by low humidity. Nevertheless, there is little evidence in plants for a specific sensor for humidity, analogous to the blue light or phytochrome photoreceptors. The detailed mechanism of the stomatal response to humidity remains unknown. Available data suggest mediation by fluxes of water vapour, with evaporation rate assuming the role of sensor. This implies that stomata respond to the driving force for diffusional water loss, leaf-air vapour pressure difference. Induction of metabolic stomatal response to humidity may involve signal metabolites, such as abscisic acid, that are present in the transpiration stream. These materials may accumulate in the vicinity of guard cells according to the magnitude and location of cuticular transpiration, both of which could change with humidity. Such a mechanism remains hypothetical, but is suggested to account for feedforward responses in which transpiration decreases with increasing evaporative demand, and for the apparent insensitivity of stomatal aperture in isolated epidermis to epidermal water status. Other responses of plants to humidity may involve similar indirect response mechanisms, in the absence of specific humidity sensors.
Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO2 and water vapour added), which we termed ‘helox’. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water-vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO2 concentration at the evaporating surfaces (ci), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant ci, suggesting heterogeneous (patchy) stomatal closure.
Prediction of forest evaporation with the Penman equation that includes a surface resistance term, is still hampered by the lack of understanding of this resistance term. A well-established, steady-state, multi-layer model was used to simulate evaporation of a pine forest in central Sweden, where previous measurements had shown a large difference between evaporation estimated by energy balance/Bowen ratio (EBBR) and water balance methods. Model input was profiles of turbulent diffusivity, boundary layer resistance, stomatal resistance, wind speed, net and global radiation and needle area density. The surface resistance, rss, as defined by the Penman equation, was always less than the bulk stomatal resistance, rst. For a projected needle area index (LAI) of around 1.5, the rss/rst quotient was 0.5 but for values of LAI > 4 or 5 it attained a constant value around 0.9. When ground evaporation was excluded, the canopy surface resistance, rsc, was very close to rst (rsc/rst ≅ 0.95) for all LAI. Bulk stomatal resistances based on point values of stomatal conductance, implied rss/rst and rsc/rst values largely differing from unity. The operational definition of average stomatal resistance was, therefore, an important point. Simulated evaporation for representative days of the “discrepancy” period gave support to EBBR measurements. It was suggested that different areal representativity might explain differences between the evaporation estimation methods. A sensitivity analysis showed that vapour pressure deficit and net radiation were equally important to determine forest evaporation but net radiation influenced it indirectly through the surface resistance.
The Penman—Monteith equation for transpiration from a uniform stand of vegetation is extended to take account of the response of stomata to the saturation deficit (SD) of air in the canopy. The main assumption is that stomatal conductance throughout the canopy decreases linearly with a value of the SD calculated from the SD at a reference height adjusted to allow for vertical gradients of temperature and vapour pressure. In most circumstances, stomata can achieve an equilibrium conductance satisfying both the physics and the physiology of the system. In others, equilibrium is unachievable: stomata cannot remain open because air within the canopy becomes drier as they close — a microclimatic form of feedback.Calculations for an arable crop and for deciduous and coniferous forest show the extent to which transpiration rate is overestimated (a) when the response of stomata to SD is ignored; and (b) when the vertical gradient of SD is ignored. Large errors are associated with (a), and (b) is important when stomatal conductance is small because of drought.
A simple scheme is developed to describe how vegetation and the convective boundary layer (CBL) interact during daylight in terms of water and sensible heat exchange. The response of vegetation to a prescribed atmospheric state is defined by a quadratic equation obtained by combining the Penman-Monteith equation with a new relation between surface conductance and transpiration rate based on laboratory evidence. The complementary response of the CBL to the input of sensible and latent at the ground surface is summarized by a simple relation between the Priestley-Taylor coefficient and surface conductance that fits daily mean values for diverse types of vegetation. The two responses are combined to provide a measure of ‘accommodation’ between vegetation and the atmosphere such that mean daily values of transpiration rate and saturation deficit can be obtained as functions of surface conductance and the net input of radiant heat.
To investigate methods of estimating the surface conductance for use
in the Penman-Monteith equation, four models were evaluated. In the
simplest model the surface conductance was independent of all environmental
variables, whereas in the most complex model the surface conductance
was a non-linear function of solar radiation, specific humidity deficit,
temperature and soil moisture deficit. The 584 hours of energy budget
Bowen ratio measurements of evaporation made over Thetford Forest,
which were used to derive the hourly values of the surface conductance
of the forest under dry canopy conditions, were split into two independent
sets by taking alternate days. One set was used to determine the
parameters in the four models using multivariate optimisation, while
the other set was used to assess the accuracy of the estimates of
surface conductance and transpiration. As the complexity of the models
increased, the difference in the total transpiration decreased from
22% to less than 1%. Also, bias at low and high values of the measured
surface conductance decreased. However, investigation of the success
of the models for estimating the surface conductance in different
years showed the results were much poorer. The parameters in the
most complex model were determined using the 1976 data and the 1974
and 1975 data sets used to evaluate the accuracy of the estimates.
The difference in the total transpiration was 14 and 11 per cent
for the two years. Also the estimated surface conductance was too
large at low values of the measured surface conductance and too small
at high values. © 1988.
We analysed Ball's empirical model of stomatal conductance (Ball, Woodrow and Berry, 1987). The original interpretation was found to be flawed, and a new one is proposed which views the model as a description of the relationship between CO 2 flux rate and stomatal conductance, rather than as a model of stomatal conductance alone. It is shown that this model is useful for describing changes in intercellular CO 2 concentration. The model was tested against data from experiments in which the responses of stomatal conductance and CO 2 flux density to humidity and temperature were measured in leaves of Hedera helix . It was found that the responses to temperature and humidity are not treated in a satisfactory way in the model whereas the response of the model to other variables is realistic. An alternative to Ball's model is described and tested. It is concluded that Ball's model is a useful starting point for development of simulation models to be used as submodels in canopy and regional models. However, as any empirical model, it is of no use for defining causal relationships. Copyright 1993, 1999 Academic Press
Stomata of corn (Zea mays L.) and sorghum (Sorghum bicolor L.) responded to changes in leaf water potential during the vegetative growth phase. During reproductive growth, leaf resistances were minimal and stomata were no longer sensitive to bulk leaf water status even when leaf water potentials approached -27 bars. Stomata of corn, cotton (Gossypium hirsutum L.), and sorghum appear to respond to changes in the humidity deficit between the leaf and air and in this manner, regulated transpirational flux to some degree. Distinct differences in water transport efficiency were observed in the three species. Under nonlimiting soil water conditions, sorghum exhibited the greatest efficiency of water transport while under limiting soil moisture conditions, cotton appeared most efficient. Corn was the least efficient with respect to nonstomatal regulation of water use. Differences in drought tolerance among the three species are partially dependent on stomatal regulation of water loss, but efficiency of the water transport system may be more related to drought adaptation. This is particularly important since stomata of all three species did not respond to bulk leaf water status during a large portion of the growing season.
Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO(2) assimilation, stomatal conductance (g(s)), quantum yield of photosystem II (phi(II)), and nonphotochemical quenching of chlorophyll fluorescence (q(N)) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO(2) assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g(s) initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40 degrees C, CO(2) assimilation and g(s) were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g(s). The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO(2) and O(2)) versus nonphotorespiring (700 microbars CO(2) and 2% O(2)) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O(2) increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO(2) in the leaf (C(i)) where C(i) was estimated from gas exchange measurements. However, estimates of C(i) from measurements of phi(II) and q(N) suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C(i). The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C(i) from gas exchange measurements. Under low VPD, 30 degrees C, high light, and saturating CO(2), castor bean (C(3) tropical shrub) has a rate of photosynthesis (61 micromoles CO(2) per square meter per second) that is about 50% higher than that of tobacco (C(3)) or maize (C(4)) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.
Stomatal response to environ-ment with Sesamum indicum L A critical appraisal of a combined stomatal-photo-synthesis model for C3 plants
- Hall A E Kaufmann
Hall A.E. & Kaufmann M.R. (1975) Stomatal response to environ-ment with Sesamum indicum L. Plant Physiology 55,455^59. Leuning R. (1995) A critical appraisal of a combined stomatal-photo-synthesis model for C3 plants. Plant, Cell and Environment 18, 000-000 (this issue).
An analysis of Ball's empirical model of stomatal conductance An analysis of stomatal conductance A model predicting stomatal conductance and its contribution to the control of photo-synthesis under different environmental conditions
- P J Aphalo
- P G Jarvis
Aphalo P.J. & Jarvis P.G. (1993) An analysis of Ball's empirical model of stomatal conductance. A«na/.s-o/Boto/iy 72, 321-327. Ball J.T. (1988) An analysis of stomatal conductance. Ph.D. thesis, Stanford University, USA. Ball J.T.. Woodrow I.E. & Berry J.A. (1987) A model predicting stomatal conductance and its contribution to the control of photo-synthesis under different environmental conditions. In Progress in Photosynthesis Research, Vol. IV (ed. J. Biggins), pp. 221-234.
Physiological and environmental regulation of stomatal conductance, photo-synthesis and transpiration
- G J Collatz
- S Griver
- J T Ball
- J A Berry
Collatz G.J., Griver S.. Ball J.T., & Berry J.A. (1991) Physiological and environmental regulation of stomatal conductance, photo-synthesis and transpiration. Agricultural and Forest Meteorology 54, 107-136.