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Wind speed affects the rate and kinetics of stomatal conductance
Abstract
Understanding the relationship between wind speed and gas exchange in plants is a longstanding challenge. Our aim was to investigate the impact of wind speed on maximum rates of gas exchange and the kinetics of stomatal responses. We conducted experiments in different angiosperm and fern species using an infrared gas analyzer equipped with a controlled leaf fan, enabling precise control of the boundary layer conductance. We first showed that the chamber was adequately mixed even at extremely low wind speed (<0.005 m s-1) and evaluated the link between fan speed, wind speed, and boundary layer conductance. We observed that higher wind speeds led to increased gas exchange of both water vapor and CO₂, primarily due to the increase in boundary layer conductance. This increase in transpiration subsequently reduced epidermal pressure, leading to stomatal opening. We documented that stomatal opening in response to light was 2.5 times faster at a wind speed of 2 m s-1 compared to minimal wind speed in Vicia faba, while epidermal peels in a buffer with no transpiration exhibited a similar opening rate. The increase in stomatal conductance under high wind was also observed in four angiosperm species under field conditions, but it was not observed in Boston fern (Nephrolepis exaltata), which lacks epidermal mechanical advantage. Our findings highlight the significant impact of boundary layer conductance on determining gas exchange rates and the kinetics of gas exchange responses to environmental changes.
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Stomatal pores, vital for CO2 uptake and water loss regulation in plants, are formed by two specialized guard cells. Despite their importance, there is limited understanding of how guard cells sense and respond to changes in vapor pressure difference (VPD). This study leverages a selection of CO2 hyposensitive and abscisic acid (ABA) signaling mutants in Arabidopsis, including heterotrimeric G protein mutants and RLK (receptor-like kinase) mutants, along with a variety of canola cultivars to delve into the intracellular signaling mechanisms prompting stomatal closure in response to high VPD. Stomatal conductance response to step changes in VPD was measured using the LI-6800F gas exchange system. Our findings highlight that stomatal responses to VPD utilize intracellular signaling components. VPD hyposensitivity was particularly evident in mutants of the ht1 (HIGH LEAF TEMPERATURE1) gene, which encodes a protein kinase expressed mainly in guard cells, and in gpa1-3, a null mutant of the sole canonical heterotrimeric Gα subunit, previously implicated in stomatal signaling. Consequently, this research identifies a nexus in the intricate relationships between guard cell signal perception, stomatal conductance, environmental humidity, and CO2 levels.
Citation: Tomasella, M.; Calderan, A.; Mihelčič, A.; Petruzzellis, F.; Braidotti, R.; Natale, S.; Lisjak, K.; Sivilotti, P.; Nardini, A. Best Procedures for Leaf and Stem Water Potential Measurements in Grapevine: Cultivar and Water Status Matter. Abstract: The pressure chamber is the most used tool for plant water status monitoring. However, species/cultivar and seasonal effects on protocols for reliable water potential determination have not been properly tested. In four grapevine cultivars and two times of the season (early season, Es; late season, Ls, under moderate drought), we assessed the maximum sample storage time before leaf water potential (Ψ leaf) measurements and the minimum equilibration time for stem water potential (Ψ stem) determination, taking 24 h leaf cover as control. In 'Pinot gris', Ψ leaf already decreased after 1 h leaf storage in both campaigns, dropping by 0.4/0.5 MPa after 3 h, while in 'Refosk', it decreased by 0.1 MPa after 1 and 2 h in Es and Ls, respectively. In 'Merlot' and 'Merlot Kanthus', even 3 h storage did not affect Ψ leaf. In Es, the minimum Ψ stem equilibration was 1 h for 'Refošk' and 10 min for 'Pinot gris' and 'Merlot'. In Ls, 'Merlot Kanthus' required more than 2 h equilibration, while 1 h to 10 min was sufficient for the other cultivars. The observed cultivar and seasonal differences indicate that the proposed tests should be routinely performed prior to experiments to define ad hoc procedures for water status determination.
Measurement of leaf carbon gain and water loss (gas exchange) in planta is a standard procedure in plant science research for attempting to understand physiological traits related to water use and photosynthesis. Leaves carry out gas exchange through the upper (adaxial) and lower (abaxial) surfaces at different magnitudes, depending on the stomatal density, stomatal aperture, cuticular permeability, etc., of each surface, which we account for in gas exchange parameters such as stomatal conductance. Most commercial devices measure leaf gas exchange by combining the adaxial and abaxial fluxes and calculating bulk gas exchange parameters, missing details of the plant's physiological response on each side. Additionally, the widely used equations to estimate gas exchange parameters neglect the contribution of small fluxes such as cuticular conductance, adding extra uncertainties to measurements performed in water-stress or low-light conditions. Accounting for the gas exchange fluxes from each side of the leaf allows us to better describe plants' physiological traits under different environmental conditions and account for genetic variability. Here, apparatus and materials are presented for adapting two LI-6800 Portable Photosynthesis Systems to work as one gas exchange system to measure adaxial and abaxial gas exchange simultaneously. The modification includes a template script with the equations to account for small fluxes. Instructions are provided for incorporating the add-on script into the device's computational sequence, display, variables, and spreadsheet results. We explain the method to obtain an equation to estimate boundary layer conductance to water for the new setup and how to embed this equation in the devices' calculations using the provided add-on script. The apparatus, methods, and protocols presented here provide a simple adaptation combining two LI-6800s to obtain an improved system to measure leaf gas exchange on adaxial and abaxial surfaces.
Graphical overview
Figure 1.
Diagram of the connection of two LI-6800s
Figure adapted from Márquez et al. (2021).
Sustaining crop productivity and resilience in water-limited environments and under rising temperatures are matters of concern worldwide. We investigated the leaf anatomical traits that underpin our recently identified link between leaf width (LW) and intrinsic water use efficiency (iWUE), as traits of interest in plant breeding. Ten sorghum lines with varying LW were grown under three temperatures to expand the range of variation of both LW and gas exchange rates. Leaf gas exchange, surface morphology and cross-sectional anatomy were measured and analysed using structural equations modelling. Narrower leaves had lower stomatal conductance (gs ) and higher iWUE across growth temperatures. They also had smaller intercellular airspaces, stomatal size, percentage of open stomatal aperture relative to maximum, hydraulic pathway, mesophyll thickness, and leaf mass per area. Structural modelling revealed a developmental association among leaf anatomical traits that underpinned gs variation in sorghum. Growing temperature and LW both impacted leaf gas exchange rates, but only LW directly impacted leaf anatomy. Wider leaves may be more productive under well-watered conditions, but consume more water for growth and development, which is detrimental under water stress.
Elevated [CO2] (eCO2) and water stress (WS) reduce leaf stomatal conductance (gs), which may affect leaf thermoregulation during heat waves (HS). Two sorghum lines, with different leaf width (LW) were grown in a glasshouse at mean day temperature of 30°C, under different [CO2] and watering levels, and subjected to HS (43°C) for six days at the start of the reproductive stage. We measured leaf photosynthetic and stomatal responses to light transients, before harvesting the plants. Photosynthesis at growth conditions (Agrowth) and biomass accumulation were enhanced by eCO2 under control conditions. HS increased gs, especially in wider leaves, and reduced the time-constant of stomatal opening (kopen) at aCO2 but not eCO2. However, HS reduced photosynthesis under WS and eCO2 due to increased leaf temperature and reduced evaporative cooling. eCO2 prevented the reduction of biomass under WS and HS, possibly due to improved plant and soil water status as a result of reduced gs. Our results suggest that the response of the C4 crop sorghum to future climate conditions depends on the trade-off between low gs needed for high water use efficiency and drought tolerance, and the high gs needed for improved thermoregulation and heat tolerance under an eCO2 future.
Olive trees are known for their capability to endure and recover from low stem water potentials (Ψstem <-6 MPa) and even maintain substantial gas exchange under such conditions. In the current experiment, we tested the physiological responses of mature olive trees in-situ while enduring such Ψstem under field conditions for seven months of rainless Israeli summer. To characterize olive trees' seasonal response to drought, we followed the annual patterns (a measurement day once a month) of Ψstem, gas exchange, osmotic adjustment, and hydraulic conductance of mature bearing olive trees that were disconnected from irrigation. We found that through the dry summer, the trees gradually dehydrated to a minimum of-8.1 MPa in October. An increase of 1320 mmol/kg in the leaf osmotic concentration through the summer was part of the tree's acclimation to the low Ψstem. Surprisingly, despite the low Ψstem that have probably led to substantial xylem cavitation, the trees maintained stomatal conductance of at least 0.023 mol m-2 s-1 and positive assimilation throughout the year. Following the winter rains, the plants recovered their Ψstem, gas exchange, and hydraulic conductance to maximal values. The results demonstrate olives' remarkable ability to tolerate Ψstem while maintaining above minimal gas exchange and producing fruits.
Leaf water potential (ψ leaf), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of ψ leaf (ψ leafPC) are most typical, yet, the practical complexity of the technique and of the underlying theory has led to ambiguous understanding of the conditions to optimize measurements. Consequently, specific techniques and precautions diversified across the global research community, raising questions of reliability and repeatability. Here, we surveyed specific methods of ψ leafPC from multiple laboratories, and synthesized experiments testing common assumptions and practices in ψ leafPC for diverse species: (i) the need for equilibration of previously transpiring leaves; (ii) leaf storage before measurement; (iii) the equilibration of ψ leaf for leaves on bagged branches of a range of dehydration; (iv) the equilibration of ψ leaf across the lamina for bagged leaves, and the accuracy of measuring leaves with artificially “elongated petioles”; (v) the need in ψ leaf measurements for bagging leaves and high humidity within the chamber; (vi) the need to avoid liquid water on leaf surfaces; (vii) the use of “pulse” pressurization versus gradual pressurization; and (viii) variation among experimenters in ψ leafPC determination. Based on our findings we provide a best practice protocol to maximize accuracy, and provide recommendations for ongoing species‐specific tests of important assumptions in future studies. This article is protected by copyright. All rights reserved.
Despite its importance for crop water use and productivity, especially in drought-affected environments, the underlying mechanisms of variation in intrinsic water use efficiency (iWUE = net photosynthesis/stomatal conductance for water vapour, g sw) are not well understood, especially in C4 plants. Recently, we discovered that leaf width (LW) correlated negatively with iWUE and positively with g sw across several C4 grasses. Here, we confirmed these relationships within 48 field-grown genotypes differing in LW in Sorghum bicolor, a C4 crop adapted to dry and hot conditions. We measured leaf gas exchange and modelled leaf energy balance three times a day, alongside anatomical traits as potential predictors of iWUE. LW correlated negatively with iWUE and stomatal density, but positively with g sw, interveinal distance of longitudinal veins (IVDL) and the percentage of stomatal aperture relative to maximum. Energy balance modelling showed that wider leaves needed to open their stomata more to generate a more negative leaf-to-air temperature difference, especially at midday when air temperatures exceeded 40ºC. These results highlight the important role that LW plays in shaping iWUE through coordination of vein and stomatal traits and by affecting stomatal aperture. Therefore, LW could be used as a predictor for higher iWUE among sorghum genotypes.
This article is protected by copyright. All rights reserved.
The physiological traits that set the tradeoff between productivity and drought adaptation in plants are still under debate. To reveal these traits, we compared the water relations of two olive (Olea europaea) cultivars: 'Barnea'- a highly productive modern cultivar; and 'Souri'- a drought-adapted traditional cultivar. We hypothesized that Souri has lower hydraulic conductivity and lower hydraulic vulnerability.
The hypothesis was tested at the leaf level. The soil volumetric water content (θ), stem water potential (ΨS) and gas exchange were measured in both cultivars while they dried until a significant reduction in their maximal photochemical potential (Fv/Fm < 0.6) was obtained. Additionally, pressure-volume relations, leaf hydraulic vulnerability and the petiole xylem architecture were evaluated.
To our surprise, Souri's leaf hydraulic conductivity was more vulnerable to low ΨS, approaching zero at -8 MPa compared with <-10 MPa in 'Barnea'. At the same time, Souri's higher osmotic content and cell rigidity enabled it to sustain 1.4 MPa lower ΨS, while maintaining near optimal (Fv/Fm). However, both cultivars significantly reduced their Fv/Fm (< 0.6) at the same θ, suggesting that the capability to sustain a low θ is not the issue. Instead, Souri's lower transpiration enabled it to withstand a longer drought while avoiding low θ. Barnea's larger xylem vessels and hydraulic conductivity supported higher stomatal conductance (gs) and assimilation rate, which nurtured its higher productivity but resulted in quick depletion of θ.
These results suggest that hydraulic resistance or the ability to sustain low θ do not set the tradeoff between productivity and drought adaptation in olive leaves.
Due to water shortages in several places in the world, alternative water sources such as atmospheric water and greywater have been studied. Dew water harvesting by passive radiative cooling is an unconventional water source that is easy to use, install, and shows great potential in several places in the world. This paper aims to experimentally evaluate the dew yield through passive radiative cooling in Vicosa city, Minas Gerais, Brazil by using standard white plastic for dew harvesting, developed by the International Organization for Dew Utilization (OPUR), polypropylene plastic, black plastic, packaging tape, and anodized aluminum as condensing surfaces during two different periods. The polypropylene plastic and packing tape materials used in this present research have not been researched before in the literature. However, they have demonstrated potential for harvesting dew. As a result, the average water collected was, respectively, 0.151, 0.135, 0.140, 0.127, and 0.046 mm/night using the OPUR standard plastic, polypropylene plastic, black plastic, and packaging tape, and anodized aluminum as condensing surfaces. Although relatively small water volumes were harvested, this water should not be neglected since it can supplement the water demand for irrigation, human and animal consumption, among other uses in drought periods. HIGHLIGHTS
Water vapor can be harvested to supplement traditional water sources using low-cost materials.;
We analyzed two low-cost materials that have not been studied for dew harvesting yet.;
There is no statistically significant difference between the amount harvested by each low-cost material and the amount harvested by the OPUR standard plastic.;
Models of guard cell dynamics, built on the OnGuard platform, have provided quantitative insights into stomatal function, demonstrating substantial predictive power. However, the kinetics of stomatal opening predicted by OnGuard models were 3‐ to 5‐fold slower than observed in vivo. No manipulations of parameters within physiological ranges yielded model kinetics substantially closer to these data, thus highlighting a missing component in model construction. One well‐documented process influencing stomata is the constraining effect of the surrounding epidermal cells on guard cell volume and stomatal aperture. Here we introduce a mechanism to describe this effect in OnGuard2 constructed around solute release and a decline in turgor of the surrounding cells and its subsequent recovery during stomatal opening. The results show that this Constraint‐Relaxation‐Recovery mechanism in OnGuard2 yields dynamics that are consistent with experimental observations in wild‐type Arabidopsis and it predicts the altered opening kinetics of ost2 H⁺‐ATPase and slac1 Cl‐ channel mutants. Thus, incorporating solute flux of the surrounding cells implicitly through their constraint on guard cell expansion provides a satisfactory representation of stomatal kinetics, and it predicts a substantial and dynamic role for solute flux across the apoplastic space between the guard cells and surrounding cells in accelerating stomatal kinetics.
Photosynthesis is limited by three main factors: stomatal conductance (g s), mesophyll conductance (g m) and maximum capacity for Rubisco carboxylation (V cmax). It is unclear how limiting factors vary under stress, particularly during long-term stress acclimation. In this work, we compared for the first time photosynthesis limitation resulting from long-term acclimation to three major abiotic stresses: drought, salinity and temperature. We used saplings of Ziziphus spina-christi, a thermophilic and drought-tolerant tree, which recently became more abundant in the Mediterranean, presumably due to increased winter temperatures. Stress acclimation was investigated by measuring growth, gas exchange, chlorophyll fluorescence and leaf structure. For each stress, photosynthesis-limiting factors were compared. We developed an integrative stress index that allowed us to precisely define stress level, enabling a comparison between stress types. Photosynthesis under all stresses was limited mostly by g s and g m (80-90%); whereas biochemistry (V cmax) made a minor contribution (10-20%). The relative contribution of g s and g m on photosynthetic limitation was influenced by stress type. During acclimation to drought or salinity, photosynthesis was limited by a decline in g s , while intolerance to low temperatures was driven by decline in g m. In all the stresses, g m decreased only under progressive reduction in leaf physiological func-tionality and was associated with low turgor under drought, an increase in leaf Na + under salinity and low leaf hydraulic conductance (K leaf) at low temperatures. Mesophyll structure (mesophyll surface area exposed to the intercellular air spaces, leaf thickness, % intercellular air spaces) did not explain g m acclimation to stress. Current work gives methodology for stress studies, and defines the main factors underlying the plant response to climate change. The ability to minimize mesophyll-imposed limitations on photosynthesis was found as a strong indicator of progressive stress tolerance. Moreover, the results demonstrate how warming climate benefits the photosynthetic function in thermophilic species, such as Ziziphus spina-christi.tr
Contents
Summary
693
I.
Introduction
693
II.
Comparison of various definitions and measurement techniques of minimum conductance
694
III.
Cuticular conductance
695
IV.
Contribution of stomata
696
V.
Environmental and ecological variation in minimum conductance
696
VI.
Use of minimum conductance in models
698
VII.
Conclusions
703
Acknowledgements
703
References
703
Summary
When the rate of photosynthesis is greatly diminished, such as during severe drought, extreme temperature or low light, it seems advantageous for plants to close stomata and completely halt water loss. However, water loss continues through the cuticle and incompletely closed stomata, together constituting the leaf minimum conductance (gmin). In this review, we critically evaluate the sources of variation in gmin, quantitatively compare various methods for its estimation, and illustrate the role of gmin in models of leaf gas exchange. A literature compilation of gmin as measured by the weight loss of detached leaves is presented, which shows much variation in this trait, which is not clearly related to species groups, climate of origin or leaf type. Much evidence points to the idea that gmin is highly responsive to the growing conditions of the plant, including soil water availability, temperature and air humidity – as we further demonstrate with two case studies. We pay special attention to the role of the minimum conductance in the Ball–Berry model of stomatal conductance, and caution against the usual regression‐based method for its estimation. The synthesis presented here provides guidelines for the use of gmin in ecosystem models, and points to clear research gaps for this drought tolerance trait.
Contents
Summary
93
I.
Introduction
93
II.
Influence of the speed of gs responses on A and Wi
93
III.
Determinants of the rapidity of gs responses
95
IV.
Conclusion
97
Acknowledgements
97
References
97
Summary
Stomatal movements control CO2 uptake for photosynthesis and water loss through transpiration, and therefore play a key role in plant productivity and water use efficiency. The predicted doubling of global water usage by 2030 mean that stomatal behaviour is central to current efforts to increase photosynthesis and crop yields, particularly under conditions of reduced water availability. In the field, slow stomatal responses to dynamic environmental conditions add a temporal dimension to gaseous fluxes between the leaf and atmosphere. Here, we review recent work on the rapidity of stomatal responses and present some of the possible anatomical and biochemical mechanisms that influence the rapidity of stomatal movements.
Soil water transported via the petiole is a primary rehydration pathway for leaves of water stressed plants. Leaves may also rehydrate by absorbing water via their epidermal surfaces. The mechanisms and physiological relevance of this water pathway, however, remain unclear, as the associated hydraulic properties are unknown. To gain insight into the foliar water absorption process, we compared rehydration kinetics via the petiole and surface of Prunus dulcis and Quercus lobata leaves. Petiole rehydration could be described by a double exponential function suggesting that two partly isolated water pools exist in leaves of both species. Surface rehydration could be described by a logistic function, suggesting that leaves behave as a single water pool. Whereas full leaf rehydration via the petiole required approximately 20 minutes, it took over 150 and 300 minutes via the surface of P. dulcis and Q. lobata, respectively. Such differences were attributed to the high resistance imposed by the leaf surface and especially the cuticle. The minimum resistance to surface rehydration was estimated to be 6.6 x 102 (P. dulcis) and 2.6 x 103 MPa m2 s g-1 (Q. lobata), which is remarkably higher than estimated for petiole rehydration. These results are discussed in a physiological context.
Stomatal oscillations are cyclic opening and closing of stomata, presumed to initiate from hydraulic mismatch between leaf water supply and transpiration rate. To test this assumption, mismatches between water supply and transpiration were induced using manipulations of vapour pressure deficit (VPD) and light spectrum in banana ( Musa acuminata ). Simultaneous measurements of gas exchange with changes in leaf turgor pressure were used to describe the hydraulic mismatches.
An increase of VPD above a certain threshold caused stomatal oscillations with variable amplitudes. Oscillations in leaf turgor pressure were synchronized with stomatal oscillations and balanced only when transpiration equaled water supply. Surprisingly, changing the light spectrum from red and blue to red alone at constant VPD also induced stomatal oscillations – while the addition of blue (10%) to red light only ended oscillations. Blue light is known to induce stomatal opening and thus should increase the hydraulic mismatch, reduce the VPD threshold for oscillations and increase the oscillation amplitude. Unexpectedly, blue light reduced oscillation amplitude, increased VPD threshold and reduced turgor pressure loss. These results suggest that additionally, to the known effect of blue light on the hydroactive opening response of stomata, it can also effect stomatal movement by increased xylem–epidermis water supply.
Stomatal responses to changes in vapour pressure deficit (VPD) constitute the predominant form of daytime gas exchange regulation in plants. Stomatal closure in response to increased VPD is driven by the rapid up-regulation of foliar ABA biosynthesis and ABA levels in angiosperms, however very little is known about the physiological trigger for this increase in ABA biosynthesis at increased VPD. Using a novel method of modifying leaf cell turgor by the application of external pressures, we test whether changes in turgor pressure can trigger increases in foliar ABA levels over 20 minutes, a period of time most relevant to the stomatal response to VPD. We found in angiosperm species that the biosynthesis of ABA was triggered by reductions in leaf turgor; and in two species tested, that a higher sensitivity of ABA synthesis to leaf turgor corresponded with a higher stomatal sensitivity to VPD. In contrast, representative species from non-flowering plant lineages did not show a rapid turgor triggered increase in foliar ABA levels, which is consistent with previous studies demonstrating passive stomatal responses to changes in VPD in these lineages. Our method provides a new tool for characterising the response of stomata to water availability.
A widespread perception is that, with increasing wind speed, transpiration from plant leaves increases. However, evidence suggests that increasing wind speed enhances carbon dioxide (CO2) uptake while reducing transpiration due to more efficient convective cooling (under high solar radiation loads). We provide theoretical and experimental evidence that leaf water use efficiency (WUE, carbon uptake per water transpired) commonly increases with increasing wind speed, thus improving plants' ability to conserve water during photosynthesis. Our leaf-scale analysis suggests that the observed global decrease in near-surface wind speeds could have reduced WUE at a magnitude similar to the increase in WUE attributed to global rise in atmospheric CO2 concentrations. However, there is indication that the effect of long-term trends in wind speed on leaf gas exchange may be compensated for by the concurrent reduction in mean leaf sizes. These unintuitive feedbacks between wind, leaf size and water use efficiency call for re-evaluation of the role of wind in plant water relations and potential re-interpretation of temporal and geographic trends in leaf sizes.
Bioassays are commonly used to study stomatal phenotypes. There are multiple options in the choice of plant materials and species used for observation of stomatal and guard cell responses in vivo. Here, detailed procedures for bioassays of stomatal responses to abscisic acid (ABA) in Arabidopsis thaliana are described, including ABA promotion of stomatal closure, ABA inhibition of stomatal opening, and ABA promotion of reaction oxygen species (ROS) production in guard cells. We also include an example of a stomatal bioassay for the guard cell CO2 response using guard cell-enriched epidermal peels from Brassica napus. Highly pure preparations of guard cell protoplasts can be produced, which are also suitable for studies on guard cell signaling, as well as for studies on guard cell ion transport. Small-scale and large-scale guard cell protoplast preparations are commonly used for electrophysiological and -omics studies, respectively. We provide a procedure for small-scale guard cell protoplasting from A. thaliana. Additionally, a general protocol for large-scale preparation of guard cell protoplasts, with specifications for three different species, A. thaliana, B. napus, and Vicia faba is also provided.
The efficiency and stress tolerance of leaf water transport are key indicators of plant function, but our ability to assess these processes is constrained by gaps in our understanding of the water transport pathway in leaves. A major challenge is to understand how different pools of water in leaves are connected to the transpiration stream and, hence, determine leaf capacitance (Cleaf) to short- and medium-term fluctuations in transpiration. Here, we examine variation across an anatomically and phylogenetically diverse group of woody angiosperms in two measures of Cleaf assumed to represent bulk-leaf capacitance (Cbulk) and the capacitance of leaf tissues that influence dynamic changes in leaf hydration (Cdyn). Among species, Cbulk was significantly correlated with leaf mass per unit area, whereas Cdyn was independently related to leaf lignin content (%) and the saturated mass of leaf water per unit dry weight. Dynamic and steady-state measurements of leaf hydraulic conductance (Kleaf) agreed if Cdyn was used rather than Cbulk, suggesting that the leaf tissue in some species is hydraulically compartmentalised and that only a proportion of total leaf water is hydraulically well connected to the transpiration stream. These results indicate that leaf rehydration kinetics can accurately measure Kleaf with knowledge of the capacitance of the hydraulic pathway.
In leaves, the transpirational flux of water exits the veins as liquid, and travels toward the stomata in both the vapor and liquid phases before exiting the leaf as vapor. Yet whether most of the evaporation occurs from the vascular bundles ('perivascular'), from the photosynthetic mesophyll cells, or within the vicinity of the stomatal pore ('peristomatal'), remains in dispute. Here a 1D model of the competition between liquid and vapor transport is developed from the perspective of non-isothermal coupled heat and water molecule transport in a composite media of airspace and cells. An analytical solution to the model is found in terms of the energy and transpirational fluxes from the leaf surfaces and the absorbed solar energy load, leading to mathematical expressions for the proportions of evaporation accounted for by the vascular, mesophyll, and epidermal regions. The distribution of evaporation in a given leaf is predicted to be variable, changing with the local environment, and to range from dominantly perivascular to dominantly peristomatal depending on internal leaf architecture, with mesophyll evaporation a subordinate component. Using mature Quercus rubra L. (red oak) trees, we show that the model can be solved for a specific instance of a transpiring leaf by combining gas exchange data, anatomical measurements, and hydraulic experiments. We also investigate the effect of radiation load on the control of transpiration, the potential for condensation on the inside of an epidermis, and the impact of vapor transport on the hydraulic efficiency of leaf tissue outside the xylem.
Motivated by studies suggesting that the stomata of ferns and lycophytes do not conform to the standard "active" ABA-mediated stomatal control model, we examined stomatal behaviour in a conifer species (Metasequoia glyptostroboides), phylogenetically midway between the fern and angiosperm clades. Similar to ferns, daytime stomatal closure in response to moderate water stress appeared to be a passive-hydraulic process in Metasequoia, immediately alleviated by rehydrating excised shoots. Only after prolonged exposure to more extreme water stress did active ABA-mediated stomatal closure become important, as foliar ABA production was triggered after leaf turgor loss. The influence of foliar ABA on stomatal conductance and stomatal aperture was highly predictable and additive with the passive-hydraulic influence. Metasequoia thus occupies a stomatal behaviour type intermediate between the passively controlled ferns and the characteristic ABA-dependent stomatal closure described in angiosperm herbs. These results highlight the importance of considering phylogeny as a major determinant of stomatal behaviour.
Leaves within a canopy may experience rapid and extreme fluctuations in ambient conditions. A shaded leaf, for example, may become exposed to an order of magnitude increase in solar radiation within a few seconds, due to sunflecks or canopy motions. Considering typical time scales for stomatal adjustments, (2 to 60 minutes), the gap between these two time scales raised the question whether leaves rely on their hydraulic and thermal capacitances for passive protection from hydraulic failure or over-heating until stomata have adjusted. We employed a physically based model to systematically study effects of short-term fluctuations in irradiance on leaf temperatures and transpiration rates. Considering typical amplitudes and time scales of such fluctuations, the importance of leaf heat and water capacities for avoiding damaging leaf temperatures and hydraulic failure were investigated. The results suggest that common leaf heat capacities are not sufficient to protect a non-transpiring leaf from over-heating during sunflecks of several minutes duration whereas transpirative cooling provides effective protection. A comparison of the simulated time scales for heat damage in the absence of evaporative cooling with observed stomatal response times suggested that stomata must be already open before arrival of a sunfleck to avoid over-heating to critical leaf temperatures. This is consistent with measured stomatal conductances in shaded leaves and has implications for water use efficiency of deep canopy leaves and vulnerability to heat damage during drought. Our results also suggest that typical leaf water contents could sustain several minutes of evaporative cooling during a sunfleck without increasing the xylem water supply and thus risking embolism. We thus submit that shaded leaves rely on hydraulic capacitance and evaporative cooling to avoid over-heating and hydraulic failure during exposure to typical sunflecks, whereas thermal capacitance provides limited protection for very short sunflecks (tens of seconds).
Drought stress is known to limit photosynthesis rates and to inflict photo-oxidative damage in grapevines. Grapevines, which are considered drought-tolerant plants, are characterized by diverse hydraulic and photosynthetic behaviors, depending on the cultivar. This research compared the photosynthesis and the photorespiration of Cabernet Sauvignon (Cs) (isohydric) and Shiraz (anisohydric) in an attempt to acquire a wider perspective on the iso/anisohydric phenomenon and its implications. Shiraz and Cs were subjected to terminal drought in the greenhouse. Soil water content (θ), leaf water potential (Ψ(l) ) and stomata conductance (g(s) ) were measured to determine the cultivars' hydraulic behavior. Gas exchange and fluorometry measurements were taken at 21 and 2% O(2) to acquire photosynthesis and photorespiration characteristics. Cs was found to behave in a near isohydric manner whereas Shiraz behaved in a near anisohydric manner. Compared to Shiraz, the reduced stomata conductance values of Cs were accompanied by higher water use efficiency and photorespiration rates, as well as photosystem II photochemical potential (F(v) /F(m) ). As compared with Shiraz, Cs compensated for lower stomata conductance by higher photosynthesis and photorespiration. These two processes contributed to higher electron flow rates that might have a role in photoinhibition avoidance, which was observed in the stability of F(v) /F(m) under drought stress.
Humidity in a small area of a Vicia faba L. leaf was perturbed with a flow of dry air from an 80 µm (inside diameter) needle, while the remainder of the leaf was maintained at high and constant humidity. The influence of the needle flow on the humidity at the leaf surface was quantified by using a spatially explicit dewpoint hygrometer to observe condensation patterns. When the dry air from a needle was applied to the leaf, stomata within the influence of the needle opened within the first few minutes of the perturbation, and local epidermal turgor pressure declined within the same time frame. When the needle flow was removed from the leaf, these responses were reversed, but with more variable kinetics. Stomata and epidermal cells outside the influence of the needle flow, which were exposed to a constant and high humidity, showed similar, but smaller, responses when the needle flow was applied to the leaf. Since the opening of these stomata should have had only a small effect on transpiration (because of the high humidity), it is likely that the reduction in epidermal turgor was the cause (rather than the result) of the stomatal opening. The magnitude of the turgor response was only loosely related to the distance from the needle flow up to distances of almost 400 µm. The data support the idea that neighbouring stomata can interact through the influence of transpiration on epidermal turgor.
Abstract According to computer energy balance simulations of horizontal thin leaves, the quantitative effects of stomatal distribution patterns (top vs. bottom surfaces) on transpiration (E) were maximal for sunlit leaves with high stomatal conductances (gs) and experiencing low windspeeds (free or mixed convection regimes). E of these leaves decreased at windspeeds > 50 cm s−1, despite increases in the leaf-to-air vapour density deficit. At 50 cm s−1 wind-speed, rapidly transpiring leaves had greater E when one-half of the stomata were on each leaf surface (amphistomaty; 10.16 mmol H2O m−2 s−1) than when all stomata were on either the top (hyperstomaty; 9.34 mmol m−2s−1) or bottom (hypostomaty; 7.02 mmol m−2s−1) surface because water loss occurred in parallel from both surfaces. Hyperstomatous leaves had larger E than hypostomatous leaves because free convection was greater on the top than on the bottom surface. Transpiration of leaves with large g, was greatest at windspeeds near zero when ∼60–75% of the stomata were on the top surface, while at high windspeeds E was greatest with, 50% of the stomata on top. For leaves with low gs, stomatal distribution exerted little influence on simulated E values. Laboratory measurements of water loss from simulated hypo-, hyper-, and amphistomatous leaf models qualitatively supported these predictions.
Stomatal opening in the light, observed in nearly all vascular land plants, is essential for providing access to atmospheric CO2 for photosynthesis. The speed of stomatal opening in the light is critical for maximizing carbon gain in environments in which light intensity changes, yet we have little understanding of how other environmental signals, particularly evaporative demand driven by vapour pressure deficit (VPD) influences the kinetics of this response. In angiosperms, and some fern species from the family Marsileaceae, a mechanical interaction between the guard cells and the epidermal cells determines the aperture of the pore. Here we examine whether this mechanical interaction influences the speed of stomatal opening in the light. To test this, we investigated the speed of stomatal opening in response to light across a range of VPDs in seven plant species spanning the evolutionary diversity of guard cell and epidermal cell mechanical interactions. We found that stomatal opening speed is a function of evaporative demand in angiosperm species and Marsilea, which have guard cell and epidermal cell mechanical interactions. Stomatal opening speeds did not change across a range of VPD in species of gymnosperm and fern, which do not have guard cell mechanical interactions with the epidermis. We find that guard cell and epidermal cell mechanical interactions may play a key role in regulating stomatal responsiveness to light. These results provide valuable insight into the adaptive relevance of mechanical advantage.
The goal of this study was to check the feasibility of the use of dendrometers to monitor water stress levels in avocado trees (Persea americana Mill, cv. Hass) during the period of 2015-2017. This water stress was generated naturally during events of high vapor pressure deficit (VPD) that normally take place during the Israeli spring (May) and late autumn (September). It was also generated artificially by considering two irrigation regimes, one providing the crop water needs estimated using lysimeter data, and one applying 75% of that amount. The different water regimes and climatic conditions allowed us to investigate the impact of seasonal atmospheric changes on transpiration (T) and on the maximum trunk daily shrinkage (MDS) of the trees. The MDS captured well the differences between the irrigation treatments, showing over 100 µm increase at peak summer in response to the treatment providing 75% of the irrigation dose applied to the well-watered treatment. We found that T increased linearly with the daily average value of VPD up to around 1.3 kPa, above which the slope of the linear relationship decreased with the increase in VPD. This reduction of T induced moderate changes in MDS under high VPD. The relationships between MDS and VPD and between MDS and T depended on the season, exhibiting substantially lower MDS during the spring and early summer. The hysteretic nature of these relationships indicates that MDS values are related to tree phenology: a VPD of 1.3 kPa corresponds to an MDS of 50 µm in the first half of the year (flowering and fruit set) and to an MDS of 150 µm during the second half of the year (fruit growth and maturation). Finally, we found that MDS was highly correlated with the stem water potential, regardless of the irrigation treatment or time of year, and is, therefore, a reliable stress indicator in avocado trees as long as VPD and phenological stage of the tree are considered.
In this paper, we investigate quantitatively the coupling between gradients of temperature and of chemical or water potential under steady-state conditions in the vapor phase. This coupling is important for the measurement and modeling of the dynamics of water in unsaturated environments such as soils and plants. We focus on a simple nonequilibrium scenario in which a gradient of temperature exists across an air-filled gap that separates two aqueous phases with no net transfer of water. This scenario is relevant for measurements of the water potential in environmental and industrial contexts. We use a tool, a microtensiometer, to perform these measurements. We observed variations of the water potential with a difference of temperature across the air gap of −7.9±0.3MPaK−1, in agreement with previous measurements. Our result is close to a first-order theoretical prediction, highlighting that most of the effect comes from the variation of saturation pressure with temperature. We then show that thermodiffusion (Soret effect) coupled to natural convection could occur in our experiment and discuss how these effects could explain the small discrepancy observed between measurements and first-order theoretical predictions.
Trees typically experience large diurnal depressions in water potential, which may impede carbon export from leaves during the day because the xylem is the source of water for the phloem. As water potential becomes more negative, higher phloem osmotic concentrations are needed to draw water in from the xylem. Generating this high concentration of sugar in the phloem is particularly an issue for the ~50% of trees that exhibit passive loading. These ideas motivate the hypothesis that carbon export in woody plants occurs predominantly at night, with sugars that accumulate during the day assisting in mesophyll turgor maintenance or being converted to starch. To test this, diurnal and seasonal patterns of leaf non-structural carbohydrates, photosynthesis, solute, and water potentials were measured, and carbon export was estimated in leaves of five mature (> 20 m tall) red oak (Quercus rubra) trees, a species characterized as a passive loader. Export occurred throughout the day at equal or higher rates than at night despite a decrease in water potential to -1.8 MPa at midday. Sucrose and starch accumulated over the course of the day, with sucrose contributing approximately 50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always greater than 0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of NSCs in leaf turgor maintenance.
By controlling the opening and closure of the stomatal pores through which gas exchange occurs, guard cells regulate two of the most important plant physiological processes: photosynthesis and transpiration. Accordingly, guard cells have evolved exquisite sensory systems. Here we summarize recent literature on guard cell sensing of light, drought (via the phytohormone abscisic acid (ABA)), and CO2. New advances in our understanding of how guard cells satisfy the energetic and osmotic requirements of stomatal opening and utilize phosphorylation to regulate the anion channels and aquaporins involved in ABA-stimulated stomatal closure are highlighted. Omics and modeling approaches are providing new information that will ultimately allow an integrated understanding of guard cell physiology.
The effect of pressure-increase rate (PIR) using the pressure-chamber technique on stem water potential measurements was studied in field-grown trees of apple (Malus domestica), nectarine (Prunus persica) and pear (Pyrus communis). Two aspects were examined: 1. The effects of five continuous PIRs from 0.03 up to 0.3 MPa s-1; 2. The effects of various PIRs at the beginning of the measurement (0.03, 0.14 and 0.3 MPa s-1) followed by a low PIR (0.03 MPa s-1) close to the endpoint. Midday stem water potential readings decreased with increasing PIR in apple, pear and nectarine. Stem water potentials at the lowest PIR were lower by about 0.03 MPa than the values obtained by extrapolating to zero PIR. Stem water potential increased with increasing initial PIR followed by 0.03 MPa s-1 towards the endpoint. Our data suggest that a PIR of 0.03 MPa s-1 is acceptable for practical irrigation scheduling in deciduous trees, and that this value should not be exceeded. High PIR at the beginning of the measurement followed by a low PIR prior to the endpoint should be avoided in deciduous trees because of an unacceptably high error. Use of a standard, continuous PIR will reduce the impact of the small inaccuracy associated with a PIR of 0.03 MPa s-1 if the thresholds for irrigation scheduling are also determined at that PIR.
Stomata1 movement is a manifestation of strain in the epidermis, associated with change in the hydraulic pressure in the epidermal cells. This chapter discusses the role and behavior of stomata in the hydrology of the soil–plant–atmosphere system. Stomata operate in the light in such a way as to maintain positive turgor in the leaves, in the majority of crop plants and other species. The transient response of stomata to changes in environment which cause rapid changes in plant–water relations is quite dramatic. The nature of the stomatal response to change in water potential must involve the mechanical and hydraulic attributes of the stomatal apparatus. There is only a tenuous relationship between the potential of water in leaf tissue in bulk and the local potential of water to which an individual stoma responds in ways, which are also discussed. Thus the transient response of stomata to change in rate of evaporation may be a device which is designed to enhance the speed of the response to light.
Contents 'Summary'I.II.III.IV.V. ReferencesSummaryStomata are an attractive experimental system in plant biology, because the responses of guard cells to environmental signals can be directly linked to changes in the aperture of stomatal pores. In this review, the mechanics of stomatal movement are discussed in relation to ion transport in guard cells. Emphasis is placed on the ion pumps, transporters, and channels in the plasma membrane, as well as in the vacuolar membrane. The biophysical properties of transport proteins for H+, K+, Ca2+, and anions are discussed and related to their function in guard cells during stomatal movements. Guard cell signaling pathways for ABA, CO2, ozone, microbe-associated molecular patterns (MAMPs) and blue light are presented. Special attention is given to the regulation of the slow anion channel (SLAC) and SLAC homolog (SLAH)-type anion channels by the ABA signalosome. Over the last decade, several knowledge gaps in the regulation of ion transport in guard cells have been closed. The current state of knowledge is an excellent starting point for tackling important open questions concerning stress tolerance in plants.
Since 2005 an unresolved debate has questioned whether R-shaped vulnerability curves (VCs) might be an artifact of the centrifuge method of measuring VCs. VCs with R-shape show loss of stem conductivity from approximately zero tension and if true this suggests that some plants either refill embolized vessels every night or function well with a high percentage of vessels permanently embolized. The R-shaped curves happen more in species with vessels greater than half the length of the segments spun in a centrifuge. Many have hypothesized that the embolism is seeded by agents (bubbles or particles) entering the stem end and traveling towards the axis of rotation in long-vessels causing premature cavitation. VCs were measured on Robinia pseudoacacia L. by the three different techniques to yield three different VCs; R-shaped: Cavitron P50 = 0.30 MPa and S-shaped: Air Injection P50 = 1.48 MPa and bench dehydration P50 = 3.57 MPa. Stem conductivity measured in the Cavitron was unstable and a function of vessel length when measured repeatedly with constant tension and this observation is discussed in terms of stability of air bubbles drawn into cut-open vessels during repeated Cavitron measurement of conductivity; hence R-shaped curves measured in a Cavitron are probably invalid.
Turgor (Ψp) and osmotic potential (Ψs) in epidermal and mesophyll cells, in-situ xylem water potential (Ψ-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (Ψp-epi, Ψp-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.Δg/Ψp-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.
Night-time transpiration (Enight) is potentially an important factor affecting whole-plant water balance and, thus, water use efficiency. The aims of the present study were: to evaluate night-time changes of stomatal conductance (gnight) and transpiration under different soil water availability conditions for seven grapevine cultivars and to compare leaf-level estimates of night-time water losses with more realistic whole-plant estimates in plants growing outdoors. Two experiments were conducted on seven grapevine cultivars (Vitis vinifera L.) growing in pots maintained at field capacity and drought stress conditions. Night transpiration was evaluated by leaf gas exchange and plant mass measurements. Results showed that Enight and gnight were far above cuticular values, suggesting sustained stomatal aperture during night-time which was reduced under drought stress. Differences between cultivars were found in the extent of drought stress-induced reduction of Enight (from a 36% reduction in Escursac to 82% in Malvasia). However, transpiration losses calculated on the basis of leaf gas exchange measurements often over-estimated the actual whole plant water loss, suggesting the presence of some water inputs to pots/plants during the night, presumably due to dew deposition. Significant stomatal opening persists during the night in irrigated grapevines, leading to substantial water losses by transpiration. Those water losses are significantly reduced under drought stress. It is remarkable that night-time plant water losses can be partially or fully compensated by dew deposition. Therefore, instantaneous leaf gas exchange measurements can result in an over-estimation of the night water losses. The present study shows for the first time that night transpiration due to partially open stomata can be compensated by dew deposition.
The relationship between stomatal aperture (a) and guard cell pressure (Pg) was measured directly in four different species (Vicia faba, Tradescantia virginiana, Ginkgo biloba and Nephrolepis exaltata) using a special cell pressure probe technique. The effect of epidermal turgor (Pep) on this relationship was also measured in T. virginiana. The relationship was sigmoidal for V. faba and T. virginiana, but entirely convex for G. biloba and N. exaltata. Epidermal turgor was found to have a pronounced closing effect on stomata of T. virginiana. Maximum aperture with full epidermal turgor (0·92 MPa) was about half that with zero epidermal turgor. Also, with full epidermal turgor stomata of T. virginiana did not begin to open until Pg was more than 1·25 MPa. These characteristics were used to develop an expression for a as a function of Pg and Pep. Results for the different species are compared and discussed in terms of possible advantages and limitations of water economy.
The method described depends on the fact that in adult leaves inspiration is stomatal rather than cuticular, so that, other things eing equal, the yield of watery vapour depends on the degree to Inch the stomata are open, and may be used as an index of their condition. In principle, it is the same as the methods of Merget and tahl. These observers used hygroscopic papers impregnated with eagents which change colour according as they are dry or damp, and tahl, who employed paper soaked in cobalt chloride, has obtained excellent results.
Leaf form and stomatal characteristics play an important part in the strategy of tall fescue (Festuca arundinacea Schreb) to limit water losses. Transpiration rates of artificially rolled leaves measured by potometer at varying wind velocities
support this assertion.
IN the recent discussion in Nature under this heading1–3, Rufelt1,3 has adopted Ivanoff's4 hypothesis that the transient increase in transpiration rate observed when a leaf is detached in air is caused “by a sudden release of the water stress in the conducting elements, which means that it is caused by an increase in the water supply to the leaf”1. He has further supposed that the increase in transpiration obtained after bathing the roots of intact plants with mannitol solution5, or 1 M sodium chloride1, is also due to the release of water stress in the plant, making the subsidiary hypothesis that the mannitol or sodium chloride causes an increase in the water permeability of the roots1. He concludes that this last effect is peculiar to the roots1, and since he writes of “the permeability barrier in the root”3 he must postulate that the transpiration stream here passes through the cells, though in the whole of the rest of the plant he supposes it to move in the cell walls only, except when it is in the xylem. He would appear to have obtained no data for the behaviour of the stomata in his experiments; his discussion of the supposed absence of ‘mechanical effects’ and the different permeabilities of guard cells and epidermal cells therefore seems too great an extrapolation from his results—transpiration rate can only be taken as a measure of stomatal diffusive conductance when there is a constant potential difference across the stomata.
The limitations on carbon dioxide assimilation by plants caused by stomata, particularly when the plant is under stress, are discussed. Mechanisms by which stomatal movement is integrated with photosynthesic requirements are described.
The experiments and simulations reported in this paper show that, for stomata sensitive to both CO2 and water vapour concentrations, responses of stomatal conductance (gws) to boundary layer thickness have two components, one resulting from changes in intercellular CO2 concentration (χci) and another from changes in leaf surface water vapour saturation deficit (Dws). The experiments and simulations also show that the boundary layer conductance (gwb) can significantly alter the apparent response of gws to ambient air CO2 mole fraction (χca) and water vapour mole fraction (χwa). Because of the feedback loop involved the responses of gws for χca and χwa each include responses to both χci and Dws. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χci and Dws—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of gws for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of gws to χci and Dws, the apparent responses of gws to Dwa and χca depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.
Illuminated leaf discs of Vicia faba were brought into equilibrium with a series of mannitol solutions. The width of stomatal aperture and the osmotic potential of guard cells and epidermal cells were determined. It was found that the maximal aperture was obtained when epidermal cells were at about incipient plasmolysis and that any increase in their turgor pressure brought about a decrease in stomatal aperture. These findings emphasize the importance of epidermal cells in determining the width of the stomatal pore.
A new method is described for determining the boundary layer resistance over wet filter paper exposed within a leaf cuvette, based on the energy balance of the filler paper. The boundary layer resistance is calculated by an iterative procedure from measurements of the relative humidity and temperature of the air in the cuvette. Comparisons between the new and the conventional method, involving measurement of the filter paper temperature, show close agreement.
To simplify the method further, a graph has been constructed for the relationship between boundary layer resistance and cuvette relative humidity at temperatures from 15 to 35°C, determined at one value of the ratio of the flow rate through the cuvette to the filter paper area.
An analysis of errors suggests that the new method is less sensitive than the conventional to errors in temperature and humidity.