No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Soil air content effects on the water status of avocado trees
Abstract
Avocado trees evolved in Andisoils, which are considered the optimum type of soils for tree growth due to their physical properties; mainly low bulk density (0.5 - 0.8 g cm-3) and high macro porosity (∼ 46%). In Chile, avocado orchards are mostly located in fine textured soils, with bulk densities between 1.3 and 1.5 g cm-3 and macro porosities close to 15%. Due to this fact, there are severe problems of poor root aeration which, in part, are responsible for the low production levels of the crop. The objective of this research was to study the effect of soil aeration in the root zone on avocado water status. The ultimate goal of this study was to generate information for developing irrigation management strategies for avocado orchards that optimize the air and water contents in the soil. The study was conducted during the 2004/05 season. Two-year-old avocado var. 'Hass' on Mexicola root stock were planted in 50 L pots using four soils of different texture. The treatments were TO: loam soil; T1: sandy soil; T2: sandy loam soil; T3: clay loam soil. Soil air content between 5% and 18% affected stomatal conductance, but not stem water potential. Soil air content lower than 17% restricted the oxygen diffusion rate to less than 20 μg cm-2 min-1, the threshold value for normal avocado tree development.
... Various management techniques to improve aeration of the root zone of avocado trees have been studied, as for example reducing irrigation frequency (Ferreyra et al. 2008). Other example is the use of soil amendments, such as the addition of earthworms, whose actions are related to soil porosity and fertility, and allow for the formation of humus, which promotes the availability of certain nutrients (López-Hernandez et al. 2004). ...
... Currently, some avocado orchards established on clay soils in the central zone of Chile are managed by applying high rates of Sulfuric Acid (SA) to the soil, because it supposed to promote a positive effect on tree physiology and crop production (Gardiazábal et al. 2007;Hussain et al. 2001); however, the effects of SA applications have been shown to increase soil electrical conductivity (Ferreyra et al. 2008) and to negatively affect soil biology (Mara and Horan 2003). So far, there is little evidence of the real effect of high doses of SA to the soil in avocado orchards, in terms of its physical and chemical characteristics. ...
... The experiment consisted of a completely randomized design with four treatments and four replications. The treatments were as follows: Control: 2 L of distilled water per pot; SA: application of 0.3 cc H 2 SO 4 (18 M) L −1 plus 1 L of distilled water every 30 days to achieve a pH of 2.5 in the solution (Ferreyra et al. 2008); HA: application of 9 cc HA in 2 L of distilled water every 15 days (commercial dose for vermicompost humic acid applications); and SA + HA: Sulfuric acid + Humic acid every 30 days plus Humic acid every 15 days (0.3 cc of H 2 SO 4 (18 M) L −1 plus 9 cc of HA Humic acid diluted in 500 ml of distilled water plus 500 ml of distilled water, every 30 days, alternated with 9 cc Humic acid in 2 L of distilled water every 15 days). Chemical composition of vermicompost humic acid used in the experiment was pH (soil:water, 1:2.5) 10.15; CE (soil:water, 1:2.5) (Orion STAR A 215, Thermo Scientific) 5.09 mS cm −1 ; organic matter content (Walkley-Black method) 33.09%, C 40.22%, N 3.49%, C/N 5.4; Cu 0.91 mg/kg; Mn (DTPA Method) 1.5 mg/kg; Zn 5 mg/kg, B 5 mg/kg; Ca 0.03%; Na 5231 mg/kg; 95.38% moisture, 4.62% dry matter. ...
Avocado crop is very sensitive to root asphyxiation. Among the agricultural management techniques, soil applications of humic and sulfuric acids became increasingly popular. In a trial with potted 'Hass' avocado trees grown on a silty-clay soil, different soil treatments were applied to determine their effects on tree physiology and soil characteristics: Control, deionized water; SA, H2SO4 application; HA, humic acid application; SA+HA, combination of both treatments. Treatments were applied during a 9-month period. SA reduced soil pH, rised electrical conductivity and reduced saturated soil hydraulic conductivity (Ks) in the upper layers of the potted soil. On the other hand, HA increased electrical conductivity and Ks compared with Control. None of the treatments improved CO2 assimilation, stomatal conductance, stem water potential, chlorophyll fluorescence, growth or nutrient content. Our findings suggest that a rapid and positive response to the application of acids is unlikely to be observed under conditions of restricted oxygen in the root zone.
... Feeder roots are mostly found 30 to 40 cm deep, and are distinguished by a whitish unsuberized color with few or no absorbing hairs (Chanderbali et al., 2013). Roots can penetrate 60 cm or more in favorable soils (Chanderbali et al., 2013;Wolstenholme, 2013), with adequate oxygenation, porous and rich in organic matter (Wolstenholme, 2013), such as andosols (Ferreyra et al., 2008). ...
... Twenty-two samples were taken, at biweekly intervals, and each shoot was labeled and its length and diameter (both in cm) were measured. 60 cm o más en suelos favorables (Chanderbali et al., 2013;Wolstenholme, 2013), con adecuada oxigenación, porosos y ricos en materia orgánica (Wolstenholme, 2013), como los andosoles (Ferreyra et al., 2008). ...
... Feeder roots are mostly found 30 to 40 cm deep, and are distinguished by a whitish unsuberized color with few or no absorbing hairs (Chanderbali et al., 2013). Roots can penetrate 60 cm or more in favorable soils (Chanderbali et al., 2013;Wolstenholme, 2013), with adequate oxygenation, porous and rich in organic matter (Wolstenholme, 2013), such as andosols (Ferreyra et al., 2008). ...
... Twenty-two samples were taken, at biweekly intervals, and each shoot was labeled and its length and diameter (both in cm) were measured. 60 cm o más en suelos favorables (Chanderbali et al., 2013;Wolstenholme, 2013), con adecuada oxigenación, porosos y ricos en materia orgánica (Wolstenholme, 2013), como los andosoles (Ferreyra et al., 2008). ...
Avocado is the third most produced crop in the State of Mexico, with 11,296 ha, where the predominant variety is ‘Hass.’ Due to a lack of knowledge about its development in different environments, its agronomic management is highly heterogeneous, since it is based on experiences in other states. The objective of this study was to analyze, describe and quantify the phenological development of ‘Hass’ avocado in three environments in the State of Mexico. The vegetative, flowering, root and fruit development of ‘Hass’ avocado was recorded during the 2011-2012 cycle. Two periods were distinguished for vegetative growth (December-April and October-November), flowering (December-February and August-October), harvest (November-February and August-October) and root growth (April-July and October-December). The vegetative growth (0.40 and 0.06 cm increase in shoot length and diameter, respectively) and root growth (36 and 24 g fresh weight and dry matter, respectively) were lower than fruit growth (70.1 mm increase in diameter) in Coatepec Harinas (temperate with andosol soil and isotherms from 14-18 °C). In contrast, the same growth measurements were higher in the localities with cambisol-luvisol soil and isotherms from 16-20 °C: Ixtapan del Oro (temperate/semi-warm, with 0.69 and 0.12 cm in shoot, and 56 and 48.8 g in root) and Temascaltepec (semi-warm, with 0.78 and 0.23 cm in shoot, and 69.3 and 31.3 g in root), but lower increases in fruit (59.4 and 56.6 mm, respectively). The phenological differences observed among environments will be useful for the technical management of the crop.
... In Chile, avocado production is located primarily in the Central Zone, a region where soil physical characteristics vary considerably [1]. Although most avocado orchards in this region are established in clay loam or clay soils, others are planted in sandy loam or sandy soils [2]. ...
... Some variables related to water status (gs, SWP) were significantly and positively associated with Ca in shoots in sandy and sandy loam soils but not in clay soil ( Figure 5). This confirms previous observations that absorption of Ca in woody trees is related to xylem sap movement, which in the case of avocado, is highly dependent on the air capacity of the soil [1,4,8,9]. This general overview of the influence of soil type on tree nutrition, water status and growth variables suggests that Ca content and partitioning is highly dependent on soil macroporosity and its interaction with plant physiological and growth responses. ...
A two-year study was conducted to determine how soil texture affects calcium (Ca) absorption and partitioning in potted ‘Hass’ avocado trees. Trees were planted in 200 L pots in one of four soil types: clay (C), clay loam (CL), sandy loam (SL) or sand (S). Prior to planting, Ca content in each soil was in the normal range of availability, although the Ca concentration was highest in C soil. After two years of tree development, dry weights of shoots and roots were significantly higher in the SL and S soils than in C soil. Trees in the C soil had higher wood dry weight than trees in SL or S soils. The Ca contents (absolute quantities, not concentrations) in the roots, shoots and whole tree were significantly lower in the C soil than in the SL or S soils. The K/Ca ratio of trees in the C soil (K/Ca = 1.5) was significantly higher than that in the other soil types. Stem water potential was significantly lower for trees in the C soil compared to the other soils. These results indicate that Ca absorption and partitioning in young avocado trees varies with soil texture, probably associated with soil effects on root growth and/or plant water status.
... Second, previous studies found that soil organic carbon (SOC), soil total nitrogen (TN), soil available nitrogen (AN), soil available phosphorus (AP), soil water content (SM) and soil pH had significant influences on bacterial communities during secondary forest succession (Jiang et al. 2021;Wang et al. 2019;Xu et al. 2021b). In addition, Liu et al. (2015) found that soil texture changed significantly during secondary forest succession; the variations in soil texture have significant influences on plant properties, root exudate composition (Vieira et al. 2020), soil oxygen content (Ferreyra et al. 2008) and soil water (Li et al. 2014), thereby further affecting soil bacterial diversity and community composition (Karimi et al. 2018;Xia et al. 2020). However, the effects of soil texture on the soil bacterial community structure during secondary forest succession have often been overlooked (Jiang et al. 2021;Liu et al. 2020;Qiang et al. 2021;Xu et al. 2021b). ...
Background and aimsCommunity dynamics, functions and driving factors of rhizosphere and bulk soil bacteria during secondary forest succession remain poorly understood in subalpine regions.Methods
Three typical successional stages (grassland, shrubland and secondary forest) were selected to analyse bacterial communities, functions and interactions in the rhizosphere and bulk soils using high-throughput sequencing technology.ResultsThe results showed no significant difference in the bacterial α-diversity in the rhizosphere soil, whereas the bacterial α-diversity in the bulk soil of the grassland was significantly lower than that of the shrubland and secondary forest. Bacterial β-diversity in the rhizosphere soil differed significantly among the three succession stages, while the bacterial β-diversity in the bulk soil in the shrubland and secondary forest was significantly different from that in the grassland. However, the potential bacterial functions of the carbon, nitrogen and sulfate cycles revealed a consistent response in the rhizosphere and bulk soils to secondary forest succession. The soil total phosphorus, ammonium nitrogen, ratio of carbon to phosphorus and pH were the main factors affecting bacterial communities and potential functional groups. Bacterial network complexity was highest in the secondary forest rhizosphere soil and the shrubland bulk soil. Different keystone bacteria were detected in the rhizosphere and bulk soils among the three successional stages; they play major role in maintaining ecosystem function and community structure.Conclusion
Our results demonstrate that the bacterial communities and interactions in the rhizosphere and bulk soils respond differently to secondary forest succession, while the bacterial functional groups revealed a consistent response.
... Careful water management is critical in the productivity of an avocado crop, as overwatering or under-watering can significantly reduce yields [16,[43][44][45]. Determining when and how much to irrigate is difficult for a grower by visual monitoring or by hand sample. ...
The irrigated agriculture sector has been facing an increased scarcity of good quality water worldwide. Consequently, the sustainability of water intensive crops, such as avocado, is threatened when water becomes scarce and expensive, or when growers must use saline water supplies that reduce crop yields. A variety of irrigation technologies and water management practices are now recommended to help growers through times of limited water supplies and elevated salinity levels. To examine how growers adopt different practices and combinations of practices, we collected data from a sample of avocado growers in California. We used Kohonen self-organizing maps, and developed logit models to identify the most common bundles of technologies and management practices that growers are using to deal with water scarcity. We test the validity of the proposed bundles and factors affecting their adoption, using primary data obtained from a survey of California avocado growers at the height of the drought during 2012–2013. Results show that farm location, share of income from agricultural production, use of cooperative extension advice, and farmer characteristics, such as age and education, all play important roles in grower adoption of individual and bundled methods to adapt to water scarcity.
... A universal critical soil oxygen level for plant growth may not be defined (Armstrong & Drew 2002). Recent studies that focused on the oxygen requirements of avocado plants had defined critical values of air-to-water ratio or critical oxygen diffusion rate values (Ferreyra et al. 2007(Ferreyra et al. , 2008Gil et al. 2012); however, these values do not indicate a critical soil oxygen level, and thus are not comparable with our measurements. In early studies, in which avocado seedlings were grown for two months in containers with controlled oxygen levels above the root surface, it was demonstrated that oxygen levels below 2% were lethal to the plants while 5% of oxygen lead to severe leaf burn and hindered root and shoot growth in relations to 10% or 21% of oxygen (Valoras et al. 1964;Stolzy et al. 1967). ...
Treated wastewater (TWW) is a major source of water for agriculture in Israel; however, recent reports indicate a marked yield loss in TWW-irrigated avocado and citrus orchards planted in clayey soils. The association of the yield loss with clayey soils rather than sandy soils suggests that it is associated with conditions in the root zone, and specifically poor aeration. A three-year study (2012–2015) was conducted in an avocado orchard planted in clayey soil, comparing the oxygen and redox conditions in the root zone of TWW-irrigated plots with fresh water (FW)-irrigated plots, together with the physiological status of the trees. Soil parameters included: continuous in-situ measurement of soil-water tension (SWT), soil oxygen, and soil redox potential, and periodic measurements of soil solution composition. Physiological parameters included: mineral composition of plant tissue from the leaves, trunk xylem and roots, root growth, yield, fruit setting, plant volume, and yield. TWW-irrigated plots were found to endure longer periods of low SWT indicating higher water content, accompanied by lower oxygen levels and more reduced conditions in comparison to FW-irrigated plots. The differences in these soil parameters between treatments were greater during the irrigation season than during the rainy period. The more reduced conditions in the TWW plots did not lead to significant differences in Fe or Mn concentrations in the soil solution or in plant leaves. TWW soil solution had significantly higher Na levels compared with FW. This did not affect the leaf Na content, but was expressed in substantially higher Na content in the root and trunk xylem, with up to seven times more trunk xylem Na in TWW-irrigated plants compared with FW-irrigated plants. Root growth was significantly hindered in TWW-irrigated plots compared with FW-irrigated plots. A negative correlation was found between root growth and the duration of hypoxic conditions, and similarly between root growth and the Na levels in the roots. TWW-irrigated plants had greater fruitlet numbers at the initial fruit-setting stage, but had a smaller number of fruit and a lower yield at harvest. The yield (kg/tree) negatively correlated with the duration of hypoxic conditions in the root zone but not with the Na levels in the roots or xylem. Our findings point towards a substantial role of oxygen deprivation as a major factor leading to the damage to TWW-irrigated orchards in clayey soils. Based on the assimilation of data, we suggest that a downward cascade is instigated in the TWW-irrigated orchards by increased input of Na into the soil, leading to degradation of soil hydraulic properties and reduced aeration. Impaired physiological functioning of the roots due to limited oxygen supply results in less roots growth, lower water uptake and impaired selectivity against Na uptake, thus imposing a negative feedback to increase soil water content, reduce aeration and root-zone oxygen availability for the roots, and further impair plant resistance to the high Na levels.
... In soils with limited soil air content (15% or less), gs in the grapevine is reduced ( Table 3). The results are consistent with values reported by Ferreyra et al. (2008) on avocado, where lack of oxygen in the soil also affected root system metabolism and caused stomata closure. However, the literature has shown that avocado trees are more sensitive to the lack of air in the soil than the grapevine, which could explain the moderate response observed in stomatal conductance in the present study (Table 3). ...
Soils with unfavourable natural conditions for root development can affect both yield and fruit quality of table grapes, mainly due to an inadequate soil air/water ratio. The objective of this study was to generate information about the adaptation of own-rooted or grafted 'Thompson Seedless' vines grown in soils with different soil air capacity (ACf). The experiment was carried out in pots containing four different soil textural classes (sandy, sandy loam, clay loam and silty loam) that generated different soil air contents (AC). Plants of cultivar 'Thompson Seedless' were grafted on five different rootstocks (110 Richter, Harmony, Ramsey, 1616 Couderc and Freedom), and an own-rooted vine was used as a control. The ACf values ranged between 9-26%, and had a direct relation with oxygen diffusion rates (ODR), which ranged between 0.25 to 0.70 µg cm -2 min -1 . The leaf stomatal conductance was high in soils with AC 22.63%, but not significantly affected by low soil AC. The midday stem water potential (SWPmd) was not affected by AC, and values ranged between -0.55 and -0.63 MPa. AC values higher than 20% decreased the pruning weight of 110-R, Harmony and Freedom, while Ramsey was the less affected in a wide range of AC. Own-rooted plants were significantly affected by low soil air content. In general, in aeration stress condition soils, rootstocks particularly 110-R and Ramsey promote vegetative growth of 'Thompson Seedless' vines, although results need to be confirmed at least one more season.
... In avocado roots, N, K, and Mg concentrations were higher in trees grown in soil with 2% oxygen than in soil with 21% oxygen, whereas Na, Cl and Zn concentrations were higher in soil with low oxygen (2%) than in soil with high oxygen (21%) (Labanauskas et al., 1978). Although the effects of low soil oxygen content as a result of flooding or low soil aeration have been reported on avocado physiology (Schaffer and Ploetz, 1989; Schaffer, 1998; Schaffer et al., 1992; Schaffer and Whiley, 2002; Gil et al., 2007; Ferreyra et al., 2008 ), little is known about the effects of soil characteristics related to water-to-air ratios on growth and nutrition uptake of avocado trees. An understanding of the relationship between soil features such as the soil water-to-air ratio on avocado growth and nutrient uptake would provide valuable insight for irrigation and fertilization management of this crop in different soils, particularly in areas with poor soil aeration. ...
In Chile, expansion of avocado production has resulted in many orchards established in marginal soils that are poorly drained and have high soil water-to-air ratios when soil moisture is at field capacity. However, avocado trees are sensitive to poor soil aeration. A study was conducted to determine the effects of different soil water-to-air ratios (W/A) on biomass and nutrient content of avocado trees. Two-year-old avocado trees were grown for 2 seasons in containers in soils, with different W/A, collected from different avocado growing regions of Chile. There were five treatments corresponding to each of the five soils. At field capacity, the two-season average W/A was 1.7, 1.3, 0.6, 0.4 or 0.3 for treatments T1, T2, T3, T4, or T5, respectively. The same amount of fertilizer was applied to each soil. Mineral element concentrations and total mineral element contents in leaves, shoots, wood and roots were determined for each tree in each treatment at the end of the experimental period. Shoot and root fresh and dry weights, leaf area and leaf retention were also determined. Although all treatments showed non-limiting soil oxygen conditions for avocado root growth, trees in soils with lower W/A had greater shoot and root dry weights and longer autumn leaf retention. Macro- and micronutrient concentrations in any plant tissue were not related to soil W/A. However, total tissue contents of N, P, K, Ca, Mg, C, N and B in roots and whole plants were highest in treatments with lower soil W/A. The results indicate that soil W/A significantly affects growth and mineral nutrition of avocado trees and should be considered for avocado site selection and management.
Keywords: mineral nutrition, nutrient uptake, soil aeration, avocado.
In natural and agroecosystem, plants are continuously exposed to environmental stresses. Besides having evolved several indigenous tolerance mechanisms, proper management of agricultural practices contribute to avert deleterious impact of stressful conditions. Mineral nutrients have irreplaceable role in plant metabolism. Deficiency in any mineral nutrients impedes plant growth, which has direct association with yield potential of the plant. Mineral nutrients participate in the synthesis of essential organic molecules, such as amino acids and proteins, and nutrient imbalance can affect many biological processes. Soil water deficit disturbs mineral nutrient relations, inhibiting plant growth and development, which reflects on the final crop yield. Water deficit normally reduces mass flow-dependent mineral nutrient uptake and the translocation of these nutrients from the roots to the shoot, affecting all metabolic processes of plant physiology. On the other hand, proper supplementation of mineral elements to crop plants can contribute to avoid stress for soil water deficit through their active participation in several defense mechanisms. Global climate changes are expected to increase the occurrence and duration of soil water deficit in arid and semi-arid regions, leading farmers, and governments to face an increasing risk of food security.
The rate of CO 2 exchange of the avocado, Persea americana (Mill.) cv Edranol, was studied under varying conditions of solar radiation flux density and temperature, using the chamber method. Internal water status and stomatal resistance were monitored using a pressure bomb and diffusion porometer. It was found that Persea americana is a typical C 3 plant, with maximal CO 2 exchange of 0,40 mg m -2 s -1 occurring in the temperature range 20-24°C. The point of inflection on the photosynthesis incident radiation response curves was approximately 200 Wm -2 for temperatures between 10 and 30°C. Under increasing water stress, stomates closed rapidly at leaf water potentials of less than -900 kPa, corresponding to soil water potentials less than -70 kPa. Leaf temperatures exceeded air temperatures by up to 6°C in stressed plants. The practical significance of the results is discussed from the points of view of high density planting and irrigation scheduling. INTRODUCTION The main avocado cultivars grown in South Africa are Fuerte, Edranol and Hass. The experiments reported here were on Edranol, representative of the Guatemalan horticultural race (Bergh, 1975). Guatemalan type avocados are adapted to highland tropical or cool sub-tropical, moist and humid climates (Wolstenholme, 1977), and have the reputation of being particularly sensitive to heat and moisture stress. Apart from the recent report of Sterne, Kaufmann & Zentmyer (1977), very little critical work on photosynthetic and internal water relations of the avocado has been published. The avocado has been affected by the modern trend towards high density orchards. It is standard practice to double plant in California, removing excess trees when crowding and shading occur (Lee, 1974). Cain (1969) emphasized that production per unit area, and not solely per tree, is the most important criterion for orchard profitability. In widely spaced standard apple orchards, only 40% of the land is effectively utilized in the long term (Cain, 1970). It has even been suggested that solar radiation stress should dictate tree spacing (Fochessati, 1972).
Greenhouse studies were conducted to determine the effects of Phytophtora root rot (caused by Phytophtora cinnamomi) and flooding on avocado (Persea americana). In addition to standard disease assessments (root necrosis, root colonization, wilt, and defoliation), dry weight accumulations and gas exchange characteristics were monitored as indicators of host distress. (...)
Equilibration periods of 1h after placement were enough to achieve representative in situ samples from an agricultural silt loam soil. Vectorial variability of CO2 in the horizontal direction was occasionally significant, and in the vertical direction consistently significant. The O2 and N2 concentrations were nearly at atmospheric levels to a depth of 60 cm. -from Author
SUMMARY Photosynthesis and leaf conductances of avocado leaves (Persea americana Mill. cv. Fuerte) were determined in the laboratory over a range of photon flux density (pfd) and temperature. Maximum rates of photosynthesis of 0.30 mg m-2s-1 were measured with saturation at c. 500 µEm-2s-1 (25 percent of full sunlight) and a temperature optimum of 28°C. The relatively low rates of photosynthesis at which saturation occurs indicate that avocado foliage exhibits characteristics of a shade plant. Measurements in the field gave similar results. Some leaves inside the canopy were photosynthesising at rates of up to 0.10 mg m-2s-1 at 60 µE m-2s-1. Xylem pressure potential and leaf diffusive conductances were determined on leaves of a field-grown Fuerte avocado tree from before sunrise to after sunset on a hot summer day (maximum pfd normal to leaf 1500 µE m-2s-1, air temperature 35°C). Rapid stomatal opening at sunrise gave maximum values for leaf conductance (0.4 cm s-1 ) by 0700 hours and these slowly decreased as the vapour pressure difference from leaf to air increased, until stomatal closure at sunset. Xylem pressure potential fell to a minimum of -1300 kPa several hours after the maximum of leaf conductance and remained at this level until late afternoon when it increased until almost full recovery (-300 kPa) was attained after sunset. The importance of these findings to avocado production is discussed.
A field study was conducted to determine how atmospheric and edaphic conditions influenced the water relations of avocado trees (Persea americana Mill. cv. Bacon). With high and low levels of incident photosynthetically active radiation (PAR, 400–700 nm wave length), and either wet or dry soil, leaf conductance decreased as the absolute humidity difference from leaf to air increased. For any water stress treatment, conductance was higher at high PAR than at low PAR. Both conductance and transpiration were higher in well-watered trees than in stressed trees, and in prestressed trees levels were intermediate to unstressed and stressed trees. A model for water flux through the soil-plant-atmosphere continuum was used to examine the relationship of leaf xylem pressure potential to transpiration in well-watered trees and in trees stressed by dry soil. There was a close linkage between leaf xylem pressure potential and transpiration in unstressed and previously stressed trees with high or low PAR, i.e. similar potentials occurred with equivalent transpiration regardless of previous treatment or time of day. In stressed trees, xylem pressure potential was lower than in unstressed trees both during the day and night, and at a given transpiration rate the potential was lower after 1400 h than before that time. The model indicated that in stressed trees xylem pressure potential was uncoupled from transpiration, presumably because of altered resistance in the soil-root portion of the transport system.
The effect of various irrigation regimes on soil aeration was tested in a two-year experiment with 15 year-old apple trees growing in soil containing 67% clay. Irrigation was applied by sprinklers at four intervals ranging from 3 to 18 days and by trickle irrigation every 7 days. Each treatment received a total of 800–850 mm water from May until September. Irrigation by sprinkling at 7 day intervals appeared to be optimal for fruit growth. Less frequent irrigations resulted in smaller fruits; sprinkling at 3–4 day intervals, as well as trickle irrigation reduced the fruit growth rate in July. Leaves from plots irrigated once every 3–4 days had a low chlorophyll content and accumulated relatively large amounts of ethanol, particularly when grafted on the Khashabi rootstock, which is highly susceptible to damage caused by inadequate soil aeration. With increasing intervals between irrigations, the resistance of the leaf surface to the diffusion of water vapour measured prior to irrigation increased, and water loss relative to that from an evaporation pan decreased. Sprinkling at intervals of 14 days resulted in maximal, and at 3–4 days in minimal, air contents of the soil when calculated as averages for the total period of irrigation. The decrease in soil air content with very frequent irrigations was particularly marked in the upper soil layer; this same layer also had a relatively low air content near the emitters in trickle irrigation. After each irrigation, relatively large amounts of ethylene accumulated in the soil atmosphere, indicating inadequate soil aeration, particularly with sprinkling intervals of 3–4 days and at a depth of 30 cm. However, the influence of the irrigation treatments on the oxygen and carbon dioxide contents of the soil atmosphere was small and not consistent.