Variation In Leaf Water Potential Research Articles

Soils and water uptake by vines in Pomerol: II - Water uptake and development by vines in 1995

<p style="text-align: justify;">The soil variability that we evidenced within the Pomerol vineyard (first part) in France (near to Bordeaux) led us to compare the behaviour of Merlot noir, which is the main local cultivar in this area. This study was conducted on four sites, which belong to the main soil map units of the area. Soils (FAO) were: a deep (A) and a shallow (C) planosols with abrupt textural changes between uppest sandy-gravelly horizons and sandy clay (A) or heavy clay (C) deeper layers, a very sandy well-drained neoluvisol (B), and a very stony cambisol with a temporary watertable (D). Water uptake by vines was assessed by leaf water potential measurements undertaken from June to September. Both observations on shoot elongation and leaf water potential measurements were performed together. The year 1995 exhibited a mild winter, then followed by both temperature and insolation higher than mean values. The rainfall in summer was less than the mean value, and the climatic water balance was negative. As a consequence, the vine phenology exhibited a very early development. Various water behaviours were observed amongst the four experimental plots: some were strongly linked to c1imatic events whereas others showed regulating mechanisms attributable to inherent soil properties such as texture and structure. The vine on soil D exhibited the greatest variations in water potential, showing significant periods of droughtiness during summer. On this soil, the variations in leaf water potential were strongly linked to rainfall events in summer. The vine on soil B kept the less negative values of water potential in relation to the very deep root development which was observed in this light and well-drained sandy material. The vine on soil A had a high water uptake during the period of shoot development, but showed a significant reduction of water availability in august. The most progressive regulation of water uptake was observed on soil C, and could be attributed to the elongation of roots with shrinking during summer in the deep c1ayey horizons. The water behaviour variability was mainly linked both to textural and structural soil properties intluencing soil water retention, aeration, and rooting development. The longest shoots were observed in soil (A) where the elongation speed was the highest. On soil (B) shoot elongation was signiticantly reduced, although water supply was the highest. This was attributed to a low nutrient availability in this very sandy soil. The elongation speed in site (D) was rather chaotic, and was related to marked variations in water supply. The lowest shoot elongation was observed in the shallow planosol (C). Preliminary results on grape quality are showed, and seem to be consistent with these behaviours.</p><p style="text-align: justify;">These results are consistent with previous studies on various vines of the Bordeaux region which have already shown that various soil properties might induce a control on water behaviour in soil and thus might result in similar consequencies for water availability for vines. The discussion points out the important role played by water uptake control mechanisms on vintage quality in this area.</p>

The use of seedling leaf death score for evaluation of drought resistance of rice

Leaf death (drought score) of seedlings has been extensively used in rice breeding programs as a selection index for drought resistance. This paper investigates the factors that affect the drought score of rice cultivars exposed to water stress during the vegetative stage and the relationship between drought score and yield. Two upland experiments consisting of 21 and 8 cultivars, respectively, were subjected to water deficit during vegetative growth. In the first experiment, the plants were grown under three stress conditions and pre-dawn leaf water potential, epidermal conductance, canopy light interception and drought score were determined during the stress period. In the second experiment leaf area, water use, dry matter growth and grain yield were also examined. Large genotypic variation in plant size existed in both experiments and this affected development of water stress and drought score. The cultivars with large plant size tended to have high drought score. A significant positive correlation ( r = 0.74∗∗, n = 63) existed between drought score and light interception. However, analysis of covariance indicated that genotypic variation in drought score existed after the effect of plant size was taken into account. Significant genotypic variation in leaf water potential existed and this was negatively correlated with drought score ( r = 0.75∗∗, n = 42). Cultivars with large leaf area tended to lose more green leaves during the stress period, but this did not result in reduced dry matter growth during or after the stress period. Green leaf area remaining after the stress period, and not amount of dead leaf (drought score) determined subsequent dry matter growth ( r = 0.45∗∗, n = 32), and there was no correlation between drought score of seedlings and grain yield. Thus for use in a vegetative screening program for drought resistance, estimates of drought score should be accompanied by estimates of plant size.

Involvement of malate and mannitol in the diurnal regulation of the water status in members of Oleaceae

This study examines water status regulation in plants of the Oleaceae family and in some other co-occurring species that are exposed to high solar radiation, in the same habitat. Fraxinus excelsior L., one of the most studied Oleaceae in this field exhibited, during the growing season, a close relationship between diurnal variations in leaf water potential and changes in malate, mannitol and K+ levels, depending on the weather conditions. On sunny days, similar variations can be observed in leaves of the other Oleaceae, with a concomitant decrease in the osmotic potential between predawn and solar noon. Malate, mannitol and the well-known osmoticum K+, contribute greatly to the osmotic potential decrease. This mechanism, which can be related to the osmotic adjustment described for both drought and salt-affected plants, appears as a general response in plants of the Oleaceae family. Among the other co-occurring species investigated, only Quercus robur L. displayed a similar mechanism under the same environmental conditions, but two other organic compounds, quinic and shikimic acids, are presumably involved. Alnus glutinosa (L.) Gaertn. and Robinia pseudacacia L. responded to a vapor deficit by partial stomatal closure, as transpiration progressed through the morning.

Analysis of the diurnal change in osmotic potential in leaves ofFraxinus excelsiorL.

The daily cycle of the transpiration rate, stomatal conductance, water potential, and the concentration of the main osmoticum identified in ash leaves, malate and mannitol, were monitored in a field on the Isere river plain. On sunny days, the stomatal conductance tends to remain close to its maximun value allowing a high transpiration rate and diurnal variations in leaf water potential, ψw, which may fall as low as −2 MPa at solar noon. These variations of ψw are closely correlated with changes in malate, mannitol and the concentration of the well-known osmoticum K+, which agree with the involvement of an osmotic adjustment to counteract the evaporative demand during daylight hours. How malate, mannitol and K+ contribute to the osmotic adjustment was analysed subsequently by comparing the solute potential ψs, evaluated by the Boyle-Van't Hoff relation, to the osmotic potential ψπ measured by thermocouple psychrometry. These experiments have led us to suspect some errors in the measurement of ψπ, presumably due to experimental artefacts and the ability of Ca 2+ , present in high levels in leaves, to form chelates with malate once the cells have been decompartmented by freezing and thawing. Since significant changes of Ca2+ also occurred during the diurnal variations of ψw, the possible mechanisms by which Ca2+ may be implicated in controlling the water status of the tree are discussed.

Physiological Basis of Yield in Moth Bean [Vigna aconitifolia (Jacq.) Marechal] Under Rainfed Conditions

AbstractPhysiological analysis of yielding ability in moth bean varieties under rainfed condition revealed that in spite of sufficient amount of dry matter production, the crop is a poor seed yielder. Late varieties produced higher amounts of dry matter and seed. The low yield in this crop is owing to low net assimilation rate (NAR) during the reproductive phase and poor migration coefficient of assimilates towards seeds. Leaf area expansion was maximum during reproductive phase. Stomatal components were not evident to limit the NAR under rainfed condition. Varietal differences were observed with respect to transpiration. Temporal variations in leaf water potential, osmotic potential and pressure potential were recorded. It is concluded that in moth bean the improvement in seed yield is possible by increasing current photosynthetic rate during the reproductive phase and can be achieved by changing the crop canopy structure for better utilization of light. Effort should also be made to improve harvest index in this crop.

Leaf Water Potential and Relative Water Content Variation in Creeping Bentgrass Clones

Creeping bentgrass (Agrostis palustris Huds.) often requires daily irrigation and syringing to persist in turf areas in warmer environments with high evapotranspiration rates. Turfgrass performance could be enhanced if turgor maintenance under decreasing soil moisture levels could be incorporated into new bentgrass cultivars. The objectives of this study were to determine: (i) if genetic variation in leaf water potential (LWP) existed among creeping bentgrass clones under declining soil moisture, (ii) if LWP was associated with maintenance of growth under declining soil moisture levels, and (iii) if genotypic variation existed in water‐relation characteristics, including components of LWP, as determined by a hydraulic press. The LWP was measured on four field‐grown creeping bentgrass clones twice per day with a hydraulic press. Leaf water potential and soil moisture levels were determined in a growth room study on five creeping bentgrass clones grown in pots, 17 by 13 cm, during a 17‐d period. Waterrelation characteristics were determined on five creeping bentgrass clones grown in a greenhouse using a hydraulic press. Genetic variation existed among clones in LWP in both field and growth room studies. The most negative LWP was not associated with the greatest leaf firing or decrease in leaf extension. The relative water content at zero turgor represented a substantial difference in the degree of hydration at zero turgor. This study illustrated that genetic variation exists for LWP and relative water content in creeping bentgrass.

Remotely measured canopy temperature of greenhouse strawberries as indicator of water status and yield under mild and very mild water stress conditions

Strawberry plants were submitted to mild and very mild water stress regimes in a tunnel (simple semicircular greenhouse) by planting them in two plots fertirrigated when the soil matric potential reached −0.07 MPa and −0.01 MPa, respectively. The plants were monitored for water stress by measuring the foliage temperature with a hand-held infrared thermometer. Parallel to this, weather variables, the difference between leaf and air temperature, the derived crop water stress index (CWSI), the soil matric potential, the leaf water potential, the photosynthetic gas exchange rates, the transpiration rates, photosynthetic pigments, sugars, starch, canopy structure and accumulated yield were measured. The wet treatment (WT) presented a higher yield and higher leaf area index (LAI). In WT, leaves were disposed mostly in a monolayer oriented to the south, whereas in dry treatment (DT) leaves were distributed in a multilayer pattern and oriented to the north. During the hotter part of cloudless days and before irrigation took place, the canopy temperature of WT was about 3°C less than that of DT. Accumulated stress degree days (SDD) were then higher in DT. WT presented lower average CWSI values, between 0.045 and 0.54, while those of DT were between 0.32 and 0.70. It was concluded that leaf temperature, its difference with air temperature (δ T) and the derived indices, such as SDD and CWSI, are useful for the assessment of even these mild and very mild water stresses in strawberries under protected conditions. Regression analysis showed that under the very mild water stress conditions tested in WT, the contribution of air vapor pressure deficit to variation in leaf water potential was significant. This did not happen under the mild water stress of DT.

Salinity effects on water relations in Lycopersicon esculentum and its wild salt‐tolerant relative species L. pennellii

Cultivated tomato Lycopersicon esculentum (L.) Mill. cv. P‐73 and its wild salt tolerant relative L. pennellii (Correll) D'Arcy accession PE‐47, were grown during spring‐summer 1989 under unheated plastic greenhouse conditions. Plants were submitted to two different salt treatments using 0 and 140 mM NaCI irrigation water. In both tomato species, salinity caused a proportionally larger reduction in leaf area than in leaf weight and, in L. esculentum, a proportionally larger decrease in stem weight than in leaf weight. Daily variations in leaf water potential (Ψ1) were fundamentally due to changes in the evaporative demand of the atmosphere. Reductions in Ψ1 due to salinity were consistent only in L. esculentum. In all the conditions studied, leaf turgor was maintained. Leaf conductance (g1)was higher in L. esculentum than in L. pennellii.Salinity induced a clear reduction in g1 levels in L. esculentum whereas, in L. pennellii, this reduction was noted only in May. In both species the Ψos (leaf osmotic potential at full turgor) levels were reduced by salinity. The bulk modulus of elasticity (E) and relative water content at turgor loss point (RWCtlp) were not affected by salinity. The RWCtlp values in L. pennellii seem to be controlled by E values.

Seasonal and clonal variations in drought tolerance of Populusdeltoides

Water relations of three field-grown eastern cottonwood (Populusdeltoides Bartr.) clones were compared for the 1989 growing season. Clonal and seasonal variations in leaf water potential, leaf osmotic potential, dry weight fraction, and injury index were measured. The injury index was calculated from conductivity changes due to electrolyte leakage during rehydration of dehydrated and nondehydrated leaves. When samples were measured after dry periods, dry weight fraction increased and injury index and predawn osmotic potential declined. There were significant negative correlations between dry weight fraction and osmotic potential for all clones. There were no significant differences between clones from Nebraska (Platte) and Indiana (Tippecanoe) throughout the season in osmotic potential and injury index. The clones Platte and Tippecanoe had significantly lower osmotic potentials than a clone from Ohio (Ohio Red) on most sample dates. When injury index values increased following favorable weather conditions, Platte and Tippecanoe had a significantly lower injury index than Ohio Red. Since all clones had lower osmotic potential, higher dry weight fraction, and lower injury index during dry periods, it was concluded that all had drought hardened during the period, indicating that all clones have some degree of drought tolerance.

Water relations of stem succulent trees in north-central Baja California.

Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H2O/m3 of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H2O/m3. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m-2 s-1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata.

Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.

Leaf Water Potential Trends in Three Grasses Native to Semiarid Argentina

The natural grazing land of the semiarid region of central Argentina is subject to long and intense periods of drought during the hot season. The aim of the present study was to analyze the pattern of leaf water status for the cool-season grassesPiptochaetium napostaense and Stipa tenuis and the warm-season grass Pappophorum subbulbosum, which are important forage species in this grazing area. Leaf water potential, soil water potential, and relative humidity were measured during 1983 and 1984. At soil water potentials above -0.5 MPa, the leaf water potential of the 3 species showed high values at 0800, after nocturnal rehydratation, and minimum values at 1400, coincident with maximum atmospheric water demand. P. subbulbosum showed a greater capacity to withstand the drop in water potential at 1400 than did the cool-season species. During periods of low soil water potential the range of daily variation in leaf water potential was reduced in the 3 species because of marked decreases in the maximum and minimum values. Leaf water potential in the cool-season species correlated better with the deficit of atmospheric vapor pressure than with soil water potential. Leaf water potential of P. subbulbosum on the other hand showed a higher correlation with soil water potential than with atmospheric vapor pressure deficit.

Diurnal variation in leaf water potential, stomatal conductance, and irradiance of winter crop under different moisture levels

Mustard (Brassica juncea) Coss., chickpea (Cicer arietinum L. and barley (Hordeum vulgare) L. were grown under different moisture levels. Diurnal changes in leaf water potential showed lower values and higher fluctuation in mustard, chickpea and barley grown with no irrigation as compared to one supplemental irrigation. Diurnal maximum of adaxial stomatal conductance in mustard and barley was higher under one irrigation treatment. In mustard stomatal conductance of abaxial surface of leaf remained higher than adaxial surface of leaf throughout the day, whereas the reverse was true in barley. Also the leaf and soil temperature and reflectance were slightly higher in all the three crops under no irrigation.

Water balance in developing leaves of four tropical savanna woody species

Water balance was studied in developing leaves of Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, four widespread tropical savanna evergreen woody species that renew their leaves during the dry season. Water potential components of leaves of different ages were estimated in the laboratory by determination of pressure-volume (PV) curves. Data from PV curves were used to help interpret the significance of variations in leaf water potential (ΨL) and stomatal conductance observed in leaves of different ages in the field.Initial osmotic potential at full turgidity as well as osmotic potential at the turgor loss point changed considerably during leaf development. Values of these water potential components for mature leaves were similar to those reported for temperate mesophytic woody species. Passive osmotic adjustment defined as the change in osmotic potential between full turgidity and turgor loss averaged only 0.3 MPa, much smaller than values for temperate mesophytic and drought hardy woody species. Since environmental conditions, especially soil moisture in the rooting zone, were essentially constant during leaf development, changes in leaf water potential components were not seasonal, but rather ontogenetic responses to diurnal water stress.In all species except C. sylvestris there were differences in ΨL between adjacent leaves exposed to the same environmental conditions, with immature leaves generally maintaining higher ΨL than mature leaves. The probable causes for this were 1) lower liquid flow resistance and 2) lower transpiration rates in younger leaves. At low transpiration rates liquid flow resistance was flux-dependent in all species, but became nearly independent of transpiration flux density (E) as E increased. This and their lower flow resistance permit high E to be sustained in developing leaves without excessive ψ drop across the leaf.During two typical dry season days E was high and even though complete or nearly complete turgor loss was sustained, these water deficits were not severe enough to cause complete stomatal closure. The possible relationship between a high stomatal conductance and the maintenance of a favorable carbon balance in these species is discussed.All of the water relations features observed suggested that in spite of their scleromorphic appearance, leaves of the four species under the conditions studied were adapted only to short term diurnal water deficits.

Diurnal and Spatial Variation in Leaf Water Potential and Leaf Conductance of Irrigated Peach Trees

Leaf water potential, conductance and relative water content were measured in three layers of the canopy of large peach trees during imgation cycles. In the top layer, leaf water potential decreased from early morning until the early afternoon reaching values ranging from about -1.8 MPa soon after irrigation to -2 5 MPa at the end of a drying cycle; leaf conductance (mean about 0.42 cm s-1) decreased slightly during this period but did not appear to be influenced by water potential at values higher than about - 1 .9 MPa. From early afternoon to late evening, leaf conductance continued to decrease and water potential increased. Similar cycles were noted in the middle and lower layers except that water potential was always higher and conductance lower. A high degree of exposure of leaves to insolation appeared to result in low potentials and high conductances. The adequacy of current imgation recommendations for high productivity is discussed.

Diurnal variations in leaf water potential and stomatal conductance of pigeon pea (Cajanus cajan (L.) Millsp.) cultivars as affected by irrigation levels

Two cultivars of pigeon pea (Cajanus cajan (L.)Millsp.) UPAS-120 and Parbhat were grown in the field under two irrigation treatments: no irrigation and irrigation when cumulative pan evaporation was equal to 50 per cent depletion of available water content in one metre root zone depth. Diurnal changes in leaf water potential and stomatal conductance were recorded on two daysi.e. October 1, 1979 and October 28, 1979 which corresponded to reproductive growth stage of the crop. Plant water potential decreased rapidly during the day up to about 15.00 and increased during evening hours. Higher leaf water potential was recorded in irrigated treatment on both dates. Adaxial and abaxial stomata differed in their response to water stress. Adaxial stomatal conductance started declining during early morning hours, however, abaxial conductance firstly increased up to 09.00 then decreased and increased again in the afternoon except in irrigated crop of cv. UPAS-120 on 28th October, where conductance never increased after 09.00. The reduced rate of stomatal conductance during day hours may be identified as one of the characteristics responsible for drought tolerance in pigeon pea.

Measurement of Leaf Water Potential Using the J14 Press

The diurnal variation in leaf water potential of wheat under field conditions is determined by means of the Scholander pressure chamber and the J14 press. Experimental work was carried out on several days during both the vegetative and reproductive growth stages. An exponential relationship (v = 4.09 e0.05x), with coefficient of determination r2 = 0.81, is found to exist between the J14 press and Scholander pressure chamber measurements. Both the Scholander pressure chamber and J14 press appear capable of following hourly variations in leaf water potential provided 8 and 16 leaves per sample are used for the two instruments respectively.

Seasonal and Diurnal Variations in Abscisic Acid, Water Potential, and Diffusive Resistance in Leaves from Irrigated and Non-irrigated Peach Trees1

Abstract Water potential, diffusive resistance, and abscisic acid (ABA) were measured at 10-12 day intervals from May to October in leaves from irrigated and non-irrigated peach (Prunus persica L. cv. Fay Elberta) trees, and measurements were taken at intervals from sunrise to sunset on September 8. Leaf water potential, before sunrise, was between −5 and −8 bars in irrigated trees during the entire season whether drip irrigated at 100% evapotranspiration (ET) or 50% ET. Non-irrigated trees showed a decrease in pre-dawn leaf water potential with time, following a pattern similar to that of decreasing soil moisture. Leaf water potential values taken during the afternoon were not associated with soil moisture and did not reflect the stressed condition of the trees. In non-irrigated trees stomatal resistance at mid-day increased rapidly after mid-summer as leaf water potential decreased. ABA concentration in leaves from irrigated trees ranged from 30 to 80 ng/g fresh wt during the entire season. In non-irrigated trees the ABA concentration increased sharply after mid-summer; this was associated with an increase in leaf diffusive resistance and a decrease in leaf water potential. Diurnal variations in leaf water potential were associated with changes in soil moisture, air temperature, relative humidity, and stomatal resistance. Leaf diffusive resistances were similar for all treatments until 1100 hr after which a notable increase occurred with increasing stress, ultimately leading to stomatal closure. ABA concentrations in leaves from irrigated and non-irrigated trees increased as leaf diffusive resistance increased; however in stressed trees, high levels of ABA in the morning were not associated with closed stomata.

Response in Plant Water Status to Integrated Values of Soil Matric Potential Calculated From Soil Water Depletion by a Field Bean Crop

Water deficits were induced in a field bean crop using Polythene rainout shelters to study the crop response to changes in soil matric potential integrated over the zone of soil water extraction. Integrated values of soil matric potential (Ψm,s) were derived from the values of matric potential (Ψm,s) at four separate depths weighted according to the corresponding rates of water depletion at the time of measurement. Linear relationships existed between Ψm,s and variations in leaf water potential before sunrise (Ψd) and in the afternoon (Ψa). The total resistance to daily water flow in the soil-plant system was found to increase linearly with falling Ψm,s. The water flux from separate soil layers indicated that the deeper the layer, the greater the resistance to water uptake from soils at a given value of soil water potential. Such an increase in resistance with soil depth, which considerably reduced the availability of soil water in the deeper soil layers, was attributed mainly to plant factors.

Genotypic Variation in Leaf Water Potential, Stomatal Conductance and Abscisic Acid Concentration in Spring Wheat Subjected to Artificial Drought Stress

A technique for studying variation in the accumulation of abscisic acid (ABA) in response to drought stress is described. Two experiments, each testing 26 spring wheat genotypes, were carried out using pot grown plants in controlled environment cabinets with nutrient solution culture, though the results of only one experiment are described in detail. Plants were subjected to water stress by withholding water as the fifth or sixth leaf on the main stem was emerging. Two stressed plants of each genotype were harvested 5 and 7 days after the treatment commenced and measurements of leaf water potential, stomatal conductance and ABA concentration were taken. There was considerable genotypic variation in the rate at which water potential decreased, partly explained by variation in plant size. Inia 66 (a genotype common to both experiments) had consistently much lower water potentials than the other genotypes. Stomatal conductances of all genotypes decreased rapidly and after 5 and 7 days they were negatively correlated with the changes in water potential. ABA concentrations varied considerably between genotypes after both 5 and 7 days without water, the variation being associated with genotypic differences in water potential on these occasions. The overall relationship between ABA concentration and water potential was highly significant. Significant differences between the slopes of the regressions for individual genotypes were found. The cultivar Sirius accumulated the most ABA at any water potential and Pelissier, Wascana and Hybrid 46 accumulated the least. The significance for drought resistance of variation in ABA accumulation is discussed.