Effects of water stress and rewatering on leaf water relations of lemon plants

Potted two-year-old lemon plants (Citrus limon (L.) Burm. fil.) cv. Fino, growing under field conditions were subjected to drought by withholding irrigation for 13 d. After that, plants were re-irrigated and the recovery was studied for 5 d. Control plants were daily irrigated maintaining the soil matric potential at about -30 kPa. Young leaves of control plants presented higher leaf conductance (g1) and lower midday leaf water potential (Ψmd) than mature ones. Young leaves also showed higher leaf water potential at the turgor loss point (Ψtlp) than mature leaves. In both leaf types g1 decreased with increased vapour pressure deficit of the atmosphere. From day 1 of the withholding water, predawn and midday leaf water potentials (Ψpd and Ψmd) decreased, reaching in both cases minimum values of -5.5 MPa, with no significant differences between mature and young leaves. Water stress induced stomatal closure, leaf rolling and partial defoliation. No osmotic adjustment was found in response to water stress in either leaf type, but both were able to enhance the cell wall elasticity (elastic adjustment). After rewatering, leaf water potential recovered quickly (within 2 d) but g1 did not. This revised version was published online in July 2006 with corrections to the Cover Date.     

The gradient between pre-dawn rhizoplane and bulk soil matric potentials, and its relation to the pre-dawn root and leaf water potentials of four species

Uptake of soil water by plants may result in significant gradients between bulk soil and soil in the vicinity of roots. Few experimental studies of water potential gradients in close proximity to roots, and no studies on the relationship of water potential gradients to the root and leaf water potentials, have been conducted. The occurrence and importance of pre-dawn gradients in the soil and their relation to the pre-dawn root and leaf water potentials were investigated with seedlings of four species. Pre-germinated seeds were grown without watering for 7 and lid in a silt loam soil with initial soil matric potentials of -0.02, -0.1 and -0.22 MPa. Significant gradients, independent of the species, were observed only at pre-dawn soil matric potentials lower than -0.25 MPa; the initial soil matric potentials were -0.1 MPa. At an initial bulk soil matric potential of -0.22 MPa, a steep gradient between bulk and rhizoplane soil was observed after 7 d for maize (Zea mays L. cv. Issa) and sunflower (Helianthus annuus L. cv. Nanus), in contrast to barley (Hordeum vulgare L. cv. Athos) and wheat (Triticum aestivum L. cv. Kolibri). Pre-dawn root water potentials were usually about the same as the bulk soil matric potential and were higher than the rhizoplane soil matric potential. Pre-dawn root and leaf water potentials tended to be much higher than rhizoplane soil matric potentials when the latter were lower than -0.5 MPa. It is concluded that plants tend to become equilibrated overnight with the wetter bulk soil or with wetter zones in the bulk soil. Plants can thus circumvent negative effects of localized steep pre-dawn soil matric potential gradients. This may be of considerable importance for water uptake and growth in drying soil.     

Complete turgor maintenance at low water potentials in the elongating region of maize leaves

Leaf elongation rate, water potential, and osmotic potential were measured in the fifth leaf of maize (Zea mays L.) plants growing in soil from which water was withheld for varying times. Elongation occurred in the basal region, which was enclosed by other leaf sheaths. When water was withheld from the soil, leaf elongation decreased and eventually ceased even though enough solutes accumulated in the elongating region to maintain turgor virtually constant. In the exposed blade, however, turgor was lost and wilt symptoms developed. If the night was prolonged, the elongating region lost much of its ability to accumulate solute, which suggests that the accumulating solutes were of recent photosynthetic origin. Under these conditions, leaf elongation was restricted to higher water potentials than under the usual photoperiodic regime.     

Leaf water characteristics and drought acclimation in sunflower genotypes

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuus L.) genotypes. Osmotic potential at full water saturation (π¹⁰⁰), apoplastic water fraction (AWF) and bulk elastic modulus (BEM) were determined by pressure-volume curve analysis on well watered or on water-stressed plants (−1.0 MPa Ψ₁ < −1.5 MPa) previously drought-pretreated or not. The drought-pretreated plants were subjected to a 7-day drought period (predawn leaf water potential reached −0.9 MPa) followed by 8 days of rewatering. In well watered plants, all genotypes in response to drought acclimation displayed a significantly decreased π¹⁰⁰ associated with a decrease in the leaf water potential at the turgor-loss point (decrease in Ψ_tlp was between 0.15 and 0.21 MPa, depending on the genotype). In two genotypes, drought acclimation affected the partitioning of water between the apoplastic and symplastic fractions without any effect on the total amount of water in the leaves. As a third genotype displayed no modification of AWF and BEM after drought acclimation, the decreased π¹⁰⁰ was only due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiency observed previously in this genotype in response to drought acclimation. In water-stressed plants, the osmotic adjustment (OA) can increase further beyond that observed in response to the drought pretreatment. However, the maintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on the extent of osmotic adjustment, but also on the interaction between OA and AWF or BEM. Adaptative responses of leaf water parameters to drought are thus quite contrasted in sunflower genotypes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Osmotic adjustment in indoor grown cassava in response to water stress

The water content-water potential relation in stressed and unstressed cassava ( Man-ihot species) was examined to ascertain (i) the magnitude of osmotic adjustment in response to water stress and (ii) the mechanisms of such adjustments.
Water stress resulted in a displacement of the water content-potential relation such that at any leaf water potential the water content was higher in the stressed plants. The osmotic potentials of turgid leaves (100% relative water content) were -0.97 and -1.00 MPa in the unstressed cultivars CMC 9 and MCOL 113 respectively. In the stressed plants, the values were-1.13 MPa (CMC 9) and-1.14 MPa (MCOL 113). The 0.14 to 0.16 MPa osmotic potential difference between the stressed and unstressed plants suggests that a stress-induced osmotic adjustment occurred in both cultivars. The biiSk volumetric elastic moduli at turgor pressures above 0.10 MPa were 9.84 MPa (CMC 9) and 13.58 MPa (MCOL 113) in the unstressed plants. Tbe higher values found in the stressed plants, 14.56 MPa in CMC 9 and 16.91 MPa in MCOL 113, suggest a stress-induced decrease in cell wall elasticity. Hence, the observed shift in the wafer content-potential relations in the cassava involved both an osmotic adjustment and a decrease in cell wall elasticity. Increasing the number of stress cycles per plant did not cause a further displacement of the water content-potential curves.     

Effects of water stress on the water relations of Phaseolus vulgaris and the drought resistant Phaseolus acutifolius

The tepary bean ( Phaseolus acutifolius Gray var. latifolius ), a drought resistant species, was compared under water stress conditions with the more drought susceptible P. vulgaris L. cvs Pinto and White Half Runner (WHR). In order to better understand the basis for the superior drought resistance of tepary, this study was designed to determine the relationships among leaf water potential, osmotic potential, turgor potential, and relative water content (RWC).
Plants were prestressed by withholding irrigation water. These stress pretreatments changed the relation between leaf water potential and relative water content of both species so that prestressed plants had lower water potentials than controls at the same leaf RWC. Tepary had lower water potentials at given RWC levels than Pinto or WHR; this can account for part of the superior resistance of tepary. In all genotypes, prestressed plants maintained osmotic potentials approximately 0.2 MPa lower than controls. Tepary reached osmotic potentials that were significantly lower (0.15 to 0.25 MPa) than Pinto or WHR. Both control and prestressed tepary plants had 0.05 to 0.25 MPa more turgor than Pinto or WHR at RWC values between 65 and 80%. Both prestressed and control tepary plants had greater elasticity (a lower elastic modulus) than Pinto or WHR. This greater turgor of tepary at low RWC values could be caused by several factors including greater tissue elasticity, active accumulation of solutes, or greater solute concentration.
Tepary had significantly lower osmotic potentials than the P. vulgaris cultivars, but there was little difference in osmotic potential between Pinto and WHR. Knowledge of differences in osmotic and turgor potentials among and within species could be useful in breeding for drought resistance in Phaseolus.     

Insect responses to plant water deficits. I. Effect of water deficits in soybean plants on the feeding preference of Mexican bean beetle larvae

Abstract. 1. We examined the preference of larvae of the Mexican bean beetle, Epilachna varivestis Mulsant (Coleoptera: Coccinellidae), for foliage of soybean ( Glycine max [L.] Merr.) grown under several levels of water deficit. Third instar larvae were exposed simultaneously to excised foliage from plants that were either well-watered (control) or had experienced water deficits (treatment).
2. Water-deficient plants were re-watered 12 h prior to initiating feeding trials to eliminate physical differences between control and treatment foliage such as leaf water potential, diffusive resistance, relative water content, and foliage toughness.
3. Examination of the free amino acid contents of re-watered and excised foliage indicated that amino acid concentrations increased markedly in foliage grown under water deficits, and that this increase persisted during the preference tests.
4. Larvae preferred control foliage, but shifted preference to treatment foliage under mild water deficits. When the leaf water potential of water-deficient treatment foliage was lower than – 1.13 MPa or when it was more than 0.5 MPa lower than that of control foliage, larvae preferred to feed on foliage from well-watered control plants.
5. The expression of preference for well-watered control foliage was coincident with increases in the concentrations of total free amino acids and individual free amino acids in the water-deficient treatment foliage.
6. These results are inconsistent with White's (1974) hypothesis because Mexican bean beetle larvae avoid plants grown under water deficits that have increased concentrations of free amino acids.     

干旱-复水对两种石斛属植物叶水势的影响

石斛属植物多附着在其他植物体或岩石上,水分获取困难,其特殊的水分利用策略是其生存和发展的重要保证.为弄清石斛属植物对干旱胁迫的适应能力和机制,该文选用3年生金钗石斛和铁皮石斛,通过盆栽控水进行干旱胁迫和复水处理,探讨在不同干旱历时和干旱后复水条件下两种石斛的叶水势变化情况.结果表明:随着干旱时间的延长,两种石斛叶水势均...     

Contribution of osmotic adjustment to the dehydration tolerance of water-stressed pigeon pea (Cajanus cajan (L.) millsp.) leaves

Abstract Water-stressed pigeonpea leaves have high levels of osmotic adjustment at low leaf water potentials. The possible contribution of this adjustment of dehydration tolerance of leaves was examined in plants grown in a controlled environment. Osmotic adjustment was varied by withholding water from plants growing in differing amounts of soil, which resulted in different rates of decline of leaf water potential. The level of osmotic adjustment was inversely related to leaf water potential in all treatments. In addition, at any particular water potential, plants that had experienced a rapid development of stress exhibited less osmotic adjustment than plants that experienced a slower development of stress. Leaves with different levels of osmotic adjustment died at water potentials between –3.4 and –6.3 MPa, but all leaves died at a similar relative water content (32%). Consequently, leaves died when relative water content reached a lethal value, rather than when a lethal leaf water potential was reached. Osmotic adjustment delayed the time and lowered the leaf water potential when the lethal relative water content occurred, because it helped maintain higher relative water contents at low leaf water potentials. The consequences of osmotic adjustment for leaf survival in water-stressed pigeonpea are discussed.     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

不同土壤水分条件下容重对玉米生长的影响

用玉米作为实验材料。进行分根实验研究不同土壤水分条件下容重对玉米生长的影响,种子根平分在装有塿土的分隔的白铁皮桶中,土壤容重分4种处理：低容重(两边容重都为1．20g·cm^-3)、中容重(两边容重都为1．33g·cm^-3)、高容重(两边容重都为1．45g·cm^-3)和混合容重(一边为1．20g·cm^-3,另一边为1．45g·cm^-3),土壤水分控制在高基质势(-0．17MPa)和低基质势(-0．86MPa)两个水平,结果表明,当植株生长在紧实土壤或土壤基质势从-0．17MPa降到-0．86MPa时。根长、根干重和地上部干重都显著降低,并且地上部干重的降幅更大,紧实土壤使根长降低的同时还使根的直径增大,无论是容重增大还是土壤水分含量降低所引起的高土壤阻力都使叶片扩展速度降低和植株变小,生长在紧实土壤中的植株变小不仅是因为叶片扩展速度降低,同时是成熟叶片叶面积缩小的结果。然而,当植株生长在混合容重土壤中时,处在低容重土壤中的根系生长得到加强,补偿甚至超补偿高容重土壤中根系生长的不足,整个植株的生长状况与低容重土壤中生长的植株接近。     

Growth response of Halimium halimifolium at four sites with different soil water availability regimes in two contrasted hydrological cycles

The relationship between water availability and plant growth response in Halimium halimifolium (L.) Willk throughout two years with contrasted precipitation (300 and 850 mm) was examined by measuring vegetative growth and midday leaf water potential in four sandy soil sites with different water availability in Doñana National Park, Spain. H. halimifolium, Cistaceae is mostly restricted to sandy substrates close to coastal Mediterranean areas. At Doñana National Park this species is the main component of stable sand vegetation. Vegetative growth started in March, ending in July. The maximum shoot elongation rate (110 mm/year) and number of branches (8.3 branches/year) occurred in plants growing in the most hygrophytic site (MN) in both the wet and dry years. Plants at this site exhibited higher midday water potentials throughout the year. In contrast, the minimum shoot elongation rate (40 mm/year) and midday water potential (–4.0 MPa) occurred in Monte Intermedio plants (MI) in the dry year, although the water table was shallower than in Monte Blanco (MB). In the wet year the minimum shoot and branch elongation were recorded in MB. The number of leaves per branch for a single main shoot was higher (55 leaves/branch) in the driest area (MB), but these leaves had the smallest area. ANOVA showed significant differences in growth response and midday water potential between the four sites. A stepwise multiple linear regression showed that midday water potential, water table depth and monthly average temperature were the variables most closely associated with shoot elongation rate. We conclude that under severe dryness, the expected patterns of greater growth in sites with better water supply may differ from compared with the expected growth pattern in normal conditions.     

Osmotic adjustment in leaves of sorghum in response to water deficits

The relationships among the total water potential, osmotic potential, turgor potential, and relative water content were determined for leaves of sorghum (Sorghum bicolor [L.] Moench cvs. `RS 610' and `Shallu') with three different histories of water stress. Plants were adequately watered (control), or the soil was allowed to dry slowly until the predawn leaf water potential reached either −0.4 megapascal (MPa) (treatment A) or −1.6 MPa (treatment B). Severe soil and plant water deficits developed sooner after cessation of watering in `Shallu' than in `RS 610', but no significant differences in osmotic adjustment or tissue water relations were observed between the two cultivars. In both cultivars, the stress treatments altered the relationship between leaf water potential and relative water content, resulting in the previously stressed plants maintaining higher tissue water contents than control plants at the same leaf water potential. The osmotic potential at full turgor in the control sorghum was −0.7 MPa: stress pretreatment significantly lowered the osmotic potential to −1.1 and −1.6 MPa in stress treatments A and B, respectively. As a result of this osmotic adjustment, leaf turgor potentials at a given value of leaf water potential exceeded those of the control plants by 0.15 to 0.30 MPa in treatment A and by 0.5 to 0.65 MPa in treatment B. However, zero turgor potential occurred at approximately the same value of relative water content (94%) irrespective of previous stress history. From the relationship between turgor potential and relative water content there was an approximate doubling of the volumetric elastic modulus, i.e. a halving of tissue elasticity, as a result of stress preconditioning. The influence of stress preconditioning on the moisture release curve is discussed.     

Seasonal patterns of water relations and enzyme activity of the facultative CAM plant Portulacaria afra (L.) Jacq.

Abstract. Portulacaria afra (L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C₃ to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from -0.1 to -0.5 MPa, predawn and midday, when the C₃ pathway of photosynthesis was utilized. Osmotic potentials were constant, from -0.7 to - 0.8 MPa, indicating very little change in turgor. P. afra shifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m⁻²) despite C₃-like predawn water potentials. Midday water potentials usually decreased 0.2-0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three-to five-fold over C₃ levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other inducible CAM species.     

Vein recovery from embolism occurs under negative pressure in leaves of sunflower (Helianthus annuus)

Leaf veins undergo cavitation at water potentials (Psi(leaf)) commonly experienced by field-growing plants. Theoretically, embolism reversal should not be possible until xylem pressures rise by several kilopascals of atmospheric pressure, but recent evidence suggests that embolized conduits can be refilled even when surrounded by others at substantial tension (novel refilling). The present study reports 'novel refilling' occurring in leaf veins of sunflower (Helianthus annuus L.) while at Psi(leaf) = -0.33 MPa. Sixty per cent loss of vein hydraulic conductance (K(vein)) was recorded at Psi(leaf) < -0.65 MPa, while stem hydraulic conductance (K(stem)) was unaffected even at Psi(leaf) = -1.1 MPa. Loss of K(vein) was accompanied by stomatal closure. Water-stressed plants (Psi(leaf) = -1.1 MPa) were rehydrated overnight to different target water potentials achieved by using PEG at different concentrations as irrigation medium. K(vein) recovered by 50% at Psi(leaf) = -0.47 MPa and vein refilling was complete at Psi(leaf) = -0.33 MPa, i.e. well below the theoretical limit for conduit refilling (-0.05 MPa as calculated for sunflower minor veins). Mercurials supplied to detached leaves had no effect on the refilling process. Upon rehydration, recovery of K(vein) was not paralleled by recovery of whole-plant hydraulic conductance or leaf conductance to water vapour (g(L)), as a likely consequence of hydraulic failure of other components of the water pathway (root system or extravascular leaf compartments) and/or root-to-leaf chemical signalling. This is the first study providing experimental evidence for 'novel refilling' in a herbaceous dicot and highlighting the importance of this process in the leaf.     

Osmotic adjustment inSorghum bicolor (L. Moench) grown under moisture stress in soil and osmotically modified solution cultures

Sorghum (Sorghum bicolor L. Moench) plants were grown in solution culture and stressed at three rates of decreasing leaf water potential (−0.123, −0.068 and −0.029 MPa day⁻¹) achieved by the incremental addition of an osmoticum, polyethylene glycol (PEG) 6000 to the solutions. Plants were also grown in soil and given different amounts of water which resulted in rates of decreasing leaf water potentials of −0.130 and −0.073 MPa day⁻¹. The rate of stress and the culture system influenced the accumulation of solutes in the cell, but not cell volume. A rapid stress (−0.123 and −0.130 MPa day⁻¹) to approximately −1.6 MPa leaf water potential resulted in 0.75 and 0.16 MPa of osmotic adjustment in the PEG and soil culture respectively. At moderate stress (−0.068 and −0.073 MPa day⁻¹) respective values were 1.68 and 0.58 MPa. There were some visual symptoms in the solution grown plants characteristic of uptake of high molecular weight PEG. However the relative growth rates of these plants were equal to or greater than those of the soil grown plants. In view of the differences in plant water status of soil and PEG solution cultured plants it was concluded that the use of the latter system would not be entirely suitable for some studies of drought resistance in sorghum, as related to crop performance in the field.     

Drought until death do us part: a case study of the desiccation-tolerance of a tropical moist forest seedling-tree,Licania platypus (Hemsl.) Fritsch

Studies of the desiccation tolerance of 15-month-old Licania platypus (Hemsl.) Fritsch seedlings were performed on potted plants. Pots were watered to field capacity and then dehydrated for 23-46 d to reach various visible wilting stages from slightly-wilted to dead. Root hydraulic conductance, k(r), was measured with a high-pressure flow meter and whole-stem hydraulic conductance, k(ws), was measured by a vacuum chamber method. Leaf punches were harvested for measurement of leaf water potential by a thermocouple psychrometer and for measurement of fresh- and dry-weight. L. platypus was surprisingly desiccation-tolerant, suggesting that most species of central Panama may be well adapted to the seasonality of rainfall in the region. The slightly-wilted stage corresponded to leaf water potentials and relative water contents of -2.7 MPa and 0.85, respectively, but plants did not die until these values fell to -7.5 MPa and 0.14, respectively. As desiccation proceeded k(r) and k(ws) declined relative to irrigated controls, but k(ws) was more sensitive to desiccation than k(r). Values of k(ws) declined by 70-85% in slightly-wilted to dead plants, respectively. By comparison, k(r) showed no significant change in slightly-wilted plants and fell by about 50% in plants having severely-wilted to dead shoots.     

Gradients in water potential and turgor pressure along the translocation pathway during grain filling in normally watered and water-stressed wheat plants

The water relations parameters involved in assimilate flow into developing wheat (Triticum aestivum L.) grains were measured at several points from the flag leaf to the endosperm cavity in normally watered (Psi approximately -0.3 MPa) and water-stressed plants (Psi approximately -2 MPa). These included direct measurement of sieve tube turgor and several independent approaches to the measurement or calculation of water potentials in the peduncle, grain pericarp, and endosperm cavity. Sieve tube turgor measurements, osmotic concentrations, and Psi measurements using dextran microdrops showed good internal consistency (i.e. Psi = Psi(s) + Psi(p)) from 0 to -4 MPa. In normally watered plants, crease pericarp Psi and sieve tube turgor were almost 1 MPa lower than in the peduncle. This suggests a high hydraulic resistance in the sieve tubes connecting the two. However, observations concerning exudation rates indicated a low resistance. In water-stressed plants, peduncle Psi and crease pericarp Psi were similar. In both treatments, there was a variable, approximately 1-MPa drop in turgor pressure between the grain sieve tubes and vascular parenchyma cells. There was little between-treatment difference in endosperm cavity sucrose or osmotic concentrations or in the crease pericarp sucrose pool size. Our results re-emphasize the importance of the sieve tube unloading step in the control of assimilate import.     

Diurnal osmotic cycling in cotton leaves as an indicator of source–sink balance

Abstract Diurnal cycling of osmotic potential was studied in leaves of cotton plants (Gossypium hirsutum L.) grown in the field. Osmotic potential was determined by a pressure-volume procedure as the value coinciding with zero turgor. In plants grown under favourable conditions (no water stress or N stress), osmotic potential at zero-turgor measured at midday was initially about 0.3 MPa lower than before dawn, but this cycling disappeared during the season as the number of fruits per plant increased. In water-stressed or N-deficient plants, osmotic cycling was decreased or even eliminated. Across treatments, cycling of osmotic potential occurred only when plants carried at least 560 cm² of leaf area per fruit. The results are interpreted to mean that diurnal cycling of osmotic potential reveals a ‘sink-limited’ condition within the plant.     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.