The influence of water stress on leaf and stem photosynthesis in Spartium junceum L.

Stem and leaf photosynthetic responses to environmental parameters were studied in Spartium junceum L., a legume with chlorophyllous stems. Stem net photosynthesis (P_n) was consistently lower than leaf P_n. The low stem P_n was due to lower quantum yield, lower mesophyll conductance and lower CO₂-saturated P_n than that of leaf P_n. Stomatal limitations to leaf and stem P_n were similar (25%). Water stress caused a greater reduction in leaf P_n than that of stems. Leaf P_n was also reduced in water-stressed plants following rehydration. The reduced leaf P_n was associated with a reduced photon saturated P_n rate and a reduced CO₂ saturated P_n rate. Apparent quantum yield, mesophyll conductance and stomatal limitation of leaves were unaffected by water-stress. Stem P_n following rehydration was not influenced by the water-stress treatment. In general, leaf P_n was more responsive to environmental parameters and more sensitive to water stress than stem P_n. These data support the hypothesis that stem P_n has greater tolerance of water stress, but is limited to low P_n by biochemical means compared to leaves.     

Xylem acoustic emissions and water relations of Calluna vulgaris L. at two climatological regions of Britain

The acoustic emission rate (as a result of xylem cavitation), stem water content, stomatal conductance, and leaf water potential of heather (Calluna vulgaris L. Hull) were compared over a period of 18 months at two contrasting sites in Britain. The sites were Thursely Common in south-east England, and Flanders Moss in central Scotland (average rainfall of 600 mm per year and 1500 mm per year, respectively). In the first year of study (1992) the natural difference between the sites was amplified by a severe drought affecting south-east England. The relative water content of the xylem (RWC) did not differ between the sites, and did not fluctuate seasonally, despite the drought at Thursely. Acoustic emissions were detected at both sites, but the incidence was low. The same range of foliage water potential and stomatal conductance was found at both sites. The leaf area to sapwood area ratio at the dry site was half that at the wet site, and it is suggested that this adjustment of the transpiring area relative to the water conducting area facilitates the close physiological state of the plants at the two sites. The possible occurrence of aerial refilling and freezing induced xylem cavitation is also discussed.     

Responses of sap flux and stomatal conductance of Pinus taeda L. trees to stepwise reductions in leaf area

Herbivory or artificial foliage removal has been shown to affect gas exchange and canopy water relations. In this study, canopy architecture and water relations in response to progressive defoliation were examined in a stand of 8-year-old loblolly pine (Pinus taeda L.) trees, a shade-intolerant, pioneer species common in the south-eastern USA. Sap flux was measured with constant heat sap flow gauges in order to estimate canopy stomatal conductance (Gs) while foliage in the 6 m high stand was harvested in 1 m increments from the bottom up. Leaf-level stomatal conductance and water potential data were also collected. Profiles of silhouette area ratio and specific leaf area showed no trends with crown height, reflecting an open canopy (leaf area index = 1.55). Therefore, short-term changes in Gs with foliage removal were attributed to hydraulic effects rather than influences of changes in mean microclimate conditions on Gs of remaining foliage. A large increase in Gs was observed during the 6 h pruning period which fully compensated for the reductions in foliage area down to 45%. Canopy stomatal conductance and whole plant liquid phase conductance as calculated from sap flux were both influenced by the rate of growth as indicated by the annual basal area increment.     

Midday stomatal conductance is more related to stem rather than leaf water status in subtropical deciduous and evergreen broadleaf trees

Midday depressions in stomatal conductance (g_s) and photosynthesis are common in plants. The aim of this study was to understand the hydraulic determinants of midday g_s, the coordination between leaf and stem hydraulics and whether regulation of midday g_s differed between deciduous and evergreen broadleaf tree species in a subtropical cloud forest of Southwest (SW) China. We investigated leaf and stem hydraulics, midday leaf and stem water potentials, as well as midday g_s of co‐occurring deciduous and evergreen tree species. Midday g_s was correlated positively with midday stem water potential across both groups of species, but not with midday leaf water potential. Species with higher stem hydraulic conductivity and greater daily reliance on stem hydraulic capacitance were able to maintain higher stem water potential and higher g_s at midday. Deciduous species exhibited significantly higher stem hydraulic conductivity, greater reliance on stem capacitance, higher stem water potential and g_s at midday than evergreen species. Our results suggest that midday g_s is more associated with midday stem than with leaf water status, and that the functional significance of stomatal regulation in these broadleaf tree species is probably for preventing stem xylem dysfunction.     

Stomatal factors and vulnerability of stem xylem to cavitation in poplars

The relationships between the vulnerability of stem xylem to cavitation, stomatal conductance, stomatal density, and leaf and stem water potential were examined in six hybrid poplar (P38P38, Walker, Okanese, Northwest, Assiniboine and Berlin) and balsam poplar (Populus balsamifera) clones. Stem xylem cavitation resistance was examined with the Cavitron technique in well-watered plants grown in the greenhouse. To investigate stomatal responses to drought, plants were subjected to drought stress by withholding watering for 5 (mild drought) and 7 (severe drought) days and to stress recovery by rewatering severely stressed plants for 30 min and 2 days. The clones varied in stomatal sensitivity to drought and vulnerability to stem xylem cavitation. P38P38 reduced stomatal conductance in response to mild stress while the balsam poplar clone maintained high leaf stomatal conductance under more severe drought stress conditions. Differences between the severely stressed clones were also observed in leaf water potentials with no or relatively small decreases in Assiniboine, P38P38, Okanese and Walker. Vulnerability to drought-induced stem xylem embolism revealed that balsam poplar and Northwest clones reached loss of conductivity at lower stem water potentials compared with the remaining clones. There was a strong link between stem xylem resistance to cavitation and stomatal responsiveness to drought stress in balsam poplar and P38P38. However, the differences in stomatal responsiveness to mild drought suggest that other drought-resistant strategies may also play a key role in some clones of poplars exposed to drought stress.     

Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

The degree of plant iso/anisohydry, a widely used framework for classifying species‐specific hydraulic strategies, integrates multiple components of the whole‐plant hydraulic pathway. However, little is known about how it associates with coordination of functional and structural traits within and across different organs. We examined stem and leaf hydraulic capacitance and conductivity/conductance, stem xylem anatomical features, stomatal regulation of daily minimum leaf and stem water potential (Ψ), and the kinetics of stomatal responses to vapour pressure deficit (VPD) in six diverse woody species differing markedly in their degree of iso/anisohydry. At the stem level, more anisohydric species had higher wood density and lower native capacitance and conductivity. Like stems, leaves of more anisohydric species had lower hydraulic conductance; however, unlike stems, their leaves had higher native capacitance at their daily minimum values of leaf Ψ. Moreover, rates of VPD‐induced stomatal closure were related to intrinsic rather than native leaf capacitance and were not associated with species' degree of iso/anisohydry. Our results suggest a trade‐off between hydraulic storage and efficiency in the leaf, but a coordination between hydraulic storage and efficiency in the stem along a spectrum of plant iso/anisohydry.     

Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ceratonia siliqua L. during air‐dehydration of 3‐year‐old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Ψ_L). The threshold Ψ_L for cavitation was less negative in L. nobilis than in C. siliqua according to the known higher drought resistance of the latter species. Leaf minor vein cavitation was also quantified by infiltrating leaves with fluorescein at different dehydration levels and observing them under microscope. Distinct decreases in the functional integrity of minor veins were observed during leaf dehydration, with high correlation between the two variables. The relationship between leaf conductance to water vapour (g_L) and Ψ_L showed that stomata of L. nobilis closed in response to stem and not to leaf cavitation. However, in C. siliqua, g_L decreased in coincidence to the leaf cavitation threshold, which was, nevertheless, very close to that of the stem. The hypothesis that stem cavitation acts as a signal for stomatal closure was confirmed, while the same role for leaf cavitation remains an open problem.     

Gas-exchange characteristics,leaf water potential and chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence in oil palm (<Emphasis Type="Italic">Elaeis guineensis</Emphasis> Jacq.) seedlings under water stress and recovery

The gas-exchange characteristics, leaf water potential and chlorophyll (Chl) a fluorescence of oil palm (Elaeis guineensis Jacq.) seedlings subjected to water stress and recovery were investigated in a greenhouse experiment. At 24 days after imposition of stress, leaf water potential in water-stressed seedlings was doubled compared to that of control and there was a drastic decline in gas-exchange parameters viz. photosynthesis, transpiration, and stomatal conductance. Water stress did not irreversibly affect gas-exchange parameters and quantum efficiency of photosystem II, as seedlings exhibited total recovery of photosynthetic apparatus by 12^th day of rehydration. These findings indicate that oil palm exhibits physiological plasticity to water stress during the seedling stage.     

Biophysical properties and functional significance of stem water storage tissues in Neotropical savanna trees

Biophysical characteristics of sapwood and outer parenchyma water storage compartments were studied in stems of eight dominant Brazilian Cerrado tree species to assess the impact of differences in tissue capacitance on whole-plant water relations. The rate of decline in tissue water potential with relative water content (RWC) was greater in the outer parenchyma than in the sapwood for most of the species, resulting in tissue-and species-specific differences in capacitance. Sapwood capacitance on a tissue volume basis ranged from 40 to 160 kg m-3 MPa-1, whereas outer parenchyma capacitance ranged from 25 to only 60 kg m-3 MPa-1. In addition, osmotic potentials at full turgor and at the turgor loss point were more negative for the outer parenchyma compared with the sapwood, and the maximum bulk elastic modulus was higher for the outer parenchyma than for the sapwood. Sapwood capacitance decreased linearly with increasing sapwood density across species, but there was no significant correlation between outer parenchyma capacitance and tissue density. Midday leaf water potential, the total hydraulic conductance of the soil/leaf pathway and stomatal conductance to water vapour (gs) all increased with stem volumetric capacitance, or with the relative contribution of stored water to total daily transpiration. However, the difference between the pre-dawn water potential of non-transpiring leaves and the weighted average soil water potential, a measure of the water potential disequilibrium between the plant and soil, increased asymptotically with total stem capacitance across species, implying that overnight recharge of water storage compartments was incomplete in species with greater capacitance. Overall, stem capacitance contributes to homeostasis in the diurnal and seasonal water balance of Cerrado trees.     

Neither xylem collapse,cavitation, or changing leaf conductance drive stomatal closure in wheat

Identifying the drivers of stomatal closure and leaf damage during stress in grasses is a critical prerequisite for understanding crop resilience. Here, we investigated whether changes in stomatal conductance (g_s) during dehydration were associated with changes in leaf hydraulic conductance (K_leaf), xylem cavitation, xylem collapse, and leaf cell turgor in wheat (Triticum aestivum). During soil dehydration, the decline of g_s was concomitant with declining K_leaf under mild water stress. This early decline of leaf hydraulic conductance was not driven by cavitation, as the first cavitation events in leaf and stem were detected well after K_leaf had declined. Xylem vessel deformation could only account for <5% of the observed decline in leaf hydraulic conductance during dehydration. Thus, we concluded that changes in the hydraulic conductance of tissues outside the xylem were responsible for the majority of K_leaf decline during leaf dehydration in wheat. However, the contribution of leaf resistance to whole plant resistance was less than other tissues (<35% of whole plant resistance), and this proportion remained constant as plants dehydrated, indicating that K_leaf decline during water stress was not a major driver of stomatal closure.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Impacts of tree height on leaf hydraulic architecture and stomatal control in Douglas-fir

This study investigated the mechanisms involved in the regulation of stomatal closure in Douglas-fir and evaluated the potential impact of compensatory adjustments in response to increasing tree height upon these mechanisms. In the laboratory, we measured leaf hydraulic conductance (K(leaf)) as leaf water potential (Psi(l)) declined for comparison with in situ diurnal patterns of stomatal conductance (g(s)) and Psi(l) in Douglas-fir across a height gradient, allowing us to infer linkages between diurnal changes in K(leaf) and g(s). A recently developed timed rehydration technique was used in conjunction with data from pressure-volume curves to develop hydraulic vulnerability curves for needles attached to small twigs. Laboratory-measured K(leaf) declined with increasing leaf water stress and was substantially reduced at Psi(l) values of -1.34, -1.45, -1.56 and -1.92 MPa for foliage sampled at mean heights of approximately 20, 35, 44 and 55 m, respectively. In situ g(s) measurements showed that stomatal closure was initiated at Psi(l) values of -1.21, -1.36, -1.74 and -1.86 MPa along the height gradient, which was highly correlated with Psi(l) values at loss of K(leaf). Cryogenic scanning electron microscopy (SEM) images showed that relative abundances of embolized tracheids in the central vein increased with increasing leaf water stress. Leaf embolism appeared to be coupled to changes in g(s) and might perform a vital function in stomatal regulation of plant water status and water transport in conifers. The observed trends in g(s) and K(leaf) in response to changes in Psi(l) along a height gradient suggest that the foliage at the tops of tall trees is capable of maintaining stomatal conductance at more negative Psi(l). This adaptation may allow taller trees to continue to photosynthesize during periods of greater water stress.     

亚低温与干旱胁迫对番茄植株水分传输和形态解剖结构的影响

为探讨亚低温和干旱对植株水分传输的影响机制,以番茄幼苗为试材,利用人工气候室设置常温(昼25 ℃/夜18 ℃)和亚低温(昼15 ℃/夜8 ℃)环境,采用盆栽进行正常灌水(75%～85%田间持水量)和干旱处理(55%～65%田间持水量),分析了温度和土壤水分对番茄植株水分传输、气孔和木质部导管形态解剖结构的影响。结果表明: 与常温正常灌水处理相比,干旱处理使番茄叶水势、蒸腾速率、气孔导度、水力导度、茎流速率、气孔长度和叶、茎、根导管直径显著减小,而使叶、茎、根导管细胞壁厚度和抗栓塞能力增强;亚低温处理下番茄叶水势、蒸腾速率、气孔导度、水力导度和叶、茎、根导管直径显著降低,但气孔变大,叶、根导管细胞壁厚度和叶、茎、根抗栓塞能力显著升高。亚低温条件下土壤水分状况对番茄叶水势、蒸腾速率、气孔导度、水力导度、气孔形态、叶、根导管结构均无显著影响。总之,干旱处理下番茄通过协同调控叶、茎、根结构使植株水分关系重新达到稳态;亚低温处理下番茄植株水分关系的调控主要通过改变叶和根导管结构实现,且受土壤水分状况的影响较小。     

Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (g_s) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween g_s and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between g_s and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.     

Sensitivity of stem and petiole hydraulic conductance of deciduous trees to xylem sap ion concentration

Hydraulic conductance of stem and petioles increased in response to an increase in xylem sap ion concentration, and decreased in response to a decrease in the ion concentration in six temperate deciduous tree species. The ion sensitivity of hydraulic conductance of stem and petioles was higher than the ion sensitivity of the stem alone. The ion sensitivity was lowest in the earliest developmental stages of the xylem, and had a seasonal maximum in the second half of summer. The ion sensitivity was highest in slow-growing species and lowest in fast-growing species. The ion sensitivity correlated negatively with mean radius of xylem conduits, hydraulic conductance of stem and petioles, hydraulic conductance of leaf laminae, and stomatal conductance, and positively with response of the hydraulic conductance of leaf laminae to HgCl₂, and stomatal response to a decrease in leaf water potential or abscisic acid. It was concluded that the high ion sensitivity of xylem hydraulic conductance is a relevant characteristic of slow growth and a conservative water use strategy.     

Effects of drought stress on photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean plants

Changes in plant growth, photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean [Glycine max (L.) Merr.] plants under drought stress were studied. Total plant dry mass was reduced by 30 % compared to well-watered control plants. Leaf water potential was slightly decreased by water stress. Water stress induced daytime shrinkage and reduced night-time expansion of stem. Photosynthetic rate, stomatal conductance and transpiration rate were significantly declined by water stress, while the intercellular CO₂ concentration was changed only slightly at the initiation of stress treatment. The maximum photochemical efficiency of photosystem 2 and apparent photosynthetic electron transport rate were not changed by water stress.     

豆科复叶和单叶树种的光合-水分关系分析

以豆科（Fabaceae）11个复叶树种和6个单叶树种为材料,测定他们的气孔导度、叶片水力导度、水势、相对含水量等指标,分析叶型对枝叶光合水分关系的影响。结果显示,复叶树种正午叶轴水势（-0.91 MPa）与单叶树种正午枝条水势（-0.88 MPa）间无显著差异,但正午枝条水势（-0.60 MPa）显著高于单叶树种。复叶树种正午气孔导度降低的百分比（55.3%）显著高于单叶树种（34.1%）。叶片、叶轴和枝条正午水势两两之间均显著正相关,但与正午气孔导度之间均不存在相关性。本研究中,17个树种的正午叶片水力导度与气孔导度间显著正相关（r=0.79,P<0.001）,但他们与气孔导度降低百分比间呈负相关（r=-0.81,P<0.001）,说明叶片导水率对日间气孔导度的维持具有重要作用。研究结果表明单叶和复叶树种在光合水分关系上存在明显差异,说明他们对环境条件具有不同的适应策略。     

Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO₂ concentrations in the proximity of a natural CO₂ spring. Trees at the spring site have been growing for generations in elevated atmospheric CO₂ concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO₂ spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO₂ was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO₂ spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO₂ spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO₂ spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO₂ on plant water use at the organ and whole-tree scale.     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

Influence of Rate of Development of Leaf Water Deficits upon Photosynthesis, Leaf Conductance, Water Use Efficiency, and Osmotic Potential in Sorghum

This study reports the effect of rate of development of leaf water deficits in soil-grown sorghum (Sorghum bicolor) on the relationship of net photosynthesis, leaf conductance, and water use efficiency to leaf water potential, and on the degree of solute accumulation (osmotic adjustment). Recovery of these processes on rewatering, and responses during a second stress cycle were also studied. The most rapid rate of stress (1.2 MPa day^?1) resulted in no solute accumulation and the lowest rate of net photosynthesis and leaf conductance for any given leaf water potential during stress. Stress at 0.7 and 0.15 MPa day^?1 led to equal solute accumulations of approximately 0.6 MPa, but net photosynthesis, leaf conductance, and water use efficiency at a given leaf water potential were lower with the faster rate of stress (0.7 MPa day^?1). Additionally, leaf conductance at a given leaf turgor potential was lowest at the 1.2 MPa day^?1 stress rate, slightly higher at the intermediate rate of stress, and clearly highest at the slowest rate of stress. Recovery of both net photosynthesis and leaf conductance upon rewatering was rapid, taking less than 3 days, but full recovery of osmotic potential took between 6 and 11 days. One slow stress cycle had no influence on relationships during a second cycle. The concept of a threshold leaf water potential for stomatal closure is discussed and the conclusion reached that stomatal closure occurs slowly over a wide range of leaf water potential (> 1.0 MPa), the range being greater for slower rates of stress.