Mechanisms of drought response in Thuja occidentalis L. I. Water stress conditioning and osmotic adjustment

We investigated the extent of osmotic adjustment and changes in transpiration rate that occur in response to repeated cycles of water deficit stress in 6-year-old Thuja occidentalis L. (eastern white cedar) trees. Groups of trees were water-stress conditioned by repeated exposure to predetermined thresholds of nonlethal water stress by withholding water until the predawn water potential fell to -0.9 (mild conditioning) or -1.4 MPa (moderate conditioning). Both the mild and moderate conditioning treatments resulted in a decrease in osmotic potential of 0.08 to 0.20 MPa and 0.11 to 0.28 MPa, respectively, relative to the well-watered controls. Mildly and moderately conditioned trees exhibited an approximately 35 and 50% reduction in cumulative transpiration, respectively, following at least two stress cycles. Transpiration rates of conditioned trees remained below those of the well-watered controls even when water potential integrals were similar. We conclude that the decrease in transpiration rate was more important than osmotic adjustment as a mechanism of response to repeated water stress.     

Adaptations to soil drying in woody seedlings of African locust bean, (Parkia biglobosa (Jacq.) Benth.)

Stomatal conductance, transpiration and xylem pressure potential of African locust bean (Parkia biglobosa (Jacq.) Benth.) seedlings subjected from the sixth week after emergence to four weeks of continuous soil drought did not differ from those of well-watered, control plants until two-thirds of the available soil water had been used. In both well-watered and drought-treated plants, stomatal conductance was highest early in the day when vapor pressure deficits were low, but decreased sharply by midday when evaporative demand reached its highest value. There was no increase in stomatal conductance later in the day as vapor pressure deficit declined. The relationship between transpiration rate and xylem pressure potential showed non-linearity and hysteresis in both control and drought-treated plants, which seems to indicate that the plants had a substantial capacity to store water. The rate of leaf extension in African locust bean seedlings subjected to six consecutive 2-week cycles of soil drought declined relative to that of well-watered, control plants, whereas relative root extension increased. It appears that African locust bean seedlings minimized the impact of drought by: (1) restricting transpiration to the early part of the day when a high ratio of carbon gain to water loss can be achieved; (2) utilizing internally stored water during periods of rapid transpiration; (3) reducing the rate of leaf expansion and final leaf size in response to soil drought without reducing the rate of root extension, thereby reducing the ratio of transpiring leaf surface area to absorbing root surface area.     

Effects of phenology,water availability and seed source on loblolly pine biomass partitioning and transpiration

One-year-old loblolly pine (Pinus taeda L.) seedlings from four seed sources (Arkansas, Georgia, Texas and Virginia) grown in 1-m-deep sand-filled pits in two water regimes (well-watered and drought) were studied, to gain insight into the process of seedling establishment. Whole-plant transpiration was measured biweekly from July to December. Whole-plant harvests were conducted at 6-week intervals from April to December. Whole-plant transpiration and transpiration per unit leaf and root area were affected by treatment, seedlot and phenology. Seedlings of the Arkansas seedlot maintained significantly higher transpiration rates per unit leaf and root area during drought than seedlings of the Virginia, Georgia or Texas seedlots, but did not accumulate greater biomass. The high transpiration rates of the Arkansas seedlings were attributed to their deep root systems. Allometric relationships indicated that, relative to the whole plant, biomass allocation to needles of drought-treated seedlings was enhanced during the summer (allometric ratio 1.09), whereas allocation to roots was enhanced in the spring and fall (allometric ratios of 1.13 and 1.09, respectively). Relative to the whole plant, biomass allocation to needles of well-watered seedlings was enhanced throughout the experiment (allometric ratio of 1.16 declining to 1.05), whereas the allometric ratio of root to total biomass was 0.89 or less throughout. Allometric relationships also indicated variation in biomass partitioning to roots in three soil layers (0-30, 30-60 and 60-100 cm), which differed among harvests in each soil layer. Root growth in both well-watered and drought-treated seedlings was concentrated in the top soil layer in the spring, shifted to the middle and bottom soil layers in the summer, and then increased in the top soil layer in the fall. Compared with well-watered seedlings, drought-treated seedlings had higher rates of root growth in the bottom soil layer in the fall, a characteristic that would confer tolerance to future periods of limited soil water availability.     

Effects of drought and rewatering on growth and transpiration in European beech seedlings late in the growing season

Drought stress is one of the most important environmental factors affecting plant growth and survival. To date, most studies aim at understanding of post-stress physiological and anatomical adaptation to drought stress; however only few studies focus on plant recovery. In the present study, transpiration, shoot water potential, and anatomical and morphological measurements were performed on 4-year-old European beech seedlings with fully developed leaves. The seedlings were exposed to three levels of soil water potential (well-watered, moderate drought stress and severe drought stress) and followed by rewatering under greenhouse conditions. Reduced transpiration rates were observed in the stressed seedlings as a response to drought stress, whereas anatomical and morphological variables remained unchanged. Three days after rewatering, transpiration rates in both moderately and severely stressed seedlings recovered to the levels of those of well-watered seedlings. Drought stress promoted leaf budding, resulting in higher shoot dry mass of stressed seedlings. Our findings indicate that anatomical and morphological adaptations of European beech seedlings to drought stress are visibly limited during late-season growth stages. These results will help us to further understand factors involved in drought adaptation potential of European beech seedlings faced with expected climate-related environmental changes. To complete our findings, further experiments on plant recovery from drought stress should be focused on different periods of growing season.     

Contrasting physiological responses of two co-occurring eucalypts to seasonal drought at restored bauxite mine sites

This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.     

Growth, leaf morphology, water use and tissue water relations of Eucalyptus globulus clones in response to water deficit

Changes in leaf size, specific leaf area (SLA), transpiration and tissue water relations were studied in leaves of rooted cuttings of selected clones of Eucalyptus globulus Labill. subjected to well-watered or drought conditions in a greenhouse. Significant differences between clones were found in leaf expansion and transpiration. There was a significant clone x treatment interaction on SLA. Water stress significantly reduced osmotic potential at the turgor loss point (Pi0) and at full turgor (Pi100), and significantly increased relative water content at the turgor loss point and maximum bulk elastic modulus. Differences in tissue water relations between clones were significant only in the mild drought treatment. Among clones in the drought treatments, the highest leaf expansion and the highest increase in transpiration during the experiment were measured in those clones that showed an early and large decrease in Pi0 and Pi100.     

Water relations of red spruce seedlings treated with acid mist

Pressure-volume curves, day and night transpiration rates, needle drying curves, and shoot water potentials were determined for 2-year-old red spruce trees that had been exposed for three months to a range of acid mists (pH 2.5 to pH 5.0) containing equimolar (NH(4))(2)SO(4) and HNO(3). No effect of acid mist was observed on cuticular resistance or on the rates of day and night transpiration, although trees exposed to acid mist exhibited symptoms of mild water stress. Significant decreases in maximum turgor, the relative water content (RWC) associated with zero turgor, and bulk volumetric elastic modulus occurred as the pH of the mist decreased from 5.0 to 2.5. At all RWC values, there was an increase in solute potential as mist pH decreased. Shoot water potential declined with a decrease in pH of the mist.     

Effects of water stress cycles on turgor maintenance processes in pear leaves (Pyrus communis)

Effects of water deficits on leaf turgor maintenance processes were analyzed for pear trees (Pyrus communis L. cv. "Barlett") grown in 120-liter containers. Four irrigation treatments were applied: a well-watered control treatment, a spring water stress cycle (Sp), a summer water stress cycle (Su), and a spring plus summer water stress cycle (Sp + Su). For the Sp treatment, water application was progressively reduced from 100 to 20% of the control dose over a period of 27 days in spring. For the Su treatment, water application was progressively reduced over 23 days in summer, from 100 to 20% of the control dose. The Sp + Su treatment comprised both the spring and summer drought stress cycles. Pressure-volume (P-V) curves were constructed and stomatal conductances were determined for pear leaves from each treatment during the spring and summer stress cycles. Leaf water potential (Psi(pi) (0)) and relative water content (R(0)) at the turgor loss point of control leaves tended to decrease from spring to summer. Changes in leaf osmotic water potential at full turgor (Psi(pi) (100)) and in symplast water fraction (R(s)) did not explain the seasonal decrease in Psi(pi) (0). The water stress treatments had no effect on Psi(pi) (100), but R(s) was reduced by the water stress treatments, particularly during the summer stress cycle of the Su and Sp + Su treatments. The decrease in R(s) was correlated with an increase in the slope of the linear region of the P-V curve. Such a coupled adjustment would lead to increased water uptake capacity of water-stressed trees only under non-turgor conditions. Furthermore, pear leaves did not actively accumulate solutes. We conclude, therefore, that changes in leaf tissue water relations as a result of leaf acclimation to water stress are unlikely to facilitate maintenance of fruit productivity under drought.     

Drought-induced oxidative stress in Canarian laurel forest tree species growing under controlled conditions

We studied photoprotection and antioxidative protection in the three major species of the Canarian laurel forest (Laurus azorica (Seub.) Franco, Persea indica (L.) K. Spreng and Myrica faya Aiton). Trees were exposed to drought under controlled conditions by withholding water until leaf relative water content (RWC) reached 50-55%. Drought reduced photosynthetic rate (P(N)) and was associated with decreased quantum yield of photosystem II (PSII) electron transport and increased non-photochemical quenching (NPQ) in L. azorica and M. faya, but did not increase NPQ in P. indica. Drought-treated trees of L. azorica had the highest de-epoxidation state (DPS) of the xanthophyll cycle and the highest zeaxanthin (Z) concentration, suggesting that this species had more effective photoprotective mechanisms than M. faya and P. indica. Moreover, beta-carotene remained unaltered in L. azorica trees during drought, suggesting that the chloroplasts of this species are better protected against oxidative stress than those of M. faya and P. indica. Increased antioxidation by ascorbate peroxidase, superoxide dismutase and glutathione reductase in L. azorica removed activated oxygen species (AOS) generated during drought treatment. Although M. faya was able to increase its energy dissipation rate by forming Z and thus increasing the DPS of the xanthophyll cycle, it did not respond to drought-induced oxidative stress with the result that beta-carotene degradation occurred. Persea indica did not activate an energy dissipation mechanism in response to drought treatment, hence formation of AOS was likely high in the drought-treated trees. In general, L. azorica was most resistant and P. indica most sensitive to photoinhibition and oxidative stress during drought.     

Shoot water relations of mature black spruce families displaying a genotype x environment interaction in growth rate. II. Temporal trends and response to varying soil water conditions

Pressure-volume curves and shoot water potentials were determined for black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families at the Petawawa Research Forest, Ontario, Canada. Trees were sampled from a dry site in 1992 and from the dry site and a wet site in 1993. Modulus of elasticity (epsilon), osmotic potential at turgor loss point (Psi(tlp)) and relative water at turgor loss point (RWC(tlp)) all decreased during the growing season. Osmotic potential at saturation (Psi(sat)) and turgor displayed no general temporal trend. Across a range of environmental conditions, Female 59 progeny had equal or lower Psi(sat), and higher or similar epsilon, mean turgor pressure (P(x)) and predawn turgor pressure (P(pd)) compared with Female 63 progeny. Osmotic potential at saturation decreased as water stress increased from mild to moderate and increased as water stress increased from moderate to severe. Stable genetic differences in Psi(sat) were maintained by the same rate of osmotic adjustment from low to moderate water stress. Modulus of elasticity and RWC(tlp) decreased with decreasing water availability, whereas Psi(tlp) showed no response. The combined effects of Psi(sat) and epsilon resulted in no change in P(pd) as water stress increased from low to moderate values, but turgor declined sharply as water stress increased from moderate to high values. We conclude that drought tolerance traits strongly influence the growth of these black spruce families across sites of varying water availability.     

Diurnal changes in water conduction in loblolly pine (Pinus taeda) and Virginia pine (P. virginiana) during soil dehydration

We studied diurnal changes in water conduction during soil dehydration in 37-month-old seedlings of one Virginia pine (Pinus virginiana Mill.) and two loblolly pine (P. taeda L.) sources, one from North Carolina (NC) and the other from the "Lost Pines" areas of Texas (TX), in an environmentally controlled growth chamber. For seedlings of similar biomass, the TX source had higher values of transpiration, needle conductance, and plant hydraulic conductivity under well-watered conditions than the NC source. Under dry soil conditions, the TX source had lower values of water conduction than the NC source. The Virginia pine source responded similarly to the TX source under both well-watered and dry soil conditions. For all three pine sources, gradients between soil and needle water potentials were greatest when the seedlings were moderately stressed. The TX and Virginia pine sources had higher gradients and lower daytime needle water potentials under moderate stress conditions than the NC source. Predawn needle water potentials did not differ among the pine sources. We conclude that the TX and Virginia pine sources have decreased daytime needle water potentials and increased water potential gradients during the daytime under moderate stress conditions, but with no disruption of recovery at predawn. The greater rates of transpiration and water conduction by the TX source compared with the NC source under well-watered conditions suggest a means by which growth can be maximized prior to the onset of drought, thereby enhancing survival of loblolly pines in drought-prone environments.     

Growth CO2 concentration modifies the transpiration response of Populus deltoides to drought and vapor pressure deficit

Cottonwood (Populus deltoides Bartr. ex Marsh.) trees grown for 9 months in elevated carbon dioxide concentration ([CO2]) showed significant increases in height, leaf area and basal diameter relative to trees in a near-ambient [CO2] control treatment. Sample trees in the CO2 treatments were subjected to high and low atmospheric vapor pressure deficits (VPD) over a 5-week period at both high and low soil water contents (SWC). During these periods, transpiration rates at both the leaf and canopy levels were calculated based on sap flow measurements and leaf-to-sapwood area ratios. Leaf-level transpiration rates were approximately equivalent across [CO2] treatments when soil water was not limiting. In contrast, during drought stress, canopy-level transpiration rates were approximately equivalent across [CO2] treatments, indicating that leaf-level fluxes during drought stress were reduced in elevated [CO2] by a factor equal to the leaf area ratio of the two canopies. The shift from equivalent leaf-level transpiration to equivalent canopy-level transpiration with increasing drought stress suggests maximum water use rates were controlled primarily by atmospheric demand at high SWC and by soil water availability at low SWC. Changes in VPD had less effect on transpiration than changes in SWC for trees in both CO2 treatments. Transpiration rates of trees in both CO2 treatments reached maximum values at a VPD of about 2.0 kPa at high SWC, but leveled off and decreased slightly in both canopies as VPD increased above this value. At low SWC, increasing VPD from approximately 1.4 to 2.5 kPa caused transpiration rates to decline slightly in the canopies of trees in both treatments, with significant (P = 0.004) decreases occurring in trees in the near-ambient [CO2] treatment. The transpiration responses at high VPD in the presence of high SWC and throughout the low SWC treatment suggest some hydraulic limitations to water use occurred. Comparisons of midday leaf water potentials of trees in both CO2 treatments support this conclusion.     

Water loss regulation in mature Hevea brasiliensis: effects of intermittent drought in the rainy season and hydraulic regulation

Effects of soil and atmospheric drought on whole-tree transpiration (E(T)), leaf water potential (Ψ(L)) and whole-tree hydraulic conductance (K(T)) were investigated in mature rubber trees (Hevea brasiliensis, clone RRIM 600) during the full canopy stage in the rainy season in a drought-prone area of northeast Thailand. Under well-watered soil conditions, transpiration was tightly regulated in response to high evaporative demand, i.e., above reference evapotranspiration (ET(0)) ~2.2 mm day(-1) or maximum vapor pressure deficit ~1.8 kPa. When the trees experienced intermittent soil drought E(T) decreased sharply when relative extractable water in the top soil was?相似文献   

Atmospheric and soil drought reduce nocturnal conductance in live oaks

Nocturnal and daytime whole-canopy transpiration rate (E) and conductance (g = E/VPD, where VPD is leaf to air vapor pressure difference) were assessed gravimetrically in drought-treated and well-watered 3-year-old saplings of live oak species (Quercus series Virentes Nixon) from the southeastern USA (Quercus virginiana Mill.) and Central America (Q. oleoides Cham. and Schlecter). Our objectives were to: (1) quantify nocturnal and daytime E and g in a controlled environment; (2) determine the impact of severe drought on nocturnal E and g; and (3) examine whether unavoidable water loss through the epidermis could account for nocturnal water loss. We calculated daytime E during peak daylight hours (between 0930 and 1330 h) and nocturnal E during complete darkness (between 2200 and 0500 h). In addition to reducing E and g during the daytime, drought-treated plants reduced nocturnal E and g on a whole-canopy basis by 62-64% and 59-61%, respectively, and on a leaf-level basis by 27-28% and 19-26%, respectively. In well-watered plants, nocturnal g declined with increasing VPD, providing evidence for stomatal regulation of nocturnal transpiration. In drought-treated plants, g was low and there was no relationship between nocturnal g and VPD, indicating that water loss could not be reduced further through stomatal regulation. Both daytime and nocturnal g declined curvilinearly with predawn water potential for all plants, but nocturnal g was unrelated to predawn water potentials below -1 MPa. The reductions in daytime and nocturnal E and g during drought were associated with decreases in whole-plant and leaf hydraulic conductances. Observed nocturnal g was within the same range as epidermal conductance for oak species determined in previous studies under a range of conditions. Nocturnal E rose from 6-8% of daytime E for well watered plants to 19-20% of daytime E for drought-treated plants. These results indicate that, during drought, saplings of live oak species reduce g to a minimum through stomatal closure, and experience unavoidable water loss through the epidermis.     

Geographical variation in water relations, hydraulic architecture and terpene composition of Aleppo pine seedlings from Italian provinces

Ecotypic variations in leaf conductance, soil-to-leaf hydraulic conductance, components of tissue water potential, hydraulic architecture parameters and xylem embolism were examined in greenhouse-grown two-year-old Aleppo pine (Pinus halepensis Mill.) seedlings from six origins representing the geographic range of the species in Italy. Cortical resin composition of the seedlings was also determined. Measurements were made on well-watered seedlings and on seedlings subjected to recurring severe drought. Drought-stressed seedlings had lower mean leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than well-watered seedlings. They also exhibited more negative osmotic potentials, higher relative water deficit at incipient plasmolysis, but a similar maximum modulus of elasticity. Drought-stressed seedlings showed a higher degree of xylem embolism, a lower Huber value, lower leaf specific conductivity and lower specific conductivity than well-watered seedlings. Drought-stressed seedlings of provenances from more xeric habitats (Tremiti, Porto Pino and Mottola) had greater leaf conductances, transpiration rates and soil-to-leaf hydraulic conductances than drought-stressed seedlings of provenances from more mesic habitats (Imperia, Otricoli and Vico del Gargano). They also showed higher osmotic adjustment and a lower degree of xylem embolism. Among provenances, there were no significant differences in hydraulic architecture parameters in response to the drought treatment; however, Tremiti and Porto Pino seedlings displayed smaller drought-induced reductions in specific conductivity and leaf specific conductivity, respectively, than seedlings from other provenances. These differences suggest that seedlings from xeric provenances, especially Tremiti, have greater resistance to desiccation than seedlings from mesic provenances. No clear association was found between terpene variability and the other traits investigated, although terpene composition was related to the geographical distribution of the provenances. We conclude that the drought-tolerance responses of Tremiti make it a more suitable provenance than the others for establishment on sites prone to severe soil water deficits.     

Influence of drought stress and low irradiance on plant water relations and structural constituents in needles of Pinus ponderosa seedlings

The influence of low light on tolerance to prolonged drought was tested on unshaded and shaded seedlings of ponderosa pine (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Unshaded seedlings of P. ponderosa var. ponderosa were also drought stressed to compare varietal responses to drought. The maximum irradiance received by shaded seedlings was 10% of full light. Seedlings were progressively drought stressed until predawn water potentials (Psi(x)) were -5.0 MPa. Relative water content (RWC) and the reciprocal of Psi(x) were analyzed by means of an unusual application of the pressure-volume relationship for determination of RWC of the apoplast (RWC(a)), osmotic potential at full turgor (Psi(oft)), and ratio of fully turgid weight to dry weight. Major varietal differences in drought response were in RWC(a) and needle cellulose content. The shaded seedlings showed tissue damage at relative water contents < 60%, and were killed by water deficits from which unshaded seedlings recovered. Correspondingly, shaded plants had significantly higher cell volume/cell mass ratio, Psi(oft), less cellulose in needle tissue, and lower RWC(a) than unshaded plants. These differences suggest that low irradiance restricts drought adaptation in ponderosa pine.     

Seasonal change in the drought response of wood cell development in poplar

Field-grown poplar trees (Populus nigra L. x P. maximowiczii Henry, clone Kamabuchi) were exposed to severe drought twice during the growing season to evaluate the impact on wood cell development. The drought treatment caused a reduction in leaf water potential, leaf wilting and a decreased concentration of osmotically active solutes in the cambial zone. Drought-induced changes in the anatomy of developing xylem cells were examined in stem sections and macerated wood samples. In early summer, drought significantly reduced the length and cross-sectional area of newly formed fibers, whereas no such effects were observed in late summer. In well-watered trees, fiber cross-sectional area declined between early and late summer. Similarly, drought reduced the cross-sectional area of vessel elements in early summer but not in late summer, whereas in both control and drought-treated trees, the cross-sectional area of vessel elements decreased between early and late summer. The vessel area to xylem area ratio was unaffected by drought because the drought-induced decrease in vessel size was matched by an increase in the number of newly formed vessel cells. In contrast to its effect in early summer, late-summer drought had no significant effect on fiber and vessel cell development, indicating that sensitivity of wood cell development to drought varies seasonally.     

PEG模拟干旱胁迫对红松幼苗生理生化指标的影响

采用PEG-6000人工模拟干旱胁迫的方法对红松幼苗针叶相对含水量、可溶性糖、可溶性蛋白质、丙二醛、脯氨酸、超氧化物歧化酶、过氧化物酶的变化进行研究,应用主成分分析方法对生理生化指标进行分析。结果表明：过氧化物酶、相对含水量、可溶性糖和脯氨酸综合反映了红松幼苗的抗旱性。随干旱胁迫加剧,过氧化物酶含量呈下降趋势;相对含水量逐渐降低,可溶性糖含量逐渐升高,脯氨酸呈现先下降后上升的趋势。     

干旱胁迫与彩色树生长状况关系研究

以金叶榆、金枝梾木和俄罗斯红叶李3种彩色树2a生苗木为试材,测定了不同程度干旱胁迫下净光合速率、蒸腾速率、细胞膜透性、叶片含水量以及高生长等指标,分析了3种彩色树的抗旱性。结果表明:在轻度干旱胁迫条件下,金叶榆和金枝梾木受到的影响小于俄罗斯红叶李;重度干旱条件下,金叶榆表现出较强的抗旱性,而金枝梾木和俄罗斯红叶李受到的影响较大。3种彩色树的抗旱性强弱排序为:金叶榆金枝梾木俄罗斯红叶李。     

Xylem sap flow and drought stress of Fraxinus excelsior saplings

We report on diurnal and seasonal variations in sap flow rate and stem water potential of Fraxinus excelsior L. saplings growing at the edge of a Fraxino-Aceretum forest in western Germany. Because of shallow soil, the trees were subjected to drought in summer. When soil water availability was not limiting, sap flow rate was related to changes in solar radiation and vapor pressure deficit. Maximum transpiration rates per leaf area were 3.5-7.4 mmol m-2 s-1, and maximum daily totals were 1.7-3.3 kg m-2 day-1. Under drought conditions, stem water potential dropped to midday minima of -2.6 to -3.5 MPa and sap flow rate was strongly related to this parameter. After the drought period, reduced apparent (whole-plant) hydraulic conductance was observed, which was attributed to a continued reduction in stomatal conductance after the drought stress had ceased. A model was developed that linked sap flow rate directly to climatic variables and stem water potential. Good correlation between measured and simulated sap flow rates allowed the model to be used for data interpretation.