Can stomatal closure caused by xylem ABA explain the inhibition of leaf photosynthesis under soil drying?

Effects of leaf water deficit and increase in endogenous ABA on photosynthesis of two tropical trees, t Acacia confusa and t Leucaena leucocephala, were investigated with two soil-drying methods, i.e. half or whole root system was subjected to soil drying. Half-root drying was achieved by allowing upper layer of soil column to dry and lower layer of soil column to remain watered. Half-root drying had little effect on leaf water potential, but when compared to the well-watered control, both methods of soil drying substantially increased the ABA concentration in xylem and reduced leaf conductance in both species. There was a significant relationship between leaf conductance and xylem ABA concentrations in both species, which was comparable to the same relationship that was generated by feeding ABA to excised twigs. The rate of photosynthesis was inhibited substantially in both soil-drying treatments and in both species, but photochchemical efficiency, measured as a ratio of variable fluorescence to a peak fluorescence emission of a dark-adapted leaf (F_v/F_m), was not reduced except in the whole root-dried t L. leucocephala plants where leaf water potential was reduced to –2.5 MPa. In all the cases where photosynthesis was inhibited, there was a concomitant reduction in both leaf conductance and calculated internal CO₂ concentration. After two days of rewatering, leaf water potential and xylem ABA concentration rapidly returned to pre-treatment levels, but leaf conductance and photosynthesis of both whole-root and half root dried t L. leucocephala remained inhibited substantially. Rewatering led to a full recovery of both stomatal conductance and photosynthesis in soil-dried t A. confusa, although its photosynthesis of whole-root dried plants did not recover fully but such difference was not significant statistically. These results suggest that drought-induced decline of photosynthesis was mainly a result of the stomatal factor caused by the increase of ABA concentration in the xylem sap. Non-stomatal factors, e.g. reduced photochemical activity and/or carbon metabolic activity, were species-specific and were brought about only at very low water potential.     

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.     

Changes in stomatal conductance along grass blades reflect changes in leaf structure

Identifying the consequences of grass blade morphology (long, narrow leaves) on the heterogeneity of gas exchange is fundamental to an understanding of the physiology of this growth form. We examined acropetal changes in anatomy, hydraulic conductivity and rates of gas exchange in five grass species (including C(3) and C(4) functional types). Both stomatal conductance and photosynthesis increased along all grass blades despite constant light availability. Hydraulic efficiency within the xylem remained constant along the leaf, but structural changes outside the xylem changed in concert with stomatal conductance. Stomatal density and stomatal pore index remained constant along grass blades but interveinal distance decreased acropetally resulting in a decreased path length for water movement from vascular bundle to stomate. The increase in stomatal conductance was correlated with the decreased path length through the leaf mesophyll. A strong correlation between the distance from vascular bundles to stomatal pores and stomatal conductance has been identified across species; our results suggest this relationship also exists within individual leaves.     

Perturbed lignification impacts tree growth in hybrid poplar--a function of sink strength, vascular integrity, and photosynthetic assimilation

The effects of reductions in cell wall lignin content, manifested by RNA interference suppression of coumaroyl 3'-hydroxylase, on plant growth, water transport, gas exchange, and photosynthesis were evaluated in hybrid poplar trees (Populus alba x grandidentata). The growth characteristics of the reduced lignin trees were significantly impaired, resulting in smaller stems and reduced root biomass when compared to wild-type trees, as well as altered leaf morphology and architecture. The severe inhibition of cell wall lignification produced trees with a collapsed xylem phenotype, resulting in compromised vascular integrity, and displayed reduced hydraulic conductivity and a greater susceptibility to wall failure and cavitation. In the reduced lignin trees, photosynthetic carbon assimilation and stomatal conductance were also greatly reduced, however, shoot xylem pressure potential and carbon isotope discrimination were higher and water-use efficiency was lower, inconsistent with water stress. Reductions in assimilation rate could not be ascribed to increased stomatal limitation. Starch and soluble sugars analysis of leaves revealed that photosynthate was accumulating to high levels, suggesting that the trees with substantially reduced cell wall lignin were not carbon limited and that reductions in sink strength were, instead, limiting photosynthesis.     

Effects of iron chlorosis and iron resupply on leaf xylem architecture, water relations, gas exchange and stomatal performance of field-grown peach (Prunus persica)

There is increasing evidence suggesting that iron (Fe) deficiency induces not only leaf chlorosis and a decline of photosynthesis, but also structural changes in leaf morphology, which might affect the functionality of leaves. In this study, we investigated the effects of Fe deficiency on the water relations of peach ( Prunus persica (L.) Batsch.) leaves and the responses of previously chlorotic leaves to Fe resupply via the root or the leaf. Iron deficiency induced a decline of maximum potential photosystem II (PSII) efficiency (F _V/F _M), of rates of net photosynthesis and transpiration and of water use efficiency. Iron chlorosis was associated with a reduction of leaf xylem vessel size and of leaf hydraulic conductance. In the course of the day, water potentials in chlorotic leaves remained higher (less negative) than in green leaves. In chlorotic leaves, normal stomatal functioning was disturbed, as evidenced by the lack of opening upon withdrawal of external CO₂ and stomatal closure after sudden illumination of previously darkened leaves. We conclude that the Fe deficiency induced limitations of xylem conductivity elicited a water saving strategy, which poses an additional challenge to plant growth on high pH, calcareous soils. Fertilisation with Fe improved photosynthetic performance but the proper xylem structure and water relations of leaves were not fully restored, indicating that Fe must be available at the first stages of leaf growth and development.     

Relations between stomatal closure, leaf turgor and xylem vulnerability in eight tropical dry forest trees

This study examined the linkage between xylem vulnerability, stomatal response to leaf water potential (Ψ_L), and loss of leaf turgor in eight species of seasonally dry tropical forest trees. In order to maximize the potential variation in these traits species that exhibit a range of leaf habits and phenologies were selected. It was found that in all species stomatal conductance was responsive to Ψ_L over a narrow range of water potentials, and that Ψ_L inducing 50% stomatal closure was correlated with both the Ψ_L inducing a 20% loss of xylem hydraulic conductivity and leaf water potential at turgor loss in all species. In contrast, there was no correlation between the water potential causing a 50% loss of conductivity in the stem xylem, and the water potential at stomatal closure (Ψ_SC) amongst species. It was concluded that although both leaf and xylem characters are correlated with the response of stomata to Ψ_L, there is considerable flexibility in this linkage. The range of responses is discussed in terms of the differing leaf‐loss strategies exhibited by these species.     

Drought-induced increase in xylem malate and mannitol concentrations and closure of Fraxinus excelsior L. stomata

Changes in the malate and mannitol composition of ash leaf (Fraxinus excelsior L.) xylem sap were studied in response to water deficit. Xylem sap was collected by the pressure method from the petiole of leaves sampled on irrigated and non-irrigated ash seedlings. As the leaf water potential decreased from -0.3 to -3.0 MPa, there was a significant increase in malate and mannitol xylem concentrations, and a concomitant decrease in maximal stomatal conductance. The functional significance of the increased malate and mannitol concentrations was investigated by using a transpiratory bioassay with mature detached leaves which exhibited, for stomatal conductance, the typical pattern showed by expanded leaves during dark/light transitions. Supplying detached leaves with mannitol in a range of concentrations found in the xylem sap had no effect on stomatal movements, but malate, for concentrations between 0.5 and 3 mM, was effective in preventing stomatal opening. The ability of malate to inhibit stomatal opening appeared to be rather non-specific. Two structural malate analogues, citrate and aspartate or an unrelated anion, shikimate, also inhibited this process. Given the drought-induced increase in xylem malate concentrations, and the fact that the range of malate levels required to close stomata was very similar to that of the concentrations found in the xylem sap, it has been hypothesized that malate is involved in the stomatal closure of ash leaves under drying conditions.Key words: Fraxinus excelsior: L., malate, mannitol, xylem sap, stomata, water deficit.      

Contrasting hydraulic strategies in <Emphasis Type="Italic">Salix psammophila</Emphasis> and <Emphasis Type="Italic">Caragana korshinskii</Emphasis> in the southern Mu Us Desert,China

Salix psammophila and Caragana korshinskii are two common shrubs in the southern Mu Us Desert, China. Their hydraulic strategies for adapting to this harsh, dry desert environment are not yet clear. This study examined the hydraulic transport efficiency, vulnerability to cavitation, and daily embolism refilling in the leaves and stems of these two shrubs during the dry season. In order to gain insight into water use strategies of whole plants, other related traits were also considered, including daily changes in stomatal conductance, leaf mass per area, leaf pressure–volume parameters, wood density and the Huber value. The leaves and stems of S. psammophila had greater hydraulic efficiency, but were more vulnerable to drought-induced hydraulic dysfunction than C. korshinskii. The difference between leaf and stem water potential at 50 % loss of conductivity was 0.12 MPa for S. psammophila and 0.81 MPa for C. korshinskii. Midday stomatal conductance decreased by 74 % compared to that at 8:30 in S. psammophila, whereas no change occurred in C. korshinskii. Daily embolism and refilling occurred in the stems of S. psammophila and leaves of C. korshinskii. These results suggest that a stricter stomatal regulation, daily embolism repair in stems, and a higher stem water capacitance could be partially compensating for the greater susceptibility to xylem embolism in S. psammophila, whereas higher leaf elastic modulus, greater embolism resistance in stems, larger difference between leaf and stem hydraulic safety, and drought-induced leaf shedding in C. korshinskii were largely responsible for its more extensive distribution in arid and desert steppes.     

热带喀斯特森林常绿和落叶榕树的水力特征和水分关系与抗旱策略

以热带喀斯特地区的直脉榕(Ficus orthoneura)和豆果榕(F.pisocarpa)为实验材料,研究了常绿和落叶树木枝条和叶片的解剖结构特征、光合水分特征和耐旱性的差异,目的在于探讨不同生活型榕树适应干旱生境的策略.直脉榕和豆果榕的叶片都有两层栅栏组织、游离状的海绵组织和钟乳体等旱生结构,同时叶片角质层蒸腾速率(gmin)和气孔导度(gs)相对较低.但与落叶的豆果榕相比,常绿的直脉榕的枝条木质部失去50％传导率的水势(P50)和gmin更低,表现出更保守的水分利用策略.总体上,两种榕树都表现出了对喀斯特干旱生境的良好适应,但是它们的适应策略表现出一定的差别.豆果榕通过落叶度过旱期,而直脉榕在结构和功能上比豆果榕更耐旱.抗旱策略和水分利用策略的不同导致两种榕树的生态位分异,减少了彼此间的水分竞争,有利于它们在喀斯特生境中共存.     

Hydraulic conductance as well as nitrogen accumulation plays a role in the higher rate of leaf photosynthesis of the most productive variety of rice in Japan

An indica variety Takanari is known as one of the most productive rice varieties in Japan and consistently produces 20-30% heavier dry matter during ripening than Japanese commercial varieties in the field. The higher rate of photosynthesis of individual leaves during ripening has been recognized in Takanari. By using pot-grown plants under conditions of minimal mutual shading, it was confirmed that the higher rate of leaf photosynthesis is responsible for the higher dry matter production after heading in Takanari as compared with a japonica variety, Koshihikari. The rate of leaf photosynthesis and shoot dry weight became larger in Takanari after the panicle formation and heading stages, respectively, than in Koshihikari. Roots grew rapidly in the panicle formation stage until heading in Takanari compared with Koshihikari. The higher rate of leaf photosynthesis in Takanari resulted not only from the higher content of leaf nitrogen, which was caused by its elevated capacity for nitrogen accumulation, but also from higher stomatal conductance. When measured under light-saturated conditions, stomatal conductance was already decreased due to the reduction in leaf water potential in Koshihikari even under conditions of a relatively small difference in leaf-air vapour pressure difference. In contrast, the higher stomatal conductance was supported by the maintenance of higher leaf water potential through the higher hydraulic conductance in Takanari with the larger area of root surface. However, no increase in root hydraulic conductivity was expected in Takanari. The larger root surface area of Takanari might be a target trait in future rice breeding for increasing dry matter production.     

Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO₂ concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO₂ concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.     

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。     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Effect of Evaporative Demand on Water Relations of Citrus limon

Conductance, transpirational flux and xylem pressure potentialwere measured in leaves of well-watered 5-year-old lemon trees(Citrus limon (L.) Burm. f.) subjected to different levels ofevaporative demand. Increased leaf-to-air absolute humiditydifference generally decreased stomatal conductance and increasedxylem pressure potential, with a good correlation between thelast two parameters; but this trend was reversed on days withvery high evaporative demand, when stomata opened in spite ofthe low humidity. Citrus limon (L.) Burm. f., lemon, water stress, stomatal conductance, leaf water potential, transpiration, air humidity     

Abscisic acid in apoplastic sap can account for the restriction in leaf conductance of white lupins during moderate soil drying and after rewatering

We studied the effects of drought on leaf conductance (g) and on the concentration of abscisic acid (ABA) in the apoplastic sap of Lupinus albus L. leaves. Withholding watering for 5d resulted in complete stomatal closure and in severe leaf water deficit. Leaf water potential fully recovered immediately after rewatering, but the aftereffect of drought on stomata persisted for 2d. ABA and sucrose were quantified in pressurized leaf xylem extrudates. We assumed that the xylem sucrose concentration is negligible and hence that the presence of sucrose in leaf extrudates indicated that they were contaminated by phloem. To eliminate this interference, the concentration of ABA in leaf apoplast was estimated by extrapolation to zero sucrose concentration, using the regression between ABA and sucrose concentrations. The estimated apoplastic ABA concentration increased by 100-fold with soil drying and did not return to pre-stress values immediately following rewatering. g was closely related to the concentration of ABA in leaf apoplast. Furthermore, the feeding of exogenous ABA to leaves detached from well-watered plants brought about the same degree of depression in g as resulted from the drought-induced increase in ABA concentration. We therefore conclude that the observed changes in the concentration of ABA in leaf apoplast were quantitatively adequate to explain drought-induced stomatal closure and the delay in stomatal reopening following rewatering.     

Changes with seasonal flooding in sap flow of the tropical flood-tolerant tree species, <Emphasis Type="Italic">Campsiandra laurifolia</Emphasis>

In order to determine how flooding affects sap flow and hydraulic conductivity of the tolerant species, Campsiandra laurifolia, trees growing in a tropical seasonally flooded forest in Venezuela were studied. We hypothesized that trees respond to rising-waters with a decrease in root-water absorption, caused by hypoxia, and stomatal conductance, and that this is reverted later on through a process of acclimation that involves improvement in water absorption. We followed the seasonal changes, of trees with the whole or part of the canopy exposed to air, in sap flow density, leaf stomatal conductance, leaf transpiration rate and xylem water potential. The highest daytime sap flow density occurred at noon and its proportion relative to the yearly maximum (drainage at falling-waters) was 41 (dry season), 15 (flooding by rising-waters for 2 weeks), 54 (2 months of flooding) and 41% (6 months of flooding). Since at rising-waters dawn xylem water potential remained high, it became apparent that the initial stages of flooding imposed a restriction to sap flow unrelated to water deficit. The decrease at rising-waters in highest daytime sap flow density was due to reduced leaf-specific hydraulic conductivity, whereas the recovery observed 1.5 months later was correlated to an increase in hydraulic conductivity, and attributed to acclimation. Sap flow density was highly and positively correlated with radiation at all seasons but rising-waters; also, the relationship with air water vapor saturation deficit was high and significant on dates other than at rising-waters. Results suggest that early flooding inhibited water absorption by roots and that this inhibition was overcome later on at a higher water column through an acclimation process involving the improvement of internal aeration by adventitious roots.     

Physiological responses of beech and sessile oak in a natural mixed stand during a dry summer

Responses of CO2 assimilation and stomatal conductance to decreasing leaf water potential, and to environmental factors, were analysed in a mixed natural stand of sessile oak (Quercus petraea ssp. medwediewii) and beech (Fagus svlvatica L.) in Greece during the exceptionally dry summer of 1998. Seasonal courses of leaf water potential were similar for both species, whereas mean net photosynthesis and stomatal conductance were always higher in sessile oak than in beech. The relationship between net photosynthesis and stomatal conductance was strong for both species. Sessile oak had high rates of photosynthesis even under very low leaf water potentials and high air temperatures, whereas the photosynthetic rate of beech decreased at low water potentials. Diurnal patterns were similar in both species but sessile oak had higher rates of CO2 assimilation than beech. Our results indicate that sessile oak is more tolerant of drought than beech, due, in part, to its maintenance of photosynthesis at low water potential.     

Gas exchange and hydraulic properties in the crowns of two tree species in a Panamanian moist forest

Hydraulic properties and gas exchange were measured in branches of two tropical tree species (Simarouba amara Aubl. and Tapirira guianensis Aubl.) in a moist lowland forest in Panama. Branch-level sapflow, leaf-level stomatal conductance, and water potential measurements, along with measurements of specific hydraulic conductivity of stems in crown tops, were used to relate hydraulic parameters to leaf conductance in two individuals of each species. Branches of the taller trees for each species (28 m, 31 m) showed much higher leaf-specific hydraulic conductance and leaf vapor-phase conductance than those of the smaller trees (18m, 23m). This was probably related to the leaf-to-sapwood area ratio in branches of taller trees, which was less than half that in branches of smaller trees. Dye staining showed evidence of massive cavitation in all trees, indicating that stomata do not control leaf water potential to prevent xylem cavitation in these species. Stomatal conductance of intact leaves also appeared to be insensitive to leaf area removal treatment of nearby foliage. Nevertheless, a simple mass-balance model of water flux combining hydraulic and vapor transport was in close agreement with observed maximal vapor-phase conductance in the four trees (r²=0.98, P=0.006). Our results suggest that the major organismal control over water flux in these species is by structural (leaf area) rather than physiological (stomatal) means.     

Correlated evolution of stem and leaf hydraulic traits in Pereskia (Cactaceae)

Recent studies have demonstrated significant correlations between stem and leaf hydraulic properties when comparing across species within ecological communities. This implies that these traits are co-evolving, but there have been few studies addressing plant water relations within an explicitly evolutionary framework. This study tests for correlated evolution among a suite of plant water-use traits and environmental parameters in seven species of Pereskia (Cactaceae), using phylogenetically independent contrasts. There were significant evolutionary correlations between leaf-specific xylem hydraulic conductivity, Huber Value, leaf stomatal pore index, leaf venation density and leaf size, but none of these traits appeared to be correlated with environmental water availability; only two water relations traits - mid-day leaf water potentials and photosynthetic water use efficiency - correlated with estimates of moisture regime. In Pereskia, it appears that many stem and leaf hydraulic properties thought to be critical to whole-plant water use have not evolved in response to habitat shifts in water availability. This may be because of the extremely conservative stomatal behavior and particular rooting strategy demonstrated by all Pereskia species investigated. These results highlight the need for a lineage-based approach to understand the relative roles of functional traits in ecological adaptation.     

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.