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

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

Relationships between sap flow and water potential in woody or perennial plants on islands of the Great Barrier Reef

Abstract. This paper describes studies on trees of Pisonia grandis , bushes of Argusia argentea , and the perennial herb Melanthera biflora , growing on One Tree Island, a coral cay of the Great Barrier Reef with 'soil' of coarse coral rubble. Water potential (Ψ_b, measured on small shoots with a pressure chamber), sap flow, stomatal conductance, vapour pressure deficit and photon flux density were monitored over day/night cycles. Sap flow and Ψ_b responded to changes in light and humidity. From these experiments good linear correlations were found between sap flow in a shoot and Ψ_b of similar adjacent shoots. The linearity suggests that the resistance to sap flow is constant as Ψ_b varies. The correlation, however, does not indicate a causal relationship between Ψ_b of an individual shoot on the plant and its sap flow. Ψ_b was only slightly different in shaded shoots from those in sunshine, although sap flow would be expected to differ between them. Enclosing shoots and so reducing their transpiration and sap flow to very low rates resulted in only small changes in Ψ_b of the enclosed shoots; Tb of such enclosed shoots should closely approximate that of the xylem at the point of shoot attachment. From these results it is suggested that the resistance to water flow in shoot and leaf xylem is small compared to the resistance further down the plant, in the root or at the root/soil interface. Shoot xylem water potential would be similar for all parts of the plant, and in such plants the water potential of shoots in the shade would be determined by the overall water use of the plant.     

Changes in leaf hydraulics and stomatal conductance following drought stress and irrigation in Ceratonia siliqua (Carob tree)

Changes in leaf hydraulic conductance (K) were measured using the vacuum chamber technique during dehydration and rehydration of potted plants of Ceratonia siliqua . K of whole, compound leaves as well as that of rachides and leaflets decreased by 20–30% at leaf water potentials (Ψ_L) of −1.5 and −2.0 MPa, i.e. at Ψ_L values commonly recorded in field-growing plants of the species. Higher K losses (up to 50%) were measured for leaves at Ψ_L of −2.5 and −3.0 MPa, i.e. near or beyond the leaf turgor loss point. Leaves of plants rehydrated while in the dark for 30 min, 90 min and 12 h recovered from K loss with characteristic times and to extents inversely proportional to the initial water stress applied. Leaf conductance to water vapour of plants dehydrated to decreasing Ψ_L and rehydrated at low transpiration was inversely related to loss of K, thus suggesting that leaf vein embolism and refilling (and related changes in leaf hydraulics) may play a significant role in the stomatal response.     

Investigation of the limitations to photosynthesis induced by leaf water deficit in field-grown sunflower (Helianthus annuus L.)

Abstract. The diurnal cycling of leaf water potential (Ψ_leaf) in field-grown sunflower ( Helianthus annuus ) was used to investigate the cause of water deficitinduced limitation of net photosynthesis. Daily midafternoon decreases in Ψ_leaf of up to 1.5 MPa and in net photosynthesis of up to 50% were typical for irrigated sunflower during seed filling. These midafternoon values were lowered an additional 0.6 to 0.8 MPa by prolonged drought treatment. There was a nearly linear relationship between the decline in net photosynthesis and reductions in leaf conductance over the course of the day. Thus, it was unexpected to find that the low, midafternoon rates of photosynthesis were associated with the highest intercellular CO₂ concentrations. These and other observations suggest that the daily decline in photosynthesis represents a 'down regulation' of the biochemical demand for CO₂ that is coordinated with the diurnally developing need to conserve water, thus establishing a balanced limitation of photosynthesis involving both stomatal and non-stomatal factors. There were no indications that either short term (i.e. diurnal declines in Ψ_leaf) or long term (i.e. drought treatment) water deficits caused any damage or malfunctioning of photosynthesis. Rather, both the daily declines in photosynthesis and the nearly 25% decrease in leaf area induced by prolonged drought appeared to be well-controlled adaptive responses by field-grown sunflower plants to limited water availability.     

Limitation of plant water use by rhizosphere and xylem conductance: results from a model

Hydraulic conductivity ( K ) in the soil and xylem declines as water potential ( Ψ ) declines. This results in a maximum rate of steady-state transpiration ( E _crit) and corresponding minimum leaf Ψ ( Ψ _crit) at which K has approached zero somewhere in the soil–leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area ( A _R: A _L), the loss of rhizosphere K limited E _crit and Ψ _crit. Above the threshold, loss of xylem K from cavitation was limiting. The A _R: A _L threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0·20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their A _R: A _L threshold. Corollary predictions are (1) within a soil type the A _R: A _L should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the A _R: A _L should increase and maximum cavitation resistance should decrease.     

Water potential and plant metabolism: comments on Dr P. J. Kramer's article 'Changing concepts regarding plant water relations', Volume 11, Number 7, pp. 565–568, and Dr J. B. Passioura's Response, pp. 569–571

Abstract. Kramer (1988) and Passioura (1988) highlight an important principle in plant physiology, namely that roots affect the activity of shoots. It is safe to say that anyone who has observed plants could cite many examples. What is in contention, however, is the interpretation of the water potential (Ψ_w) after some investigators (Bates & Hall, 1981; Blackman & Da vies, 1985; Eavis & Taylor, 1979; Gollan, Passioura & Munns, 1986; Termaat, Passioura & Munns, 1985; Turner, Schulze & Gollan, 1985) observed that roots in water deficient media can affect shoot physiology in ways not involving the Ψ_w of the shoot. As a result, some investigators (Sinclair & Ludlow, 1985; Passioura, 1988) question the Ψ_w concept on the grounds indicated by Passioura (1988): if roots affect shoot activity by means other than by changing shoot Ψ_w, the Ψ_w concept must be wrong. This is unfortunate. It has been known for years that metabolic reactions generally do not respond directly to the Ψ_w or its components. Nevertheless, the shoot Ψ_w, and its components play important roles in the physiology and metabolism of plants. Often these roles are interactive with roots. In what follows, I will review some of the evidence for these principles.     

Effect of air and soil temperature on water balance of jojoba growing under controlled conditions

Jojoba [ Simmondsia chinensis (Link) Schneider] cuttings were grown in pots under constant light intensity and vapour pressure deficit at wir temperatures of 18 and 27°C in climate-controlled cabinets. Leaf conductance and transpiration rate decreased exponentially as the xylen water potential (Ψ_x) decreased concurrently with the drying out of the soil. At high Ψ_x'leaf conductance and transpiration rate were much higher at the higher air temperature, and as Ψ_x declined both parameters decreased more rapidly at 27°C than at 18°C. When soil temperatures were decreased from 27 to 13°C, leaf water potential was not affected at either air temperatures, but transpiration rate was reduced. A linear negative correlation was found between transpiration rates and soil temperatures. It is suggested that the low soil temperature may restrict reducion of water flux in turn reduces stomatal conductance and transpiration without affecting the water potential in the shoot. The releavance of the response to changes in soil or air temperature to the performance of the plant in its semi-arid habitat is discussed.     

Accumulation rate of ABA in detached maize roots correlates with root water potential regardless of age and branching order

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA ( S _ABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. S _ABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections). All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. S _ABA measured in turgid roots changed with root age and among root categories. This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of S _ABA to changes in root water potential ( Ψ _root) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of S _ABA to RWC were due to different RWC values among categories at a given Ψ _root, and not to differences in the response of S _ABA to Ψ _root.     

Proteolysis and nitrogen fixation in faba-bean (Vicia faba) nodules under water stress

Nodulated faba-beans ( Vicia faba L. var. minor) exhibiting high rates of N₂ fixation (133 μmol C₂H₄ g⁻¹ dry weight h⁻¹), were subjected to water restriction. A loss of C₂H₂ reduction due to water stress was always associated with a decline of the leghemoglobin content for each of the 4 decreasing values of Ψ_mod. Electron micrographs showed ultrastructural alterations of the fixing tissue, which affected both partners and increased with the severity of water stress. In the nodule cytosol, the alkaline proteolysis approximately doubled when Ψ_mod decreased from −0.55 MPa to −1.55 MPa. Concomitantly, an increase of the nodule intracellular pH from 6.3 to 7.0 was observed. Proteolysis was due to serine proteases, exhibiting a pH-optimum of 8 and which actively degraded purified leghemoglobin in vitro (K_m=100 μ M ). The degradation of leghemoglobin during water stress may contribute to the loss of C₂H₂ reduction and may affect the pattern of recovery upon rewatering.     

Transport constraints on water use by the Great Basin shrub, Artemisia tridentata

As soil and plant water status decline, decreases in hydraulic conductance can limit a plant's ability to maintain gas exchange. We investigated hydraulic limitations for Artemisia tridentata during summer drought. Water use was quantified by measurements of soil and plant water potential ( Ψ ), transpiration and leaf area. Hydraulic transport capacity was quantified by vulnerability to water stress-induced cavitation for root and stem xylem, and moisture release characteristics for soil. These data were used to predict the maximum possible steady-state transpiration rate ( E _crit) and minimum leaf xylem pressure ( Ψ _crit). Transpiration and leaf area declined by ~ 80 and 50%, respectively, as soil Ψ decreased to –2·6 MPa during drought. Leaf-specific hydraulic conductance also decreased by 70%, with most of the decline predicted in the rhizosphere and root system. Root conductance was projected to be the most limiting, decreasing to zero to cause hydraulic failure if E _crit was exceeded. The basis for this prediction was that roots were more vulnerable to xylem cavitation than stems (99% cavitation at –4·0 versus –7·8 MPa, respectively). The decline in water use during drought was necessary to maintain E and Ψ within the limits defined by E _crit and Ψ _crit.     

Water stress induced changes in concentrations of proline and total soluble sugars in nodulated alfalfa (Medicago sativd) plants

Susceptibility of alfalfa ( Medicago saliva L. cv. Aragón) nodules and leaves to water stress has been investigated. Nodule acetylene reduction activity (ARA), leaf CO₂ exchange rate (CER) as well as soluble protein, proline and total soluble sugar (TSS) contents were determined during drought. Water status was estimated as water potential (Ψ_w) and Relative water content (RWC) of the respective tissues. Maximum rates of ARA required higher Ψ_w than CER. Nodules had lower RWC for a given Ψ_w than leaves. Water stress reduced soluble protein content in both tissues; however, the decline in soluble protein content was detected at greater Ψ_w in nodules than in leaves. Proline and TSS increased in leaves and nodules, and again the threshold Ψ_w triggering such accumulation was higher in nodule tissues. Oior results suggest that alfalfa nodules are more susceptible to water shortage than leaves. Effects of accumulated TSS and proline upon leaf and nodule physiology are discussed in relation to protein stability (proline), pH control (proline) and osmotic adjustment (proiine and TSS). The TSS accumulation induced by water stress suggests that substrate shortage would not be the primary effect of drought on nodule activity.     

Alfalfa leaf senescence induced by drought stress: photosynthesis, hydrogen peroxide metabolism, lipid peroxidation and ethylene evolution

Exchange rates of CO₂ and H₂O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa ev. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψ_leaf) down to,-2.8 MPa reduced CO₂ availability due to stomatal closure and inhibited the rate of photosynthesis. Leaf water potential lower than,-2.8 MPa directly affected CO₂ fixation, although CO₂ was not limiting. Transpiration was more affected by stornatal closure than photosynthesis, which led to am apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψ_leaf lower than,-2.0 MPa had an increased quantum requirement, probably due to the severe stress effect on photoenergetic reactions.
Ethylene evolution from alfalfa leaves increased when they were subjected to Ψ_leaf of,- 1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethyiene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (maloridialdehyde content) and an increased peroxide content. Superoxide disinutase activity (SOD; EC 1.15.1.1) was not affected by drought stress. Catalase (EC 1.11.1.6) was inhibited at slight stress, but significantly increased at a Ψ_leaf of -2.0 MPa. Peroxidase (EC 1.11.1.7) was progressively inhibited as drought stress developed. The possible implication of reactive O₂ intermediates in drought stress-induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems.     

Competitive strategies for water availability in two Mediterranean Quercus species

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour ( g _L), water potential ( Ψ _L) and relative water content ( RWC ) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area ( K _RL). Quercus cerris behaved as a drought-tolerant species, with strong reductions of K _RL, Ψ _L, and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing g _L, maintaining Ψ _L and RWC high and K _RL at the same level as that measured in the spring. A 'conductance ratio' ( CR ) was calculated in terms of the ratio of g _L to K _RL. Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought-tolerant with drought-avoiding species can be generalized is also discussed.     

Phosphoenolpynivate carboxylase, malate and alcohol dehydrogenase activities in alfalfa (Medicago sativa) nodules under water stress

The effect of drought upon phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31), malate ddiydrogenase (MDH; EC 1.1.1.37), alcohol dehydrogenase (ADH; EC 1.1.1.1) and β -hydroxybulyrate dehydrogenase ( β -OH-BDH; EC 1.1.1.30) enzyme activities as well as the leghemoglobin (Lb), malate and ethanol contents of alfalfa nodules ( Medicago sativa L. cv. Aragon) were examined. Both the ieghemoglobin (Lb) content and the Lb/soluble protein ratio were significantly reduced at a nodule water potential (Ψ_nod) of—1.3 MPa. At lower Ψ_nod, Lb content decreased further, but the ratio remained unchanged. Slight stress (—1.3 MPa) drastically affected acetylene reduction activity (ARA; 60% reduction) whereas in vitro PEPC activity was main-tained at relatively constant values. As stress progressed (—2.0 MPa), a simultaneous reduction in both activities was observed. Severe stress (Ψ_nod lower than —2.0 MPa) stimulated in vitro PEPC. Bacteroid β -J-OH-BDH activity was stimulated by slight (—1.3 MPa) and moderate (—2.0 MPa) drought. MDH activity rose in slightly stressed nodules (Ψ_nod—1.3 MPa). Greater water deficits sharply decreased MDH activity to values significantly lower than those found in control nodules. Nodule malate content followed the same pattern as MDH. The plant fraction of the nodule showed constitutive ADH activity and contained ethanol. ADH was stimulated at slight (— 1.3 MPa) and moderate drought levels (—2.0 MPa). Ethanol content showed similar behavior to ADH activity. Inhibition of ARA, reduction of Lb content and stimulation of the fermentative metabolism induced by water stress suggest some reduction ira O₂ availability within the nodule.     

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets ( Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO₂ partial pressure (C_i), relative water content (RWC), water content/dry weight (H₂O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and C_i were determined on non-detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non-rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas C_i decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H₂O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H₂O/DW decrease and the final turgor flexion documented.     

Relations between carbohydrate, water status and adventitious root formation in leafy pea cuttings rooted under various levels of atmospheric CO2 and relative humidity

Three levels of atmospheric CO₂ and 2 levels of relative humidity (RH) during the rooting period were tested for their effect on several factors presumed to influence adventitious root formation in leafy pea ( Pisum sativum L. cv. Alaska) cuttings. Compared to normal CO₂ levels (350 μl l⁻¹), neither 1800 nor 675 μl l⁻¹ CO₂ affected the rooting percentage or the number of roots per cutting. However, 1800 μl l⁻¹ CO₂ increased root and shoot dry weight, root length, carbohydrate levels in the base of the cuttings and water potential (Ψ_w) of cuttings compared to normal levels of CO₂. Compared to 87% RH. 55% RH decreased all of the above parameters, including the number of roots per cutting. A polyvinyl chloride antitranspirant (which partially blocks stomata and slows photosynthesis) applied simultaneously with 87% RH increased Ψ_w and root length but lowered all of the other above parameters, compared to 87% RH without antitranspirant. Increasing current photosynthate (products of photosynthetic activity after excision), carbohydrate, or Ψ_w either alone or together was associated with increased root system size but not necessarily with increased rooting percentage or root number. The data are consistent with a hypothesis that the number of roots per cutting increased with increasing current photosynthate and carbohydrate until some other factor became limiting. Also, the effect of Ψ_w on rooting percentage and root number was mediated through its effect on current photosynthate and carbohydrate.     

Drought-induced variations of water relations parameters in Olea europaea

The effects of water stress on water potential components, tissue water content, mean elastic modulus and the osmoregulation capacity of olive (Olea europaea L. cv. Coratina) leaves was determined. Artificial rehydration of olive leaf tissues altered the P-V relationships so that a plateau phenomenon occurred. Points in the P-V curve in the region affected by the plateau, generally up to –0.5 MPa, were corrected for all the samples analyzed. In the corrected P-V relationship, an osmotic adjustment was found in drought-stressed leaf tissues. Osmotic potentials at full turgor (_{0 (sat)}) and osmotic potential at turgor-loss (_{0 (TVT)}) decreased from –2.06±0.01 MPa and –3.07±0.16 MPa in controls to –2.81±0.03 MPa and –3.85±0.12 MPa in most stressed plants. Osmotic adjustment values obtained from the P-V curves agreed with those obtained using an osmometer. An active osmotic adjustment of 1.42 MPa was also observed in 1–4 mm- diameter roots. Mannitol is the main carbohydrate involved in osmotic potential decrease in all treatments. The maximum elastic modulus increased from 11.6±0.95 MPa in the controls to 18.6±0.61 MPa in the most stressed plants.     

Field studies of water relations and photosynthesis in Scots pine.

Needle water potential at noon and diurnal variation in needle water potential was measured with a pressure chamber during the growth seasons (1974, 1975 and 1976) in a 20-year-old stand of Scots pine (Pinus sylvestris L.), subjected to daily irrigation and nutrient treatments at Jädraås, in central Sweden (60°50'N).
In unstressed conditions there was little difference in water potential amongst the treatments. However, during a dry spell in 1976 the diurnal curves of water potential in the control and combined irrigated and fertilized treatment diverged by up to 6 bar in the middle of the day. Irrigation alone resulted in higher midday water potentials (Ψ_n) than in the controls only during this period. At other times in 1974, 1975 and early 1976, the values of Ψ_n in the irrigated treatment were similar to the controls. Fertilization alone resulted in higher Ψ_n than in the controls in both 1975 and 1976 during both wet and dry periods. However, the largest difference between treatment and control occurred in the combined, irrigated and fertilized treatment during the dry spell in 1976.
Several possible explanations for the effect of fertilization on leaf water potential are discussed, including changes in hydraulic conductivity and water storage. The most likely explanation is a reduced transpiration rate as a result of more effective stomatal control of water loss.     

Inositol 1,4,5-trisphosphate formation in leaves of winter oilseed rape plants in response to freezing, tissue water potential aed abscisic acid

Time courses of formation of inositol 1,4,5-trisphosphate (IP₃) were followed in the leaves of non-acclimated and cold (2°C)-acclimated winter oilseed rape ( Brassica napus L. var. oleifera ) plants, subjected to different freezing temperatures or to polyethylene glycol 8000 (PEG) and abscisic acid (ABA) treatments. Changes in water potential (Ψ_w) and in ABA level in the frost- and PEG-treated tissues were also determined. Results obtained indicate that temperatures sligthly higher than LT₅₀ induced a transient and substantial increase in IP₃ level, both in non-acclimated and cold-acclimated tissues. At comparable freezing temperature (–5°C) the response of cold-acclimated leaves was lower than that of non-acclimated ones. The PEG-depedent decrease in Ψ_w to –0.9 MPa or ABA (0.1 m M ) treatment gave rise to a transient increase in IP₃ content in non-acclimated tissues only. Collectively, the data indicate that cold acclimation of plants may lead to lower cell responsiveness to the factors studied in terms of induction of IP₃ formation. Changes in the IP₃ content, observed in the present experiments, support our previous suggestion that non-killing freezing temperatures may induce the phosphoinositide pathway, both in non-acclimated and cold-acclimated tissues. Lowering of tissue water potential to some threshold value or a high exogenous ABA supply may mimic the freezing-dependent reaction in the non-acclimated leaves.     

Leaf hydraulics and drought stress: response, recovery and survivorship in four woody temperate plant species

Efficient conduction of water inside leaves is essential for leaf function, yet the hydraulic-mediated impact of drought on gas exchange remains poorly understood. Here we examine the decline and subsequent recovery of leaf water potential ( Ψ _leaf), leaf hydraulic conductance ( K _leaf), and midday transpiration ( E ) in four temperate woody species exposed to controlled drought conditions ranging from mild to lethal. During drought the vulnerability of K _leaf to declining Ψ _leaf varied greatly among the species sampled. Following drought, plants were rewatered and the rate of E and K _leaf recovery was found to be strongly dependent on the severity of the drought imposed. Gas exchange recovery was strongly correlated with the relatively slow recovery of K _leaf for three of the four species, indicating conformity to a hydraulic-stomatal limitation model of plant recovery. However, there was also a shift in the sensitivity of stomata to Ψ _leaf suggesting that the plant hormone abscisic acid may be involved in limiting the rate of stomatal reopening. The level of drought tolerance varied among the four species and was correlated with leaf hydraulic vulnerability. These results suggest that species-specific variation in hydraulic properties plays a fundamental role in steering the dynamic response of plants during recovery.