Comparison of continuously recorded plant-based water stress indicators for young lemon trees

Continuously recorded plant-based water stress indicators (sap flow and parameters derived from trunk diameter fluctuations) were compared in potted young lemon trees (Citrus limon (L.) Burm. fil, cv. Verna) grafted on sour orange (C. aurantium L.) rootstock submitted to deficit irrigation. Daily maximum (MXTD) and minimum (MNTD) trunk diameters and daily trunk diameter shrinkage (MDS) were directly influenced by the water supply to the trees from the soil. The continuously recorded plant-based water stress indicators presented different degrees of sensitivity when used to estimate the water status of the plants. Sap flow (SF) and MDS were more immediate and sensitive than MXTD and MNTD. However, the higher signal intensity: noise for SF and the fact that its signal intensities remained clearly above unity during the stress period, indicating that the soil water was depleted, point to the greater reliability of this indicator. Also, the possibility of developing further baseline relationships between SF and air vapour pressure deficit in fully irrigated trees in field conditions increases the feasibility of using this parameter in automatic irrigation systems.     

Stem and leaf water potentials, gas exchange, sap flow, and trunk diameter fluctuations for detecting water stress in lemon trees

The sensitivity of continuous (on a whole-day basis) and discretely (at midday) measured indicators of the plant water status in adult lemon trees in response to a cycle of water deprivation and recovery, and the feasibility of obtaining baselines for tree water status indicators was investigated in 30-year-old Fino lemon trees (Citrus limon (L.) Burm. fil.) grafted on sour orange (C. aurantium L.) rootstocks. Control plants (T0) were irrigated daily above their crop water requirements in order to obtain non-limiting soil water conditions, while T1 plants were subjected to water stress by withholding irrigation for 50 days, after which time irrigation was restored and plant recovery was studied for 16 days. In T0 plants the water relations and the plant symptoms confirmed that they had not suffered waterlogging. In contrast, T1 plants showed a substantial degree of water stress, which developed very slowly. Maximum daily trunk shrinkage (MDS) increased in response to water stress during the first 15 days of the experiment, but when the stem water potential (Ψ_stem) fell below −1.8 MPa, the MDS signal intensity decreased. However, Ψ_stem and sap flow (SF) signal intensities progressively increased during the water stress period. The results showed that MDS is a very suitable plant-based indicator for precise irrigation scheduling in adult lemon trees. Reference or baseline relationships for MDS, Ψ_stem, and SF measurements as a function of several parameters related to the evaporative demand of the atmosphere were obtained. This fact open up the possibility of considering a plant-based indicator measurement at a given time relative to the expected value under non-limiting water conditions, which can be calculated from the reference relationships.     

Relationships Between Climatic Variables and Sap Flow, Stem Water Potential and Maximum Daily Trunk Shrinkage in Lemon Trees

The feasibility of obtaining sap flow (SF), maximum daily trunk shrinkage (MDS) and midday stem water potential (Ψ_stem) baselines or reference values for use in irrigation scheduling was studied in adult Fino lemon trees (Citrus limon (L.) Burm. fil.) grafted on sour orange (C. aurantium L.) rootstocks. Plants were irrigated daily above their water requirements in order to obtain non-limiting soil water conditions. The results indicated that baselines for plant-based water status indicators (MDS, SF and Ψ_stem) can be obtained, even though there was a certain scattering of the data points representing the relations between the plant-based measurements and the environmental variables (reference evapotranspiration, solar radiation, vapour pressure deficit and temperature). SF was more closely associated with changes in the studied evaporative demand variables than were MDS and Ψ_stem. SF and Ψ_stem were more closely correlated with changes in reference evapotranspiration (ETo) (r ² = 0.93 and 0.79, respectively), while MDS behaviour was best correlated with mean daily air temperature (T _m) (r ² = 0.76). Increases in the evaporative demand induced more negative Ψ_stem values and, as a consequence, SF increased, which, in turn, was translated into an increase in MDS. This confirmed that SF and MDS were very good predictors of the plant water status during the observation period and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in lemon trees.     

Physiological Indicators of Plant Water Status of Irrigated and Non-irrigated Grapevines Grown in a Low Rainfall Area of Portugal

Water is a key resource in commercial wine production and both large excesses and deficits have undesirable effects upon the amount and quality of the wine produced. A balance between the water requirements of a fully developed canopy and the induced stress necessary for the commercial quality of the wine must be reached. Thus we need a physiological indicator that integrates both soil and climatic conditions to use as a management tool. An experimental field was established in the eastern part of the Demarcated Region of Douro – Portugal, to study the effect of water supply on the quality of the musts produced and we need a physiological indicator that relates to the water use and stress of the grapevines (Vitis vinifera L.) and to the later evaluation of the effect of irrigation practices upon the quality of the musts. We chose as indicators sap flow, leaf water potential at pre-dawn (0600 h), mid-morning (1000 h), solar noon (1400 h) and sunset (1900 h), stomatal conductance and leaf transpiration both measured at mid-morning and at solar noon, and related them to our experimental treatments that induce differences in soil water content, evaluated with time-domain reflectometry probes, with the objective of selecting the indicator that best describes the plant water status under different amounts of available water. Sap flow, leaf water potential and leaf transpiration rate measured at solar noon had highly significant correlations with soil water content and their regression on soil water content was also highly significant. Each of these parameters has shortcomings and none has a clear advantage over the other two as an integrator of the environmental conditions under these experimental conditions. Further studies of the parameters and their relationship with the quality characteristics of the produced musts are needed to achieve the ultimate objective of manipulating the soil water content.     

Daily variations in water relations of apricot trees under different irrigation regimes

Mature apricot (Prunus armeniaca L. cv. Búlida) trees, growing under field conditions, were submitted to two drip irrigation treatments: a control (T1), irrigated to 100 % of seasonal crop evapotranspiration (ETc), and a continuous deficit (T2), irrigated to 50 % of the control throughout the year. The behaviour of leaf water potential and its components, leaf conductance and net photosynthesis were studied at three different times during the growing season, when they revealed a diurnal and seasonal pattern in response to water stress, evaporative demand of the atmosphere and leaf age. The deficit-irrigated trees showed, among other effects, a pronounced decrease in leaf water potential (ψ_w), decreased in leaf conductance (g_s) and no osmotic adjustment. For this reason, g_l and ψ_w can be considered good indicators of mature apricot tree water status and can therefore be used for irrigation scheduling.     

Detection of Quantitative Trait Loci for Yield and Drought Tolerance Traits in Soybean Using a Recombinant Inbred Line Population

To investigate the genetic basis of drought tolerance in soybean ( Glycine max L. Merr.) a recombinant inbred population with 184 F_2:7:11 lines developed from a cross between Kefeng1 (drought tolerant) and Nannong1138-2 (drought sensitive) were tested under water-stressed and well-watered conditions in field and greenhouse trials. Traits measured included leaf wilting coefficient, excised leaf water loss and relative water content as indicators of plant water status and seed yield. A total of 40 quantitative trait loci (QTLs) were identified: 17 for leaf water status traits under drought stress and 23 for seed yield under well-watered and drought-stressed conditions in both field and greenhouse trials. Two seed yield QTLs were detected under both well-watered and drought-stressed conditions in the field on molecular linkage group H and D1b, while two seed yield QTLs on molecular linkage group C2 were found under greenhouse conditions. Several QTLs for traits associated with plant water status were identified in both field and greenhouse trials, including two leaf wilting coefficient QTLs on molecular linkage group A2 and one excised leaf water loss QTL on molecular linkage group H. Phenotypic correlations of traits suggested several QTLs had pleiotropic or location-linked associations. These results will help to elucidate the genetic basis of drought tolerance in soybean, and could be incorporated into a marker-assisted selection breeding program to develop high-yielding soybean cultivars with improved tolerance to drought stress.     

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

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

Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.     

Comparison of proline accumulation in two mediterranean shrubs subjected to natural and experimental water deficit

The effect of water stress on proline accumulation was tested in two contrasted species of Mediterranean scrub: Halimium halimifolium (L.) Willk and Pistacia lentiscus L. Leaf water potential, stomatal resistance and proline content have been measured both in experimental and in natural water stress conditions. Both species accumulated proline in their leaves when leaf water potential dropped below a threshold value of –3.0 MPa, under natural as well as under experimental conditions. In the field, however, a time-lag between decrease of leaf water potential and proline accumulation could be observed. In Halimium halimifolium, proline accumulation appeared to be associated with severe stress conditions as most plants with high proline contents suffered irreversible wilting, especially in the greenhouse. P. lentiscus showed a different pattern, accumulating proline at two different times of the year, as a response to cold or to drought. The results of our study indicated that the role of proline in this species, rather than an osmotic agent, seems to be more related to a protective action in cases of severe stress conditions.     

玉米叶片水分利用效率的保守性

水分利用效率是植物个体或生态系统水分利用过程的重要特征参数,可表征不同时空尺度的植物碳-水耦合关系,对植物适应气候变化研究具有重要意义。以玉米为例,利用中国气象局固城农业气象野外科学试验基地2013—2014年玉米不同灌溉方案模拟试验资料,对不同叶位叶片的水分利用效率特征及其影响因素进行分析。结果表明:植株顶部第1片叶片水分利用效率在拔节期和乳熟期呈现明显的峰值,反映出明显的周期变化规律及其与叶片生理生态特征的紧密相关。在相同环境条件下,不同叶位叶片的水分利用效率不存在显著性差异,即玉米叶片水分利用效率具有空间稳定性与叶龄保守性。同时,研究指出叶片光合速率和蒸腾速率在叶位之间的协调变化是导致空间稳定性和叶龄保守性的主要原因。研究结果可为植物水分关系研究提供参考,也可为水分利用效率的尺度化研究提供依据。     

Physiological and Anatomical Basis of Differential Tolerance to Soil Flooding of Lotus corniculatus L. and Lotus glaber Mill

Lotus corniculatus L. and Lotus glaber Mill. are warm-season legume species adapted to many kinds of environmental stress, including flooding conditions, whereas other popular forage legumes, like alfalfa or white clover, cannot thrive. This study evaluates the relationship between root aerenchyma, water relations and leaf gas exchange and the differential tolerance to soil flooding of L. corniculatus and L. glaber. Adult plants of these species, established independently in grasslands mesocosms, were subjected to 40 days of early spring flooding at a water depth of 6 cm. Both species presented constitutive aerenchyma tissue in the roots. Under flooding conditions, this parameter was 26.2% in L. glaber and 15.3% in L. corniculatus. In addition, flooded plants of L. glaber presented a leaf biomass 47.5% higher above water while L. corniculatus showed a leaf biomass 59.6% lower in the same layer, in comparison to control plants. Flooded plants of L. glaber maintained the stomatal conductance (g _s) and transpiration rate (E) for 25 days, although these parameters reduce slightly to 40–60% in comparison to controls after 40 days of flooding. In this species, a reduction in photosynthesis (A) in flooding conditions was detected only on the last day of measurement. In L. corniculatus, the same parameters (g _s, E and A) were affected by flooding since day 18 of treatment, and values reached 25–40% in comparison to control plants after 40 days of flooding. Flooding did not affect above-ground biomass in L. glaber; while in L. corniculatus, above-ground biomass was 35% lower than in control plants. Our results confirmed that L. glaber is more able to cope with flooding stress than L. corniculatus, even in the presence of natural competitors. On the whole, this experiment provides information that can aid in the identification of anatomical and physiological parameters associated with flood-tolerance in this forage legume species, with economic potential for the agricultural areas subject to periodic flooding.     

Growth and Gas Exchange of Three Sorghum Cultivars Under Drought Stress

A field study was conducted to evaluate the drought tolerance of three sorghum [Sorghum bicolor (L.) Moench] cultivars, Gadambalia, Arous elRimal and Tabat, and quantify the physiological bases for differences in their drought tolerance. Water stress reduced shoot dry mass of Gadambalia, Arous elRimal and Tabat by 43, 46 and 58 %, respectively. The respective reduction in leaf area of the three cultivars was 28, 54 and 63 %. The reduction in net photosynthetic rate, stomatal conductance and transpiration rate due to water stress was lowest in Gadambalia and highest in Tabat. The leaf water potentials and relative water contents of Gadambalia under wet and dry treatments were similar, while those of Tabat were significantly reduced by water stress. The lowest and highest liquid water flow conductance was displayed by Tabat and Gadambalia, respectively. Drought tolerance in Gadambalia is associated with its smaller leaf area, higher liquid water flow conductance, and ability to maintain high leaf water potential, relative water content, stomatal conductance, transpiration rate and photosynthetic rate under drought stress. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of Cynodon dactylon to prolonged water deficits under saline conditions

Competition for available water resources in both arid and semi-arid locations has led to greater scrutiny of turfgrass irrigation. Irrigation management strategies, including deficit irrigation, need further investigation. The objective of this research was to determine the physiological response of bermudagrass (Cynodon dactylon (L.) Pers.) to prolonged water deficits under saline conditions. Bermudagrass was grown in large columns packed with three different soil types (sandy loam, silt loam and clay). Synthesized saline irrigation water was applied at three different salinity levels (1.5, 3.0 and 6.0 dS/m). Two previous experiments that were conducted with these columns over a 3.5 year period led to differential profile salinization in all 27 columns. At the end of this 3.5 year period, all irrigations were terminated and plant growth and water status were monitored over a 95 day dry-down period. Midday stomatal conductance, leaf water potential, canopy temperature, soil water in storage and stolon elongation were measured over the experimental period. On day 95, above ground tissue was harvested for dry weight and elemental tissue analysis. Midday stomatal conductance decreased around day 30 in all columns regardless of soil salinity. This decrease was not associated with a threshold leaf water potential, as midday leaf water potentials remained constant over a 60 day period. Stolon elongation also ceased before any deviation in the midday leaf water potential occurred. A concomitant reduction in evapo ranspiration was not associated with the measured decrease in stomatal conductance. This would suggest that bermudagrass may have regulated stomatal activity to compensate for lower conductances during periods of greater stress and/or that measured midday stomatal conductances cannot always be directly coupled to extended time evapotranspiration measurements.     

Growth and nitrogen fixation ofAeschynomene under water stressed conditions

Summary Studies on the tolerance ofAeschynomene americana L. to periods of flooding or soil moisture deficit were conducted in an attempt to elucidate nitrogen fixation as affected by soil moisture. Nitrogenase activity was not reduced significantly in pot-grown Aeschynomene plants subjected to flooding in greenhouse conditions. After 20 days of withholding water from the soil, nitrogenase activities of the drought-stressed plants were much lower than those of either the well-watered or flooded plants. Leaf water potentials were similar in flooded and control plants; however, the droughted plants had leaf water potentials that were 4 bars lower than those of the control plants. Aeschynomene plants were tolerant to long-term periods of flooding, but exhibited a reduction in nitrogenase activity and leaf water status when subjected to soil moisture deficits.     

Water relations determine short time leaf growth patterns in the mangrove Avicennia marina (Forssk.) Vierh.

High‐resolution leaf growth is rarely studied despite its importance as a metric for plant performance and resource use efficiency. This is in part due to methodological challenges. Here, we present a method for in situ leaf growth measurements in a natural environment. We measured instantaneous leaf growth on a mature Avicennia marina subsp. australasica tree over several weeks. We measured leaf expansion by taking time‐lapse images and analysing them using marker tracking software. A custom‐made instrument was designed to enable long‐term field studies. We detected a distinct diel growth pattern with leaf area shrinkage in the morning and leaf expansion in the afternoon and at night. On average, the observed daily shrinkage was 37% of the net growth. Most of the net growth occurred at night. Diel leaf area shrinkage and recovery continued after growth cessation. The amount of daily growth was negatively correlated with shrinkage, and instantaneous leaf growth and shrinkage were correlated with changes in leaf turgor. We conclude that, at least in this tree species, instantaneous leaf growth patterns are very strongly linked to, and most likely driven by, leaf water relations, suggesting decoupling of short‐term growth patterns from carbon assimilation.     

Transgenic tobacco plants expressingBoRS1 gene fromBrassica oleracea var.acephala show enhanced tolerance to water stress

Water stress is by far the leading environmental stress limiting crop yields worldwide. Genetic engineering techniques hold great promise for developing crop cultivars with high tolerance to water stress. In this study, theBrassica oleracea var.acephala BoRS1 gene was transferred into tobacco throughAgrobacterium- mediated leaf disc transformation. The transgenic status and transgene expression of the transgenic plants was confirmed by polymerase chain reaction (PCR) analysis, Southern hybridization and semi-quantitative one step RT-PCR analysis respectively. Subsequently, the growth status under water stress, and physiological responses to water stress of transgenic tobacco were studied. The results showed that the transgenic plants exhibited better growth status under water stress condition compared to the untransformed control plants. In physiological assessment of water tolerance, transgenic plants showed more dry matter accumulation and maintained significantly higher levels of leaf chlorophyll content along with increasing levels of water stress than the untransformed control plants. This study shows thatBoRS1 is a candidate gene in the engineering of crops for enhanced water stress tolerance.     

三峡消落带落羽杉人工幼林叶片分解及磷释放特征

为探究三峡消落带人工重建植被落羽杉（Taxodium distichum）幼林叶片在不同水文条件下的分解特征及其对土壤-水体磷的贡献潜力,本实验在控制条件下,模拟三峡库区消落带土壤水分变化设置了常规生长水分条件（CK）、轻度干旱水分胁迫（T1）、潮湿（T2）、2 cm水淹（T3）、10 cm水淹（T4）5个不同处理,研究落羽杉叶片分解及磷释放特征。研究结果表明：（1）在整个试验期间（90 d）,T3、T4两个水淹处理的叶片失重率分别达51%（T3）和55%（T4）,显著高于CK、T1、T2三个未水淹处理;（2）未分解残留叶片中的全磷含量在CK、T1、T2三个未水淹的处理组呈现增加趋势,而水淹处理（T3、T4）呈减少趋势;（3）土壤全磷含量在试验过程中呈现波动性变化特征,但至试验结束时,各处理中土壤全磷含量与CK相比均无显著差异;（4）T3、T4处理中叶片添加显著增加了上覆水体中总磷含量,试验过程中呈现出在分解初期迅速上升,在分解10 d时达到峰值,之后逐渐降低并趋于稳定,但试验结束时仍显著较高,分别是对应无叶片组的17.15倍（T3）和5.81倍（T4）。这些结果说明水淹通过促进落羽杉叶片的分解从而增加上覆水体中磷的含量,因此有必要在水淹前对消落带的落羽杉幼林叶片进行适时采收,以尽可能减少其对库区水体的磷负荷。     

Leaf responses to soil water deficits: Comparative sensitivity of leaf expansion rate and leaf conductance in field-grown sunflower (Helianthus annuus L.)

The relative importance of changes in leaf expansion rate (LER) and leaf conductance (g₁) in the control of crop transpiration depends primarily on their sensitivity to soil water deficits. The aim of this paper was to quantify the responses of LER and g₁ to soil water deficits in sunflower (Helianthus annuus L.) under conditions of moderate (spring) and high (summer) evaporative demand. Soil water content, g₁, and LER were measured in dryland (DRY) and daily-irrigated (WET) crops established on a deep sandy-loam (Typic Xerofluvent) in a Mediterranean environment. There was no difference between g₁ of DRY and WET plants (p>0.20) in contrast with a highly significant difference in LER (p<0.001). Even under the harsh conditions of the summer experiment, g₁ did not respond to water deficit in a ten-day period in which LER of DRY plants was reduced to approx. 30% of that measured in WET controls. This field study indicates that g₁ plays at most a minor role in the control of sunflower transpiration in the pre-anthesis period and confirms the importance of leaf expansion in the regulation of gas exchange of expanding canopies subjected to soil water deficits.     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

Photosynthesis and water-relation traits of the summer annual C4 grasses, Eleusine indica and Digitaria adscendens, with contrasting trampling tolerance

Two summer annual C₄ grasses with different trampling susceptibilities were grown as potted plants, and diurnal leaf gas exchange and leaf water potential in each grass were compared. The maximum net photosynthetic rate, leaf conductance and transpiration rate were higher in the trampling-tolerant Eleusine indica (L.) Gaertn. than in trampling sensitive Digitaria adscendens (H. B. K.) Henr. Leaf water potential was much lower in E. indica than in D. adscendens. There were no differences in soil-to-leaf hydraulic conductance and leaf osmotic potential at full turgor as obtained by pressure–volume analysis. However, the bulk modulus of elasticity in cell walls was higher in E. indica leaves than in D. adscendens leaves. This shows that the leaves of E. indica are less elastic. Therefore, the rigid cell walls of E. indica leaves reduced leaf water potential rapidly by decreasing the leaf water content, supporting a high transpiration rate with high leaf conductance. In trampled habitats, such lowering of leaf water potential in E. indica might play a role in water absorption from the compacted soil. In contrast, the ability of D. adscendens to colonize dry habitats such as coastal sand dunes appears to be due to its lower transpiration rate and its higher leaf water potential which is not strongly affected by decreasing leaf water content.