不同水分条件下两个树种木质部栓塞对P素添加的响应

 在两种水分供给(干旱胁迫和适宜水分，土壤含水量分别为田间持水量的30%～40%和70%～80%)下，研究了耐旱树种元宝枫(Acer truncatum)和 中生树种女贞(Ligustrum lucidum )木质部栓塞(以导水率(Percentage loss of hydraulic conductivity, PLC)损失程度衡量)对P素添加的 响应。结果发现，两个树种PLC的日变化均呈现出先上升后降低的规律，表明木质部栓塞的形成与恢复是植物体的一种平常事件；除适宜水分条 件的女贞外，P素可以显著提高元宝枫和遭受干旱胁迫时女贞的PLC；两种水分条件下，干旱胁迫时元宝枫木质部栓塞明显高于适宜水分供给时 。女贞的PLC在两种水分状况下无显著差异；树种间，干旱胁迫促进了元宝枫木质部的栓塞形成，明显高于同等水分条件下的女贞。该研究结果 证实了“木质部限流耐旱假设”。     

基于氘示踪剂和热扩散技术的栓皮栎水分运输速率与效率研究

利用同位素示踪和热扩散技术研究了不同胁迫处理栓皮栎的水分运输和储存差异。结果表明,注射氘同位素后,充分灌溉、轻度和重度胁迫处理的最大氘同位素比率分别升高到586.67‰、997.33‰和1364.89‰,处理间差异显著。轻度和重度胁迫处理的示踪速率分别为0.10 m/h和0.07 m/h,显著低于充分灌溉处理,但半减期和残留期显著高于充分灌溉。轻度和重度胁迫处理的枝条栓塞程度(PLC)比充分灌溉显著增加,液流通量、水势和蒸腾速率则显著减小。统计分析表明蒸腾作用强弱决定树体水分运输速率,PLC的增加和枝条水势的降低阻碍木质部水分运输。半减期和残留期内,轻度和重度胁迫处理的累积液流量显著高于充分灌水处理,运载相同体积的示踪剂,胁迫处理栓皮栎需要的水量增加,表明胁迫环境下受到PLC、蒸腾以及与树体储水交换的影响,水分运输效率下降。栓皮栎通过栓塞和储水交换来降低水分运输速率和效率,调控水分的收支平衡来适应干旱的环境。     

8种木本植物木质部栓塞变化与生理生态指标关系的研究Ⅰ.与植物木质部水势的关系

以自然状况下生长良好的耐旱树种刺槐(Robinia pseudoacacia L.)、元宝枫(Acer truncatum Bge.)、沙棘(Hippophae rhamnoides L.)、白榆(Ulmus pumila L.)、油松(Pinus tabulaeformis Carr.)、白皮松(Pinus bungeana Zucc.ex Endl.)及中生树种女贞(Ligustrum lucidum Ait.)、柳树(Salix matsudana Koidz. f. pendula Schneid.)为研究对象,用压力室法测定木质部水势,用冲洗法测定木质部栓塞程度,研究不同生长季节木质部栓塞与水势间的关系.结果表明针叶树油松、白皮松在各季节水势均较高,水势变化幅度较小,木质部不易发生栓塞,这与其木质部由管胞构成,对木质部栓塞不敏感,在干旱时采用高水势延迟脱水的耐旱策略有关.阔叶树刺槐、元宝枫、沙棘、白榆、女贞和柳树的木质部栓塞现象是其在每天正常生长过程中不可避免的 "平常事件",是它们适应干旱的一种方式.它们的木质部栓塞程度与水势表现出了相反的变化趋势,即同一树种在同一季节内水势值越低,木质部栓塞程度越大,但在不同树种及同一树种的不同季节不存在这种关系.由此可见,植物木质部栓塞对水势的敏感程度(即木质部栓塞脆弱性)主要由树种的木质部结构决定,同时受到树种特性、树木生长发育时期、外界环境因子的影响,木质部栓塞的脆弱性也具有季节变化特征.     

O_3和干旱胁迫对元宝枫叶片气孔特征的复合影响

利用开顶式气室(OTC)研究了O3胁迫(空气O3浓度加118μg·m-3)、干旱胁迫(土壤水分保持在田间持水量的40%~50%)以及O3与干旱复合胁迫(空气O3浓度加118μg·m-3和土壤水分保持在田间持水量的40%~50%)对元宝枫(Acer truncatum Bunge.)叶片气孔特征的影响。结果表明:O3胁迫41 d时气孔密度变化不明显,62和110 d时显著增加(P0.05);干旱胁迫和复合胁迫下气孔密度增加显著(P0.05);O3、干旱及复合胁迫处理110 d气孔面积、周长、长度和宽度明显减小(P0.05),气孔指数明显增加(P0.05);各胁迫气孔开度均明显减小(P0.05),O3和干旱对气孔开度影响存在协同作用,干旱减小了O3胁迫下的气孔开度。O3和干旱胁迫下气孔特征响应不同表明影响气孔的机制不同,O3对气孔影响具有累积效应,干旱可限制O3摄入量从而减小了O3对植物的伤害。     

干旱胁迫对金花茶幼苗光合生理特性的影响

以金花茶一年生实生苗为材料,采用盆栽控水试验,研究不同水分处理(CK、T1、T2、T3,土壤含水量分别为田间持水量的85%~90%、65%~70%、50%~55%、35%~40%)对金花茶幼苗光合生理指标的影响。结果显示:(1)在重度干旱胁迫(T3处理)下,金花茶植株都因干旱而死亡,表明金花茶幼苗对干旱胁迫的耐受极限为田间持水量的50%~55%(土壤含水量为15.04%~16.54%)。(2)随着干旱胁迫程度的加剧,金花茶幼苗叶片净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr)、气孔限制值(Ls)和PSⅡ实际光化学量子效率(ΦPSⅡ)均显著降低,胞间CO2浓度(Ci)和水分利用效率(WUE)无显著变化,干旱胁迫下金花茶光合速率降低的主要原因是由非气孔因素所致。(3)与CK相比,T1处理下叶片初始荧光(F0)、最大荧光(Fm)、PSⅡ最大光化学效率(Fv/Fm)和丙二醛(MDA)含量均无显著变化,而T2处理下F0、MDA含量显著升高,Fm、Fv/Fm显著降低;T1处理下金花茶的PSⅡ反应中心还未受到伤害,而T2处理下其光合机构发生了不可逆的破坏。(4)叶片叶绿素总量(Chl)、叶绿素a(Chl a)、叶绿素b(Chl b)、类胡萝卜素(Car)含量、Chl a/Chl b、Car/Chl、叶片相对含水量(RWC)均随干旱胁迫的增强而降低,脯氨酸(Pro)含量随干旱胁迫的加剧呈现出先升高后降低的趋势。研究表明,金花茶对干旱胁迫极为敏感,水分稍有亏缺,便会明显抑制其光合作用;金花茶不耐干旱的生理特性可能是限制其种群扩散的一个重要原因。     

干旱胁迫对持绿性高粱叶片渗透调节及叶绿体超微结构的影响

在盆栽条件下,研究了开花期和灌浆期干旱胁迫(土壤含水量为田间最大持水量的45%~50%)对持绿性高粱(B35)和非持绿性高粱(三尺三)叶片水分、渗透调节物质以及叶绿体超微结构的影响.结果表明: 干旱胁迫下,两高粱品系叶片自由水含量下降,束缚水含量增加,相对含水量降低,水分饱和亏缺增加,相对电导率增大,但三尺三各指标的变化幅度均大于B35.对于渗透调节物质,干旱胁迫下,三尺三可溶性糖含量的增幅大于B35,脯氨酸含量的增幅小于B35,可溶性蛋白含量的降幅大于B35.干旱胁迫下,B35与三尺三的叶绿体超微结构均受到一定程度的破坏,但B35叶绿体结构保持相对完好,受损程度明显小于三尺三.在干旱胁迫下,持绿性高粱通过较强的渗透调节表现出更好的干旱适应能力.
     

水分胁迫对银中杨耗水特征与水分利用的影响

在不同供水条件下,采用LI-6400光合作用系统(LI-COR,1995,USA)和电子天平等仪器,对银中杨苗木气体交换、蒸腾耗水量和水分利用效率及其有关环境因子进行了测定。结果表明,在研究的诸环境因子中,土壤含水量是制约苗木蒸腾耗水的主导因子;苗木的蒸腾耗水量随土壤干旱胁迫的加重而减少,并且当土壤含水量低于田间持水量的50%时,气象因子对苗木蒸腾耗水的影响不再明显;土壤含水量为田间持水量的70%时,银中杨苗木的净光合速率和蒸腾速率下降不显著,而水分利用效率有所提高,为2.579μmol.mmol-1,在土壤含水量大于田间持水量的70%的水分条件下,银中杨苗木可以正常生长。     

8种木本植物木质部栓塞变化与生理生态指标关系的研究 I.与植物木质部水势的关系

以自然状况下生长良好的耐旱树种刺槐（Robinia pseudoacacia L.）、元宝枫（Acer truncatum Bge）、沙棘（Hippophae rhamnoides L.）、白榆（Ulmus pumila L.）、油松（Pinus tabulaeformis Carr.）、白皮松（Pinus bungeana Zucc.ex Endl.）及中生树种女贞（Ligustrum lucidum Ait.）、柳树（Salix matsudana Koidz.f.pendula Schneid.）为研究对象,用压力室法测定木质部水势,用冲洗法测定木质部栓塞程度,研究不同生长季节木质部栓塞与水势间的火系。结果表明：针叶树油松、白皮松在各个季节水势均较高,水势变化幅度较小,木质部不易发生栓塞,这与其木质部由管胞构成,对木质部栓塞不敏感,在干旱时采用高水势延迟脱水的耐旱策略有关。阔叶树刺槐、元宝枫、沙棘、白榆、女贞和柳树的木质部栓塞现象是其在每天正常生长过程中不可避免的“平常事件”,是它们适应干旱的一种方式。它们的木质部栓塞程度与水势表现出了相反的变化趋势,即同一树种在同一季节内水势值越低,木质部栓塞程度越大,但在不同树种及同一树种的不同季节不存在这种关系。由此可见,植物木质部栓塞对水势的敏感程度（即木质部栓塞脆弱性）主要由树种的木质部结构决定,同时受到树种特性、树木生长发育时期、外界环境因子的影响,木质部栓塞的脆弱性也具有季节变化特征。     

6个耐旱树种木质部结构与栓塞脆弱性的关系

木质部栓塞脆弱性对干旱响应的研究已成为全球气候变化背景下的热点和重要内容。该文以6个耐旱树种刺槐(Robinia pseudoacacia)、沙棘(Hippophae rhamnoides)、榆树(Ulmus pumila)、元宝枫(Acer truncatum)、旱柳(Salix matsudana)、榛(Corylus heterophylla)为研究对象,采用Cochard Cavitron离心机技术建立木质部栓塞脆弱曲线,计算木质部栓塞脆弱性,利用染色法、硅胶注射法等测定木质部导管直径、导管内径跨度、导管连接度、导管密度、导管长度和木质部密度,探究木质部结构与栓塞脆弱性的关系,区分6个耐旱树种木质部结构在抗栓塞性上的差异,以期建立6个耐旱树种在木质部结构方面的抗栓塞性指标。结果表明:6个耐旱树种木质部栓塞脆弱性大小为刺槐榆树沙棘旱柳元宝枫榛,其中,刺槐、沙棘和榆树的栓塞脆弱曲线为"r"形,而元宝枫、旱柳和榛的栓塞脆弱曲线为"s"形,脆弱曲线为"r"形的树种与脆弱曲线为"s"形的树种栓塞脆弱性差异极显著(p0.01)。线性分析表明:木质部结构影响各树种的栓塞脆弱性,其中,木质部密度影响最大(t=0.702),导管直径次之(t=0.532),导管长度影响最小(t=0.010)。     

土壤水分胁迫对设施番茄叶片气孔特性的影响

为了研究土壤水分胁迫对设施番茄叶片气孔特性的影响,本研究以"金粉2号"为试材,于2013年5—8月设计4个土壤水分梯度试验[正常灌溉(田间持水量的70%~80%)、轻度胁迫(田间持水量的60%~70%)、中度胁迫(田间持水量的50%~60%)和重度胁迫(田间持水量的30%~40%)],利用数码显微成像系统和数码测距软件测定了叶片气孔参数。结果表明:番茄叶片气孔长度、宽度、气孔开张度和开张比均随水分胁迫程度的增加而减小,且随着胁迫时间的延长,其减小幅度不断增大;番茄气孔长宽的减小具有同步性,但气孔长度下降更为显著;随着水分胁迫程度的增加,气孔逐渐由长卵形转变为近圆形;随着土壤水分胁迫强度的增加,气孔上表皮气孔密度逐渐增大,而气孔下表皮密度呈先减小后增大的趋势。该研究揭示了土壤水分胁迫对设施番茄叶片气孔发展的影响规律,可为设施番茄水分管理提供科学依据。     

The relations of stomatal closure and reopening to xylem ABA concentration and leaf water potential during soil drying and rewatering

Two tropical tree species, Acacia confusa and Leucaena leucocephala, were used to study the relationships among stomatal conductance, xylem ABA concentration and leaf water potential during a soil drying and rewatering cycle. Stomatal conductance of both A. confusa and L. leucocephala steadily decreased with the decreases in soil water content and pre-dawn leaf water potential. Upon rewatering, soil water content and pre-dawn leaf water potential rapidly returned to the control levels, whereas the reopening of stomata showed an obvious lag time. The length of this lag time was highly dependent not only upon the degree of water stress but also on plant species. The more severe the water stress, the longer the lag time. When A. confusa and L. leucocephala plants were exposed to the same degree of water stress (around –2.0 MPa in pre-dawn leaf water potential), the stomata of A. confusa reopened to the control level 6 days after rewatering. However, it took L. leucocephala about 14 days to reopen fully. A very similar response of leaf photosynthesis to soil water deficit was also observed for both species. Soil drying resulted in a significant increase in leaf and xylem ABA concentrations in both species. The more severe the water stress, the higher the leaf and xylem ABA concentrations. Both leaf ABA and xylem ABA returned to the control level following relief from water deficit and preceded the full recovery of stomata, suggesting that the lag phase of stomatal reopening was not controlled by leaf and/or xylem ABA. In contrast to drying the whole root system, drying half of the root system did not change the leaf water relations, but caused a significant increase in xylem ABA concentration, which could fully explain the decrease of stomatal conductance. After rewatering, the stomatal conductance of plants in which half of the roots were dried recovered more rapidly than those of whole-root dried plants, indicating that the leaf water deficit that occurred during the drying period was related to the post-stress stomatal inhibition. These results indicated that the decrease in stomatal conductance caused by water deficit was closely related to the increase in xylem ABA, but xylem ABA could not fully explain the reopening of stomata after relief of water stress, neither did the leaf ABA. Some unknown physiological and/or morphological processes in the guard cells may be related to the recovery process.     

Elevated [CO2] alleviates the impacts of water deficit on xylem anatomy and hydraulic properties of maize stems

Plants can modify xylem anatomy and hydraulic properties to adjust to water status. Elevated [CO₂] can increase plant water potential via reduced stomatal conductance and water loss. This raises the question of whether elevated [CO₂], which thus improves plant water status, will reduce the impacts of soil water deficit on xylem anatomy and hydraulic properties of plants. To analyse the impacts of water and [CO₂] on maize stem xylem anatomy and hydraulic properties, we exposed potted maize plants to varying [CO₂] levels (400, 700, 900, and 1,200 ppm) and water levels (full irrigation and deficit irrigation). Results showed that at current [CO₂], vessel diameter, vessel roundness, stem cross-section area, specific hydraulic conductivity, and vulnerability to embolism decreased under deficit irrigation; yet, these impacts of deficit irrigation were reduced at elevated [CO₂]. Across all treatments, midday stem water potential was tightly correlated with xylem traits and displayed similar responses. A distinct trade-off between efficiency and safety in stem xylem water transportation in response to water deficit was observed at current [CO₂] but not observed at elevated [CO₂]. The results of this study enhance our knowledge of plant hydraulic acclimation under future climate environments and provide insights into trade-offs in xylem structure and function.     

Native root xylem embolism and stomatal closure in stands of Douglas-fir and ponderosa pine: mitigation by hydraulic redistribution

Hydraulic redistribution (HR), the passive movement of water via roots from moist to drier portions of the soil, occurs in many ecosystems, influencing both plant and ecosystem-water use. We examined the effects of HR on root hydraulic functioning during drought in young and old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine (Pinus ponderosa Dougl. Ex Laws) trees growing in four sites. During the 2002 growing season, in situ xylem embolism, water deficit and xylem vulnerability to embolism were measured on medium roots (2–4-mm diameter) collected at 20–30 cm depth. Soil water content and water potentials were monitored concurrently to determine the extent of HR. Additionally, the water potential and stomatal conductance (g_s) of upper canopy leaves were measured throughout the growing season. In the site with young Douglas-fir trees, root embolism increased from 20 to 55 percent loss of conductivity (PLC) as the dry season progressed. In young ponderosa pine, root embolism increased from 45 to 75 PLC. In contrast, roots of old-growth Douglas-fir and ponderosa pine trees never experienced more than 30 and 40 PLC, respectively. HR kept soil water potential at 20–30 cm depth above –0.5 MPa in the old-growth Douglas-fir site and –1.8 MPa in the old-growth ponderosa pine site, which significantly reduced loss of shallow root function. In the young ponderosa pine stand, where little HR occurred, the water potential in the upper soil layers fell to about –2.8 MPa, which severely impaired root functioning and limited recovery when the fall rains returned. In both species, daily maximum g_s decreased linearly with increasing root PLC, suggesting that root xylem embolism acted in concert with stomata to limit water loss, thereby maintaining minimum leaf water potential above critical values. HR appears to be an important mechanism for maintaining shallow root function during drought and preventing total stomatal closure.     

干旱胁迫对油松和侧柏水分运输安全性和有效性的影响

通过采用改良的冲洗法测定5年生苗木油松(Pinus tabulaeformis)和侧柏(Platycladus orientalis)在不同干旱胁迫时期的水力结构参数,结果表明随着干旱胁迫增加,不同分枝级和茎段所在区域的导水率损失(PLC)增加,比导率(Ks)减少。油松和侧柏在0级,1级和2级分枝级的发生栓塞的水势阈值分别为-0.55、-0.49、-0.43和-0.90、-0.78、-0.74MPa。随着相对分枝级的增加,油松和侧柏的水势阈值增大,栓塞脆弱性变大。非限速区PLC大而Ks小,限速区PLC小而Ks大。油松和侧柏相对分枝级和茎段所在区域的栓塞脆弱性大小为2级1级0级,限速区非限速区,且油松大于侧柏。油松和侧柏在不同干旱胁迫,不同相对分枝级,不同茎段所在区域采取不同的方式来适应由水势降低而引起的栓塞变化。其采取的生态策略包括:保持较高的水分安全性;减轻安全性而对有效性的折衷;同时降低有效性和安全性但不终止任何生产力或树高的组织生长所需水的限制。     

荒漠河岸林植物木质部导水与栓塞特征及其对干旱胁迫的响应

 以同处于干旱区的塔里木河下游(铁干里克)和黑河下游(乌兰图格)断面为研究区, 比较了荒漠河岸林主要建群种胡杨(Populus euphratica)、柽柳(Tamarix spp.)、疏叶骆驼刺(Alhagi sparsifolia)和花花柴(Karelinia caspia)在长期遭受不同干旱胁迫下的根、枝条木质部导水力和栓塞化程度的变化特征, 并分析了木质部导水对干旱胁迫的响应及适应策略。结果表明: 1) 黑河下游荒漠河岸林植物的导水能力显著高于塔里木河下游, 其中柽柳、胡杨、疏叶骆驼刺和花花柴根木质部的初始比导率(K_s0)分别高11.97、6.74、7.10和3.73倍, 枝条的K_s0分别高9.48、3.65、2.07和1.88倍, 地下水埋深导致的干旱胁迫程度不同是诱发荒漠植物导水能力差异的根本原因; 2)柽柳耐干旱能力最强, 适应范围较宽, 而花花柴、疏叶骆驼刺的耐旱性相对较弱, 适生范围较窄, 这可能与植物的根系分布有关; 3)干旱胁迫较轻时, 枝条木质部是荒漠河岸林植物水分传输的主要阻力部位, 干旱胁迫严重时, 根木质部是限制植株水流的最大阻碍部位; 4)荒漠河岸林植物主要通过调节枝条木质部的水流阻力来适应干旱胁迫, 且其适应策略与干旱胁迫程度有关, 干旱胁迫轻时, 植物通过限制枝条木质部水流来协调整株植物的均匀生长; 干旱胁迫严重时, 植物通过牺牲劣势枝条、增强优势枝条水流来提高植株整体生存的机会。     

Is desiccation tolerance and avoidance reflected in xylem and phloem anatomy of two coexisting arid‐zone coniferous trees?

Plants close their stomata during drought to avoid excessive water loss, but species differ in respect to the drought severity at which stomata close. The stomatal closure point is related to xylem anatomy and vulnerability to embolism, but it also has implications for phloem transport and possibly phloem anatomy to allow sugar transport at low water potentials. Desiccation‐tolerant plants that close their stomata at severe drought should have smaller xylem conduits and/or fewer and smaller interconduit pits to reduce vulnerability to embolism but more phloem tissue and larger phloem conduits compared with plants that avoid desiccation. These anatomical differences could be expected to increase in response to long‐term reduction in precipitation. To test these hypotheses, we used tridimensional synchroton X‐ray microtomograph and light microscope imaging of combined xylem and phloem tissues of 2 coniferous species: one‐seed juniper (Juniperus monosperma) and piñon pine (Pinus edulis) subjected to precipitation manipulation treatments. These species show different xylem vulnerability to embolism, contrasting desiccation tolerance, and stomatal closure points. Our results support the hypothesis that desiccation tolerant plants require higher phloem transport capacity than desiccation avoiding plants, but this can be gained through various anatomical adaptations in addition to changing conduit or tissue size.     

Hydraulic failure and tree dieback are associated with high wood density in a temperate forest under extreme drought

Catastrophic hydraulic failure will likely be an important mechanism contributing to large‐scale tree dieback caused by increased frequency and intensity of droughts under global climate change. To compare the susceptibility of 22 temperate deciduous tree and shrub species to hydraulic failure during a record drought in the southeastern USA, we quantified leaf desiccation, native embolism, wood density, stomatal conductance and predawn and midday leaf water potential at four sites with varying drought intensities. At the two driest sites, there was widespread leaf wilting and desiccation, and most species exhibited predawn leaf water potentials of ≤3 MPa and >60% loss of xylem conductivity in branches. Although species with high wood density were more resistant to cavitation, they had higher levels of native embolism and greater canopy dieback than species with low wood density. This unexpected result can be explained by the failure of species with dense wood to avert a decline in water potential to dangerous levels during the drought. Leaf water potential was negatively correlated with wood density, and the relationship was strongest under conditions of severe water deficit. Species with low wood density avoided catastrophic embolism by relying on an avoidance strategy that involves partial drought deciduousness, higher sensitivity of stomata to leaf water potential and perhaps greater rooting depth. These species therefore maintained water potential at levels that ensured a greater margin of safety against embolism. These differences among species may mediate rapid shifts in species composition of temperate forests if droughts intensify due to climate change.     

Hydraulic vulnerability, vessel refilling, and seasonal courses of stem water potential of Sorbus aucuparia L. and Sambucus nigra L.

Differences in the seasonal variation in stem water potential between the two shrub species Sorbus aucuparia and Sambucus nigra were related with their vulnerability to xylem cavitation. It was also demonstrated indirectly that the two species differ in the extent to which they reverse cavitation. Seasonal variation in stem water potential was investigated during three growing seasons with in situ stem psychrometers. Sorbus experienced wide water potential variations and reached a minimum of -4.2 MPa during drought. Under the same microclimatic conditions, Sambucus experienced consistent stem water potentials with a minimum of -1.7 MPa. The relationship between percentage loss in hydraulic conductivity (PLC) and water potential (hydraulic vulnerability curve) of the two species differed in shape: a flat curve with nearly total loss of conductivity at -6 MPa was found for SORBUS: Sambucus showed a steep vulnerability curve with 90% loss conductivity at -2.2 MPa. Thus, Sambucus is extremely vulnerable to cavitation, but Sorbus is an almost invulnerable species. This different cavitation resistance adjusted the ranges of field stem water potential that the species experienced. Finally, seasonal courses of naturally occurring (native) embolism were compared with calculated PLC courses. This comparison indicates that Sorbus did not refill embolized xylem vessels whereas Sambucus reversed embolism. It was concluded that species which are highly vulnerable to cavitation and drought-induced embolism need refilling of embolized vessels as well as isohydric water potential patterns as two strategies of survival.