模拟岩溶水分供应分层的干旱胁迫对青冈栎光合特性和生长的影响

根据喀斯特地质特点,建立水分供应分层模拟柱(土壤层-岩石层-岩溶水层),并设置不同土壤水分梯度:重度干旱、中度干旱、水分适宜和水分饱和(相当于土壤田间持水量的30％～40％、50％ ～ 60％、70％ ～ 80％和90％～100％),研究2年生青冈栎(Cyclobalanopsis glauca)苗木在模拟喀斯特生境下地经生长量、枝条生长量、光合曲线特征以及光合参数变化.结果表明:土壤干旱胁迫能显著降低青冈栎地径和枝条生长量,但在岩溶层有水的条件下,地径生长量不受土壤干旱胁迫影响;青冈栎的光合作用-光响应曲线符合非直角双曲线函数模型,不同处理光响应曲线的变化趋势基本一致;最大净光合速率和表观量子效率均表现为水分饱和＞水分适宜＞中度干旱＞重度干旱;土壤干旱胁迫使青冈栎光饱和点降低、光补偿点升高,但岩溶水层有水时情况发生改变.     

岩溶干旱胁迫下青冈栎水分参数变化

根据岩溶生境特点设计表层岩溶水-岩石(灰岩)-土壤水分供应分层模拟柱,对青冈栎(Cyclobalanopsis glauca)进行水分控制试验;运用压力容积(P-V)技术,研究了在水分胁迫下青冈栎的日水势(P)、饱和含水量时的最大渗透势(ψsats)、初始质壁分离时的渗透势(ψtlps)、初始质壁分离时渗透水相对含量(ROWCtlp)、初始质壁分离时相对含水量(RWCtlp)、质外体水的相对含量(AWC),以及饱和含水量时最大渗透势与初始质壁分离时的渗透势之差(△P)的变化.结果表明,在无表层岩溶水-岩石-土壤处理中,随着土壤干旱胁迫程度的增加,ROWCtlp、RWCtlp、ψsats、ψtlp呈明显下降,而AWC值上升;在有表层岩溶水-岩石-土壤处理下,青冈栎受土壤水分胁迫不明显,水分参数变化不显著.表层岩溶缺水时,土壤水分含量是影响青冈栎水分参数变化的主要因素,在一定的干旱胁迫范围内,随着干旱程度的提高,青冈栎耐旱性不断增加.     

接种菌根真菌对青冈栎幼苗耐旱性的影响

利用丛枝菌根真菌摩西球囊霉(Glomus mosseae)、根内球囊霉(Glomus intraradices)和外生菌根真菌彩色豆马勃(Pisolithus tinctorius)对石漠化地区造林树种青冈栎(Cyclobalanopsis glauca)幼苗进行接种试验。在大棚盆栽条件下模拟土壤干旱胁迫,研究菌根真菌对青冈栎生长和耐旱性的影响。结果表明:在土壤干旱条件下,接种菌根处理植株生物量显著高于未接种处理(P0.05),菌根依赖性随土壤水分含量降低而升高;未接种处理植株叶绿素含量在土壤干旱条件下显著降低(P0.05),除接种Pisolithus tinctorius处理外,其它接种处理叶绿素含量无显著变化。土壤干旱使植株体内脯氨酸和可溶性糖含量上升,在中度干旱条件下,接种处理可溶性糖含量均显著高于对照处理,接种Glomus intraradices、Pisolithus tinctorius处理脯氨酸含量显著低于对照(P0.05);在重度干旱条件下,接种Glomus mosseae和Glomus intraradices处理可溶性糖含量显著高于对照处理(P0.05),而相应的脯氨酸含量显著低于对照处理。当土壤水分含量在田间持水量55%—65%时,接种处理植株SOD、POD和CAT酶活性显著高于未接种处理(P0.05),在土壤水分含量降至35%—45%时,Glomus mosseae和Glomus intraradices处理SOD酶活性显著高于对照,并且所有接种处理POD酶活性均显著高于对照。此外,在水分干旱条件下,植株全磷和全钾含量也显著高于未接种处理(P0.05)。研究表明,丛枝菌根真菌和外生菌根真菌均能够侵染青冈栎幼苗根系;在干旱胁迫条件下,接种菌根真菌能够提高青冈栎植株生物量、抗氧化酶活性、增加植株可溶性糖含量和促进植株养分吸收,提高植株耐旱性,从而使青冈栎幼苗在岩溶干旱环境下更容易存活。     

干旱胁迫对喀斯特生境蚬木幼苗光合特性及抗性生理的影响

为了解蚬木(Excentrodendron hsienmu)幼苗对喀斯特干旱生境的生理适应,基于构建的模拟喀斯特生境装置,以2年生蚬木幼苗为供试材料,研究岩溶水分和土壤水分两个因素对蚬木幼苗叶片光合及抗性生理的影响。结果表明:蚬木的光合作用-光响应曲线符合非直角双曲线函数模型,岩溶水层2种处理下的光响应曲线变化基本一致。土壤水分显著影响叶片最大净光合速率和暗呼吸速率,2个因素及其交互作用极显著影响叶片相对叶绿素含量。最大净光合速率和暗呼吸速率随土壤干旱胁迫加剧而下降,相对叶绿素含量则表现为增加趋势。土壤水分极显著影响游离脯氨酸、显著影响丙二醛和过氧化物酶,干旱胁迫组的游离脯氨酸含量显著高于对照,重度干旱胁迫下丙二醛含量显著高于对照。过氧化物酶活性均随干旱加剧先升后降,峰值出现在中度胁迫。研究表明,蚬木具有较强的耐旱潜力,渗透调节物质的主动适应是其耐旱的主要原因。     

弄拉典型峰丛岩溶区青冈栎叶片形态特征及对环境的适应

选取广西弄拉典型峰丛洼地生态系统中不同地貌类型的青冈栎 (Cyclobalanopsisglauca)叶片进行形态解剖特征比较分析 ,结果表明 :( 1 )峰丛洼地生态系统中不同地貌青冈栎叶片形态解剖特征差异显著。山顶青冈栎叶片部分表皮结构 (角质膜、表皮毛、表皮细胞、气孔 )趋向旱化。山顶青冈栎下表皮毛比山腰青冈栎的浓密且长。两者叶片的厚度和宽度、上表皮细胞个数、气孔指数、下表皮厚度、栅栏组织厚度、海绵组织厚度存在着显著性差异 ;位于山顶的青冈栎受到水分的胁迫 ,在形态解剖上呈现出中生偏旱的结构。 ( 2 )青冈栎的各种形态解剖指标说明在岩溶区青冈栎的抗旱性主要是通过抵御干旱来适应水分的胁迫 ,并主要取决于减少蒸腾失水和维持水分吸收能力两个方面     

接种丛枝菌根真菌对青冈栎幼苗生长和光合作用的影响

利用菌根真菌摩西球囊霉和根内球囊霉,对喀斯特地区造林树种青冈栎进行接种试验,测定菌根真菌对青冈栎幼苗生长、生物量和光合作用的影响。结果表明:接种丛枝菌根真菌能显著促进青冈栎幼苗株高、地径、叶面积和幼苗生物量的增长,并且能提高幼苗成活率和改善幼苗的光合能力。摩西球囊霉和根内球囊霉处理的青冈栎幼苗生物量分别是未接种处理的2.1和1.9倍;摩西球囊霉和根内球囊霉处理下的水分利用效率分别比对照处理提高了33.6%和8.8%;摩西球囊霉对青冈栎幼苗株高、地径、生物量的促进作用好于根内球囊霉,而根内球囊霉处理的幼苗叶面积、主根长、根冠比大于摩西球囊霉处理。总体而言,接种丛枝菌根真菌特别是摩西球囊霉能促进青冈栎幼苗的生长和生物量增长,在石漠化植被恢复中具有潜在应用价值。     

不同类型喀斯特植物的荧光特征及抗旱性比较

利用叶绿素荧光技术观测了五个不同类型的喀斯特植物翅荚香槐、大盔凤仙、红背叶、牛耳朵和青冈栎在不同作用光强下的光能利用特征,并且对这五种类型植物在PEG诱导水分胁迫下的抗旱性作了比较。结果表明,随着作用光强的增加,这五种植物的光化学反应能力(qP)逐渐降低,非光化学耗散作用(NPQ)明显增加,同时PSⅡ有效光化学效率(Fv′/Fm′)随之下降,导致PSⅡ电子传递量子效率(ΦPSⅡ)也明显下降,但在相同作用光强下这五个类型植物的荧光特征无明显差异。在PEG诱导水分胁迫的实验中,牛耳朵、青冈栎在PEG处理后Fv/Fm变化不明显,而红背叶Fv/Fm值下降程度最大,其次为翅荚香槐、大盔凤仙。由此推测,牛耳朵和青冈栎的耐干旱能力最强,红背叶抗旱能力最弱,翅荚香槐、大盔凤仙居中。     

四种不同生活型植物幼苗对喀斯特生境干旱的生理生态适应性

喀斯特石漠化是我国西南喀斯特地区最严重的生态环境问题, 生境干旱是限制该地区植物生长的主要因素之一, 掌握喀斯特植被不同演替阶段不同生活型植物对干旱胁迫的适应策略有助于提高植被恢复的成功率。通过人工模拟4种干旱强度, 测定叶片水势、气体交换、叶绿素荧光、光合色素含量、渗透调节物质浓度、抗氧化酶活性以及生物量, 研究了喀斯特地区4种不同生活型植物幼苗对干旱胁迫的适应策略。这4种植物为常绿灌木火棘(Pyracantha fortuneana)、落叶灌木小果蔷薇(Rosa cymosa)、常绿乔木猴樟(Cinnamomum bodinieri)和落叶乔木圆果化香树(Platycarya longipes)。结果表明: 随着干旱程度的加深, 4种植物幼苗的叶片水势、光合能力、叶绿素含量、生物量增长、叶重比(LMR)、叶面积比(LAR)和比叶面积(SLA)逐渐下降, 而热耗散(NPQ)、类胡萝卜素与叶绿素含量比值、丙二醛含量和根重比(RMR)逐渐上升; 圆果化香树和猴樟的水分利用效率(A_n/g_s)、渗透调节物质浓度和抗氧化酶活性呈先升高后降低的趋势, 而火棘和小果蔷薇的A_n/g_s、脯氨酸含量和超氧化物歧化酶活性呈上升趋势。严重干旱下, 火棘和小果蔷薇幼苗的叶片水势和叶绿素含量下降较少, 具有较高的光合能力和生物量增长, 这主要是由于它们具有较低的SLA和LAR、较高的NPQ和A_n/g_s以及较高的渗透调节能力和抗氧化保护能力。中度干旱下, 猴樟幼苗叶片水势下降很少, LMR和LAR也较高, 脯氨酸含量和抗氧化酶活性非常高。但在严重干旱下, 其叶片水势、LMR、LAR和生物量增长大幅度下降, 最大光化学效率和光合速率也非常低, 渗透调节能力与抗氧化酶活性大幅度下降至正常水平以下。水分好的条件下, 圆果化香树幼苗具有较高的RMR以吸收充足的水分, 具有较高的LAR和叶绿素含量, 保证了生物量的大量积累。然而, 干旱胁迫致使其生物量大幅度下降, 主要是由于LMR、LAR、气体交换和叶绿素含量的大量下降以减少蒸腾面积、水分散失和对光能的吸收。研究结果表明, 火棘、小果蔷薇和猴樟幼苗主要采用耐旱策略, 其中猴樟抗严重干旱的能力较弱; 圆果化香树幼苗对干旱胁迫更为敏感, 主要采取避旱策略。     

狭叶红景天幼苗对水分及遮阴的生长及生理生化响应

研究植物对水分和遮阴胁迫的响应及其生理机制对制定合理的栽培管理措施十分必要。以红景天属植物为研究对象,设置土壤含水量分别为田间持水量的80%(过湿水分)、70%(正常水分)、60%(轻度干旱)、40%(中度干旱)、20%(重度干旱)5个水分梯度;设置2个遮阴处理,以全光照(遮阴率为0)为对照、黑色遮阴网遮阴(遮阴率为85%),研究狭叶红景天生长及生理生化指标的变化特征。结果表明:在不同水分处理下,与对照相比,叶绿素含量、茎干重和茎重比(SMR)显著增加(P0.05),株高、总生物量、叶面积、叶干重、叶重比(LMR)、比叶面积(SLA)、叶面积比(LAR)和叶面积根干重比(LARMR)增加,根冠比和根重比(RMR)减少;随着干旱程度加剧,丙二醛(MDA)、脯氨酸(Pro)和可溶性糖(Ss)含量增加,超氧化物歧化酶(SOD)活性总体呈先增加后减小的趋势。在遮阴处理下,株高、SMR、SLA、LAR和LARMR显著增加(P0.05),叶绿素SPAD值和叶面积增加,总生物量、根干重、根冠比和LMR显著减少(P0.05),茎干重和叶干重减少,MDA含量显著增加,Pro含量略有下降,Ss含量减少。在水分胁迫下,狭叶红景天中度干旱时通过增加酶活性抵御伤害,重度干旱超过其阈值,SOD活性下降,植物体受到伤害,Ss可能是主要的渗透调节物质。在遮阴处理下,狭叶红景天通过增加SLA避免遮阴伤害。狭叶红景天在受到环境胁迫时会通过形态改变、调节MDA含量、抗氧化酶活性和渗透调节物质来保证自身正常的生长发育。     

干旱胁迫下巨尾桉的形态可塑性和生理响应特征

以1月龄巨尾桉组培苗为材料,通过模拟雨季和旱季的土壤水分条件试验探讨干旱胁迫下巨尾桉幼苗形态的可塑性及生理响应特征.结果表明:与高水和中水处理相比,低水处理(13%左右土壤体积水分含量,模拟旱季土壤水分)导致巨尾桉幼苗生物量减少,叶片形态发生明显变化,总叶面积、叶片数、平均叶面积和比叶面积减少,而且叶片可溶性糖含量增加;但水分胁迫下巨尾桉生物量分配模式不变,叶片脯氨酸含量变化也不明显,叶片最大光化学量子效率(Fv/Fm)仍维持在正常水平.研究发现,干旱并未对巨尾桉光合能力和水分吸收策略造成过多影响,巨尾桉能通过限制生长、减少水分消耗来应对干旱胁迫,保证植株安全度过干旱同时又不会过度消耗当地环境水分,以利于维持当地旱季的水分平衡.     

Transpiration of <Emphasis Type="Italic">Cyclobalanopsis glauca</Emphasis> (syn. <Emphasis Type="Italic">Quercus glauca</Emphasis>) stand measured by sap-flow method in a karst rocky terrain during dry season

Seasonal drought may have a high impact on the karst ecosystem. The transpiration from Cyclobalanopsis glauca (syn. Quercus glauca) stand on a rocky hilly slope in South China was measured during the dry period of 2006 by using the Granier’s sap-flow method. During the experimental period, maximum sap flux density (J _s) ranged from 20 to 40 g H₂O m⁻² s⁻¹ according to diameter of breast height (DBH) of individual trees. On sunny days, daily transpiration varied between 3.4 and 1.8 mm day⁻¹. Transpiration of C. glauca was closely correlated to the radiation, air temperature, and vapor pressure deficit (VPD). Soil moisture was a very important factor influencing transpiration. The very low soil water content might result in low stand transpiration even when VPD is high, but high soil water content might also result in low transpiration if it was low VPD. However, VPD rather than soil moisture, affected largely the stand transpiration under high soil water content. The amount of transpiration was much more than that of the total soil moisture loss during the continuous sunny days, indicating that the dry shallow soils were probably not the only source for root-uptake water. C. glauca grows deep roots through the rock fissures of epikarst, indicating that epikarst might be another main source for sustaining transpiration in response to dry demand in autumn. Therefore, a large amount of deep roots of karst species would be a very important hydraulic connecting from the epikarst to above ground by transpiration, which would promote the biogeochemical process in a karst system.     

Responses to drought preconditioning in<Emphasis Type="Italic"> Eucalyptus globulus</Emphasis> Labill. provenances

Shoot water relations and morphological responses to drought preconditioning were studied by subjecting 5-month-old seedlings of three provenances of Eucalyptus globulus to different water regimes for 36 days in a greenhouse pot study. Moderately stressed plants were watered every 6 days and severely stressed plants were watered every 9 days. Control plants were watered daily. Drought cycles induced significant changes in morphological and physiological characteristics. Preconditioned seedlings were smaller in size, root collar diameter, height, and leaf area than control seedlings. Shoot/root ratio was not affected by drought. Osmotic potential at full turgor (ψπ_FT) and osmotic potential at turgor loss point (ψπ_TLP) were significantly lower and the magnitude of osmotic adjustment was significantly higher under the severe than under the moderate stress treatment. In severely stressed plants a decrease of turgid mass/dry mass contributed to osmotic adjustment. In a subsequent acclimation test, preconditioned seedlings showed higher values of stomatal conductance, predawn relative water content and water potential and lower mortality than control plants. These variables were significantly related to ψπ_FT. We assume that the reduced leaf area and osmotic adjustment observed in preconditioned seedlings contributed to drought acclimation in the selected E. globulus provenances leading to better rates of gas exchange and improved water status than non-conditioned plants. Provenances exhibited differences in their responses to drought, albeit mainly morphological differences. E. globulus subsp. bicostata from Tumbarumba grew more quickly (larger diameter and height relative growth rate) than the other provenances, implying a greater ability to tolerate water stress. It can be expected that preconditioned seedlings will display greater tolerance of water stress than non-conditioned plants and perform better during early establishment (higher survival and early growth).     

喀斯特土壤上香樟幼苗接种不同AM真菌后的耐旱性效应

为探索喀斯特土壤适生植物香樟幼苗在接种不同AM真菌后的耐旱适应性,进行了香樟幼苗接种幼套球囊霉(Glomus etunicatum)和层状球囊霉(Glomus lamellosum)后水分胁迫处理试验。结果表明：（1）接种AM真菌显著提高了香樟幼苗的生物量积累,AM促进植株生物量效应依次为中度>轻度>正常>重度,同一水分胁迫处理下生物量幼套球囊霉>层状球囊霉。（2）中度干旱下香樟幼苗菌根依赖性最大,幼套球囊霉接种植株的菌根依赖性较层状球囊霉大。（3）接种AM真菌显著提高了植株叶片可溶性糖、可溶性蛋白质和脯氨酸含量,并降低了丙二醛含量;在正常供水下植株叶片可溶性糖、可溶性蛋白质和脯氨酸含量层状球囊霉接种>幼套球囊霉接种>对照,干旱胁迫下表现为幼套球囊霉接种>层状球囊霉接种>对照;干旱胁迫下的幼套球囊霉接种植株丙二醛含量低于层状球囊霉接种植株。（4）总体上,可溶性糖与脯氨酸相关性极显著,可溶性蛋白质与丙二醛之间呈显著负相关性。幼套球囊霉接种香樟幼苗的耐旱性高于和层状球囊霉接种香樟幼苗。     

Effect of an Epichloë endophyte on adaptability to water stress in Festuca sinensis

As water stress, including drought and waterlogging, can severely affect plant growth, this study investigated the effects of an endophyte from the genus Epichloë on two different ecotypes of Festuca sinensis grass under five soil water conditions in a controlled greenhouse experiment. Changes in F. sinensis plants grown with (E+) and without the endophyte (E−) were evaluated as they were subjected to different water treatments (20%, 35%, 50%, 65% and 80% relative saturation moisture content, RSMC). Growth parameters such as plant height, number of tillers, blade width, stem diameter, root length, total biomass, root-shoot ratio and relative water content were determined. The results showed that drought and waterlogging significantly (P < 0.05) inhibited the growth of F. sinensis. The presence of the endophyte significantly (P < 0.05) increased plant growth and root-shoot ratio under drought and waterlogged conditions. In addition, the plant height, number of tillers, blade width, stem diameter and total biomass in seedlings of both ecotypes reached the maximum at 65% RSMC, which suggests the optimal water condition. These findings also show that moderate drought (35% and 50% RSMC) could promote root growth of grass seedlings. Therefore, endophytic infections can result in enhanced host plant resistance to drought and waterlogged conditions.     

Variable Chlorophyll a Fluorescence and CO(2) Uptake in Water-Stressed White Spruce Seedlings

Regulation of photosynthetic activity can contribute to the prevention of photodamage in stress resistant plants during exposure to drought or low temperatures. Responses to increasing levels of water stress were examined in seedlings of the stress resistant forest conifer, white spruce (Picea glauca [Moench.] Voss). Some seedlings were grown under aseptic in vitro conditions and others in pots. In relatively resistant in vivo seedlings, photosynthetic activities changed slowly in response to increasing water stress. Highly sensitive in vitro seedlings responded to water deficits similarly to in vivo seedlings but over a much shorter time scale. Fluorescence, CO₂ exchange, and stomatal conductance data reported here suggest possible mechanisms for the regulation of photochemical activity in these plants.     

Plant ecology of tropical and subtropical karst ecosystems

Substantial areas of tropical forests, including those within nine tropical biodiversity hotspots, contain karst landscapes that have developed on soluble carbonate rocks. Here, we review how the ecology of karst forest trees is influenced by hydrological, edaphic, and topographic factors that exhibit fine spatial heterogeneity. Comparative analysis of drought tolerance traits including wood density contributes to the assessment of whether karst tree species are more drought‐tolerant compared to non‐karst trees. Although karst ecosystems are generally considered to have low phosphorus availability, foliar nitrogen‐to‐phosphorus ratios exhibit wide variation across karst regions without a clear difference from non‐karst ecosystems. According to the analyses of leaf phenology, stem water storage, and isotopic signatures from xylem sap, water use strategies of karst trees can be classified into five types: (a) soil water dependent, (b) epikarst water dependent (mainly use water stored in fine pores and gaps within the epikarst rock during the dry season), (c) groundwater dependent, (d) fog water dependent, and (e) drought‐deciduous (shed leaves during the dry season). Overall, published data suggest that only a subset of karst tree species are exclusively distributed within karst hilltops where water availability is limited. The diverse resource acquisition and utilization strategies of karst plants across edaphic habitats must be considered when developing effective strategies to conserve and restore biodiversity in karst landscapes, which are under increasing anthropogenic pressure.     

Application of photochemical parameters and several indices based on phenotypical traits to assess intraspecific variation of oat (<Emphasis Type="Italic">Avena sativa</Emphasis> L.) tolerance to drought

Functionality of the photosynthetic system under water stress is of major importance in drought tolerance. Oat (Avena sativa L.) doubled haploid (DH) lines obtained by pollination of F ₁ oat crosses with maize were used to assess the differences in plant genotypic response to soil drought. The investigations were based on the measurements of gas exchange and chlorophyll a fluorescence kinetics. Drought was applied to 17-day-old seedlings by withholding water for 14 days and subsequent plant recovery. Non-stressed optimally watered plants served as controls. Yield components were determined when plants reached full maturity. It was shown differences among the oat lines with respect to drought stress susceptibility (SI) and stress tolerance index mean productivity and drought susceptibility index. Sensitivity to drought of individual DH lines was significantly different, as demonstrated by the correlation between drought susceptibility index and yield components, such as dry weight (GW) or grain number (GN) of the harvested plants. GW and GN were lower in drought-sensitive genotypes exposed to drought stress compared to those resistant to drought. The principal component analysis allow to separate three groups of lines differing in their sensitivity to drought stress and indicated that tolerance to drought in oat has a common genetic background.     

岩溶区青冈栎整树蒸腾的季节变化

应用Granier热消散树干液流技术,在裸露岩溶区坡地上对青冈栎样树的树干液流和整树蒸腾过程变化及其驱动因子进行了研究.结果表明:青冈栎树干液流密度与树木胸径大小的关系是随机的,日间液流密度峰值出现在13:30—14:30;日液流密度峰值夏季最大,为56.00 g H₂O·m^-2·s^-1,春季最小,在35.86 g H₂O·m^-2·s^-1.岩溶区单树日蒸腾量随着天气变化起伏较大,单树日蒸腾量与水汽压亏缺和太阳辐射呈显著的幂函数相关关系(R=0.97,P<0.01).平均整树日蒸腾量变化格型为夏季高冬春低,秋季(旱季)随土壤水分的减少由高到低变化.与其他地区的树种相比,即使受旱季的干燥少土双重胁迫,裸露岩溶区坡地上的青冈栎整树日蒸腾量仍然较高,推断在岩溶区旱季青冈栎的水分来源可能很大程度上依赖于富水的表层岩溶带.     

Turgor Regulation via Cell Wall Adjustment in White Spruce

Turgor regulation at reduced water contents was closely associated with changes in the elastic quality of the cell walls of individual needles and shoots of naturally drought-resistant seedlings of white spruce (Picea glauca [Moench] Voss.) and of seedlings of intermediate resistance that had been pretreated with paclobutrazol, a stress-protecting, synthetic plant-growth regulator. Paclobutrazol-treated seedlings showed marked increases in drought resistance, and pressure-volume analysis combined with Chardakov measurements confirmed observations that water stress was ameliorated during prolonged drought. Turgor was maintained in the paclobutrazol-treated and in the naturally resistant drought-stressed seedlings despite water contents near or below the turgor-loss volumes of well-watered controls. The maintenance of turgor in these seedlings was in large part a function of the dynamic process of cell wall adjustment, as reflected by marked reductions in both the saturated and turgor-loss volumes and by large increases in the elastic coefficients of the tissues. Shear and Young's moduli, calculated from pressure-volume curves and the radii and wall thicknesses of mesophyll cells, also confirmed observed changes in the elastic qualities of the cell walls. Elastic coefficients of well-watered, paclobutrazol-treated seedlings were consistently larger than those in well-watered controls and several times larger than the values in untreated plants, which succumbed rapidly to drought. In contrast, untreated seedlings that withstood prolonged drought without wilting displayed elastic coefficients similar to those in seedlings that had been treated with paclobutrazol but that had not been exposed to drought.