避雨环境下苹果幼树水分状态指标对干旱胁迫的响应

在避雨环境下进行土壤水势渐进式下降处理,研究了苹果树体水分状态指标对土壤干旱胁迫响应的敏感性,分析了不同水分状态指标与树体水分平衡之间的关系.结果表明: 树干直径日较差(MDS)及中午树干水势(Ψ_stem)对干旱胁迫最敏感.MDS对参考蒸散(ET₀)有明显的响应,且对干旱胁迫比较敏感,与ET₀呈显著正相关,相对树干直径日较差(MDS_r)与相对土壤水势(Ψ_{r soil})呈显著负相关,树干直径可实现连续性测量及自动化记录.Ψ_stem对土壤干旱胁迫较敏感,且与ET₀呈显著负相关,相对中午树干水势(Ψ_{r stem})与Ψ_{r soil}呈显著相关,目前叶水势和树干水势难以实现自动化连续性观测.其他树体水分状态指标,如黎明前叶水势(Ψ_pd)、树干直径日生长量(DG)和气孔导度(g_s)等对中度或重度干旱胁迫也有不同程度的响应,但总体上对土壤水势变化的响应不敏感.     

不同光强对两种桤木幼苗光合特性和抗氧化系统的影响

通过搭建荫棚设置3种不同的光强, 模拟森林幼苗生长的旷地(砍伐迹地)、林窗和林下光照环境(分别为100%、56.2%和12.5%的全光照), 比较研究了外来种台湾桤木(Alnus formosana)和乡土种桤木(A. cremastogyne)幼苗的叶形态、光合能力、热耗散和抗氧化酶的活性, 探讨了两树种幼苗对光强的适应及光保护策略。结果表明: 在3种光强下, 一定光强范围内随着光强的增加, 两种桤木幼苗的比叶重(LMA)、类胡萝卜素(Cars)、类胡萝卜素/叶绿素(Cars/Chl)和抗氧化酶(超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX))活性升高, 最大净光合速率(P_max)、光饱和点(LSP)、光补偿点(LCP)和非光化学猝灭系数(NPQ)具有升高的趋势; Chl含量和瞬时光能利用效率(LUE)降低; 净光合速率(P_n)、气孔导度(G_s)、气孔限制值(L_s)升高, 胞间CO₂浓度(C_i)降低, 推测P_n降低的主要因素是非气孔限制, 表明两种桤木幼苗均能适应不同的生长光强。生长在相同光强下, 桤木幼苗光抑制现象比台湾桤木幼苗严重, 台湾桤木幼苗对光强适应能力较强。随着光照强度的增加, 台湾桤木幼苗NPQ增加不显著, 热耗散较少, 相同光强下P_max和抗氧化酶活性显著高于桤木幼苗, 而桤木幼苗随着光强的增加热耗散显著, 表明在光抑制时, 台湾桤木幼苗主要是通过提高P_max利用光能和抗氧化酶系统进行保护性调节, 桤木幼苗则通过天线系统非辐射耗散将过剩的光能以热能的形式消耗掉。     

环境因子对常绿阔叶树种脱耦联系数及冠层气孔导度估算的影响

精确模拟冠层气孔导度(G_S)对于评估区域蒸散具有重要意义。该研究选择两种常见的人工阔叶树种尾叶桉(Eucalyptus urophylla, 外来种)和木荷(Schima superba, 本地种)作为研究对象, 利用K?stner法和修订的Penman-Monteith公式计算冠层平均气孔导度(分别定义为G_S1和G_S2)。研究还分析了环境因子对冠层脱耦联系数(Ω)的影响, 并用其来评价两种方法模拟的冠层气孔导度的合理性。结果表明, 两个树种冠层气孔导度均与气象条件耦合较好(尾叶桉: Ω = 0.10 ± 0.03, 木荷: Ω = 0.17 ± 0.03)。主成分分析显示, 光合有效辐射(PAR)以及水汽压亏缺(D)显著影响Ω的大小, 而风速(u)的影响较小。单因素分析则发现各环境因子与Ω之间的相关性并不显著。边界线分析表明D和PAR的增加使得Ω最终趋向于一个与树种有关的稳定值(木荷≈ 0.20, 尾叶桉≈ 0.05), 而Ω随u的增加呈幂指数下降。与木荷相比, 尾叶桉具有更高的气孔导度(尾叶桉和木荷的G_S2年平均值分别为(33.42 ± 9.37) mmol·m ^-2·s ^-1和(23.40 ± 2.03) mmol·m ^-2·s ^-1), 并且尾叶桉和木荷的G_S1与G_S2的线性拟合斜率分别为0.92 (R ² ≈ 0.70)和0.98 (R ² ≈ 0.76) , 表明G_S1比G_S2高估了冠层气孔导度。另外, G_S1和G_S2对水汽压亏缺的敏感性与参比气孔导度(G_Siref, D = 1 kPa时的气孔导度)的比值Pi与Ω紧密相关。根据统计, 尾叶桉和木荷的G_S1估计值在Ω = 0.05-0.15 (83.1%的数据)和0.10-0.20 (47.8%的数据)之间时是相对可靠的。     

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。     

控水条件下侧柏冠层气孔导度对土壤水的响应

建立了不同控水条件下(无降水、一半降水、自然降水和二倍降水)的侧柏样地,于2016年8月—2017年8月监测了样地土壤含水量(SWC)、降水量、液流密度(J_s)、叶面积指数(LAI)和水汽压亏缺(VPD)等因子,分析SWC对侧柏冠层气孔导度(g_s)的影响。结果表明: 一半、自然和二倍降水样地的SWC与降水量呈正相关,SWC变化范围分别为4.9%～16.0%、7.2%～22.9%、7.4%～29.6%,无降水样地的SWC在8—10月下降50%;7月的日g_s在14:00达到峰值(166.64 mmol·m^-2·s^-1),显著高于其他月份,且出现双峰现象, 1月的日g_s在12:00达到峰值(54.1 mmol·m^-2·s^-1);3个降水条件下,侧柏g_s与SWC呈负二次相关关系,且g_s达到峰值,对应的SWC分别为8.5%、12.5%和18.5%,均趋近于年平均SWC。不同控水样地内侧柏g_s对VPD的敏感性(δ)/参比冠层气孔导度(g_sref)均≥0.6,表明不同控水条件下土壤水分状况较适合侧柏蒸腾用水的需求。当SWC在3.7%～7.5%时,δ和g_sref值迅速增大,说明气孔调节能力更好,植物气孔对VPD的响应更敏感;当SWC上升到11%时,SWC变化对g_sref和g_s对VPD响应敏感性的影响不显著。可能存在侧柏产生适应状态的SWC阈值,植物体在自身的生命活动中关闭或减小气孔开度,降低叶片水势以适应过高的VPD,保护植物不会引起过度蒸腾,从而对蒸腾的调控更加有效。     

Impact of environmental factors on the decoupling coefficient and the estimation of canopy stomatal conductance for ever-green broad-leaved tree species

精确模拟冠层气孔导度(G_S)对于评估区域蒸散具有重要意义。该研究选择两种常见的人工阔叶树种尾叶桉(Eucalyptus urophylla, 外来种)和木荷(Schima superba, 本地种)作为研究对象, 利用K?stner法和修订的Penman-Monteith公式计算冠层平均气孔导度(分别定义为G_S1和G_S2)。研究还分析了环境因子对冠层脱耦联系数(Ω)的影响, 并用其来评价两种方法模拟的冠层气孔导度的合理性。结果表明, 两个树种冠层气孔导度均与气象条件耦合较好(尾叶桉: Ω = 0.10 ± 0.03, 木荷: Ω = 0.17 ± 0.03)。主成分分析显示, 光合有效辐射(PAR)以及水汽压亏缺(D)显著影响Ω的大小, 而风速(u)的影响较小。单因素分析则发现各环境因子与Ω之间的相关性并不显著。边界线分析表明D和PAR的增加使得Ω最终趋向于一个与树种有关的稳定值(木荷≈ 0.20, 尾叶桉≈ 0.05), 而Ω随u的增加呈幂指数下降。与木荷相比, 尾叶桉具有更高的气孔导度(尾叶桉和木荷的G_S2年平均值分别为(33.42 ± 9.37) mmol·m ^-2·s ^-1和(23.40 ± 2.03) mmol·m ^-2·s ^-1), 并且尾叶桉和木荷的G_S1与G_S2的线性拟合斜率分别为0.92 (R ² ≈ 0.70)和0.98 (R ² ≈ 0.76) , 表明G_S1比G_S2高估了冠层气孔导度。另外, G_S1和G_S2对水汽压亏缺的敏感性与参比气孔导度(G_Siref, D = 1 kPa时的气孔导度)的比值Pi与Ω紧密相关。根据统计, 尾叶桉和木荷的G_S1估计值在Ω = 0.05-0.15 (83.1%的数据)和0.10-0.20 (47.8%的数据)之间时是相对可靠的。     

晋西黄土丘陵区若干树种水分生理及抗旱性量化研究

1996年林木生长季节(5~10月)野外条件下测定了12个树种的P-V曲线水分参数。研究了水分参数的季节变化及与林木抗旱性的关系。提出树种在水分胁迫条件下以水分参数Ψ_i^sat和Ψ_i^tlp所定义的渗透调节能力。并研究了树种的这种渗透调节能力的季节变化规律及与环境因素的关系。应用数量分析的方法对林木的抗旱性予以综合评价。     

地下水埋深对胡杨叶片光合作用及抗氧化物质积累的影响

以塔里木荒漠生态系统建群种—胡杨（Populus euphratica Oliv.）为试材,研究胡杨光合气体交换参数、抗氧化酶活性及渗透调节物质沿地下水埋深（GWD）梯度的变化规律,探讨胡杨适应干旱荒漠环境的生理生态机制。结果表明：（1）不同GWD条件下胡杨净光合速率（P_n）、蒸腾速率（T_r）、气孔导度（G_s）的日变化均呈单峰型,而胞间CO₂摩尔分数（C_i）日变化呈“V”型,P_n与G_s变化同步,峰值均出现在12：00,而T_r峰值滞后P_n、G_s 2 h。不同GWD间P_n峰值差异显著（P<0.05）,T_r、G_s峰值和C_i谷值在GWD为5.5 m极显著降低（P<0.01）。胡杨P_n、T_r、G_s、C_i、水分利用效率（WUE）和光能利用效率（LUE）均随GWD增加而降低,其中G_s、LUE日均值在GWD为5.5 m显著降低（P＜0.05）,但不同GWD条件下P_n、T_r、C_i、WUE日均值均无显著差异（P＞0.05）;（2）12：00—16：00胡杨P_n下降主要受气孔因素限制,16：00—20：00 P_n下降主要受非气孔因素限制;（3）通过对不同GWD条件下胡杨P_n、T_r与生理生态因子进行相关、偏相关、逐步回归分析发现,G_s是影响胡杨P_n、T_r的主要因子;不同GWD条件下胡杨调控P_n、T_r的因子不同,GWD增加使胡杨P_n与光合有效辐射（PAR）、T_r与G_s之间的相关性增强,表明GWD直接调控胡杨叶片水汽交换（G_s）过程;（4）胡杨叶片丙二醛（MDA）含量、超氧化物歧化酶（SOD）活性、过氧化物酶（POD）活性和游离脯氨酸（Pro）含量均随GWD增加而增大,而可溶性蛋白质（SP）含量、可溶性糖（SS）含量则降低,表明随GWD增加,胡杨叶片细胞膜透性和光合碳同化受抑增强,胡杨通过提高保护酶活性（POD、SOD）和渗透调节（Pro）能力协同抵御地下水位降低所带来的干旱胁迫,以维持基本正常的生理活动,这是胡杨适应荒漠区干旱生境的生理生态策略。     

干旱胁迫下亚精胺对玉米幼苗抗旱性影响的生理生化机制

为探究外源亚精胺(Spd)在增强玉米干旱胁迫耐受性中的作用,以‘先玉335’(干旱不敏感型)和‘丰禾1’(干旱敏感型)为试验材料,采用营养液水培法,研究了15%聚乙二醇(PEG-6000)模拟干旱胁迫下,外源Spd (0.1 mmol·L^-1)对玉米幼苗生长、光合特性、叶绿素含量、渗透调节物质、活性氧生成、膜质过氧化及根系活力的影响.结果表明:干旱胁迫下,外源Spd处理可显著促进干旱胁迫下玉米幼苗的生长;提高叶绿素含量、净光合速率(P_n)、气孔导度(g_s)、蒸腾速率(T_r)和水分利用效率(WUE),减缓‘丰禾1’叶片中胞间CO₂浓度(C_i)的升高,有效减轻干旱胁迫对玉米幼苗叶片光合作用的气孔限制和非气孔限制;增加脯氨酸和可溶性糖的含量;降低O₂^-·生成速率、H₂O₂和丙二醛(MDA)含量、细胞膜透性,有效缓解了胁迫对膜的伤害;增强幼苗根系活力;其中干旱敏感品种‘丰禾1’变化幅度大于耐旱品种‘先玉335’.表明在干旱胁迫下,外源Spd能促进玉米幼苗对光能的捕获与转换,增强光合作用,促进幼苗的生长,并能够通过减少玉米幼苗体内活性氧的产生,增加渗透调节物质的积累以稳定细胞膜系统,提高根系活力,从而增强幼苗对干旱逆境的适应性,且对干旱敏感品种‘丰禾1’的效果更显著.     

干旱胁迫对转PtPIP2;8基因84K杨苗木光合、生长和根系结构的影响

为研究水通道蛋白PtPIP2;8基因功能, 了解其不同表达水平的转基因84K杨(Populus alba × P. glandulossa)应对干旱胁迫的响应, 该文以转PtPIP2;8 84K杨抑制表达株系(抑制表达)、野生型(WT)和转PtPIP2;8 84K杨超表达株系(超表达)为试验材料, 测定PtPIP2;8表达水平、根系导度、光响应曲线、气体交换参数、生长及根系形态指标。结果显示: (1) WT植株PtPIP2;8仅在根系表达; 超表达植株PtPIP2;8除在根部显著表达外, 在茎和叶片中也显著表达; 抑制表达植株PtPIP2;8仅在根部有微量表达, 表达量分别是WT和超表达植株的1/20和1/80。(2)根系结构分析发现, 超表达植株总根长、总根表面积、总根体积、总根尖数显著低于WT和抑制表达植株, 根系导水率显著高于WT和抑制表达植株, 表明PtPIP2;8参与了植物根系水分运输, 提高了水分运输效率。(3)正常水分条件下, 抑制表达植株苗高、叶面积显著低于WT和超表达植株, 根冠比显著高于WT和超表达植株。干旱胁迫后, 抑制表达植株净光合速率(P_n)、气孔导度(G_s)下降幅度小, 仍能维持较高的P_n。气体交换参数显示抑制表达植株P_n、G_s日变化为“单峰”型, 属气孔因素引起的净光合速率下降;WT和超表达植株P_n、G_s日变化为“双峰”型, 干旱胁迫后, 抑制表达植株P_n略微下降, WT和超表达植株P_n均下降, 尤其是13:00、15:00下降显著, 表明WT和超表达植株对干旱胁迫更加敏感, 干旱对其影响更大。(4)干旱胁迫后, 抑制表达植株相对生长速率、总生物量降低的最少, 根冠比最高; 总根表面积、总根体积、总根尖数显著高于WT植株。表明PtPIP2;8直接参与水分运输并提高水分运输效率, 其转化影响了植株根系发育和生长。超表达植株根系发育的下降和叶面积的增大减弱了它的抗旱性, 而抑制表达植株矮小, 降低的叶面积, 增加的根系生长和根冠比提高了它的抗旱能力。从研究结果来看, 水通道蛋白提高了水分跨膜运输效率, 而非水通道蛋白导水机制对干旱有较强的耐受性。     

Does the turgor loss point characterize drought response in dryland plants?

The water potential at turgor loss point (Ψ_tlp) has been suggested as a key functional trait for determining plant drought tolerance, because of its close relationship with stomatal closure. Ψ_tlp may indicate drought tolerance as plants, which maintain gas exchange at lower midday water potentials as soil water availability declines also have lower Ψ_tlp. We evaluated 17 species from seasonally dry habitats, representing a range of life‐forms, under well‐watered and drought conditions, to determine how Ψ_tlp relates to stomatal sensitivity (pre‐dawn water potential at stomatal closure: Ψg_s0) and drought strategy (degree of isohydry or anisohydry; ΔΨ_MD between well‐watered conditions and stomatal closure). Although Ψg_s0 was related to Ψ_tlp, Ψg_s0 was better related to drought strategy (ΔΨ_MD). Drought avoiders (isohydric) closed stomata at water potentials higher than their Ψ_tlp; whereas, drought tolerant (anisohydric) species maintained stomatal conductance at lower water potentials than their Ψ_tlp and were more dehydration tolerant. There was no significant relationship between Ψ_tlp and ΔΨ_MD. While Ψ_tlp has been related to biome water availability, we found that Ψ_tlp did not relate strongly to stomatal closure or drought strategy, for either drought avoiders or tolerators. We therefore suggest caution in using Ψ_tlp to predict vulnerability to drought.     

强旱生小灌木绵刺劈裂生长过程中的水分特征

绵刺(Potaninia mongolica)是西鄂尔多斯-东阿拉善地区特有的单种属残遗植物。选取内蒙古磴口县境内具有绵刺群落的草原化荒漠区为研究样区,于2002~2003年每年8月1~5日采集未劈裂、正在劈裂和已劈裂植株,运用PV技术对不同劈裂生长状态绵刺的多种水分关系参数(${φ_{s}}^{sat}$、${φ_{s}}^{tlp}$、ROWC^tlp、RWC^tlp、Δφ、ε^max等)进行了测定,从绵刺保持膨压的能力和途径两方面进行了深入探讨;同时结合同一项目研究中绵刺劈裂生长过程中抗氧化酶系统和内源激素方面的研究成果,综合分析并探讨了绵刺劈裂生长的发生机理及其环境适应性。结果表明:1)未劈裂绵刺主要通过增加细胞内溶质(如脯氨酸),减少细胞内的水分丧失来进行渗透调节,从而在干旱胁迫下能够维持正常的膨压。2)已劈裂绵刺通过渗透调节和高的组织弹性两条途径来共同保持膨压,以抵抗不良的生存环境;同时对环境水分胁迫具有较高的敏感性。3)3种状态绵刺保持膨压的能力由强到弱依次为:未劈裂绵刺、正在劈裂绵刺、已劈裂绵刺。4)劈裂的发生导致绵刺保持膨压能力的降低,同时耐旱方式和途径发生了变化。     

Global analysis of plasticity in turgor loss point,a key drought tolerance trait

Many species face increasing drought under climate change. Plasticity has been predicted to strongly influence species' drought responses, but broad patterns in plasticity have not been examined for key drought tolerance traits, including turgor loss or ‘wilting’ point (π_tlp). As soil dries, plants shift π_tlp by accumulating solutes (i.e. ‘osmotic adjustment’). We conducted the first global analysis of plasticity in Δπ_tlp and related traits for 283 wild and crop species in ecosystems worldwide. Δπ_tlp was widely prevalent but moderate (?0.44 MPa), accounting for 16% of post‐drought π_tlp. Thus, pre‐drought π_tlp was a considerably stronger predictor of post‐drought π_tlp across species of wild plants. For cultivars of certain crops Δπ_tlp accounted for major differences in post‐drought π_tlp. Climate was correlated with pre‐ and post‐drought π_tlp, but not Δπ_tlp. Thus, despite the wide prevalence of plasticity, π_tlp measured in one season can reliably characterise most species' constitutive drought tolerances and distributions relative to water supply.     

Most stomatal closure in woody species under moderate drought can be explained by stomatal responses to leaf turgor

Reduced stomatal conductance (g_s) during soil drought in angiosperms may result from effects of leaf turgor on stomata and/or factors that do not directly depend on leaf turgor, including root‐derived abscisic acid (ABA) signals. To quantify the roles of leaf turgor‐mediated and leaf turgor‐independent mechanisms in g_s decline during drought, we measured drought responses of g_s and water relations in three woody species (almond, grapevine and olive) under a range of conditions designed to generate independent variation in leaf and root turgor, including diurnal variation in evaporative demand and changes in plant hydraulic conductance and leaf osmotic pressure. We then applied these data to a process‐based g_s model and used a novel method to partition observed declines in g_s during drought into contributions from each parameter in the model. Soil drought reduced g_s by 63–84% across species, and the model reproduced these changes well (r² = 0.91, P < 0.0001, n = 44) despite having only a single fitted parameter. Our analysis concluded that responses mediated by leaf turgor could explain over 87% of the observed decline in g_s across species, adding to a growing body of evidence that challenges the root ABA‐centric model of stomatal responses to drought.     

Effects of flooding and drought on stomatal activity, transpiration, photosynthesis, water potential and water channel activity in strawberry stolons and leaves

Transpiration, xylem water potential and water channel activity were studied in developing stolons and leaves of strawberry (Fragaria × ananassa Duch.) subjected to drought or flooding, together with morphological studies of their stomata and other surface structures. Stolons had 0.12 stomata mm^–2 and a transpiration rate of 0.6 mmol H₂O m^–2 s^–1, while the leaves had 300 stomata mm^–2 and a transpiration rate of 5.6 mmol H₂O m^–2 s^–1. Midday water potentials of stolons were always less negative than in leaves enabling nutrient ion and water transport via or to the strawberry stolons. Drought stress, but not flooding, decreased stolon and leaf water potential from –0.7 to –1 MPa and from –1 to –2 MPa, respectively, with a concomitant reduction in stomatal conductance from 75 to 30 mmol H₂O m^–2 s^–1. However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from stolons of flooded strawberry plants showed no change in water channel activity. In these stolons, turgor may be preserved by maintaining root pressure, an electrochemical and ion gradient and xylem differentiation, assuming water channels remain open. By contrast, water channel activity was reduced in stolons of drought stressed strawberry plants. In every case, the effect of flooding on water relations of strawberry stolons and leaves was less pronounced than that of drought which cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. This stomatal closure under flooding may be due to release of stress ethylene. In the relative absence of stomata from the stolons, cellular (apoplastic) water transport in strawberry stolons was primarily driven by water channel activity with a gradient from the tip of the stolon to the base, concomitant with xylem differentiation and decreased water transport potential from the stolon tip to its base. Reduced water potential in the stolons under drought are discussed with respect to reduced putative water channel activity.     

Measurement of Leaf Hydraulic Conductance and Stomatal Conductance and Their Responses to Irradiance and Dehydration Using the Evaporative Flux Method (EFM)

Water is a key resource, and the plant water transport system sets limits on maximum growth and drought tolerance. When plants open their stomata to achieve a high stomatal conductance (g_s) to capture CO₂ for photosynthesis, water is lost by transpiration^1,2. Water evaporating from the airspaces is replaced from cell walls, in turn drawing water from the xylem of leaf veins, in turn drawing from xylem in the stems and roots. As water is pulled through the system, it experiences hydraulic resistance, creating tension throughout the system and a low leaf water potential (Ψ_leaf). The leaf itself is a critical bottleneck in the whole plant system, accounting for on average 30% of the plant hydraulic resistance³. Leaf hydraulic conductance (K_leaf = 1/ leaf hydraulic resistance) is the ratio of the water flow rate to the water potential gradient across the leaf, and summarizes the behavior of a complex system: water moves through the petiole and through several orders of veins, exits into the bundle sheath and passes through or around mesophyll cells before evaporating into the airspace and being transpired from the stomata. K_leaf is of strong interest as an important physiological trait to compare species, quantifying the effectiveness of the leaf structure and physiology for water transport, and a key variable to investigate for its relationship to variation in structure (e.g., in leaf venation architecture) and its impacts on photosynthetic gas exchange. Further, K_leaf responds strongly to the internal and external leaf environment³. K_leaf can increase dramatically with irradiance apparently due to changes in the expression and activation of aquaporins, the proteins involved in water transport through membranes⁴, and K_leaf declines strongly during drought, due to cavitation and/or collapse of xylem conduits, and/or loss of permeability in the extra-xylem tissues due to mesophyll and bundle sheath cell shrinkage or aquaporin deactivation^5-10. Because K_leaf can constrain g_s and photosynthetic rate across species in well watered conditions and during drought, and thus limit whole-plant performance they may possibly determine species distributions especially as droughts increase in frequency and severity^11-14.We present a simple method for simultaneous determination of K_leaf and g_s on excised leaves. A transpiring leaf is connected by its petiole to tubing running to a water source on a balance. The loss of water from the balance is recorded to calculate the flow rate through the leaf. When steady state transpiration (E, mmol • m^-2 • s^-1) is reached, g_s is determined by dividing by vapor pressure deficit, and K_leaf by dividing by the water potential driving force determined using a pressure chamber (K_leaf= E /- Δψ_leaf, MPa)¹⁵.This method can be used to assess K_leaf responses to different irradiances and the vulnerability of K_leaf to dehydration^14,16,17.     

Reversible decreases in the bulk elastic modulus of mature leaves of deciduous Quercus species subjected to two drought treatments

Changes in leaf water relations under water stress were examined. In experiment 1, water stress was imposed by withholding irrigation to potted seedlings of deciduous oak, Quercus crispula and Q. serrata. Changes in the pressure–volume (P–V) curve in mature leaves were followed. The leaf water potential at turgor loss (Ψ_l,tlp) significantly decreased after 13 d of drought treatment. The bulk elastic modulus (?) significantly decreased, which contributed to the maintenance of cell turgor together with the decrease in osmotic potential. In experiment 2, water stress was imposed by notching a branch of a Q. serrata tree. After the notching, the daily minimum leaf water potential (Ψ_l) decreased, and a significant decrease in Ψ_l,tlp was observed 15 d after notching. The osmotic potential at water saturation (Ψ_π,sat) did not decrease significantly until 25 d after notching whereas, ? had already decreased significantly within 15 d after notching and increased promptly after substantial precipitation. It was confirmed that ? of mature leaves decreased reversibly in water stress. This response of ? was more rapid than that of the osmotic potential and, thus, effectively maintained cell turgor when water stress was suddenly imposed on the leaves.     

Leaf water relations characteristics of differently potassium fertilized and watered field grown barley plants

Seasonal leaf water relations characteristics were studied in fully irrigated spring barley (Hordeum distichum L. cv. Gunnar) fertilized at low (50 kg K ha⁻¹) or high (200 kg K ha⁻¹) levels of potassium applied as KCl. The investigation was undertaken from about 14 days before anthesis until the milk ripe stage in leaves of different position and age. Additionally, the effects of severe water stress on leaf water relations were studied in the middle of the grain filling period in spring barley (cv. Alis). The leaf water relations characteristics were determined by the pressure volume (PV) technique. Water relations of fully irrigated plants were compared in leaf No 7 with the water relations of slowly droughted plants (cv. Alis). Leaf osmotic potential at full turgor (ψ _π ¹⁰⁰ ) decreased 0.1 to 0.3 MPa in droughted leaves indicating a limited osmotic adjustment due to solute accumulation. The leaf osmotic potential at zero turgor (ψ _π ⁰ ) was about −2.2 MPa in fully irrigated plants and −2.6 MPa in droughted plants. The relative water content at zero turgor (R⁰) decreased 0.1 unit in severely droughted leaves. The ratio of turgid leaf weight to dry weight (TW/DW) tended to be increased by drought. The tissue modulus of elasticity (ε) decreased in droughted plants and together with osmotic adjustment mediated turgor maintenance during drought. A similar response to drought was found in low and high K plants except that the R⁰ and ε values tended to be higher in the high K plants. Conclusively, during drought limited osmotic adjustment and increase in elasticity of the leaf tissue mediated turgor maintenance. These effects were only slightly modified by high potassium application. The seasonal analysis in fully irrigated plants (cv. Gunnar) showed that within about 14 days from leaf emergence ψ _π ¹⁰⁰ decreased from about −0.9 to −1.6 MPa in leaf No 7 (counting the first leaf to emerge as number one) and from about −1.1 to −1.9 MPa in leaf No 8 (the flag leaf) due to solute accumulation. A similar decrease took place in ψ _π ⁰ except that the level of ψ _π ⁰ was displaced to a lower level of about 0.2 to 0.3 MPa. Both ψ _π ¹⁰⁰ and ψ _π ⁰ tended to be 0.05 to 0.10 MPa lower in high K than in low K plants. R⁰ was about 0.8 to 0.9 and was independent of leaf position and age, but tended to be highest in high K plants. The TW/DW ratio decreased from about 5.5 in leaf No 6 to 4.5 in leaf No 7 and 3.8 in leaf No 8. The TW/DW ratio was 4 to 10% higher in high K than in low K plants indicating larger leaf cell size in the former. The apoplastic water content (V_a) at full turgor constituted about 15% in leaf No 7. ε was maximum at full turgor and varied from about 11 to 34 MPa. ε tended to be higher in high K plants. Conclusively, in fully watered plants an ontogenetically determined accumulation of solutes (probably organic as discussed) occurred in the leaves independent of K application. The main effect of high K application on water relations was an increase in leaf water content and a slight decrease in leaf ψ_π. The effect of K status on growth and drought resistance is discussed.     

Aquaporin regulation in roots controls plant hydraulic conductance,stomatal conductance,and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction in K_root‐r caused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease in K_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root‐r and g_s. Our results demonstrated that the reduction in K_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant‐l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.     

A comparison of pressure–volume curves with and without rehydration pretreatment in eight woody species of the semiarid Loess Plateau

Pressure–volume (P–V) curves are frequently used to analyze water relation properties of woody plants in response to transpiration-induced tissue water loss. In this study, P–V analyses were conducted on eight woody species growing in the semiarid Loess Plateau region of China during a relatively dry summer season using both the recently recommended instantaneous measurement and the traditional method with rehydration pretreatment. Generally, P–V-derived parameters in this study reflected conditions in a dry growth environment. Species-specific differences were also found among P–V parameters, suggesting each species uses different mechanisms to respond to drought. Based on the results from instantaneous measurements, a descending sequence for drought tolerance ranked by water potentials at the turgor loss point (Ψ^tlp) was Rosa hugonis > Syringa oblata = Armeniaca sibirica > Caragana microphylla > Pyrus betulaefolia > Acer stenolobum > Quercus liaotungensis > Robinia pseudoacacia. The first five species also showed lower levels of osmotic potential at full turgor (Ψ _π ^sat ) and higher symplastic osmotic solute content per dry weight, suggesting they possess advantages in osmotic adjustment. Also, this study supports previous reports noting rehydration pretreatment resulted in shifts in P–V parameters. The magnitude of the shifts varied with species and water conditions. The effect of rehydration was stronger for species with higher drought tolerance or subjected to the influence of drought. Differences in the parameters among species were mitigated as a result of rehydration. Those with a lower Ψ^tlp or midday water potential were more deeply affected by rehydration. Application of instantaneous measurements was strongly recommended for proper analysis of P–V curves particularly in arid and semiarid areas.