木本植物应对干旱胁迫的响应机制：基于水力学性状视角

干旱最显著的影响表现在区域尺度的森林死亡事件中,可以在短时间内杀死数百万棵树木。鉴于未来极端干旱事件的频率和强度可能随温度的升高而增加,迫切需要明确树木对干旱胁迫的响应对策以及衰退死亡机理,揭示木本植物在干旱环境中存活和死亡的生理机制,了解树木在未来气候下的适应机制,提高预测树木对干旱反应的准确性。在常用植物功能性状的基础上,重点纳入与植物水分运输能力及耐旱性相关的水力学性状,系统总结了:1)植物木质部水分运输的物理机制;2)植物应对干旱胁迫的水力响应过程:3)干旱胁迫下木本植物水分利用对策;以及4)干旱胁迫下木本植物衰退/死亡机理。最后,提出3个尚待解决的主要问题:1)加强纳入水力性状阐明植物对干旱胁迫的响应和调节机制;2)加强从全株植物的角度考虑植物不同组织性状间的关系;3)深入探究树木干旱致死机理。     

木本植物木质部的冻融栓塞应对研究进展

冻融栓塞在中高纬度地区木本植物中普遍存在。抗冻融栓塞能力对在寒冷环境中木本植物的生长和安全越冬十分关键, 这直接决定植物分布范围。冻融栓塞是由于冰中气体溶解度低, 木质部水分在低温下冷冻, 使之前水中溶解的气体逸出到导管中, 随后木质部中的冰融化又使气泡扩张而引发的栓塞现象。木质部解剖结构的差异会影响植物的抗冻融栓塞能力, 植物还可以通过调节木质部正压、代谢耗能等方式主动修复冻融栓塞, 也可通过增加树液溶质含量等逃避冷冻, 以减少低温损伤。然而, 与干旱栓塞相比, 目前对木质部冻融栓塞的形成以及植物响应和调节机制的理解不足。为此, 该文首先综述了木质部冻融栓塞的形成机制和植物的逃避、忍耐、修复等3种冻融栓塞的应对策略, 然后总结了木质部抗低温胁迫能力的生理表现、影响因子和评价指标, 并在此基础上讨论了低温抗性、干旱抗性和水力效率之间的多元权衡关系, 最后提出今后该领域中的5个优先研究问题: (1)不同植物冰冻的最低温度阈值; (2)是否存在应对低温胁迫的水力脆弱性分割机制; (3)冻融栓塞修复与代谢消耗的关系; (4)低温抗性、干旱抗性和水力效率之间的权衡关系; (5)抗冻融栓塞性状是否能够纳入经济性状谱系。     

木本植物木质部的冻融栓塞应对研究进展

冻融栓塞在中高纬度地区木本植物中普遍存在。抗冻融栓塞能力对在寒冷环境中木本植物的生长和安全越冬十分关键,这直接决定植物分布范围。冻融栓塞是由于冰中气体溶解度低,木质部水分在低温下冷冻,使之前水中溶解的气体逸出到导管中,随后木质部中的冰融化又使气泡扩张而引发的栓塞现象。木质部解剖结构的差异会影响植物的抗冻融栓塞能力,植物还可以通过调节木质部正压、代谢耗能等方式主动修复冻融栓塞,也可通过增加树液溶质含量等逃避冷冻,以减少低温损伤。然而,与干旱栓塞相比,目前对木质部冻融栓塞的形成以及植物响应和调节机制的理解不足。为此,该文首先综述了木质部冻融栓塞的形成机制和植物的逃避、忍耐、修复等3种冻融栓塞的应对策略,然后总结了木质部抗低温胁迫能力的生理表现、影响因子和评价指标,并在此基础上讨论了低温抗性、干旱抗性和水力效率之间的多元权衡关系,最后提出今后该领域中的5个优先研究问题:(1)不同植物冰冻的最低温度阈值;(2)是否存在应对低温胁迫的水力脆弱性分割机制;(3)冻融栓塞修复与代谢消耗的关系;(4)低温抗性、干旱抗性和水力效率之间的权衡关系;(5)抗冻融栓塞性状是否能够纳入经济性状谱系。     

南亚热带10种造林树种的水力结构和水力安全

南亚热带地区人工纯林面积大, 但是结构简单, 对气候变化响应敏感。在区域气候干旱化的背景下, 造林树种的生理生态策略及其对季节性干旱的响应亟待研究。该研究选择南亚热带地区10种造林树种(包括6种乡土种和4种外来种), 测定这些树种的平均生长速率、水力学性状以及经济学性状, 分析性状与生长速率之间的相关关系, 并比较水力安全边际和气孔安全边际的种间差异。结果发现: (1)造林树种的生长速率与木质部导水率显著正相关, 但与木材密度、比叶面积以及水力安全性指标无显著相关性。(2)造林树种的水分传导效率性和安全性之间没有权衡关系, 外来树种Acacia crassicarpa和Eucalyptus grandis × urophylla同时具有较高的木质部导水率和较强的抗栓塞能力。(3)造林树种的水力安全边际和气孔安全边际的种间差异显著, 大叶相思(Acacia auriculiformis)、红锥(Castanopsis hystrix)、壳菜果(Mytilaria laosensis)和阴香(Cinnamomum burmannii)在干季发生水力失败的风险较高。建议南亚热带人工林的生态监测指标体系中应包括树木水力学性状, 进而为人工林的可持续经营管理提供重要参考。     

植物木质部水力学研究进展

植物为适应陆地环境进化出木质部维管系统,通过水力学机制高效安全的向光合器官长距离运输水分,木质部水分运输对蒸腾、气孔运动、光合碳同化等生理过程有调控和协调作用,被称为植物生理学的支柱。植物水力学作为木质部水分运输的研究内容和手段,已成为整合植物与生态系统功能的中心枢纽。该文首先概述了植物水分运输的水力学机制、运输系统的局限性,以及木质部结构与功能之间的关系;其次,阐述了木质部栓塞的形成机制并详细介绍了栓塞的诱导方法和测试技术,分析了水分运输系统安全与效率之间的权衡关系,总结了植物对环境的响应和干旱致死的预测模型,讨论了测试技术问题及其引发的当前木质部逆压力修复和指数型木质部栓塞脆弱性曲线有效性的争议;最后,总结了目前植物木质部水力学研究的成果,提出了尚待解决的主要问题,探讨了研究机会与方向。     

植物气孔气态失水与SPAC系统液态供水的相互调节作用研究进展

讨论了植物气孔气态失水与SPAC系统液态供水相互作用研究领域的一些重要现象和行为.当植物水力信号和化学信号共同作用促进气孔对叶水势的调节时,植物对叶水势的调节表现为等水行为.气孔对环境湿度变化响应的反馈机制可用来解释土壤干旱条件下气孔和光合的午休现象,以及气孔导度和水流导度之间的相关关系;而气孔对环境湿度变化响应的前馈机制,则可用来解释气孔导度对大气 叶片间水汽饱和差的滞后反应.植物最大限度地利用木质部传输水分的策略,要求气孔快速响应以避免木质部过度气穴化和短时间内将气穴逆转的相应机制.     

丛枝菌根真菌对杨树生长、气孔和木质部微观结构的影响

植物气孔与木质部导管及纤维的功能直接关系着植物的水分利用, 进而影响植物的生长。为研究丛枝菌根真菌(AMF)对杨树抗旱性的影响, 采用温室盆栽的方法, 研究两种水分条件下, 接种根内球囊霉(Rhizophagus irregularis)对速生杨107 Populus × canadensis (P. nigra × P. deltoides) ‘Neva’气孔及木质部微观结构的影响。结果表明: AMF的侵染显著提高了杨树幼苗地上和地下部分生物量, 对叶片气孔长度、茎部导管细胞直径和纤维细胞长度也有促进作用。AMF对生物量和导管细胞直径的增加幅度表现出干旱条件下>正常水分条件下, 而对气孔长度的提高幅度表现出干旱条件下<正常水分条件下。正常水分条件下, AMF增加了杨树叶片的气孔密度, 减小了纤维细胞直径, 对相对水分饱和亏缺无影响; 干旱条件下, AMF增加了纤维细胞直径, 降低了相对水分饱和亏缺, 对气孔密度无影响。综上所述, 干旱条件下, AMF对导管水分传输能力的促进作用明显增加, 而对气孔蒸腾能力的促进作用有所减少, 从而更利于杨树在遭遇干旱时保持水分, 减少干旱对菌根杨树造成的水分亏缺, 提高菌根杨树对干旱的耐受性。     

异源过表达齿肋赤藓ScABI3基因改变拟南芥气孔表型并提高抗旱性

ABI3是ABA信号通路中关键的转录调控因子, 参与种子休眠、质体发育及苔藓耐干等重要生理过程, 在植物抗逆中发挥关键作用。以荒漠耐干苔藓——齿肋赤藓(Syntrichia caninervis)为材料, 克隆了抗逆基因ScABI3并获得3个独立的pCAMBIA1301-ScABI3转基因拟南芥(Arabidopsis thaliana)纯合株系。结果表明, 转基因拟南芥叶片气孔孔径增大, 单位面积气孔数量减少, 植株水分利用效率提高; 在干旱处理14天后转基因拟南芥植株存活率显著高于野生型, 离体叶片失水率显著低于野生型。进一步研究发现, ScABI3转基因拟南芥通过提高自身活性氧(ROS)清除能力增强植株抗旱性。研究结果可为开发利用荒漠植物基因资源培育抗逆作物品种奠定基础。     

施水和钾素添加对元宝枫和女贞木质部栓塞的影响

通过干旱胁迫处理(土壤含水量为田间持水量的30%~40%)和正常水分供给(土壤含水量为田间持水量的70%~80%)两种盆栽控制试验, 研究了耐旱树种元宝枫(Acer truncatum)和中生树种女贞(Ligustrum lucidum)枝条木质部栓塞(以导水率损失百分数(Percentage loss of hydraulic conductivity, PLC)衡量)和水势(Water potential, WP)对钾(K)素添加的响应。结果表明: 两个树种PLC的日变化均呈现先升后降的规律, 而WP呈现相反的变化趋势; 添加K素显著提高了两个树种的PLC, 除了干旱胁迫下的元宝枫, 其他树种和处理的PLC随K素含量的增加而增加; K素显著降低了干旱胁迫下元宝枫和女贞的WP,但对正常水分供给下两个树种的WP无显著影响; 元宝枫的平均PLC高于女贞, 平均WP低于女贞。以上结果表明, K素可能通过降低枝条的WP, 促进树种木质部栓塞的形成, 达到限制水分进一步丧失的目的。K素在调节气孔运动、渗透作用等方面的作用, 可能是树种木质部栓塞形成的诱 因。     

干旱胁迫植物个体生理响应及其生态模型预测研究进展

全球气候突变导致干旱事件频发,进而易引发严重的植物衰退甚至死亡,聚焦植物尤其是树木死亡的生理学机制并期望基于此评估及预测气候变化导致植物死亡风险已成为热点话题。植物通过调整内在生理代谢过程,例如通过调节渗透物质的含量,来平衡渗透势、维持细胞膨压、调节植物激素的信号水平,诱导植物气孔开放程度降低,有利于植物保存水分、调控植物水通道蛋白的表达,进而保持体内水分稳定并对干旱胁迫做出快速响应。这些生理过程中的每一环调节都为了确保水分运输的效率和安全性,增加植物抗旱性以及生态系统稳定性。植物的抗旱性不仅体现在生理代谢方面的调节,还表现在植物水力特性与解剖结构间相辅相成。当植物改变水力特性的同时,其茎叶会在解剖结构上做出调整以满足植物在干旱环境下水分供需平衡,从而降低植物蒸腾水分散失、增强细胞储水并提高生存能力。植物应对水分胁迫的策略通常与水分消耗和碳获取之间的平衡有关,明晰植物水分消耗与光合碳获取间存在平衡关系的性状特征便于更好地理解植物的水分利用策略。然而,植物表现出的任意单一性状特征的强弱都无法代表整个植物适应逆境的优劣,未来只有通过将植物更多性状特征进行相互关联,以具有代表植物水力功能、结...     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Interactive effects of water stress and xylem-limited bacterial infection on the water relations of a host vine

Xylella fastidiosa, a xylem-limited bacterial pathogen that causes bacterial leaf scorch in its hosts, has a diverse and extensive host range among plant species worldwide. Previous work has shown that water stress enhances leaf scorch symptom severity and progression along the stem in Parthenocissus quinquefolia infected by X. fastidiosa. The objective here was to investigate the mechanisms underlying the interaction of water stress and infection by X. fastidiosa. Using the eastern deciduous forest vine, P. quinquefolia, infection and water availability were manipulated while measuring leaf water potentials (psi(L)), stomatal conductance (g(s)), whole shoot hydraulic conductance (K(h)), per cent xylem embolism, and xylem vessel dimensions. No significant differences in any of the physiological measurements were found between control and infected plants prior to drought. Drought treatment significantly reduced psi(L) and g(s) at all leaf positions throughout the day in late summer in both years of the study. In addition, infection significantly reduced psi(L) and g(s) in the most basal leaf positions in late summer in both years. Whole shoot hydraulic conductance was reduced by both low water and infection treatments. However, per cent embolized vessels and mean vessel diameter were affected by drought treatment only. These results imply that the major effect of infection by X. fastidiosa occurs due to reduced hydraulic conductance caused by clogging of the vessels, and not increased cavitation and embolism of xylem elements. The reduced K(h) caused by X. fastidiosa infection acts additively with the water limitation imposed by Drought stress.     

九种不同材性的温带树种叶水力性状及其权衡关系

不同材性树种的解剖、叶脉分布等结构性状差异会影响树木的水分运输效率和水分利用策略, 进而限制树木的生存、生长和分布。然而, 材性对叶导水率、水力脆弱性及其潜在的权衡关系的影响尚不清楚。该研究选择东北温带森林中不同材性的9种树种(散孔材: 山杨(Populus davidiana)、紫椴(Tilia amurensis)、白桦(Betula platyphylla); 环孔材: 蒙古栎(Quercus mongolica)、水曲柳(Fraxinus mandshurica)、胡桃楸(Juglans mandshurica); 无孔材: 红皮云杉(Picea koraiensis)、樟子松(Pinus sylvestris var. mongolica)、红松(Pinus koraiensis), 测量其基于叶面积和叶质量的叶导水率(K_area和K_mass)、水力脆弱性(P₅₀)、膨压丧失点水势(TLP)及叶结构性状, 以比较不同材性树种叶水力性状的差异, 并探索叶水力效率与安全的权衡关系。结果表明: 3种材性树种的K_area、K_mass和P₅₀均差异显著(p < 0.05)。无孔材树种的K_area和K_mass最低, 而散孔材和环孔材树种差异不显著; 环孔材树种P₅₀最高, 而散孔材和无孔材树种差异不显著。K_area和K_mass均与P₅₀显著负相关(p < 0.05), 但散孔材、环孔材和无孔材树种的相关关系分别呈线性、幂函数和指数函数关系。这表明叶水力效率与安全之间存在一定的权衡关系, 但该关系受树木材性的影响。K_mass与TLP显著负相关(p < 0.01), 其中散孔材和环孔材树种呈线性负相关, 无孔材树种呈负指数函数关系; P₅₀随TLP的增加而增加, 这表明树木在面临水分胁迫时, 其质外体和共质体抗旱阻力共同协调保护叶片活细胞, 防止其水分状况到达临界阈值。K_mass与叶干物质含量、叶密度、比叶重均显著负相关, 而P₅₀与之显著正相关(p < 0.01, P₅₀与比叶重的关系除外), 表明树木叶水力特性的变化受相同叶结构特性驱动, 树木增加对水力失调的容忍需要在叶水力系统构建上增加碳投资。     

Leaf hydraulic traits and their trade-offs for nine Chinese temperate tree species with different wood properties

Aims Trees with different wood properties display variations in xylem anatomy and leaf vein structure, which may influence tree water transport efficiency and water-use strategy, and consequently constrain tree survival, growth and distribution. However, the effects of wood properties on leaf hydraulic conductance and vulnerability and their potential trade-offs at leaf level are not well understood. Our aims were to examine variations in leaf hydraulic traits of trees with different wood properties and explore potential trade-offs between leaf hydraulic efficiency and safety.
Methods Nine tree species with different wood properties were selected for measuring the leaf hydraulic traits, including three diffuse-porous species (Populus davidiana, Tilia amurensis, Betula platyphylla), three ring-porous species (Quercus mongolica, Fraxinus mandshurica, Juglans mandshurica), and three non-porous species (Picea koraiensis, Pinus sylvestris var. mongolica, Pinus koraiensis). Four dominant and healthy trees per species were randomly selected. The hydraulic traits measured included leaf hydraulic conductance on leaf area (K_area) and dry mass (K_mass) basis, leaf hydraulic vulnerability (P₅₀), and leaf water potential at turgor loss point (TLP), while the leaf structural traits were leaf dry mass content (LDMC), leaf density (LD) and leaf mass per unit area (LMA).
Important findings The K_area, K_mass, and P₅₀ differed significantly among the tree species with different woody properties (p < 0.05). Both K_area and K_mass were the lowest for the non-porous trees, and did not differ significantly between the diffuse-porous and ring-porous trees. The ring-porous trees had the highest P₅₀ values, while the diffuse-porous and non-porous trees showed no significant differences in P₅₀. Both K_area and K_mass were negatively correlated with P₅₀ (p < 0.05) for all the trees, and the relationships for the diffuse-porous, ring-porous, and non-porous trees were fitted into linear, power, exponential functions, respectively. This indicates that significant trade-offs exist between leaf hydraulic efficiency and safety. The K_mass was correlated (p < 0.01) with TLP in a negative linear function for the diffuse- and ring-porous trees and in a negative exponential function for the non-porous trees. The P₅₀ increased with increasing TLP. These results suggest that apoplastic and symplastic drought resistance are strictly coordinated in order to protect living cells from approaching their critical water status under water stresses. The K_mass was negatively correlated (p < 0.01) with LDMC, LD, or LMA, while the P₅₀ was positively correlated with LDMC and LD; this suggests that variations in K_mass and P₅₀ are driven by similar changes in structural traits regardless of wood traits. We conclude that the tree tolerance to hydraulic dysfunction increases with increasing carbon investment in the leaf hydraulic system.     

Drought stress limits the geographic ranges of two tree species via different physiological mechanisms

Range shifts are among the most ubiquitous ecological responses to anthropogenic climate change and have large consequences for ecosystems. Unfortunately, the ecophysiological forces that constrain range boundaries are poorly understood, making it difficult to mechanistically project range shifts. To explore the physiological mechanisms by which drought stress controls dry range boundaries in trees, we quantified elevational variation in drought tolerance and in drought avoidance‐related functional traits of a widespread gymnosperm (ponderosa pine – Pinus ponderosa) and angiosperm (trembling aspen – Populus tremuloides) tree species in the southwestern USA. Specifically, we quantified tree‐to‐tree variation in growth, water stress (predawn and midday xylem tension), drought avoidance traits (branch conductivity, leaf/needle size, tree height, leaf area‐to‐sapwood area ratio), and drought tolerance traits (xylem resistance to embolism, hydraulic safety margin, wood density) at the range margins and range center of each species. Although water stress increased and growth declined strongly at lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth at its range boundary. By contrast, ponderosa pine showed little elevational variation in drought‐related traits but avoided drought stress at low elevations by limiting transpiration through stomatal closure, such that its dry range boundary is associated with limited carbon assimilation even in average climatic conditions. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms – a result that has important implications for process‐based modeling approaches to tree biogeography. Further, we show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range‐limit context, helps elucidate a mechanistic understanding of range constraints.     

Cavitation resistance of peduncle,petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P₅₀) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P₅₀ was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.     

三种锦鸡儿属植物水力结构特征及其干旱适应策略

水分胁迫是干旱半干旱区限制植物生长的主要因素。以干旱半干旱区的3种锦鸡儿属植物为研究对象,从生态适应策略角度来分析3种锦鸡儿植物产生生态分离的原因。对三种锦鸡儿属植物茎干叶片的显微结构、生理功能(导水率、光合速率以及水分利用效率)进行测定,并统计了3种锦鸡儿植株的形态特征,如一、二级枝的直径、长度、末端叶面积。结果表明:三种锦鸡儿属植物都能形成较小的导管直径来适应旱生环境,但是在导水结构上又表现出一定的差异性。中间锦鸡儿的导管直径最小,次脉密度和最大净光合速率最大;柠条锦鸡儿的导管直径、叶片厚度和比叶重(LMA)最大。小叶锦鸡儿在导水率下降50%时的水势(P_(50))最大,水分胁迫时极易发生栓塞,但正是由于导管的栓塞降低了水分运输效率,使其在旱生环境中能够通过减少水分的供应来降低水分的丧失,从而保证自身生长的水分需求;而中间锦鸡儿则主要通过减小导管直径来适应旱生环境;柠条锦鸡儿的水分利用效率最高,抗栓塞能力最强,抗旱性最好,同时柠条锦鸡儿可以通过减少蒸腾面积来减少水分的丧失。植物的导管直径大小、叶片厚度、LMA、叶脉密度对植物导水速率、光合速率等生理功能都有一定的影响。     

叶片结构的水力学特性对植物生理功能影响的研究进展

叶片是植物进行光合、呼吸、蒸腾作用的主要器官, 早期的研究主要集中于水分在叶片中的运输路径, 而对叶脉结构及其生态学意义研究甚少。近年来关于叶片叶脉结构、气孔结构的功能及叶片水力学特性的意义研究已经成为植物生理生态的研究热点。该文综述了叶脉的结构性状的指标(叶脉密度、直径、间距等), 叶片水力学结构特性对植物生长、水分运输、气体交换、光合作用等生理功能的影响, 及其与植物对干旱适应性之间的关系。叶脉结构是决定叶片生理功能的基础, 因此在未来的工作中应分析比较不同种类植物叶脉结构形态与导水、光合、呼吸、同化作用之间的关系, 建立植物茎干-枝-叶系统水力传导的机理性模型, 用以探索不同植物功能结构和高效用水生理生态学机制, 据此评估不同种类植物在未来气候情景下的地位。