冬小麦叶片气孔导度模型水分响应函数的参数化

植物气孔导度模型的水分响应函数用来模拟水分胁迫对气孔导度的影响过程, 是模拟缺水环境下植物与大气间水、碳交换过程的关键算法。水分响应函数包括空气湿度响应函数和土壤湿度(或植物水势)响应函数, 该研究基于田间实验观测, 分析了冬小麦(Triticum aestivum)叶片气孔导度对不同空气饱和差和不同土壤体积含水量或叶水势的响应规律。一个土壤水分梯度的田间处理在中国科学院禹城综合试验站实施, 不同水分胁迫下的冬小麦叶片气体交换过程和气孔导度以及其他的温湿度数据被观测, 同时观测了土壤含水量和叶水势。实验数据表明, 冬小麦叶片气孔导度对空气饱和差的响应呈现双曲线规律, 变化趋势显示大约1 kPa空气饱和差是一个有用的阈值, 在小于1 kPa时, 冬小麦气孔导度对空气饱和差变化反应敏感, 而大于1 kPa后则反应缓慢; 分析土壤体积含水量与中午叶片气孔导度的关系发现, 中午叶片气孔导度随土壤含水量增加大致呈现线性增加趋势, 但在平均土壤体积含水量大于大约25%以后, 气孔导度不再明显增加, 而是维持在较高导度值上下波动; 冬小麦中午叶片水势与相应的气孔导度之间, 随着叶水势的增加, 气孔导度呈现增加趋势。根据冬小麦气孔导度对空气湿度、土壤湿度和叶水势的响应规律, 研究分别采用双曲线和幂指数形式拟合了水汽响应函数, 用三段线性方程拟合了土壤湿度响应函数和植物水势响应函数, 得到的参数可以为模型模拟冬小麦的各类水、热、碳交换过程采用。     

环境因素变化对冬小麦气孔阻力的影响

本文据同步观测到的气孔阻力和土壤水势、净辐射能、气温、叶温、水汽压、饱和差、风速等环境因素变化值,用统计方法分析了小麦叶片气孔阻力与环境因素的关系,结果表明：冬小麦叶片的气孔导性主要受土壤水势影响。叶片正反两面的气孔对环境因素变化的反应不同,正面气孔导性主要受土壤水势影响,而反面气孔导性则与气温、水汽压和饱和差关系较大。     

田间小麦叶片气孔对环境因子响应的模拟及叶片水分平衡的计算

经实测与数据分析得出田间供试小麦叶片气孔对光强、叶温、叶片水势分别作一定形式的响应,据此估算了冠层总气孔导度,进而建立了预测小麦群体蒸腾速率、冠层总气孔导度、叶温、叶片水势等的水分平衡模型。实验表明模型具有较好的预测性。供试小麦叶片一天中可溶性糖含量变化对叶片水势的影响不大。     

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.     

根源ABA参与气孔调节的数学模拟

建立了包括植物体内的水分传输,并有根源ABA参与的气孔调节模型,模拟了饱和水气压差(VPD)、气温、表层土壤含水量(θ_(s1))等环境因子对叶片水势、木质部汁液中ABA浓度([ABA]_x)及气孔导度的影响。结果显示,VPD和气温的变化能够改变叶片水势及气孔导度;[ABA]_x几乎不受VPD和气温变化的影响,却决定着叶片水势及气孔导度对VPD和气温变化的响应幅度;θ_(s1)影响[ABA]_x,并由此影响气孔导度,但相比之下对叶片水势的作用并不显著。     

树木储存水对水力限制的补偿研究进展

水力限制假说认为水分传输阻力与水势梯度协同调节气孔气体交换、影响CO₂的吸收进而限制树木的高生长,比较合理地解释了不同生境树木极限高度和高生长的差异.但该理论并未考虑到水力结构和其他生物学特征在树木向上生长的同时会进行适应性调整,以减弱逐渐增加的水分传输阻力.树木储存水对缓解木质部的水力限制、控制叶片水势的波动具有重要的生物学意义,也可能是气孔调节和水分状况变化格型的重要决定因素,其对水力限制可起到部分补偿作用.本文对储存水影响树木水分利用过程中的水力限制进行了综述,探讨了储存水补偿水力限制的可能性机理以及相对应的研究方法,并对未来的研究方向进行了展望.     

拔节期复水对玉米苗期受旱胁迫的补偿效应

拔节期恢复充分供水可使苗期受旱程度不同的玉米的株高和地上部干重恢复到或接近一直充分供水的水平;复水后叶片水势在短期内可以接近对照的水平,并在较长时间内保持较低的渗透调节能力,同时复水可降低叶片气孔阻力和蒸腾强度,提高叶片光合速率和水分利用效率,表现出一定的补偿效应。     

两种南亚热带林下灌木叶片水分特性的比较研究

鼎湖山南亚热带针叶—阔叶混交林下灌木桃金娘和三叉苦叶片气孔导度的日进程多为单峰形。一天中出现气孔导度高值的时间随季节而异,从3月到10月逐渐提前。夏季气孔导度最大,冬季最小。8月份生长于坡顶群落中的两种灌木叶片气孔出现“午睡”现象。不同生长地点中,桃金娘的气孔导度均明显高于三叉苦。叶片水势从早晨起逐渐降低,中午达最低值,随后有所回升。 3月和8月的水势较高,12月水势最低。水势与气孔导度间呈双曲线关系。压力—体积曲线的测定表明叶片水势低值时仍能维持正膨压。     

The decoupling between gas exchange and water potential of Cinnamomum camphora seedlings during drought recovery and its relation to ABA accumulation in leaves

气候变化将改变降雨格局,从而导致极端干旱事件增多。然而,树木如何协调生理和生化响应来应对干旱-恢复的机制仍不清楚。本 研究探讨了干旱-恢复过程中树木生理与生化特征的耦联关系。我们首先将香樟(Cinnamomum camphora)盆栽幼苗种植在水分充足的条件下,然后通过停止浇水以达到干旱处理的目的。当幼苗胁迫至轻度干旱(气孔关闭)和中度干旱(ψxylem = −1.5 MPa)时,分别对其进行复水处理。在干旱及复水4天过程中,我们测定了香樟叶片水势、气体交换、脱落酸以及非结构性碳水化合物的变化规律。我们发现干旱强度在很大程度上决定了香樟的生理与生化响应,并影响其干旱后恢复。轻度干旱导致气孔关闭,并引起叶片脱落酸累积和水势下降,而中度干旱可进一步引起叶片脯氨酸累积和非结构性碳水化合物的变化。干旱强度的增加会导致气体交换恢复滞后,但对水势的恢复无显著影响。另外,干旱与复水过程中水势与气体交换之间的关系存在较大差异: 即干旱过程中水势与气体交换存在很强的耦联关系,而这种耦联关系在复水过程中并不存在,其主要原因是由于叶片脱落酸累积,从而延缓了气孔导度的恢复。综上,本研究结果表明,脱落酸可能是导致香樟旱后气孔导度恢复滞后的主要影响因素。此外,干旱强度对树木生理和生化的恢复具有显著影响。     

植物应对干旱胁迫的气孔调节

气孔是植物控制叶片与大气之间碳、水交换的重要门户,植物的生长和生存都依赖于叶片气孔对碳获取和水散失的调控.因此,气孔调节机理研究与气孔导度模型研发是精确模拟陆地生态系统碳、水循环过程不可或缺的内容.近年来,随着气候变化的加剧,干旱事件愈发频繁,对植物的存活、生长和分布产生深刻影响.为了深入理解植物碳-水耦合机理过程、预测全球变化下植物及群落的动态,开展植物应对干旱胁迫的气孔调节研究尤为重要.本文综述了植物在干旱胁迫条件下气孔调节机制和模型研究进展.首先阐述了植物气孔对干旱胁迫的主动调节与被动调节,讨论了气孔调节的演化过程,包括蕨类和石松类植物的被动水力调节、被子植物的主动调节和裸子植物的双重调节机制,认为裸子植物的气孔调节方式是植物进化过程中介于蕨类、石松类植物和被子植物之间的一种重要过渡类型.然后分析了气孔调节与水力调节的关系,讨论了“植物水势和气孔导度解耦”问题中存在的争议.之后介绍了基于水分利用效率假说和最大碳增益假说所建立的气孔导度优化模型的应用,并指出后者有更强的预测能力和应用前景.最后,为了有效减少植被对气候变化响应预测中的不确定性,提出了2个亟待开展的研究问题:将植物叶片的气孔调节功能研究由个体扩展到生态系统甚至更大尺度,改进陆地生态系统碳水循环机理模型;量化气孔调节的主动水力反馈过程,修正植物气孔功能水力模型.     

A new, vapour-phase mechanism for stomatal responses to humidity and temperature

A new mechanism for stomatal responses to humidity and temperature is proposed. Unlike previously-proposed mechanisms, which rely on liquid water transport to create water potential gradients within the leaf, the new mechanism assumes that water transport to the guard cells is primarily through the vapour phase. Under steady-state conditions, guard cells are assumed to be in near-equilibrium with the water vapour in the air near the bottom of the stomatal pore. As the water potential of this air varies with changing air humidity and leaf temperature, the resultant changes in guard cell water potential produce stomatal movements. A simple, closed-form, mathematical model based on this idea is derived. The new model is parameterized for a previously published set of data and is shown to fit the data as well as or better than existing models. The model contains mathematical elements that are consistent with previously-proposed mechanistic models based on liquid flow as well as empirical models based on relative humidity. As such, it provides a mechanistic explanation for the realm of validity for each of these approaches.     

The Simulation of Leaf Conductance Responses to Changes in Environmental Water Status

A simulation model of stomatal response to change of environmental water status was set up based on the works on the mechanism of stomatal movement. The variations of leaf conductance, water potential and turgot pressure in guard cells, subsidiary cells and the other cells or tissues in leaf with leaf-air vapour pressure difference and soil water potential have been calculated by our model. The calculated results fit very well with the data from experiments. The different patterns of leaf transpiration variation with the difference between leaf-air and vapour pressure can be explained quantitatively.     

Co-ordination of vapour and liquid phase water transport properties in plants

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.     

An analytical model of the hydraulic aspects of stomatal dynamics

An analytical model of the hydraulic aspects of stomatal dynamics is formulated in this paper. The model consists of a coupled system of non-linear, ordinary differential equations, written in terms of water potentials, hydrostatic pressures, osmotic potentials, water vapor resistances and water fluxes. The model is validated by comparisons with the experimental literature. Numerical solutions of the model show qualitative agreement with most known stomatal responses.Stomatal opening in the model is dependent on the interaction of the guard and subsidiary cells in the following manner. Pore opening is initiated by a rise in the guard cell hydrostatic pressure. As the stomate opens, transpiration increases, causing the cell wall water potential to drop. The drop in cell wall water potential then causes the subsidiary cell pressure to drop, opening is accelerated, and the stomate literally “pops” open. Simulated opening proceeds in two distinct phases: a stress phase and a motor phase. During the stress phase, guard cell pressure rises but the pore remains closed. The motor phase commences when the guard cell pressure has risen sufficiently to initiate pore opening, beyond which point opening progresses rapidly.Hydropassive stomatal movements are found to be insufficient to regulate water loss at low leaf water potentials. Stable, hydraulically-based oscillations in stomatal aperture are shown in the model by the existence of a stable limit cycle. The period of these oscillations is strongly influenced by the cell membrane hydraulic conductivity. An increased conductivity results in a shorter period oscillation. Environmental conditions promoting oscillatory behavior are in qualitative agreement with the experimental literature.     

A model of stomatal conductance to quantify the relationship between leaf transpiration, microclimate and soil water stress

A model of stomatal conductance was developed to relate plant transpiration rate to photosynthetic active radiation (PAR), vapour pressure deficit and soil water potential. Parameters of the model include sensitivity of osmotic potential of guard cells to photosynthetic active radiation, elastic modulus of guard cell structure, soil‐to‐leaf conductance and osmotic potential of guard cells at zero PAR. The model was applied to field observations on three functional types that include 11 species in subtropical southern China. Non‐linear statistical regression was used to obtain parameters of the model. The result indicated that the model was capable of predicting stomatal conductance of all the 11 species and three functional types under wide ranges of environmental conditions. Major conclusions included that coniferous trees and shrubs were more tolerant for and resistant to soil water stress than broad‐leaf trees due to their lower osmotic potential, lignified guard cell walls, and sunken and suspended guard cell structure under subsidiary epidermal cells. Mid‐day depression in transpiration and photosynthesis of pines may be explained by decreased stomatal conductance under a large vapour pressure deficit. Stomatal conductance of pine trees was more strongly affected by vapour pressure deficit than that of other species because of their small soil‐to‐leaf conductance, which is explainable in terms of xylem tracheids in conifer trees. Tracheids transport water by means of small pit‐pairs in their side walls, and are much less efficient than the end‐perforated vessel members in broad‐leaf xylem systems. These conclusions remain hypothetical until direct measurements of these parameters are available.     

Interpretation of an empirical model for stomatal conductance in terms of guard cell function

An empirical model for stomatal conductance (g), proposed by Leuning (1995, this issue) as a modification of Ball, Woodrow & Berry's (1987) model, is interpreted in terms of a simple, steady-state model of guard cell function. In this model, stomatal aperture is a function of the relative turgor between guard cells and epidermal cells. The correlation between g and leaf surface vapour pressure deficit in Leuning's model is interpreted in terms of stomatal sensing of the transpiration rate, via changes in the gradient of total water potential between guard cells and epidermal cells. The correlation between g, CO₂ assimilation rate and leaf surface CO₂ concentration in Leuning's model is interpreted as a relationship between the corresponding osmotic gradient, irradiance, temperature, intercellular CO₂ concentration and stomatal aperture itself. The explicit relationship between osmotic gradient and stomatal aperture (possibly describing the effect of changes in guard cell volume on the membrane permeability for ion transport) results in a decrease in the transpiration rate in sufficiently dry air. Possible extension of the guard cell model to include stomatal responses to soil water status is discussed.     

In the light of stomatal opening: new insights into 'the Watergate'

Stomata can be regarded as hydraulically driven valves in the leaf surface, which open to allow CO2 uptake and close to prevent excessive loss of water. Movement of these 'Watergates' is regulated by environmental conditions, such as light, CO2 and humidity. Guard cells can sense environmental conditions and function as motor cells within the stomatal complex. Stomatal movement results from the transport of K+ salts across the guard cell membranes. In this review, we discuss the biophysical principles and mechanisms of stomatal movement and relate these to ion transport at the plasma membrane and vacuolar membrane. Studies with isolated guard cells, combined with recordings on single guard cells in intact plants, revealed that light stimulates stomatal opening via blue light-specific and photosynthetic-active radiation-dependent pathways. In addition, guard cells sense changes in air humidity and the water status of distant tissues via the stress hormone abscisic acid (ABA). Guard cells thus provide an excellent system to study cross-talk, as multiple signaling pathways induce both short- and long-term responses in these sensory cells.     

Guard cell apoplastic photosynthate accumulation corresponds to a phloem-loading mechanism

Apoplastic phloem loaders have an apoplastic step in the movement of the translocated sugar, prototypically sucrose, from the mesophyll to the companion cell-sieve tube element complex. In these plants, leaf apoplastic sucrose becomes concentrated in the guard cell wall to nominally 150 mM by transpiration during the photoperiod. This concentration of external sucrose is sufficient to diminish stomatal aperture size in an isolated system and to regulate expression of certain genes. In contrast to apoplastic phloem loaders and at the other extreme, strict symplastic phloem loaders lack an apoplastic step in phloem loading and mostly transport raffinose family oligosaccharides (RFOs), which are at low concentrations in the leaf apoplast. Here, the effects of the phloem-loading mechanism and associated phenomena on the immediate environment of guard cells are reported. As a first step, carbohydrate analyses of phloem exudates confirmed basil (Ocimum basilicum L. cv. Minimum) as a symplastic phloem-loading species. Then, aspects of stomatal physiology of basil were characterized to establish this plant as a symplastic phloem-loading model species for guard cell research. [(14)C]Mannitol fed via the cut petiole accumulated around guard cells, indicating a continuous leaf apoplast. The (RFO+sucrose+hexoses) concentrations in the leaf apoplast were low, <0.3 mM. Neither RFOs (<10 mM), sucrose, nor hexoses (all, P >0.2) were detectable in the guard cell wall. Thus, differences in phloem-loading mechanisms predict differences in the in planta regulatory environment of guard cells.     

Hydraulic and chemical signalling in the control of stomatal conductance and transpiration.

Abscisic acid (ABA) transported in the xylem from root to shoot and perceived at the guard cell is now widely studied as an essential regulating factor in stomatal closure under drought stress. This provides the plant with a stomatal response mechanism in which water potential is perceived in the root as an indication of soil water status and available water resources. There is also ample evidence that stomata respond directly to some component of leaf water status. This provides additional information about water potential gradients developing between root and shoot as the result of water transport, allowing for a more stable regulation of shoot water status and better protection of the transport system itself. The precise location at which leaf water status is sensed, however, and the molecular events transducing this signal into a guard cell response are not yet known. Major questions therefore remain unanswered on how water stress signals perceived at root and leaf locations are integrated at the guard cell to control stomatal behaviour.     

The guard-cell apoplast as a site of abscisic acid accumulation in Vicia faba L.

Abscisic acid (ABA) integrates the water status of a plant and causes stomatal closure. Physiological mechanisms remain poorly understood, however, because guard cells flanking stomata are small and contain only attomol quantities of ABA. Here, pooled extracts of dissected guard cells of Vicia faba L. were immunoassayed for ABA at sub‐fmol sensitivity. A pulse of water stress was imposed by submerging the roots in a solution of PEG. The water potentials of root and leaf declined during 20 min of water stress but recovered after stress relief. During stress, the ABA concentration in the root apoplast increased, but that in the leaf apoplast remained low. The ABA concentration in the guard‐cell apoplast increased during stress, providing evidence for intra‐leaf ABA redistribution and leaf apoplastic heterogeneity. Subsequently, the ABA concentration of the leaf apoplast increased, consistent with ABA import via the xylem. Throughout, the ABA contents of the guard‐cell apoplast, but not the guard‐cell symplast, were convincingly correlated with stomatal aperture size, identifying an external locus for ABA perception under these conditions. Apparently, ABA accumulates in the guard‐cell apoplast by evaporation from the guard‐cell wall, so the ABA signal in the xylem is amplified maximally at high transpiration rates. Thus, stomata will display apparently higher sensitivity to leaf apoplastic ABA if stomata are widely open in a relatively dry atmosphere.