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.     

The Hydraulic Pathways in Nicotiana glauca (Grah.) and Tradescantia virginiana (L.) Leaves, and Water Potentials in Leaf Epidermes

The hydraulic conductances of leaves of a species which exhibitsstomatal responses to humidity (Nicotiana glauca) are significantlylower than the conductances in a species which does not exhibitsuch responses (Tradescantia virginiana). This difference couldat least partly account for their difference in stomatal responseto humidity. In both species, the hydraulic conductance betweenthe leaf bulk and its epidermis is much lower than the conductancein any other part of the pathway. The apparently conflictingresults, reported in recent literature, on the hydraulic conductancesand water pathways in leaves are reinterpreted, and shown tobe due to misinterpretation of results. The recently publishedcriticisms of a technique used to measure hydraulic conductivityare commented on and refuted. An examination of the factors that influence the water potentialat the sites of evaporation from the inner walls of the epidermisnear stomatal pores showed that the water potential at thesesites is lower than the bulk epidermal water potential. Thewater potential at these sites changes in a complex way as stomatalaperture changes. As it is reduced the ratio of: ‘waterpotential at sites of evaporation on the inner walls of theepidermis near stomatal pores/bulk leaf water potential‘increases. The positive feedback effect of this phenomenon,which tends to keep stomatal water potential constant as thestomata close and therefore enhances closure, and two other‘passive’ positive feedback effects on the waterpotential at sites of evaporation near stomata that have beenreported in the literature are briefly discussed. Nicotiana glauca (Grah.), Tradescantia virginiana (L.), sub-stomatal cavities, peristomatal evaporation, stomata, humidity response, leaf hydraulic conductance, water potential     

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.     

Stomatal responses to non-local changes in PFD: evidence for long-distance hydraulic interactions

Interactions among stomata within a single areole have recently been reported, and evidence suggests that hydraulic mechanisms may be responsible for these interactions. Such interactions may play a role in patchy stomatal behaviour by coordinating stomatal behaviour within areoles. However, models suggest that longer‐distance interactions may be required to produce the large‐scale discoordination that is characteristic of stomatal patchiness. This study was undertaken to characterize long‐distance interactions between ‘artificial patches’ of stomata under varying conditions of evaporative demand and soil water stress. Gas‐exchange was monitored in two adjacent regions (‘patches’) of a wheat leaf by two independent gas mixing and analysis systems. When photon flux density (PFD) was changed in only one of these patches, stomatal conductance responded in both patches in a manner consistent with hydraulic interactions propagated by changes in xylem water potential. These data are discussed in the context of mechanisms for patchy stomatal conductance and implications for the design and analysis of gas‐exchange experiments.     

Stomatal sensing of the environment

The effects of environmental factors on stomatal behaviour are reviewed and the questions of whether photosynthesis and transpiration eontrol stomata or whether stomata themselves control the rates of these processes is addressed. Light affects stomata directly and indirectly. Light can act directly as an energy source resulting in ATP formation within guard cells via photophosphorylation, or as a stimulus as in the case of the blue light effects which cause guard cell H⁺ extrusion. Light also acts indirectly on stomata by affecting photosynthesis which influences the intercellular leaf CO₂ concentration ( C _i). Carbon dioxide concentrations in contact with the plasma membrane of the guard cell or within the guard cell acts directly on cell processes responsible for stomatal movements. The mechanism by which CO₂ exerts its effect is not fully understood but, at least in part, it is concerned with changing the properties of guard cell plasma membranes which influence ion transport processes. The C _i may remain fairly constant for much of the day for many species which is the result of parallel responses of stomata and photosynthesis to light. Leaf water potential also influences stomatal behaviour. Since leaf water potential is a resultant of water uptake and storage by the plant and transpirational water loss, any factor which affects these processes, such as soil water availability, temperature, atmospheric humidity and air movement, may indirectly affect stomata. Some of these factors, such as temperature and possibly humidity, may affect stomata directly. These direct and indirect effects of environmental factors interact to give a net opening response upon which is superimposed a direct effect of stomatal circadian rhythmic activity.     

Stomatal regulation by microclimate and tree water relations: interpreting ecophysiological field data with a hydraulic plant model

Dynamics in microclimate and physiological plant traits were studied for Pubescent oak and Scots pine in a dry inner-alpine valley in Switzerland, at a 10 min resolution for three consecutive years (2001-2003). As expected, stomata tended to close with increasing drought in air and soil. However, stomatal aperture in oak was smaller than in pine under relatively wet conditions, but larger under dry conditions. To explore underlying mechanisms, a model was applied that (i) quantifies water relations within trees from physical principles (mechanistic part) and (ii) assumes that signals from light, stomatal aperture, crown water potential, and tree water deficit in storage pools control stomata (systemic part). The stomata of pine showed a more sensitive response to increasing drought because both factors, the slowly changing tree water deficit and the rapidly changing crown water potential, closed the stomata. By contrast, the stomata of oak became less drought-sensitive as the closing signal of crown water potential was opposed by the opening signal of tree water deficit. Moreover, parameter optimization suggests that oak withdrew more water from the storage pools and reduced leaf water potentials to lower levels, without risking serious damage by cavitation. The new model thus suggests how the hydraulic water flow and storage system determines the responses in stomatal aperture and transpiration to drought at time scales ranging from hours to multiple years, and why pine and oak might differ in such responses. These differences explain why oaks are more efficient competitors during drought periods, although this was not the case in the extremely dry year 2003, which provoked massive leaf loss and, from July onwards, physiological activity almost ceased.     

Oscillations in stomatal conductance and plant functioning associated with stomatal conductance: Observations and a model

Summary Measurements of transpiration, leaf water content, and flux of water in a cotton plant exhibiting sustained oscillations, in stomatal conductance are presented, and a model of the mechanism causing this behaviour is developed. The dynamic elements, of the model are capacitors—representing the change of water content with water potential in mesophyll, subsidiary and guard cells—interconnected by resistances representing flow paths in the plant. Increase of water potential in guard cells causes an increase in stomatal conductance. Increase of water potential in the subsidiary cells has the opposite effect and provides the positive feed-back which can cause stomatal conductance to oscillate. The oscillations are shown to have many of the characteristics of free-running oscillations in real plants. The behaviour of the model has been examined, using an analogue computer, with constraints and perturbations representing some of those which could be applied to real plants in physiological experiments. Aspects of behaviour which have been simulated are (a) opening and closing of stomata under the influence of changes in illumination, (b) transient responses due to step changes in potential transpiration, root permeability and potential of water surrounding the roots, (c) the influence of these factors on the occurrence and shape of spontaneous oscillations, and (d) modulation of sustained oscillations due to a circadian rhythm in the permeability of roots.     

Cavitation induced by a surfactant leads to a transient release of water stress and subsequent 'run away' embolism in Scots pine (Pinus sylvestris) seedlings

Cavitation decreases the hydraulic conductance of the xylem and has, therefore, detrimental effects on plant water balance. However, cavitation is also hypothesized to relieve water stress temporarily by releasing water from embolizing conduits to the transpiration stream. Stomatal closure in response to decreasing water potentials in order to avoid excessive cavitation has been well documented in numerous previous studies. However, it has remained unclear whether the stomata sense cavitation events themselves or whether they act in response to a decrease in leaf water potential to a level at which cavitation is initiated. The effects of massive cavitation on leaf water potential, transpiration, and stomatal behaviour were studied by feeding a surfactant into the transpiration stream of Scots pine (Pinus sylvestris) seedlings. The stomatal response to cavitation in connection with the capacitive effect was also studied. A major transient increase in leaf water potential was found due to cavitation in the seedlings. As cavitation was induced by lowering the surface tension, the two mechanisms could be uncoupled, as the usual relation between xylem water potential and the onset of cavitation did not hold. Our results indicate that the seedlings responded more to leaf water potential and less to cavitation itself, as stomatal closure was insufficient to prevent the seedlings from being driven to 'run-away' cavitation in a manner of hours.     

Drought limitations to leaf‐level gas exchange: results from a model linking stomatal optimization and cohesion–tension theory

We merge concepts from stomatal optimization theory and cohesion–tension theory to examine the dynamics of three mechanisms that are potentially limiting to leaf‐level gas exchange in trees during drought: (1) a ‘demand limitation’ driven by an assumption of optimal stomatal functioning; (2) ‘hydraulic limitation’ of water movement from the roots to the leaves; and (3) ‘non‐stomatal’ limitations imposed by declining leaf water status within the leaf. Model results suggest that species‐specific ‘economics’ of stomatal behaviour may play an important role in differentiating species along the continuum of isohydric to anisohydric behaviour; specifically, we show that non‐stomatal and demand limitations may reduce stomatal conductance and increase leaf water potential, promoting wide safety margins characteristic of isohydric species. We used model results to develop a diagnostic framework to identify the most likely limiting mechanism to stomatal functioning during drought and showed that many of those features were commonly observed in field observations of tree water use dynamics. Direct comparisons of modelled and measured stomatal conductance further indicated that non‐stomatal and demand limitations reproduced observed patterns of tree water use well for an isohydric species but that a hydraulic limitation likely applies in the case of an anisohydric species.     

Effects of humidity on light-induced stomatal opening: evidence for hydraulic coupling among stomata

A mechanism for co-ordinating behaviour of stomata within an areole during patchy stomatal conductance has recently been proposed. This mechanism depends on hydraulic interactions among stomata that are mediated by transpiration-induced changes in epidermal turgor. One testable prediction that arises from this proposed mechanism is that the strength of hydraulic coupling among stomata should be proportional to evaporative demand and, therefore, inversely proportional to humidity. When a leaf is illuminated following a period of darkness, there is typically a period of time, termed the Spannungsphase, during which guard cell osmotic and turgor pressure are increasing, but the pore remains closed. If hydraulic coupling is proportional to evaporative demand, then variation among stomata in the duration of the Spannungsphase should be lower for leaves at low humidity than for leaves at high humidity. A similar prediction emerged from a computer model based on the proposed hydraulic mechanisms. These predictions were tested by measuring individual stomatal apertures on intact transpiring leaves at low and high humidity and on vacuum-infiltrated leaf pieces (to eliminate transpiration) as PFD was increased to high values from either darkness or a low value. Results showed that the range of Spannungsphasenamong stomata was reduced at low humidity compared to high humidities. Experiments that began at low PFD, rather than at darkness, showed no delay in stomatal opening. These results are discussed in the context of the proposed hydraulic coupling mechanisms.     

Stomatal control of xylem embolism

Abstract. The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short-term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.     

A spatially explicit model of patchy stomatal responses to humidity

Stomata of leaves can exhibit either temporally stable, spatially homogeneous behaviour or complex spatial and temporal dynamics, depending on environmental and physiological conditions. To test the ability of accepted physiological mechanisms to describe these patterns, we developed a simple, spatially explicit model of stomatal responses to humidity that incorporated hydraulic interactions among stomata. Model results showed qualitative agreement with experimental evidence for a number of phenomena: (1) at high humidities, whole-leaf steady-state conductance is a monotonic function of humidity; (2) the initial stomatal response following a perturbation in humidity is in the direction opposite to the final response, and (3) spatial dynamics include patch formation and self-organization similar to that observed in actual leaves. These comparisons do not eliminate other explanations, but do suggest that novel mechanisms need not be invoked to explain the diversity of spatial and temporal patterns of stomatal behaviour in leaves.     

Stomatal response characteristics of Tradescantia virginiana grown at high relative air humidity

Plants produced at high relative air humidity (RH) show poor control of water loss after transferring to low RH, a phenomenon which is thought to be due to their stomatal behaviour. The stomatal anatomy and responses of moderate (55%) and high (90%) RH grown Tradescantia virginiana plants to treatments that normally induce stomatal closure, i.e. desiccation, abscisic acid (ABA) application and exposure to darkness were studied using attached or detached young, fully expanded leaves. Compared with plants grown at moderate RH the transpiration rate, stomatal conductance and aperture of high RH grown plants measured at the same condition (40% RH) were, respectively, 112, 139 and 132% in light and 141, 188 and 370% in darkness. Besides the differences in stomatal size (guard cell length was 56.7 and 73.3 µm for moderate and high RH grown plants, respectively), there was a clear difference in stomatal behaviour. The stomata responded to desiccation, ABA and darkness in both moderate and high RH grown plants, but the high variability of stomatal closure in high RH grown plants was striking. Some stomata developed at high RH closed in response to darkness or to a decrease in relative water content to the same extent as did stomata from moderate RH grown plants, whereas others closed only partly or did not close at all. Evidently, some as yet unidentified physiological or anatomical changes during development disrupt the normal functioning of some stomata in leaves grown at high RH. The failure of some stomata to close fully in response to ABA suggests that ABA deficiency was not responsible for the lack of stomatal closure in response to desiccation.     

Woody plants optimise stomatal behaviour relative to hydraulic risk

Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbon‐maximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle.     

干旱下植物气孔运动的调控

概述了植物气孔对大气干旱和土壤干旱的反应,认为植物气孔对大气干旱的反应并不是一种反馈机制;并就干旱条件下植物气孔运动的水力学和化学信号调控机制进行了简要论述,认为虽然化学信号调控干旱下气孔运动更为广泛,但ABA不是唯一的化学信号,水分关系影响了信号的产生、运转和气孔对信号的敏感性,干旱条件下水力学和化学信号共同调控着植物的气孔运动。     

Localization of mechanisms involved in hydropassive and hydroactive stomatal responses of Sambucus nigra to dry air

The response of stomata to a reduction of air humidity is composed of a hydropassive opening followed by active closure. Whereas the mechanisms behind the hydropassive opening are largely understood, the location and physiological basis of the sensing mechanisms leading to active closure are not yet known. This study attempts to evaluate the importance of a single pore's transpiration on its own response and that of adjacent pores. Selected stomata on attached intact leaves of Sambucus nigra were sealed with mineral oil and the response to a reduction of humidity was continuously observed in situ. Blocking a pore's transpiration had no appreciable effect on hydropassive opening and subsequent stomatal closure. If the adjacent stomata were additionally sealed, the closing response was reduced, but not the hydropassive opening. On the other hand, sealing the entire leaf surface, except a small area including the observed stomata, also reduced stomatal closure. These results indicate that strictly local processes triggered by a pore's own transpiration are not required to induce stomatal closure. To describe the effect of one pore's transpiration on the hydropassive and hydroactive responses of neighboring stomata, a simple spatial model was constructed. It suggests that 90% of the closing effect covers an area of approximately 0.5 mm2, whereas the effect on hydropassive opening affects an area of approximately 1 mm2. This divergence may suggest mechanisms other than or in addition to those involving changes of local leaf water potential.     

Stomatal responses to long-term high vapor pressure deficits mediated most limitation of photosynthesis in tomatoes

Plants grown at high vapor pressure deficit (VPD) usually present decreased photosynthesis, but stomatal and mesophyll limitation to photosynthesis remain poorly quantified. To better understand the regulation of high VPD on photosynthesis and plant growth in tomatoes, we investigated the limitation of stomatal conductance and mesophyll conductance to photosynthesis and relative importance of stomatal morphology and function in stomatal conductance. Both the net photosynthesis rate and total biomass were significantly limited by high VPD. Meanwhile, stomatal conductance and mesophyll conductance were decreased under high VPD. The stomatal conductance limitation was responsible for 60% of the total photosynthetic limitation. Moreover, a reduction in stomatal density and stomatal size occurred under high VPD, which was significantly correlated with the down-regulation of stomatal conductance. The stomatal morphology contributed to more than half the change in stomatal conductance. Nevertheless, stomatal movement was also an important factor in regulating stomatal conductance. The decrease of hydraulic conductance and transpiration rate with no significant difference in relative water content, leaf water potential, and/or osmotic potential suggested passive hydraulic regulation in the feedforward responses of stomata to high VPD.     

An Investigation into the Stomatal Behaviour of a Wilty Mutant of Pisum sativum

The water relations and stomatal behaviour of a wilty line ofpea (JI 1069) were investigated and compared with those of severalnon-wilty lines (JI 1180, JI 1194, and JI 74). The leaves ofthe wilty line were found to have a lower percent water content,water potential and diffusive resistance and the dimensionsof the stomatal cells were larger than those of the non-wiltytypes. The aperture of stomata on epidermal samples taken from plantsin the light or dark period of a diurnal rhythm was consistentlylarger for the wilty pea than for the non-wilty lines, however,their stomatal responses on detached epidermis to light, CO₂and KC1 concentration were similar. There was no differencein response to ABA of stomata on detached epidermis of wiltyor non-wilty types of pea. Key words: Pisum sativum, Wilty mutant, Water relations, Stomatal behaviour     

Stomatal sensitivities to changes in leaf water potential, air humidity, CO2 concentration and light intensity, and the effect of abscisic acid on the sensitivities in six temperate deciduous tree species

To characterise the stomata of six temperate deciduous tree species, sets of stomatal sensitivities to all the most important environmental factors were measured. To compare the importance of abscisic acid (ABA) in the different stomatal responses, the effect of exogenous ABA on all the stomatal sensitivities was determined.Almost all the stomatal sensitivities: the sensitivity to a decrease in leaf water potential, air humidity, CO₂ concentration ([CO₂]) and light intensity, and to an increase in [CO₂] and light intensity were the highest in the slow-growing species, and the lowest in the fast-growing species. Drought increased the sensitivity to the environmental changes that induce a decrease in the stomatal conductance, and decreased the sensitivity to the changes that induce an increase in this conductance. The sensitivities of the slow-growers were most strongly affected by drought and ABA. Therefore the success of the slow-growers in their ecological niches can be based on the highly sensitive and strictly regulated responses of their stomata. The fast-growers had the highest sensitivity to an increase in leaf water potential and this sensitivity was sharply reduced by drought and ABA. Thus, the dominance of the trees in riparian areas can be based on the ability of their stomata to quickly reach high conductance in well-watered conditions and to efficiently decrease this rate during drought.Stomatal sensitivities to the hydraulic environmental factors (water potentials in plant and air) had higher values in well-watered trees and a more pronounced response to drought than the sensitivities to the photosynthetic environmental factors ([CO₂] and light intensity). Thus, the hydraulic factors most likely prevail over the photosynthetic factors in determining stomatal conductance in these species.In response to exogenous ABA, the rates of stomatal closure, following a decrease in air humidity and light intensity, and an increase in [CO₂], were accelerated. Stomatal opening following an increase in air humidity and light intensity and a decrease in [CO₂] was replaced by slow closing. The rate of stomatal opening following an increase in leaf water potential was reduced. As the sensitivities to changes in light were modified less by the ABA than the other stomatal sensitivities, the prediction of stomatal responses on the basis of the sensitivity to light alone should be excluded in stomatal models.