Stomatal Water Relations and the Control of Hydraulic Supply and Demand
Stomata have fascinated plant biologists for well over 100 years. It is difficult to think of another plant system that responds to so many factors or displays such complexity at so many levels. Indeed, when one considers the number of feedback loops invo
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Stomatal Water Relations and the Control of Hydraulic Supply and Demand By Thomas N. Buckley and Keith A. Mott
1 Introduction Stomata have fascinated plant biologists for well over 100 years. It is difficult to think of another plant system that responds to so many factors or displays such complexity at so many levels. Indeed, when one considers the number of feedback loops involving stomatal conductance and all of the potential interactions among these feedbacks, it is really quite remarkable that stomata work at all. There is now general agreement about the environmental factors to which stomata respond, and the advent of relatively low-cost, easy to use gas-exchange systems and porometers has resulted in a surfeit of data describing stomatal conductance responses in natural and laboratory situations. Despite this, the mechanisms by which stomata respond to environmental factors are largely unknown. It has been clear for some time that active regulation of guard cell osmotic pressure plays a large role in most stomatal responses, and there has been some recent progress in elucidating the signal transduction chains and ion transport processes responsible for this aspect of stomatal function. It seems likely that many of these subcellular processes will be worked out in the near future. However, in order to be useful in predicting and interpreting gas exchange, these metabolic processes must be translated into dynamics of stomatal conductance at larger scales. This in turn requires a detailed understanding of the hydraulic factors that link stomatal aperture to guard cell osmotic content and to the water relations of the epidermis. Short-term stomatal responses to perturbations in Hydraulic supply or demand are an appropriate context in which to develop this understanding, not only because of the ecological significance of these stomatal responses and their consequent need for explanation, but also because they provide a window on epidermal hydraulics: the system is prodded from different angles, one by one, producing apparently disparate responses that lend themselves to logical synthesis. Our goal in this review is a parsimonious, synthetic, mechanistic explanation of these responses.
Progress in Botany, Vol. 63 © Springer-Verlag Berlin Heidelberg 2002
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Ecology
2 Hydraulics of Stomatal Responses to Environmental Factors Although most stomatal responses are driven ultimately by changes in guard cell solute concentration, stomata are fundamentally hydraulic entities, and it is impossible to understand conductance responses to environmental perturbations without taking hydraulics into account. Stomatal dynamics emerge from a highly connected hydraulic medium: water evaporates from a mesh of mesophyll and epidermal cells to which guard cells are hydraulically slaved. Two factors ensure the dynamic and spatial significance of hydraulic interactions among stomata. First, individual stomatal apertures are controlled by both guard cell turgor and epidermal cell turgor, which affect aperture in opposite ways but
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