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How Stomata Resolve the Dilemma of Opposing Priorities

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How Stomata Resolve the Dilemma of Opposing Priorities Authors:
Raschke, K. Publication:
Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, Volume 273, Issue 927, pp. 551-560 Publication Date:
02/1976 Origin:
10.1098/rstb.1976.0031 Bibliographic Code:


Satisfaction of a leaf's need for CO_2 requires an intensive gas exchange between mesophyll and atmosphere; prevention of excessive water loss demands that gas exchange be kept low. Stomata open when a low CO_2 concentration in the guard cells triggers (a) uptake of K^+ in exchange of H^+, (b) production of organic acids, and (c) import of Cl^-. 'Hydropassive' stomatal closure (i.e. turgor loss without reduction of the solute content of the guard cell) appears insufficient to protect the plant from desiccation. An additional 'hydroactive' solute loss is necessary; it is brought about by (+)-abscisic acid (ABA) acting as feedback messenger between mesophyll and epidermis. Stomatal closure not only curbs water loss but improves water-use efficiency because transpiration is proportional to stomatal conductance (at constant temperature). In contrast, assimilation, following saturation kinetics with respect to intercellular CO_2, is relatively insensitive to changes in stomatal conductance (as long as stomata are wide open). In Xanthium strumarium, the amplitude of stomatal responses to ABA depends on the concentration of CO_2 in the guard cells; the opposite statement is also true. These interactions cause stomata to behave like 'adjustable control systems' capable of giving priority either to CO_2 assimilation or to water husbandry.
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