Stomatal Physiology

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Abstract
The response of stomata to light, water deficit and CO2 is predictable. Light and water deficit of the leaf are the major factors determining stomatal aperture. The light effect can be explained almost quantitatively by the photosynthetic reduction of the CO2 concentration in the intercellular spaces and in the guard cells. Little is known about the mechanism of action of hydrature of the leaf. The mechanism of stomatal movement is still not understood. The information is negative in the sense that a number of processes going on in the guard cells during movement have been found inadequate. CO2 occupies an important position but the mechanism of action of CO2 is not known. Because of the inadequacy of processes which will allow an explanation of stomatal movement according to the classical concepts of the water relations of cells, active transport mechanisms have been called upon. Stomatal opening, and possibly closing as well, is coupled to energy yielding processes. It is not at all certain, however, that the energy is required for the basic mechanism underlying turgor changes. The energy shortage caused by uncoupling agents could very well affect secondary phenomenai such as membrane properties, which make it possible for potential turgor changes to be realized. At present, only hypothetical interpretations of stomatal movement can be offered. There are 2 alternatives: a pathway combining the various classical theories, such as suggested by Sayre or a system incorporating some form of active transport. In the latter case, the classical pathway probably functions as a stabilizing, energy-conserving mechanism. The classical pathway, in which the starch-sugar conversion occupies a central position, does not have any obvious improbabilities. However, at present it is not supported by evidence of a quantitative nature. The major objection against this hypothesis is the lack of correlation between stomatal movement and osmotic changes. This discrepancy adds weight to the arguments of those favoring active transport of water or solute. Active movement of water, for example, will allow rapid changes in turgor without accompanying changes in osmotically active substances. No suggestions can be made with respect to the active mechanisms. Whether opening or closing is active, whether the movement of salt or water is involved, is a matter of personal preference. The various hypotheses can be tested by suitable experiments. The efforts should be concentrated on a limited number of plant species which have already been studied extensively, for example, wheat, Pelargonium zonale and Vicia faba. Experiments should deal with the guard cells directly. It will be advisable to do more work with isolated stomata or surface sections, similar to the elegant series of experiments carried out by Mouravieff. There is a bibliography of 132 references.