THE MECHANISM OF STOMATAL MOVEMENT

KETELLAPPER, H. J. (California Inst. Tech., Pasadena.) The mechanism of stomatal movement. Amer. Jour. Bot. 46(3): 225-231. Illus. 1959.-A review of the pertinent literature reveals that fairly exact knowledge is available about the effects of light, C02, and water content of the leaf on stomatal movement. Two distinct types of light action can be distinguished: an indirect effect, mediated through photosynthesis and the resulting lower CO, concentration within the leaf, and a direct effect which occurs even when the intercellular spaces are washed out with C02-free air. The CO2 concentration occupies a very central position in the regulation of stomatal movement. The action of CO2 and light will be influenced, or even counteracted, by the value of the water deficit in the leaf. A large number of changes in the guard cells, accompanying stomatal movement, has been described in the literature and some of these have been discussed in the present review: for example, the changes in osmotic value, in starch content, and in permeability. These observations led to the formulation of the classical theories of stomatal movement. It has been concluded that the classical theories are not adequate to explain stomatal movement. At present one can choose between 2 alternatives: an osmotic theory incorporating the pathways of the classical theories and bringing these together in one general scheme hinging on the sugar-starch conversion, or else a theory incorporating non-osmotic processes for the removal of water or osmotic active material. These alternatives have been discussed in some detail. It has been concluded that more experimental evidence is required before a valid theory of stomatal movement can be brought forward. DURING STOMATAL MOVEMENT turgor changes occur in the guard cells of the stomates. The construction of the wall of the guard cells causes these turgor changes to manifest themselves as changes in shape and volume. Stomatal opening is changed in this way. Stomatal movements are thus turgor movements just as are, for example, nyctinastic movements of leaves. It is not yet quite clear how the rapid turgor changes come about, particularly since they are readily reversible. Various environmental factors influence stomatal movement and the theories which try to explain the process have been based on the observed or imagined changes within the guard cells due to the action of light, C02, relative humidity, and water content of the plant (hydrature), and on the possible relation of these changes to the response of the stomates. It will, therefore, be necessary to discuss the effects of environment on stomates in some detail. LIGHT.-In darkness the stomates are closed. When a turgid leaf is exposed to light after darkness, the stomates open. During the opening, the turgor of the guard cells and the osmotic value at which incipient plasmolysis occurs (Oi) increase. The increase is dependent on light intensity. Stoalfelt (1929) calls this process "photoactive opening" and assumes that the guard cells actively respond to the light stimulus. When the light is switched off, the stomates close and, assuming that closing is an active reversal of the process leading to opening, Stalfelt calls it "photoactive closing." Paper invited by the Editorial Committee. Received for publication December 15, 1958. The light effect is proportional to the product of light intensity and length of exposure during the early part of the opening process. When the stomates open further, the linearity disappears. The light intensity which causes opening varies with species. Went (1944) found that tomatoes required a minimum of 275 ft.-c., while Heath and Russell (1954a) found 90 ft.-c. effective in Pelargonium. The action spectrum of the light reaction of stomates has been studied by Sierp (1933) and Harms (1936). Both authors agree that blue, green, yellow and orange are equally effective. The stomates are about half as sensitive to red as they are to the other colors and infrared does not have any effect. Efforts have been made to determine if chlorophyll is necessary for the light reaction. Experiments with variegated leaves showed that stomates in chlorophyll-free areas open and close under the influence of light, but the movements may be more restricted. Virgin (1957) and Shaw (1958) demonstrated with albino mutants of barley that stomates in chlorophyll-free plants do not react on light at all. In fact, these plants have no stomatal transpiration, only strong guttation. Their rate of water loss is very slow. The results with albino mutants do suggest that there is no light reaction when no chlorophyll is present. However, there are some difficulties in the evaluation of the data, because no opening has ever been observed in the albino mutants and it could well be that the stomates are unable to open. A possible reason may be that there is not sufficient carbohydrate in the guard cells to produce the required increase in tiiraor ITnless it can be shown that the stomates