Studies in Stomatal Behaviour

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Abstract
As in the previous investigation (Part I), stomatal responses in wheat were followed by means of porometer readings; between these, air was forced through the stomata to sweep out the substomatal cavities. Instead of forcing the whole of a relatively slow air-flow in through the stomata, however, a fast flow was swept across the leaf surface and a portion of this passed into the intercellular space system. This helped to maintain the desired carbon dioxide content close to the stomatal openings and the stomata could thus be treated with different carbon dioxide concentrations. A factorial experiment was carried out in which 6 carbon dioxide concentrations from zero to 0·084 per cent, were combined with 3 light intensities from 90 to 800 f.c, the 18 treatments being replicated 6 times in three ‘6×6 latin squares’. A supplementary experiment in which the same 6 carbon dioxide treatments were given in darkness was carried out in duplicate. The results confirmed previous findings in that the stomata showed no appreciable response to variation of external carbon dioxide supply between zero and 0·01 per cent, concentration and that a large and highly significant light response was found at each of these two carbon dioxide levels. This is held to indicate the existence of a light effect (direct or indirect) not operating by reduction of the internal carbon dioxide content of the guard cells. On the assumption that the rates of air flow through the stomata were adequate, the combined results of the two experiments are interpreted mainly in terms of two direct effects of light upon the stomata, one depending on photosynthetic reduction of carbon dioxide in the guard cells and the other, mentioned above, not so operating. To these must be added the indirect effect responsible for the ‘transmission’ described in Part 1, which might possibly account for the whole of the second direct effect postulated. Finally, if the rates of flow were in fact grossly inadequate, the trends attributed to the first direct effect mentioned above might have been due to indirect control by the mesophyll via the intercellular space atmosphere. Such control must account for part of the light responses which occur under natural conditions. The natural light response is thus made up of at least two and more probably four components—two direct and two indirect, in each case one operating by the internal carbon dioxide and the other not. A highly significant diurnal rhythm of stomatal movement under constant illumination and temperature is shown to occur in wheat. The measure of stomatal resistance used (log resistance per 10,000 stomata) is discussed, and it is concluded that although it is empirical a much more satisfactory measure is not at present available.

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