Studies in Stomatal Behaviour

Abstract

It was found that stomata on illuminated leaves, both of Pelargonium and wheat, opened much wider where the leaf surface was enclosed in a small volume of air, as in a normal porometer cup, than elsewhere. This was shown for both species by the infiltration method, and for Pelargonium by Lloyd's method and direct microscopical observation also. The effect was shown not to be due to pressure of the porometer cup or glass plate on the leaf, or to temperature differences, nor directly to the lack of movement or high humidity of the enclosed air. A considerable body of data was collected which appeared to support the hypothesis that the wide opening was due to accumulation of some volatile substance produced by the leaf, but all the results were also consistent with the view that it was caused by reduction in the carbon dioxide content of the enclosed air below the normal 0·03 per cent. owing to photosynthesis. Further crucial experiments with both the porometer and infiltration methods left virtually no doubt that the latter hypothesis was correct. This extreme sensitivity of stomata to carbon dioxide concentration within the range 0·03 per cent. to zero is discussed in relation to their operation in nature, and a possible biological advantage is suggested. The bearing of the effect upon porometer investigations is also discussed and it is concluded that for all quantitative or semi-quantitative experimentation it is essential to use a cup detached between readings, or at least swept with air such as surrounds the rest of the leaf, and to have the upper leaf surface above the cup area freely exposed or similarly swept. For qualitative investigation of the light response of stomata the traditional form of cup may be used. The importance is stressed of allowing porometer readings to reach equilibrium under one set of conditions before changing to another, when investigating the ‘closing’ or ‘opening’ effects of external factors. Several subsidiary effects, observed in the course of the investigation, are discussed; in particular an effect of humidity upon the rate of response to other factors.