In two series of experiments in which eight substances were applied to the leaf surface of young cotton plants all significantly reduced transpiration. The reduction in transpiration increased more or less proportionally with concentration up to levels which caused serious injury.
With one exception all compounds reduced plant growth to a greater degree than transpiration, causing an increase in the "'transpiration ratio" and hence a reduction in the efficiency of water utilization. This effect became more pronounced as the concentration of the applied compound was increased, and indicated a degree of metabolic inhibition of growth. The only compound which reduced transpiration proportionally more than growth was phenyl mercuric acetate which, while essentially being a metabolic inhibitor, appeared to act specifically on the leaf stomata rather than generally on the photosynthetic apparatus.
It is emphasised that while any compounds which reduce transpiration without serious injury to plants may have value for such applications as watershed management, efforts should be intensified to produce a material which will simultaneously improve water use efficiency. Such a substance should impose a high resistance barrier between the evaporating surfaces in the leaf. and the air. either by inducing stomatal closure, or forming a surface film. without also metabolically inhibiting photosynthesis.