effects of silicone coatings on leaves

Dimethyl silicones of viscosities 0.25, 10, and 125 St were applied to leaf surfaces and their effects on the rates of transpiration and photosynthesis were measured. The effect of the 10 and 125 St materials lasted more than 16 days, decreasing both photosynthesis and transpiration. The diffusion coefficients of water vapour and carbon dioxide through 10 St silicone were measured in a separate experiment and it is shown that the leaf results can be interpreted in terms of an almost complete silicone cover over the stomata. Calculations show that for materials with DH2O/DCO2 greater than 1.6 (free air ratio), provided they have no physiological effects, the most effective mode of action would be complete blockage of a fraction of the stomates rather than partial blockage of all. The amount of water saved by doubling the stomatal resistance of three crop types (short grass, farm crops, and trees) in two contrasting environments (Southern England and California) is calculated, and it is inferred that anti-transpirants would be most effective when applied to well-watered tall crops in regions of high evaporative demand.