Some factors responsible for varying effectiveness of stomatal closing antitranspirants

Abstract

Experimental data are presented to explain some of the varying effectiveness of phenyl mercuric acetate (PMA) as a stomatal closing antitranspirant. In a comparison of sorghum, maize, cotton, peanuts, and tomatoes, stomatal apertures were affected to different extents by PMA sprays and environmental conditions. In addition, the relationship between aperture and stomatal resistance differed between cotton and sorghum. Thus the effectiveness of PMA depends on the balance between the degree to which stomata may be closed by prevailing conditions, the extent to which PMA can bring about further closure, and what this closure means in terms of diffusion resistance. In pot experiments with these species, PLMA sprays conserved soil water, but the higher transpiration by unsprayed plants led to water deficits which soon caused stomatal closure. The balance between soil water storage, evaporative conditions, and the particular objectives of transpiration reduction would determine the value of application. Trials with sorghum, in both the field and glasshouse, showed that the effect of a single spray of PMA may be short-lived, and that successive applications may be necessary for a persistent effect. The resulting toxicity may be unacceptable.