Studies on the Mechanisms of Action of the Antitranspirant Phenylmercuric Acetate, and its Penetration into the Mesophyll

Abstract

Experiments have examined the effect of phenylmercuric acetate (PMA) on the guard cells of Commelina communis. In one series, PMA was supplied to the leaf surface; after different time intervals the epidermis was removed and the ability of the stomata to open was determined. In the other series, different concentrations of PMA were included in the medium used for inccubating epidermal strips with which ion-stimulated stomatal opening was assayed. At concentrations of 10^-54 M and above the effect of PMA was severe and the structural integrity of the guard cells was affected; they were unable to accumulate neutral red. At concentrations arpound 10^-6 M the guard cells were less affected and PMA brought about a transient stimulation of stomatal opening by releasing subsidiary-cell turgor pressure. A solution of 5 × 10^-4 M PMA applied to leaves reduced by half the photosynthetic ¹⁴CO₂ incorporation into C. communis mesophyll. In Zea mays it increased the CO₂ compensation point and also the resistance to diffusion in the gas phase (R_A, but there was a proportionately greater increase in the apparent liquid phase resistance (R_t). This direct inhibition of mesophyll photosynthesis undermines one of the major objectives of applying anatitranspirants, and for this reason it is suggested that PMA is unsuitable for general application to crops.