Nature of Specific Susceptibility to Alternaria kikuchiana in Nijisseiki Cultivar among Japanese Pears VIII.

Effects of Other Cations on Losses of K⁺ after Exposure to AK-Toxin

Abstract

The host-specific toxin (AK-toxin) of Alternaria kikuchiana caused rapid increases in efflux of K⁺ from susceptible leaves of Japanese pear (Pyrus serotina). On the contrary, toxin caused a decrease in effluxes of Na⁺, Mg²⁺ and Ca²⁺ from susceptible tissues, as compared to efflux from control tissue. Toxin-induced loss of K⁺ was much greater in the presence of Na⁺, Mg²⁺ or Ca²⁺ in the ambient solution. Stimulation of toxin-induced leakage was evident immediately after adding these cations to toxin-treated tissues; when these cations were removed, loss of K⁺ reverted to the lower level. The stimulative effect of Na⁺ and Mg²⁺ increased as the concentrations were increased from 0.01 to 10mM; the effects of Ca²⁺ reached a plateau at 0.1 to 1.0mM and then rapidly decreased. Susceptible tissues, in the presence of Mg²⁺ at 10mM, lost most of their K⁺ by 3hr after exposure to toxin. When toxin-induced K⁺ efflux was stimulated by other cations, the tissues took up such cations. Na⁺ and Mg²⁺ uptakes by toxin-treated tissues continued to increase at a constant rate, but Ca²⁺ uptake leveled off gradually with time. These data show that AK-toxin affects permeability of susceptible plasma membrane to K⁺; the loss of K⁺ may be associated with the presence of certain other inorganic cations.