Oxidative burst and electrolyte leakage induced by sulfhydryl blockers and by membrane permeabilizing reagents in different organs of Egeria densa

Sulfhydryl blockers, such as N-ethylmaleimide, iodoacetate and heavy metals induce a transitory stimulation of Or consumption and H2O2 production (oxidative burst) and a rapid release of electrolytes in leaves of various aquatic plants. The correlation between these two responses to N-ethylmaleimide or to Ag+ in separate organs and stages of leaf development was investigated in Egeria densa. Only adult leaves were able to respond to the sulfhydryl blockers with an oxidative burst, whereas this response was absent in immature growing leaves and in stem and root segments. In N-ethyl maleimide- as well as in Ag+-treated adult leaves the oxidative burst was constantly associated with a relevant electrolyte leakage. These data are consistent with a model in which the SH reagent would first interact with a plasmalemma protein, leading to an increase in passive permeability to ions and to the activation of an oxidative enzyme of the type of the superoxide synthase described for granulocytes. In its rum, active-oxygen species produced by the activated oxidase might further damage the plasma membrane, increasing its passive permeability. Digitonin and nystatin, two reagents known to cause a permeabilization of lipid membranes, induced in adult E. densa leaves a transient increase in the rate of O-2 consumption and H2O2 production and an electrolyte leakage very similar to those induced by sulfhydryl blockers. These effects, however, were nor influenced by the flavin analogues diphenylene iodonium and quinacrine, and were partially inhibited by the presence of CN- and salicylhydroxamic acid, thus suggesting the involvement of a different oxidase in the oxidative burst elicited by these reagents.