Oxidative burst and electrolyte leakage induced by sulfhydryl blockers andby 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 (oxi dative burst) and a rapid release of electrolytes in leaves of various aqua tic plants. The correlation between these two responses to N-ethylmaleimide or to Ag+ in separate organs and stages of leaf development was investigat ed in Egeria densa. Only adult leaves were able to respond to the sulfhydry l blockers with an oxidative burst, whereas this response was absent in imm ature growing leaves and in stem and root segments. In N-ethyl maleimide- a s well as in Ag+-treated adult leaves the oxidative burst was constantly as sociated with a relevant electrolyte leakage. These data are consistent wit h a model in which the SH reagent would first interact with a plasmalemma p rotein, leading to an increase in passive permeability to ions and to the a ctivation of an oxidative enzyme of the type of the superoxide synthase des cribed 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 tr ansient increase in the rate of O-2 consumption and H2O2 production and an electrolyte leakage very similar to those induced by sulfhydryl blockers. T hese effects, however, were nor influenced by the flavin analogues diphenyl ene iodonium and quinacrine, and were partially inhibited by the presence o f CN- and salicylhydroxamic acid, thus suggesting the involvement of a diff erent oxidase in the oxidative burst elicited by these reagents.