Exogenous Nitrate as a Terminal Acceptor of Electrons in Rice (Oryza sativa) Coleoptiles and Wheat (Triticum aestivum) Roots under Strict Anoxia

Abstract

To elucidate the physiological role of exogenous nitrate under anaerobic conditions, we studied the effect of 10 mM KNO3 on the mitochondrial ultrastructure in rice (Oryza sativa L.) coleoptiles and in wheat (Triticum aestivum L.) roots, detached from four-day-old seedlings, under strict anoxia. In wheat roots, following 6-h-long anoxia in the absence of exogenous nitrate, the mitochondrial membranes were partially degraded and, after 9 h under anoxia, the mitochondrial membranes and the membranes of other organelles were completely destroyed. In rice coleoptiles, the partial membrane degradation was observed only after 24 h and their complete breakdown after 48 h of anaerobiosis. In the presence of exogenous nitrate, no membrane destruction was noticed even after 9 and 48 h of anaerobiosis in wheat roots and rice coleoptiles, respectively. These results indicate that exogenous nitrate exerts protective action as a terminal electron acceptor, alternative to the molecular oxygen. Our findings are compared with the results of other researchers concerning the adverse or favorable nitrate action on plant growth, metabolism, and energy status under anaerobic stress.