Chloroplast ultrastructure and ethylene production of senescing and rust-infected cowpea leaves

An investigation of the changes in chloroplast ultrastructure induced by rust infection of cowpea leaves revealed that although initial changes in the pustule center resembled those observed in naturally senescing leaves, subsequent changes more closely resembled chromoplast development in certain ripening fruit. Ethylene treatment of healthy leaves induced changes similar to those seen during the later stages of pathogenesis and detached infected leaves released greater amounts of this gas than controls. The restriction of the chromoplast-like changes to the pustule center and the fact that the endogenous ethylene levels of newly picked infected leaves were barely higher than those of comparable healthy tissue suggested that most of the increased production of this gas was associated with the relatively small region of high fungal concentration. In contrast, naturally senescing leaves contained one-sixth, or less, of the ethylene content of infected leaves and released about half as much as infected tissue when detached. Thus it is suggested that, in many plants, increased ethylene production results in chromoplast-like development of the chloroplasts and that the degree of such development during pathogenesis or senescence may reflect the degree of ethylene production during these processes.