A relationship between plant responses to cytokinins and cyanide-resistant respiration

Cytokinin bioassay material from six sources ( Amaranthus caudatus L.; Avena sativa L.; Cucumis sativus L.; Funaria hygrometrica Hedw.; Pisum sativum L., cv. Alaska and cv. Progress No. 9; Xanthium pensylvanicum Wallr.) was analyzed for cyanide-resistant oxygen consumption (alternative respiration) during the course of a response to benzyladenine (40 μ M ). In five of the six bioassay systems, the alternative pathway was found to be present but disengaged (ϱ=0) in the cytokinin-treated material while untreated controls continued to fully utilize the pathway (ϱ=1). The moss, Funaria hygrometrica , was the exception to this pattern, with the alternative pathway fully engaged in both treated and untreated protonemata throughout the period of cytokinin-induced bud formation. Respiration via the alternative pathway was not inhibited by benzyladenine in this latter tissue, although titrations with benzyladenine in carrot ( Daucus carota L.) cell suspensions mimicked those obtained using salicylhydroxamic acid, an inhibitor of the alternative pathway. The dwarf pea, Progress No. 9, showed no response to cytokinin application in terms of increased ethylene production, and was also found to lack any capacity for the alternative pathway. Taken together, these results point to a role for the alternative pathway in responses of at least some plant tissues to cytokinins.