EVIDENCE FOR 2 CLASSES OF RESPONSES OF WATERMELON COTYLEDONS TO BENZYLADENINE

Watermelon (Citrullus vulgaris Schrad.) cotyledons were excised after 24 h of imbibition and cultured for 5 days on filter paper saturated with benzyladenine (BA) solution at concentrations ranging from 10-8 to 10-4 M. Alternatively the cotyledons were exposed to 10-4 M BA for 5 min or 4 h and then cultured in water. We examined the effects of these treatments on several parameters: growth, carotenoid level, lipid disappearance and representative enzymes for plastids, mitochondria and microbodies. Two groups of responses to BA could be distinguished. Growth and mitochondrial enzyme levels showed a dose-response curve with maximal stimulation at the highest concentration tested (10-4 M). A 5-min treatment with 10-4 M BA yielded a very small response. Other responses (increase in carotenoids and in marker enzymes for plastids and microbodies) showed a sharp optimum at 10-5 M concentration in the dose-response curve. A 5-min treatment with 10-4 M BA was sufficient to yield a large response. Irradiation of the cotyledons with white light mimicked several responses to BA belonging to the second group, but none belonging to the first group. © 1979.