Interaction of Ethylene and a Cytokinin in Promoting Hypocotyl Elongation in a Dwarf Strain of Watermelon

Experiments were conducted to study the interaction of ethylene and the cytokinin N(6)-benzyladenine (BA) in promoting hypocotyl elongation in a dwarf strain of watermelon (Citrullus lanatus [Thunb] Matsu. and Nakai). Optimum promotion of hypocotyl elongation is elicited by an apical treatment with 0.2 microgram BA. At dosages above 0.3 microgram per apex, BA-enhancement of elongation is reduced concomitant with stimulation of ethylene production and lateral expansion of hypocotyls. Application of the ethylene biogenesis inhibitor, aminoethoxyvinylglycine, at dosages from 0.3 to 10 micrograms per apex inhibited BA-induced ethylene production. In seedlings treated with 0.2 microgram BA, 10 micrograms aminoethoxyvinylglycine per apex reduced ethylene production to about one-third of control levels and reduced BA stimulation of hypocotyl elongation by 74%. Exposure of watermelon seedlings to 60 +/- 10 nanoliters per liter of ethylene in a flowing system nearly eliminated aminoethoxyvinylglycine inhibition of BA-promoted growth. The results suggest that physiological levels of internal ethylene are required for cytokinin promotion of hypocotyl elongation in watermelon.