Differential hormone responses in different growing zones of the bean hypocotyl

Differential growth responses to plant growth substances-a cytokinin (benzyl adenine), gibberellic acid (GA3), indoleacetic acid (IAA), abscisic acid (ABA), coumarin and ethylene-and to environmental factors (carbon dioxide and red light) were found in tissues of different age in a 4-cm segment of the etiolated hypocotyl of Phaseolus vulgaris L. Elongation in all zones of the hypocotyl which were growing or were about to grow was generally enhanced, but each factor or phytohormone acted specifically on a particular zone, in the order cytokinin, red light, GA3, CO2, C2H4 and IAA, going from the top of the hypocotyl. ABA inhibited elongation both in the red-light- and GA3-dependent, immature zones and in the IAA-dependent, relatively mature zones. The effects of C2H4 and CO2 were opposite and those of auxin and coumarin were similar.