Gibberellins and Heterosis in Maize : II. Response to Gibberellic Acid and Metabolism of [H]Gibberellin A(20).

Two maize inbreds, CM7 and CM49, and CM7 x CM49, their F(1) hybrid (which displayed significant heterosis), were examined with regard to response to exogenous gibberellin A(3) (GA(3)), and in their ability to metabolize GA(20), a native GA of maize. The leaf sheath elongation response to GA(3) was far greater for the imbreds than for their hybrid. The inbreds also displayed significant elongation of the leaf blades in response to GA(3), whereas the hybrid was unaffected. Promotion of cell division in the leaf sheath of CM7 and the hybrid was effected by GA(3), but no promotion of cell elongation was observed in CM49, even though significant leaf sheath elongation occurred. Shoot dry weight of both inbreds was significantly increased by GA(3), but response by the hybrid in this parameter was slight and variable. Root dry weight of CM7 was significantly increased by GA(3), but was unchanged in CM49 and the hybrid. Thus, inbred shoot dry weight increases effected by GA(3) were not at the expense of the root system. Rapid metabolism of [2,3-(3)H]GA(20) occurred in all genotypes, although genotypic differences were observed. The hybrid had the highest rates of metabolism to GA glucosyl conjugate-like substances. Oxidative metabolism was also fastest in the hybrid, followed by CM7, and slowest in CM49, the slowest-growing inbred. Thus, rate of GA(20) metabolism is under genetic control in normal (i.e. not dwarfed) maize genotypes. These results, taken together with previous reports that the hybrid has significantly enhanced levels of endogenous GA-like substances, suggest that GA play a role in the expression of heterosis in maize.