TESTS OF A MECHANISTIC MODEL OF ONE HORMONE REGULATING BOTH SEXES IN CUCUMIS-SATIVUS (CUCURBITACEAE)

A mechanistic model of sex determination in flowering plants, which as sumes that one hormone has male and female cell receptors to inhibit o ne sex and induce the other independently, was tested in cucumbers (Cu cumis sativus) by applications of hormones and their inhibitors. Appli cations of gibberellic acid (GA) and an inhibitor of its synthesis (pa clobutrazol) showed that GA had dual effects on sex expression of prom oting maleness and inhibiting femaleness. Conversely, applications of Ethrel (an ethylene release agent) and AgNO3 (an ethylene action inhib itor) indicated that ethylene induced femaleness and inhibited malenes s. Results of various combined applications of the two hormones and th eir inhibitors suggested that ethylene had overriding effects on GA an d acted more directly on sex expression in cucumber. These experiments indicated that there is only one hormone, not two, regulating sex exp ression in cucumber and that the sex hormone is likely to be ethylene. Results thus supported the assumption of the model that one hormone c an regulate both sexes by inducing one and inhibiting the other indepe ndently. Model predictions were confirmed that from a monoecious line, female plants could be induced by increasing the ethylene level, and male plants induced by decreasing the ethylene level. On the other han d, as the model predicts, applications of ethylene or its inhibitor in duced gynomonoecious, female, and trimonoecious plants from a hermaphr oditic line. In conclusion, all assumptions and predictions of the one -hormone model were confirmed in the experiments.