Brassinosteriod-induced Femaleness in Cucumber and Relationship to Ethylene Production

The Cucurbitaceae family is noted for a diversity of sex expression phenotypes. Typically, a phase of male fl owers precedes either female or bisexual fl ower production. Sex determination of individual fl owers is regulated by a combination of genetic, environ- mental, and hormonal factors. Ethylene, auxins, and gibberellins have all been shown to infl uence fl ower sex expression in cucurbits. Ethylene, which promotes femaleness, plays a predominant role. In this study, we tested whether brassinosteroids (BR), a more re- cently identifi ed class of plant hormones, also infl uences cucurbit sex expression. Applied epi-brassinolide (epi-BL) caused a signifi cant decrease in time of appearance of the fi rst female fl ower on monoecious cucumber plants, and increased total female fl owers on the main stem. Increasing concentrations had a stronger effect. Of the three species tested, cucumber, melon and zucchini, cucumber was the most responsive to BR. Application of epi-BL also caused an increase in ethylene production by cucumber and zucchini seedlings, suggesting that the BR effect may be mediated by ethylene. To investigate the possible relationship between BR and ethylene on sex expression, we identifi ed the concentration of ethephon (5 ppm) that caused an increase in ethylene production comparable to that induced by 10 µM epi-BL (approximately two-fold). Treatment with 5 ppm ethephon was suffi cient to increase femaleness of cucumber plants, but not zucchini plants, suggesting that the difference in response to epi-BL treatment may refl ect differences in sensitivity to ethylene. Collectively, our results indicate that application of brassinosteroids to cucumber cause earlier and increased female fl ower production, and that the effects may be medi- ated, at least in part, by brassinosteroid-induced production of ethylene.