There has been no systematic research on the effect of plant growth regulators and silver nitrate treatments on the control of sex expression in watermelon. In this study, we tested sex expression responses of four watermelon forms(monoecism, gynoecism, andromonoecism,and hermaphrodite) to gibberellin, ethephon and silver nitrate treatments. Results have shown that, in monoecious plants, gibberellins(GA_3)and ethephon treatments reduced the percentage of female flowers and delayed the occurrence of the first female flower, while silver nitrate induced the formation of bisexual flowers. In gynoecious plants, both ethephon and silver nitrate treatments transformed some female flowers into bisexual flowers, and treatment with ethephon resulted in a mass of abnormal flowers, while no obvious effect of treatment with GA_3 was observed. In andromonoecious plants, ethephon and GA_3 treatments delayed the occurrence of the first bisexual flower, and GA_3 reduced the percentage of bisexual flowers, while no distinct effect for silver nitrate treatment was observed. In hermaphroditic plants, ethephon treatment induced the appearance of numerous abnormal flowers, while no obvious effects for GA_3 and silver nitrate treatments were observed.We analyzed the transcription levels of all the expressed aminocyclopropane-1-carboxylic acid synthase(ACS) homologues in two gynoecious mutants and their wild types. We also tested the gene expression of CitACS4 which had been recognized as the andromonoecious gene in all treatments. All these results suggested that the best masculinizing treatment for breeding of the gynoecious line is silver nitrate, which repressed the expression of CitACS4 and induced many bisexual flowers for use in self-fertilization subsequently.
