| Sex determination in flowering plants is complicated and variable at flower, individual, population, species, and higher taxa levels, and despite its importance, basic questions have still not been clearly answered. A mechanistic model is proposed that assumes that one hormone has male and female receptors to inhibit one sex and induce the other independently. The range of hormone concentration and the two sensitivity levels of the receptors interact to regulate sex expression. The change of hormone concentration serves as a pivotal control of male and female expression. Every possible relationship among the three components matches a sex form in nature, and all sex forms in nature can be explained by these relationships. Furthermore, evolutionary transitions among breeding systems are suggested. The model is also supported by most of the best-documented studies of individual species available in the literature.; The model was tested in buffalograss (Buchloe dactyloides (Nutt.) Engelm.) in a series of experiments using exogenous gibberellin (GA), cytokinin, auxin, Ethrel, and a gibberellin inhibitor (paclobutrazol, PAC). Only GA exhibited consistent effects on sex expression, promoting maleness and inhibiting femaleness at the same time, whereas PAC had the opposite effects. Two model predictions were confirmed. GA transformed two dwarf female genotypes into neuter and near-neuter plants as is predicted for genotypes with a physiologically-wide overlapping of male and female sterile regions. High levels of GA also induced a normal female genotype to produce some male inflorescences.; Applications of GA and PAC in cucumbers (Cucumis sativus L.) showed that GA had the dual functions of promoting maleness and inhibiting femaleness. Conversely, applications of ethylene and AgNO{dollar}sb3{dollar} (an ethylene inhibitor) indicated that ethylene induced femaleness and inhibited maleness. Results of various combined applications of the hormones and their inhibitors suggested that ethylene had overriding effects on GA and acted more directly on sex expression in cucumber. These experiments indicated that there is only one hormone, not two, regulating sex expression in cucumber, and that the sex hormone is likely to be ethylene. Model predictions of the possible sex forms that can be induced from a monoecious line and a hermaphroditic line were confirmed. |
