EFFECTS OF EXOGENOUS GROWTH-REGULATORS AND A GIBBERELLIN INHIBITOR ONSEX EXPRESSION AND GROWTH FORM IN BUFFALOGRASS (BUCHLOE-DACTYLOIDES),AND THEIR ECOLOGICAL SIGNIFICANCE

Populations of buffalograss (Buchloe dactyloides (Nutt.) Engelm.) cons ist of varying proportions of plants with male inflorescences only, fe male inflorescences only, or a mixture of male and female inflorescenc es that appears to depend on environmental conditions. To investigate the degree of stability in sex expression and its potential hormonal c ontrol, we applied exogenous gibberellin, cytokinin, auxin, Ethrel, an d a gibberellin inhibitor (paclobutrazol) to monoecious, male, and fem ale plants of buffalo-grass in a series of experiments. No changes in sex type in male and female plants were observed. Gibberellin showed s ignificant effects in promoting maleness in monoecious genotypes and i ncreasing male inflorescences in male plants. Paclobutrazol greatly en hanced femaleness in monoecious genotypes. For all other hormones, the re was no consistent effect on sex expression. For all genotypes paclo butrazol greatly reduced plant height and stolon internode length and produced a dense prostrate mat by changing the tiller habit from uprig ht to prostrate. In contrast, gibberellin-treated plants had a higher growth rate and longer stolons (due to increased internode lengths) th an controls. Furthermore, the gibberellin-treated female culms were si gnificantly taller than controls, while male culms were not, also indi cating that a higher gibberellin endogenous level is associated with t he production of male inflorescences in buffalograss. There were highl y significant genotypic differences within the same sex type in number s of inflorescences and in the inflorescence sex ratio in monoecious p lants. The results are consistent with the hypothesis that a mutation( s), greatly reducing gibberellin levels or its activity and leading mo re or less simultaneously to shortened culms, contracted inflorescence s, and increased femaleness, initiated the evolution of dioecy in buff alograss.