The regulation of heterospory in Selaginella / Kenton Eugene Brooks.

Tese (Ph.D.) - University of Oregon, 1973

The manner in which heterospory is regulated in Selaginella was studied using two species: S. wallacei Heiron. and S. pallescens (Presl) Spring. The plants were grown under various environmental conditions and they were treated with plant hormones. The effects on the proportion of mega- and microsporangia produced in the strobili were compared quantitatively. S. wallacei was grown under nutritional conditions which caused a high proportion of microsporangia to be produced regularly in strobili. Specimens were sprayed with ethephon (2-chloroethylphnsphonic acid), an ethylene releasing compound, and controls were sprayed with distilled water. The controls continued to produce mostly micro-sporangia, but the ethephon treated plants produced mostly megasporangia. Two files each of megasporangia and microsporangia are regularly produced in the strobili of S. pallescens. Specimens were sprayed with ethephon and controls were sprayed with distilled water. Megasporangia continued to be produced in the normally megasporangiate files, and in addition megasporangia were produced in the two microsporangiate files. S. pallescens was sealed in a large chamber and treated with ethylene gas at selected concentrations. Ethylene gas at 1 ppm in air was effective in causing the preferential production of megasporangia. A tem-fold lower concentrations was also effective, but 0.01 ppm was ineffective. The time during the development of a sporangius at which determination of the type of spores to be produced was ascertained from an experiment in which S. pallescens was first treated with ethylene gas, causing the production of megasporangia in the two microsporan-giate files. The plant was then treated only with air, during which time a fixed number of microsporangia ware produced in the same files. Finally the specimen was returned to an ethylene atmosphere for several weeks of growth. The rate of growth of strobili was recorded during the experiment. The position of the band of microsporangia relative to the positions of the bands of megasporangia induced by ethylene treatment indicates that determination occurs when the sporangium is about six nodes from the apex. The length of a sporangium at the sixth node is between 80 and 130 microns. S. pallescens was sprayed with gibberellic and the control was sprayed with distilled water. Two files each of mega- and microsporangia continued to be produced in the control, but only microsporangia were produced in the strobiii sprayed with kinetin. S. wallacei was sprayed with gibberellic acid and control specimens with distilled water. In the control mostly microsporangia were produced while in the treated strobili mostly megasporangia were produced. Similar results were obtained when S. wallacei was watered with solutions containing gibberellic acid. In these experiments photoperiod had no effect on the proportions of mega- and microsporangia produced. Gibberellic acid did not cause the preferential production of megasporangia in S. pallescens. A computer simulation of the growth dynamics of Selaginella sporangia was developed to correlate the available information and to explore the feasibility of various hypotheses concerning growth of mega- and microsporangia. Excised strobili of S. wallacei were grown in sterile culture on a defined mineral medium. The seven pairs of pre-meiotic sporangia continued to develop and produced sporocytes which underwent meiosis to form normal appearing mega- and micrspore. No new sporangia were initiated during culture. The apex reverted to producing vegetative leaves in the vegetative phyllotaxy. Other data indicates that ethylene gas is produced and evolved by both cut and intact S. wallacei plants. Auxins had no effect on the proportions if mega- and microsporangia produced in either species. Poor mineral nutrition favors the production of microsporangia in S. wallacei, but causes the preferential abortion of megasporangia in S. pallescens.