LD2668 .T4 1947 F56 / Master of Science

Call number: LD2668 .T4 1956 B76 / Master of Science

Call number: LD2668 .T4 1978 M92 / Master of Science

Thesis (PhD (Agric.) -- Stellenbosch University, 1984. / INTRODUCTION: The study of seed dormancy and germination has for centuries occupied the minds of agronomists, physiologists, brewers, bakers and, more recently, weed scientists. The agronomist requires that the seed that he sows will germinate rapidly and uniformly and produce a vigorous healthy seedling .. The physiologist is interested in the understanding of the basic processes involved at the molecular level, and the geneticist in the inheritance of the quiescent character of the seed. Brewers seek a seed that will retain its viability at least until the following crop is harvested but which will also, on imbibition, rapidly set in motion those processes that will convert starch into sugar. The baker is concerned with the baking quality of the seed and, as far as he is concerned~ the more dormant the seed the better, as this eliminates the problem of pre-harvest sprouting which is very detrimental to baking quality. The weed scientist seeks to encourage all weed seeds present in the soil to germinate simultaneously so as to enable him to destroy the weed population with one application of herbicide or a single cultivation.

published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

published_or_final_version / Biological Sciences / Master / Master of Philosophy

Germination of seeds of desert annual plants is reduced where there are high densities of annual seedlings. This is interpreted as a response by seeds to avoid a severe biotic environment in which growth rate and fecundity are likely to be reduced by larger established competitors. This density-dependent germination response is due primarily to reduced germination of small-seeded annuals where densities of large-seeded annuals are high. Because of this germination response, and because of competition at the plant stage, large-seeded annuals could, in the absence of significant levels of predation by seed-eating rodents, dominate the annual plant community to a much greater extent than is commonly observed. By reducing densities of large-seeded annuals, rodents allow densities of small-seeded annuals to increase and thus exert a positive indirect effect on granivorous ants. Seed-eating rodents and a parasitic fungus both prey on Erodium cicutarium, a dominant annual plant. These two unrelated predators significantly influence each other's densities by their use of a common prey species. Dispersal of desert annual seeds that successfully germinate is apparently not as widespread as is suggested by observations that some desert annual seeds are redistributed throughout the year by wind and water. Removal of plants during seed set significantly reduced densities of seedlings on sample plots the next year.

Guayule (Parthenium argentatum Gray) is one of two major plants in the world grown for natural rubber and therefore, is of potential importance to the U.S. One area of research interest relates to methods of enhancing production of viable seeds. Therefore, studies were conducted in 1984, 1985, and 1986, in Tucson, Arizona to determine the effect of honey bee pollination on: seed set; seed weight; total seed yield; percentage of seed germination; yield and percentage of rubber and resin content. This experiment involved four guayule cultivars and four pollination treatments: plants caged with bees; plants caged without bees; plants open-pollinated; and plants individually covered with Delnet bags. All four cultivars responded positively to honey bee pollination. Plots with bees produced at least 195% more seeds than plots from which bees were excluded. However, there were no qualitative differences in the seed weights between the treatments. The percentage of seed germination from plots serviced by bees was significantly greater (65%) than from plots without bees (50%). Highest seed germination rates were obtained when seeds were collected in May (80%) and September (76%). June, July, and August seed collections resulted in lower seed germination rates (40%, 26%, and 63%, respectively). The plots in which bees were present gave a higher rubber yield (323 kg/ha) than plots without bees (255 kg/ha). However, there were no quantitative differences in the percentage of resin content between the treatments. These studies demonstrated that (1) honey bees can increase seed yield, seed germination, and rubber content in guayule, and (2) seeds produced during summer months had poorer germination rates and lower yields.

Artificially aged and non-aged seeds of 22 grain sorghum (Sorghum bicolor L.) F₁ hybrids and their F₂ and F₃ generations were evaluated for germination salt tolerance. Six of the hybrids, along with their F₂ and F₃ generations, were tested for emergence salt tolerance. Effects of seed production environments on germination salt tolerance were also studied using F₂ generations of 12 sorghum hybrids produced under full-season irrigation, limited irrigation, and double-cropping conditions. Germination tests were conducted in a growth chamber, on trays, while the emergence test was conducted in a greenhouse in flats filled with sand. The experiments were conducted under non-saline and saline conditions in randomized complete block designs. Artificial seed aging resulted in significant reductions in germination percentages under both non-saline and saline environments. The overall mean reduction in germination caused by seed aging was more than twice as much under salinity stress as under the non-stress conditions (48 vs. 17%), indicating differential effects of salinity on different quality seeds. Entries that appeared to be resistant to seed aging also had higher germination percentages under salinity stress. Seed production environment influenced germination performance in both non-saline and saline environments. The overall mean germination percentages of sorghum seeds produced under three different field environments were significantly different from one another. Significant differences were observed in germination and emergence percentages within F₁, F₂, and F₃ generations. Except for the F₁ entries, however, emergence index differences within the F₂ and F₃ generations were not significant. Correlations between germination and emergence percentages in the non-saline and saline treatments were generally nonsignificant. This suggests that germination and emergence responses of sorghum may vary under different salinity levels and different environments.