Different aspects of the mode of action of cytokinin in the moss Physcomitrella patens were studied. In the search for a cytokinin receptor, a particulate fraction was isolated which had cytokinin-binding activity . Cytokinin-binding a c t ivi ty is described as the amount of binding of a radioactive cytokinin that is prevented from binding by excess unlabel led cytokinin. The cytokinin-binding activity was only detected with a tritiated cytokinin of high specific activity. Two assays were used, an equil ib r ium d ia ly s i s and a c ent r i fugat ion assay. These assays were used to show that the cytokinin-binding a c t iv i ty was heatlab i le and can be solubilised by the detergent Triton X100, but not by acetone. Cytokinin-binding a c t iv i ty was higher in phosphate-starved t is sue which consists mainly of caulonemata, the target cells for cytokinin action. A 13 ,0 0 0-8 0 ,000g pel let appeared to contain a component which has some of the characteristic s expected for a cytokinin receptor that was a membrane bound protein. Mutagenised spores were screened for cytokinin nonresponding mutants. No such mutants were obtained from 25,000 plants. A temperature-sensitive mutant, ove 409, was isolated which produced normal leafy shoots at I7°C, but many abnormal buds at 2 4 ° C . T h i s mutant was found to over-produce isopentenyladenine at the higher temperature. The level of cytokinin production in wild type and ove 78 was also found to be temperature-dependent. The change in cytokinin production by ove 409 was over twice that with wild type and ov.e 78, and dropped to wi ld type levels at 17°C. By the use of protoplast fusion, ove 409 was found to be recessive to wild type and in the same complementation group as ove A78. The role of light was studied in the induction of the gametophore. In the presence of supplied cytokinin, bud induction was caused by red light around 613 to 687nm. When the cytokinin-induced bud induction occurs in the dark after exposure to red light , bud induction is reduced by a short exposure to far-red light or a dark period before the addit ion of the cytokinin- This indicates that phytochrome is involved in bud induction. However, another factor appears to be involved in bud induction as this was greater when the cytokinin was applied to the cultures 2 hours after the exposure to red light. The possible role of calcium ions in bud induction was investigated. Calcium ions have been reported to induce the first stages of buds in the moss Funaria hygrometrica. Using the calcium ionophore A23187, calcium ions were shown to induce chloronemal branching in Physcomitre 11a patens. Therefore calcium ions appear to have a role in branching, but not in the differentiation of a bud.

Ph.D. / The sensitivity of Petunia hybrida flowers to the plant hormone ethylene increases during the early stages of senescence and directly after pollination of the flowers. During these early stages .of senescence, ethylene production is very low and only increases after ethylene sensitivity had already increased. The increase in ethylene sensitivity is due to the forming of short-chain saturated fatty acids that accumulate in the petal tissue during the early stage of senescence. Following pollination, these acids are formed in the stylar tissue and transported to the corolla where it causes a rapid increase in ethylene sensitivity. The synthesis of short-chain saturated fatty acids is controlled by the enzyme acetyl-GoA carboxylase. As the acitivity of this enzyme decreases, the concentrations of short-chain fatty acids increase due to the lack of malonyl-GoA required for chain lengthening. During the early stages of senescence, the activity of this enzyme decreases simultaneously with an increase in short-chain fatty acids and ethylene-sensitivity. The results of this study indicate that acetyl-GoA carboxylase is a key enzyme involved in the control of ethylene sensitivity in petunia flowers.

An experiment was conducted to ascertain the effects of irradiation and various concentrations of an aqueous solution of ortho-chlorophenoxyacetic acid on growth rate, abscission layer formation, fruit set, yield, time of maturity, ascorbic acidj dry weight, total nitrogen, total phosphorous, titratable acidity, reducing sugars, total carbohydrates, roughage, ash weight, sodium, potassium, calcium, magnesium, copper, iron, zinc and manganese of tomatoes. Supplementary illumination beyond that of the normal day length resulted in significant increases in weekly growth rate and iron content, with slight increases in fruit set, titratable acidity, total nitrogen, ash weight and magnesium content of tomatoes. The xylem area of the abscission layer was increased as a result of the supplementary illumination. Supplementary illumination also resulted in significant decreases in ascorbic acid, total carbohydrates, sodium and potassium. There were also slight decreases in yield, reducing sugars and calcium of tomatoes. Under normal day length the ortho-chlorophenoxy-acetic acid was more effective in inducing fruit set than where supplementary light was used. The most effective concentration was 75 p.p.m. The trends of the effects of the treatments are discussed, as well as the significant findings. / Science, Faculty of / Botany, Department of / Graduate

No description available.

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Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 104-107).

Thesis (M.S.)--Texas State University--San Marcos, 2008. / Includes bibliographical references (leaves 72-77). Also available on microfilm.

Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 78-85). Also available in electronic version. Access restricted to campus users.

The work on Spiraea in vitro shoot cultures was done to determine the feasibility of using light quality to modify endogenous phytohormone balances to decrease apical dominance. Such an effect would enable a reduction in the high levels of exogenous cytokinin benzyladenine (BA) applied in culture and thus reduce potential side-effects. The Spiraea in vitro light quality response was characterized by examining the effects of different light wavelengths on growth. A mixture of red/FR induced rates of shoot proliferation with 0.25 mg/1 BA that were as high as rates obtained under white light with 0.5 mg/1 BA. Shoot quality, as determined by the proportion of shoots 1 cm or longer (useful shoots), was highest under red/FR light. The lowest shoot proliferation rate was observed under blue light. When light wavelengths intermediate between blue and red light (green, yellow, and orange) were applied to explants only minor growth modifications occurred. Green light did not inhibit shoot initiation but inhibited shoot elongation at the 0.5 mg/1 BA level. The efficacy of the light source-filter combinations in the first experiment was studied in two further experiments. With the three light sources (tungsten filament, fluorescent, and metal halide) together with a blue filter, results supported the putative blue light inhibitory effect suggested in the first light quality experiment. Under the red filter, the tungsten filament source induced the highest shoot number means at both BA levels used (0.25 and 0.5 mg/1). Two factors may have contributed to the red/FR effect observed in the first experiment; the time under an incubation light regime before transfer to the treatment regime, and the photon fluence rate of each regime. In the subsequent study to examine these factors, shoot initiation was optimized at the lower BA levels of 0.25 and 0.4 mg/1 when cultures under low fluence red/FR were transferred after four weeks to white light of a higher fluence for one more week. Glyphosate, a known promoter of IAA oxidation, was used to investigate the presumed effect of lowered IAA-cytokinin interactions. Two types of responses to glyphosate occurred, each one dependent on the glyphosate concentration. At the lower glyphosate level (0.087 mg/1), cultures under both light regimes with 0.25 mg/1 of BA, showed a strong inhibition of shoot initiation. This inhibitory effect was overcome in cultures with 0.5 mg/1 of BA and an overall stimulatory response occurred as shoot initiation rates were as much as four-fold higher than in the previous experiments. For both BA levels, changes in shoot number were greater under white light than under red/FR. At the higher glyphosate level (0.2 67 mg/1), the shoot initiation rates were greater than glyphosate-free controls for both BA levels under white light although under red/FR the rates were virtually unchanged from controls. The glyphosate effect investigated for Spiraea cultures appears to be influenced by the levels of the cytokinin BA resulting in pleiotropic effects which depend on the specific concentrations of each component. / Land and Food Systems, Faculty of / Graduate