The plant hormone, abscisic acid (ABA), has previously been shown to have an impact on the resistance or susceptibility of plants to pathogens. In this thesis, it was shown that ABA had a regulatory effect on an extensive array of plant defence responses in three different plant and pathogen interaction combinations as well as following the application of an abiotic elicitor. In unique studies using ABA deficient mutants of Arabidopsis, exogenous ABA addition or ABA biosynthesis inhibitor application and simulated drought stress, ABA was shown to have a profound effect on the outcome of interactions between plants and pathogens of differing lifestyles and from different kingdoms. The systems used included a model plant and an important agricultural species: Arabidopsis thaliana (Arabidopsis) and Peronospora parasitica (a biotrophic Oomycete pathogen), Arabidopsis and Pseudomonas syringae pathovar tomato (a biotrophic bacterial pathogen) and an unrelated plant species, soybean (Glycine max) and Phytophthora sojae (a hemibiotrophic Oomycete pathogen), Generally, a higher than basal endogenous ABA concentration within plant tissues at the time of avirulent pathogen inoculation, caused an interaction shift towards what phenotypically resembled susceptibility. Conversely, a lower than basal endogenous ABA concentration in plants inoculated with a virulent pathogen caused a shift towards resistance. An extensive suppressive effect of ABA on defence responses was revealed by a range of techniques that included histochemical, biochemical and molecular approaches. A universal effect of ABA on suppression or induction of the phenylpropanoid pathway via regulation of the key entry point gene, phenylalanine ammonia-lyase (PAL), when stimulated by biotic or abiotic elicitors was shown. ABA also influenced a wide variety of other defence-related components such as: the development of a hypersensitive response (HR), the accumulation of the reactive oxyden species, hydrogen peroxide and the cell wall strengthening compounds lignin and callose, accumulation of SA and the phytoalexin, glyceollin and the transcription of the SA-dependent pathogenesis- related gene (PR-1). The near genome-wide microarray gene expression analysis of an ABA induced susceptible interaction also revealed an yet unprecedented insight into the great diversity of defence responses that were influenced by ABA that included: disease resistance like proteins, antimicrobial proteins as well as phenylpropanoid and tryptophan pathway enzymes. Subtle differences were found in the number and type of defence responses that were regulated by ABA in each type of plant and pathogen interaction that was studied. This thesis has clearly identified in plant/pathogen interactions previously unknown and important roles for ABA in the regulation of many defence responses.

Legislation to reduce open field burning in grass seed fields within the Willamette valley of western Oregon changed established production practices. In the creeping grasses such as Kentucky bluegrass (Poa pratensis L.) and creeping red fescue (Festuca rubra L.) non-thermal management resulted in reduced yield. Studies were conducted to examine the effects of three stubble height treatments in comparison to open field burning in creeping red fescue seed production. The effects of light quality on characteristics of plant development were investigated in field and controlled environments. Exogenous applications of plant growth regulators (PGR's) were made to elucidate the causes of low seed yields observed without burning. Field plots were prepared in fall of 1994, and 1995 in creeping red fescue commercial production fields as well as at Hyslop research farm in 1995. Three cultivars were included in the trial; Shademaster and Hector, which produce many rhizomes, and Seabreeze which produces few rhizomes. The effects of stubble height, PGRs, and field burning were measured during fall regrowth and flowering. Non-structural carbohydrates available for early regrowth were reduced when stubble was removed below 5.0 cm, particularly in first-year stands. Fall tiller height was increased by stubble remaining and was negatively correlated with flowering. Rhizome development was reduced when stubble was removed mechanically or burned to the crown, whereas yield potential was increased. Fall ethylene application reduced fall tiller height, fall tiller number, and percent fertile tillers the following spring and was similar to control treatment compared with burn. Other PGRs did not produce consistent results in this study. Excess ethylene produced by decaying stubble may impact floral induction and reduce yield potential in creeping red fescue seed crops. Light quality as measured by red:far-red ratio (R:FR) was reduced by canopy closure during regrowth but not by the presence of stubble. In controlled environment studies, red light (R) promoted taller tillers, greater stage of development, and greater tiller number than far-red (FR) light. Sunlight enriched with FR completely inhibited rhizome formation. Results suggest that environments with excess reflected FR may negatively impact early development of creeping red fescue seed crops. / Graduation date: 1998

Pepper plants (Capsicum annuum L.) were grown in field and a greenhouse in Arizona to determine the effects of water stress, root temperature, and exogenously applied growth regulators on cytokinin production and the resulting growth. Research showed that vegetative plants were significantly higher in cytokinin activity and growth parameters than fruiting plants. Also, in root-pruned fruiting plants, cytokinin activity was less than that of intact fruiting plants. In vegetative plants, the competition between removed sinks and the rest of the shoot was reduced and, hence, more cytokinin came from the roots to the shoots. Besides, additional carbohydrates were available and recycled to the roots. In respect to temperature effect, elevating temperature from 15 to 30°C had a pronounced effect of increasing the growth rate and cytokinin activity. The measured parameters declined when temperature exceeded 30°C. Temperatures between 25 and 30°C were found to be optimum. Under experimental conditions, growth regulators (Cytex® and Burst®) applied to the soil or foliage had no significant effect on growth rates or cytokinin activity in roots. Also, applying Burst® or kinetin to the nutrient medium had inconsistent and statistically nonsignificant effects on photosynthesis and transpiration.

Lowther and Boll in the study of the effect of 2,4,5-TCPAA on etiolated bean leaf discs observed that inhibition of expansion at low concentrations was relieved at high concentrations where growth was equal to that of the light plus Co control. This means that at appropriate concentrations, TCPAA completely replaced Co in the expansion of light-treated leaf discs. [...]

Plant growth regulators and nitrogen fertilization have been associated with the increased incidence of fusarium head blight, a destructive disease of wheat (Triticum aestivum L.). In Canada, the major causal organism of this disease is Fusarium graminearum Schwabe, the conidial state of Gibberella zeae (Schw.) Petch. Most studies concerning the effect of plant growth regulators on fusarium head blight were conducted in fields with natural infection. The objective of this research was to evaluate the effect of growth regulators and nitrogen fertilizer on the incidence of fusarium head blight of wheat with artificial inoculations. / A survey conducted in a field trial testing the effect of the plant growth regulator Cerone on the yield components of several cultivars of spring wheat showed that Cerone treatments increased Fusarium infection only in cultivar Columbus. Further research was conducted using cultivar Max, a cultivar susceptible to fusarium head blight, which is widely grown in Quebec. In controlled-condition greenhouse trials, the growth regulators Cycocel and Cerone, as well as nitrogen fertilization did not influence the disease progress. In the 1991 field experiment, the highest incidence of seed infection was observed in Cycocel treatments when the macroconidia of F. graminearum were directly applied to the heads, but not significantly different from the non-treated control. None of the nitrogen levels affect the incidence of seed infection. In the 1992 field trial, the plots were treated with macroconidia of F. graminearum applied to the heads or with Fusarium-colonized corn applied to the rows. Both Cycocel and Cerone significantly increased the incidence of spikelet only in the colonized corn treatments. Cycocel also increased the incidence of seed infection, but only in colonized corn treatments. Cycocel also increased the incidence of seed infection in the non-inoculated treatments. Growth regulators had no effect on the disease when heads were inoculated directly with macroconidia.

No description available.

Thesis (M.S.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on January 8, 2008) Includes bibliographical references.

Thesis (Ph. D.)--Illinois State University, 1969. / Title from title page screen, viewed Aug. 24, 2004. Dissertation Committee: Robert M. Chasson, M.J. Nadakavukaren (co-chairs), Arthur D. Bond, John L. Frehn, David F. Weber. Includes bibliographical references (leaves 106-114). Also available in print.

Thesis (M.S.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 60-64). Also available on the World Wide Web.

Thesis (MScAgric)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.