Increases in wheat production due to breeding may be reaching a plateau. The application of N-fertilizer beyond optimum levels leads to lodging and to an increase in tiller death. The use of plant growth regulators (retardants) may be an answer for future increases in production. Experiments were conducted in 1983 and 1984 to apply growth retardant paclobutrazol to reduce lodging under heavy rates of nitrogen, to determine if such an approach could reduce tiller mortality, partition more dry matter to the seed, and increase yield and yield components of the semi-dwarf winter wheat variety Yamhill. The application of paclobutrazol significantly reduced lodging promoted by nitrogen fertilizer. The reduction in lodging did not affect yield in part because lodging occurred late at the hard dough stage of development. The application of paclobutrazol at floral initiation actually increased tiller death by promoting the formation of late tillers that died before reaching maturity. Investigation of applying paclobutrazol at earlier stages of development of wheat is suggested. Paclobutrazol application reduced the total dry matter production by decreasing dry matter accumulation in the stem. Dry matter lost from stems was not apparently relocated in the ear. Ear dry weight remained the same following the application of paclobutrazol. Nitrogen application in these tests seemed to reduce the capacity of the ear to accept carbohydrate by decreasing the number of florets per spikelet. It also decreased seed weight in 1983 and grain yield in 1983 and 1984. An increase in L.A.I. (leaf area index) may have increased mutual shading with negative effects on tiller development. Paclobutrazol application decreased grain yield and harvest index by decreasing seed weight and by reducing the number of grains per ear in 1984. The decrease in seed weight was attributed to competition from non-surviving tillers and to the reduction in L.A.D. (leaf area duration) due to stripe rust incidence. Further work is needed to evaluate possible paclobutrazol x N interaction on a variety of wheat susceptible to lodging. Different locations and more effective control of diseases would be required for such studies. Application of paclobutrazol at different stages of growth should be investigated. Also, an investigation of paclobutrazol interference with nitrogen uptake and utilization is suggested.

Masters Theses Listed by Discipline: Aerospace Engineering. Agricultural Economics, Sciences and Engineering. Architechtural Engineering and Urban Planning. Astronomy. Astrophysics. Ceramic Engineering. Communications Engineering and Computer Science. Cryogenic Engineering. Electrical Engineering. Engineering Mechanics. Engineering Physics. Engineering Science. Fuels, Combustion, and Air Pollution. General and Environmental Engineering. Geochemistry and Soil Science. Geological Sciences and Geophysical Engineering. Geology and Earth Science. Geophysics. Industrial Engineering. Marine and Ocean Engineering. Materials Science and Engineering. Mechanical Engineering and Bioengineering. Metallurgy. Meteorology and Atmospheric Science. 17 additional disciplines. Index.