Graduate Thesis Or Dissertation
 

Effects of plant regulators on growth development and seed production of alfalfa development

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  • Experiments were conducted to determine the effects of certain plant regulatory chemicals on seed production of alfalfa (Medicago sativa L.). Two plant regulators, SADH (succinic acid 2,2-dimethylhydrazide) and TIBA (2,3,5- triiodobenzoic acid), received particular attention. Replicated field tests of the plant regulators were carried out on the cultivar 'Talent' in southern Oregon and on 'DuPuits' in the Willamette Valley. Other experiments were conducted in the greenhouse, laboratory, and in controlled environment chambers to explore possible modes of action. In 1972 foliar sprays of SADH and an experimental formulation of succinic acid derivatives, TD-6266-R, resulted in seed yields of 'Talent' of approximately 400 kg ha⁻¹ compared to 168 kg ha⁻¹ in untreated plots. These responses were statistically significant, while lesser seed yield increases due to 2,4-DB [4-(2,4-dichlorophenoxY) butyric acid] and chlormequat [(2-chloroethyl)trimethylammonium chloride] were not significant. Similar field testing in 1973 resulted in no seed yield enhancement by any plant regulator. However, precipitation at the experimental site during the 1973 growing season was only 0.4 cm in contrast to the 30-year average of 4.3 cm. A plant regulator-environment interaction is suggested. Yield component analysis showed that the number of seeds per pod was increased significantly by the same treatments which promoted yield. Number of pods per raceme was also a strong determinant of seed yield. The relative importance of one component of yield, the number of seedbearing racemes per unit land area, is yet to be established. After treatment with SADH and TIBA in 1973, fieldgrown 'DuPuits' alfalfa accumulated more total dry matter and up to twice as much dry matter in pods as did untreated plots. Shoots treated with TIBA exhibited a mean net carbon dioxide exchange (NCE) rate of 21 mg CO₂ dm⁻² leaf area hr⁻¹ compared to 16 mg CO2 dm⁻² hr ⁻¹ for SADH-treated and control shoots. However, dry matter accumulation and NCE responses were statistically nonsignificant at the .05 probability level. Specific leaf weight exhibited an increased diurnal maximum when treated with SADH and TIBA, while the diurnal minimum remained equal to that of control plants. The principal effect of TIBA on reproductive development was via its promotion of the growth and raceme initiation of axillary branches. In a dosage response study, 10 ppm TIBA was most effective, resulting in a fourfold increase in total branch length per primary stem and a highly significant increase in total raceme production. The finding that two axillary structures generally arise from an alfalfa leaf axil may be useful in understanding the phenomena of branching and floral initiation in this crop. Alfalfa genotypes differed in their growth response to environmental conditions. It appeared possible that genotypes also differed in response to applied plant regulators. This research has shown that applied plant regulators can promote flowering in alfalfa and has suggested that the carbon budget of alfalfa may also be affected. These changes may or may not be translated into increased seed production. The net effects are a result of plant regulator interaction with environmental conditions and plant genotype. Thus, variability in the response to plant regulators can be expected.
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