Degree Type


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Degree Name

Doctor of Philosophy




Seedlings of certain soybean cultivars fail to exhibit normal hypocotyl elongation at 25C. Earlier studies at Iowa State University implicated ethylene in this phenomenon. It was also demonstrated that partial removal of cotyledons in the anomalous cultivars results in normal hypocotyl elongation. The present study was undertaken with Amsoy 71 seedlings in an attempt to (1) determine locus of the anomalous temperature sensitivity in the susceptible cultivars and (2) assess roles of seedlings parts and of putative precursors in the control of ethylene evolution in these seedlings;Ethylene evolution by intact seedlings of Amsoy 71, a short-hypocotyl cultivar, is significantly greater at 25C than at 30C. Apical 2-cm hypocotyl segments show a similar temperature dependence for ethylene evolution. Cotyledons and epicotyl, when left attached to each other, evolve ethylene at similar rates at 25C and 30C. Hypocotyl segments from seedlings of Cutler 71 also show enhanced ethylene evolution at 25C. However, segments from Corsoy, a long hypocotyl cultivar, evolve ethylene at a low rate at both 25C and 30C. Partial (50%) removal of Amsoy 71 cotyledons significantly reduces ethylene evolution at 25C but not at 30C;Neither 125 (mu)M methionine nor 10 (mu)M indole-3-acetic acid (IAA) can stimulate ethylene evolution in apical 2-cm segments of Amsoy 71 soybean seedlings when supplied alone, but together they greatly enhance ethylene evolution. In the presence of 10 (mu)M IAA, methionine concentrations of 60, 125, 353, and 1000 (mu)M stimulate ethylene evolution to a similar extent. In the presence of 100 (mu)M IAA, the exogenous methionine requirement is eliminated. The immediate precursor of ethylene, 1-aminocyclopropane-1-carboxylic acid, also stimulates ethylene evolution by hypocotyl segments at both 25C and 30C;Free IAA levels in epicotyls of seedlings grown at 25C and 30C are 294 and 140 ng per gram dry weight, respectively; those in hypocotyls are 236 and 371 ng. IAA levels in cotyledons are consistently lower than in epicotyls and hypocotyls. The levels in cotyledons are similar at 25C and 30C. IAA levels across the seedling parts are not significantly different at the two temperatures, but the interaction of temperature and seedling part is significant;Frequent hypocotyl breakage occurs at 25C in seedlings of the anomalous cultivars, Amsoy 71 and Cutler 71, but not in those of the normal cultivar, Corsoy. Breakage is not the cause of enhanced ethylene evolution, since hypocotyl wounding does not induce additional ethylene evolution.



Digital Repository @ Iowa State University,

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Nader Seyedin



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99 pages