Graduate Thesis Or Dissertation
 

The effect of osmotic moisture stress during germination on Douglas-fir (Pseudotsuga menziesii)

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  • Commercially collected Douglas-fir seed from a moist coastal ecotype (LaPush, Washington) and a dry inland ecotype (Kaibab National Forest, Arizona) were subjected to osmotic stresses ranging from 0 to -8 atmospheres using Carbowax polyethylene glycol 6000 during imbibition and germination. To determine the effect of osmotic stress on the initiation and progression of germination, daily germination was recorded during a 25 day period. Results were compared on the basis of total germination, number of days to the first germinant, and a combined index, germination value. To determine the effect of osmotic stress on growth and moisture uptake during germination, whole seed fresh weight, dry weight, and moisture content were obtained. To determine the effect on respiration and the respiratory quotient, measurements were made during germination using the direct method of Warburg. The total soluble protein content as estimated by Lowry's method and isocitratase activity as estimated by a method proposed by Jacks and Alldridge were determined from the endosperm of four-day old germinants. In addition, seedling fresh and dry weight and radicle length were measured. To determine the effect of osmotic stress on soluble nucleotide and total RNA and DNA contents, samples were obtained from the embryos during the first 12 days of germination using a modified Schmidt-Thannhauser method. The results of all determinations were compared using factorial analyses of variance. The results indicated that: 1) the start of germination was delayed and total germination reduced by osmotic stress; 2) rates of water uptake, oxygen uptake, and carbon dioxide' evolution were reduced while the respiratory quotient was unaffected by osmotic stress; 3) endosperm total soluble protein content of four-day old germinants increased initially in the inland seed source and decreased in the coastal seed source with increasing osmotic stress; 4) enzyme synthesis in the endosperm as measured by isocitratase activity was reduced by osmotic stress; 5) nucleotide and nucleic acid contents of the embryo were reduced by osmotic stress; 6) growth as measured by radicle length of four-day old seedlings was reduced by osmotic stress; and 7) the differences induced by osmotic stress tended to increase with time. The inland source generally had larger relative values than the coastal source for all factors except specific activity of isocitratase which decreased an average of 29 percent in the former and 54 percent in the latter between 0 and 4 atmospheres. This difference in response was felt to be partially due to a combination of a larger initial seed and embryo size, a higher imbibitional water content under stress, and a greater ability to maintain the specific activity of key enzymes. The changes induced by osmotic stress in both sources were felt to be the result of either reduced reactivation of protein synthesis and/or respiration. These changes were induced by a decreased level of cell hydration after imbibition and carried over from the reactivation phase to the de novo synthesis phase of germination,. The basis of resistance to moisture stress during germination appeared to be the ability to maintain the specific activity of key enzymes either by synthesis or by maintenance of the active conformation of the enzyme.
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file details StewartRonaldE1970.pdf 2017-10-20 Pubblico Scaricare