Graduate Thesis Or Dissertation

 

Physiological effects of water stress on young corn plants Public Deposited

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  • Laboratory experiments were used to investigate the mechanism of plant response to water stress by determining the sensitivity of leaf elongation, photosynthesis and transpiration in young corn plants to a decrease in leaf water potential. In initial experiments, 9 day old corn plants were grown at soil water potentials of -0.35 and -2.50 bars for 6 days using the polyethylene glycol semi-permeable membrane technique of controlling soil water potential. Leaf elongation and soluble carbohydrate content were found to be more sensitive to a reduction in soil water potential than net assimilation and transpiration. Lowering the soil water potential from -0.35 to -2.50 bars resulted in a 44 percent decrease in the rate of leaf elongation and a 42 percent increase in the soluble carbohydrate content of the plant, while the rates of net assimilation and transpiration were reduced by 26 and 24 percent respectively. The differing sensitivity of leaf elongation and photosynthesis to decreasing soil water potential was examined in detail in subsequent experiments by simultaneously monitoring the rates of net photosynthesis, transpiration, and leaf elongation and the leaf water potential of a young corn plant as it became water stressed. Leaf elongation ceased at a leaf water potential of -9 to -9.5 bars, whereas the rates of net photosynthesis and transpiration were not reduced significantly until the leaf water potential reached -12 to -13 bars. The sharp decrease in rate of net photosynthesis in the vicinity of -12 to -13 bars was due to increases in both the stomatal and mesophyll resistances to CO₂ transfer. It was concluded that the decreases in the rate of net photosynthesis due to water stress were caused by stomatal and nonstomatal effects of approximately equal magnitude. The nonstomatal or intracellular factors responsible for the decrease in the rate of net photosynthesis were not identified. It was hypothesized that the differing sensitivity of leaf elongation and photosynthesis to water stress, may result in the accumulation of photosynthate within a mildly water stressed plant. This mechanism was demonstrated in experiments where the soluble carbohydrate content of the top 3 leaves of a mildly stressed corn plant was shown to be significantly higher than in the corresponding leaves of a nonstressed control plant after a 6 hour stress period. This increase in the soluble carbohydrate content of the mildly stressed plant was accompanied by a significant decrease in the net photosynthetic rate of this plant. A similar response was obtained when leaf elongation rate was reduced by lowering the temperature of the apical meristem. The results were interpreted as evidence for the operation of a source-sink type control mechanism of photosynthesis in mildly stressed plants. The effect of water potential on the elongation rate, the adenylate energy charge, ATP content and free amino acid content of the youngest unrolled leaf of a 6 leaf corn plant was examined in a final series of experiments. These experiments sought to determine whether the decrease in photosynthate utilization, observed when cell expansion was limited by water stress, was due to a simple product inhibition feedback mechanism or to the direct effect of water stress on some aspect of cell metabolism. The ATP content of the elongating cells was found to be as sensitive as leaf elongation to small changes in leaf water potential. Adenylate energy charge did decrease with leaf water potential, but was not as sensitive as the ATP content to changes in leaf water potential. The free amino acid level was found to increase at leaf water potentials lower than -10 bars, and this may indicate that the inhibition of protein synthesis during water stress may be due to the deficiency of chemical energy within the cell. It was concluded that during water stress, the biosynthetic activity of elongating cells may be limited by a low level of available energy in the form of ATP. This decreased synthesis of ATP may be due to a direct effect of water stress on respiration and ATP formation. The sensitivity of leaf enlargement to mild water stress and its subsequent effect on photosynthesis indicates that plant growth and production may be limited by mild stress in the field situation. The vegetative growth of plants depends on both the photosynthetic rate and the rate of increase of the photosynthetic surface area. The response of leaf enlargement to water stress in the field therefore warrants thorough further investigation.
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