Graduate Thesis Or Dissertation
 

Leaf elongation as a function of soil water potential and photoperiod

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  • Leaf elongation is a major indicator of plant growth. It is controlled by complex interactions of many factors including light intensity, photoperiod, water availability, atmospheric conditions, and plant water stress. In this study, the response of leaf elongation to length of photoperiod and to plant water stress as indicated by leaf water potential was investigated. Fifteen day old seedlings of Sudan grass (Sorghum vulgare) were subjected to soil water potentials ranging from -0.35 bars to -2.50 bars using the polyethylene glycol (carbowax) semi-permeable membrane technique of controlling soil water potential. Leaf elongation rates and leaf water potential changes were measured in response to photoperiods of 5, 6, and 8 hours, in response to the initiation of lighting, and in response to continuous lighting for several days. The elongation rate appeared to increase in a sigmoid curve as the photoperiod was increased up to 8 hours. The sigmoid relationship indicated that the increase in the elongation rate as the photoperiod was increased from zero to 2 hours would be much smaller than the increase in elongation rate as the photoperiod was increased from 2 to 4 hours or from 4 to 6 hours. This was believed to be due to a larger portion of the total photosynthate produced during the shorter photoperiods being used for respiration. For one hour after the initiation of lighting, the plant experienced mild water stress, as the leaf water potential decreased, increased, and decreased again. The elongation rate was depressed during this period of water stress. It was suggested that earlier reports of light inhibition of leaf elongation should at least partly be attributed to light-induced water stress. At a carbowax osmotic potential of -2.50 bars around the soilroot slabs, and under continuous lighting, the total leaf water potential and leaf elongation rate gradually decreased until the experiments were discontinued because of microbial decomposition of the membranes. Extrapolation from these data indicated that leaf elongation ceases at a total leaf water potential of -24.0 bars. This was compared to a value for maize, obtained in an earlier experiment, of -9.2 bars. It was suggested that the ability of Sudan grass to continue leaf elongation at a lower leaf water potential than maize is due to its being able to maintain a turgor pressure sufficiently high for growth at a lower leaf sap osmotic potential.
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