The Relationship between Environmental Factors and Water Status in the Rice Plant : I. On leaf water potential, leaf water content on an areal basis and water saturation deficit in leaf blades

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In the previous report, it was considered that the slight variation of leaf water content on an areal basis (leaf water content) gave much influence upon stomatal aperture). The present study was conducted to establish other measurement methods of water status in the rice plant, i.e., leaf water potential and water saturation deficit (WSD), and to discuss which of them was satisfactory for measuring water status in the rice plant. The results obtained are as follows: Three hours for equilibration of water potential between gas phase and rice plant leaves in psychrometer chamber were required for leaf water potential measurement (Fig. 1) and full turgidity was attained in four hours after immersing leaves in water for WSD measurement (Fig. 2). Diurnal changes of water status in leaf blades were measured by leaf water potential, leaf water content and WSD. The results were that water in leaf blades was contained sufficiently in the early morning and it decreased in the daytime and again increased towards the evening (Fig. 3 and 4). Close correlation was found between leaf water potential and leaf water content, and between leaf water potential and WSD at each leaf position on the stem, and the height of those regression lines was different from each other depending on leaf position on the stem (Fig. 5). In the early morning when there was no water stress in the rice plant, leaf water potential was constant being about -2 bars through all leaf position investigated. On the contrary, leaf water content and WSD were different from each other depending on leaf position on the stem, i.e., the maximal values of leaf water content and WSD were obtained for 11th and 12th leaves, 10th and 11th leaves, respectively. The values of leaf water content and WSD gradually decreased both to upper and to lower leaves as they were away from the leaves with maximum values (Fig. 6). At the same degree of wilting of leaves or when apparent photosynthetic rate was reduced to zero due to water deficit in leaves, leaf water potential was almost constant through all leaf position investigated, but leaf water content and WSD were different from each other depending on leaf position on the stem (Table 1 and 2). From the results mentioned above and that the standard deviation of measured values of leaf water potential at the same water status was much less than that of leaf water content and WSD, it is considered that leaf water potential expresses leaf water status physiologically and it is more suitable for measurement of water status in the rice plant of which stomatal aperture is much influenced by the slight variation of water status. It is assumed that stomatal aperture changes with leaf water potential and that the stomata of the rice plant begin to close due to the slight decrease of leaf water potential.