In the previous report, a large difference was found between the leaf water potential(Ψ
1) measured by the thermocouple psychrometer and the xylem water potential (Ψ
x) measured by the pressure chamber under intense transpiration in the rice plant, and the water status in transpiring leaves was discussed. The present study was conducted to investigate the interrelations of Ψ
1 and Ψ
x among leaves on the different position of a stem or portions in a leaf blade in the rice plant and corn and to discuss the water condition of xylem and mesophyll cells in plants. The results obtained are as follows: Close agreement between Ψ
1 and Ψ
x was obtained in the dark room or under severe water stress with deficient moisture in soil, that is, under practically no transpiration, over wide range of water potential. With sufficient moisture in soil both under sunlight and artificial light, that is, under intense transpiration, Ψ
x was much lower than Ψ
1. These relations between Ψ
1 and Ψ
x were also recognized in the previous report. Furthermore, close correlation was found between transpiration rate and Ψ
1, and between transpiration rate and Ψ
x, but Ψ
x was lower than Ψ
l for a given increase of transpiration rate. With sufficient moisture in soil, independently of low Ψ
x of other leaves exposed to the sun even on a same stem, Ψ
x of the shaded leaves became as high as that when a whole plant was shaded, and independently of low Ψ
x of the apical part of a leaf blade exposed to the sun even in a same leaf, Ψ
x of the shaded basal part of a leaf blade became as high as that when a whole leaf was shaded. But under severe water stress with deficient moisture in soil, Ψ
x of the shaded leaf and the shaded basal part of a leaf blade remained as low as Ψ
x of other exposed leaves and the exposed apical part of a leaf blade, respectively. Interesting enough was the fact that even with sufficient moisture in soil, Ψ
x of the shaded apical part of a leaf blade was higher by only one or two bars than that of the exposed basal part of a leaf blade, and was considered to be affected by the latter. From these results, it is considered that negative pressure in vessels due to transpiration is hardly transmitted to the parts of a plant in reverse direction of transpiration stream, and negative pressure in vessels due to low root water potential or transpiration is transmitted to the parts of a plant in direction of transpiration stream away from the water resources. Ψ
1 of a shaded leaf of a given leaf position on a stem on which all leaves of other leaf position were exposed to the sun was lower than Ψ
1 of the leaf of the same leaf position when a whole plant was shaded. And even if any part of a leaf blade was shaded, Ψ
1 of a shaded part of a leaf blade was almost equal to Ψ
1 of the other part of a leaf blade exposed to the sun. Therefore, it is considered that Ψ
1 of organs or tissues was interrelated with that of other parts of a plant. Furthermore, it was recognized that, though Ψ
x ofleaves under intense transpiration was practically equal to Ψ
x of leaves under water stress with deficient moisture in soil, Ψ
1 of the former was much higher than Ψ
1 of the latter. From the results mentioned above, it is considered that the pressure chamber can be used only for the leaves without transpiration to estimate leaf water potential, and that using the pressure chamber to measure leaf water potential under condition of intense transpiration may give an erroneous result. [the rest omitted]
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