On Resistance to Water Transport from Roots to the Leaves at the Different Positions on a Stem in Rice Plants.

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Abstract
The degree of the midday depression in the stomatal conductance and the photosynthetic rate is larger in the lower leaves than in the upper leaves on a stem. It was clarified in the previous report that resistance to water transport correlates closely with the degree of the midday depression in the stomatal conductance. The present study was to investigate the difference among the resistances to water transport from root surface to leaves (whole resistances to leaves) at different positions on the stem and also to investigate the factors related to the difference among the whole resistances to the leaves. The whole resistance to the older leaves at the lower position on the stem was larger than that to the younger leaves at the upper position. There was no significant difference among the whole resistances to the leaves of the same age. The whole resistance to each leaf increased considerably after the leaf had fully expanded. It was concluded that the longer the time elapsed after the full leaf expansion, the higher the whole resistance to the leaf at the lower position of the stem. The increase in the resistance from the base of a stem to the leaf measured by using the excised stem was far smaller than that in the whole resistance. Since older leaves and younger leaves were closely connected with older crown roots and younger crown roots, respectively, it was suggested that an increase in the whole resistance to thc leaf with time after full leaf expansion might result from the increase in the resistance of the roots connected with the leaf, i. e. the reduction in passive water uptake capacity of the roots due to age of the roots. Also, the larger midday depression in the stomatal conductance of the lower leaves was supposed to be due to the larger whole resistance to them. Moreover, it was also found out that resistance to water transport in the stem and the leaf is the dominant resistance in the whole plant resistance when the leaf had just expanded fully. The ratio of the root resistance to the whole resistance, however increased considerably with age of the leaf.

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