Abstract
In order to make clear thc mode of the occurrence of favorable effect of root-cutting treatment on the growth behavior of direct sown rice plants, several pot experiments were carried out from the view point of stress physiology. Result of the experiments and conclusion on causes of the occurrence of beneficial effect of root-cutting treatment as obtained from the first and second reports are summarized as follows: 1. Root-cutting treatment on the plants grown by direct sowing culture in well-drained paddy field brought about decreases in stomatal aperture, transpiration, and consequently, water stress on the plant due to lack of water balance, which is seen in the decrease of water content in the leaf and wilting of leaves. The investigation revealed that ethylene production increased in the plant body due to the water stress. 2. Root-cutting inhibited the production of cytokinin in the roots and checked its translocation to the plants grown above the ground surface, resulting in the decrease of chlorophyll content and increase of ethylene production in the leaf. Further investigation revealed that increase of ethylene production by the root-cutting treatment can be prevented by treating the leaves with kinetin solution. 3. The increase in ethylene production by root-cutting is due to water stress and inhibition of cytokinin translocation. It also lowers chlorophyll content in the leaf. However, as new roots emerged seven to ten days after the root-cutting treatment, absorption of nitrogen by roots increased along with reactivated cytokinin translocation. In consequence, chlorophyll content regained its level and after that it became higher than that in the control, and number of fresh leaves increased until later growing stages. 4. After the root-cutting treatment, physiological activity of thc roots, namely, oxidizing ability of α-Naphtylamine, was greatly enhanced due to the emergence of thick new roots and branched roots from the old remaining roots, which had also been accelerated by the treatment. This root activity lasted until the later growth stages. 5. The present experiment also suggested that thc increase of breaking strength which enhanced lodging resistance, observed in the previous report was presumably caused by the increase in ethylene produced in the plant until the later growth stages. 6. It was found that in the treated plants, physiological activity of the upper leaves to adapt themselves to drought condition was enhanced at the maturing stage; this was observed in the decrease of water content in upper leaves and of transpiration rate per unit leaf area, increase of diffusion pressure deficit (D P D) in the leaf blade, and of osmotic pressure in the cell, and decline of water potential in the leaf blade and panicle. The above-mentioned morphological, physiological and ecological changes in the rice plant induced by root-cutting treatment are reflections of water stress and of the changes of plant hormonal behaviour due to the treatment, which resulted in better ripening and increased yield of the rice plants.
