Mechanisms on the increase of resistance to water transport which causes the acute wilting and death of organs in crop plants growing under sufficient siol moisture conditions.

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Even though crop plants are growing under sufficient soil moisture conditions, acute severe wilting and dehydration sometimes occur under intense transpiration. The remarkable increase in resistance to water transport is responsible for the acute severe wilting in cucumber plants and the desiccated white head in rice plants. This research was undertaken in order to investigate the mechanisms on the increase in the resistance.1.(1)The leaf wilting resulted mainly from the increase in resistance to water transport at the base of the stem. Many xylem vessels were occluded by tyloses significantly at the hypocotyl in the wilted cucumber plants compared to the non-wilted plants which means that formation of the tyloses is responsible for the increased resistance. When leaf water potential was decreased by partial excision of the root systems, the resistance at the base of the stem and the formation of tyloses in xylem vessels increased significantly. (2)The mechanism for the acute severe wilting was considered as follows : (i)stem water potential decreases under intense transpiration, (ii)tyloses form in xylem vessels of the hypocotyl at some critical low water potentials all occur as a positive feedback leading to the acute severe wilting and death of leaves.2.(1)The desiccated white head resulted maily from the remarkable increase in stem resistance to water transport from the flag leaf node to the neck node of panicle. The increase of the stem resistance might be induced by the reduction in stem water potential due to the increase of panicle transpiration under the conditions of high temperature, low humidity and high wind velocity. (2)Because the formation of tyloses in xylem vessels might be responsible for the remarkable increase of stem resistance.