Study of the tolerance for low temperature of woody plants

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In temperate evergreen tree species, lowering of the turgor loss point of shoot and of the osmotic potential at full turgor from autumn into winter have been well known. These changes in shoot water relations closely relate to the tolerance to low temperature and frost in winter. We tested a hypothesis that such seasonal change in shoot water relations is driven by root temperature by manipulating temperature of root and shoot of Cryptomeria japonica seedlings separately. In field condition, the lowering of the turgor loss point of shoot becomes clear when the temperature was under 5℃. This change in shoot water relations might represent adaptive responses to increased root hydraulic resistance at low temperature, when the viscosity of water was increased. The lowering of the turgor loss point was accelerated by root cooling and delayed by root warming. However, shoot warming did not give any effect on the shoot water relations under the low root temperature (<5℃) in winter. The change in shoot water relations by root cooling needed about 1 week and was reversible by warming of the seedling or the root. The lowering of the turgor loss point was driven much more by the increasing in cell wall elasticity than by the osmotic adjustment such as increase in sugar content. From these results, we clarified that the changes in the shoot water relations from autumn to winter was the response to the decrease of root (soil) temperature and the adaptive response to the winter cold. It was considered that frost injury of Cryptomeria japonica in late autumn and early spring was occurred under the condition when soil temperature was higher than 5℃.