Study of the tolerance for low temperature of woody plants

在温带万年青树种中，从秋季到冬季，枝条的膨压损失点和完全膨压时的渗透势降低是众所周知的。这些水分关系的变化与冬小麦对低温和霜冻的耐受性密切相关。通过分别控制柳杉幼苗的根和冠的温度，验证了根系温度是导致枝水分关系季节性变化的主要原因的假设。在田间条件下，当温度低于5℃时，新梢膨压丧失点降低的趋势明显。这种水分关系的变化可能代表了低温下根系水阻力增加的适应性反应，当水分粘度增加时。膨压丧失点的降低被根冷加速，被根暖延迟。在冬季根温较低（<5℃）的条件下，茎部增温对水分关系无明显影响。根系降温引起的地上部水分关系的变化需要1周左右，幼苗或根系加温可使其恢复。膨压损失点的降低更多地是由细胞壁弹性的增加而不是由渗透调节如糖含量的增加驱动的。从这些结果中，我们阐明，从秋季到冬季的变化，在地上部水分关系是根（土壤）温度下降的反应和适应性反应的冬季寒冷。认为柳杉晚秋早春冻害发生在地温高于5℃的条件下。

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℃.