1975 Volume 43 Issue 4 Pages 377-382
1. Relationship between the resistance to the leaf burn and the drought resistance of detached leaves was studied, since the leaf burn seemed to be caused by dehydration. The drought resistance is devided into two components: (a) drought avoidance, which means ability to prevent water loss under a drought condition, and (b) drought tolerance, which means ability to remain alive at a low water content. The authors have made an attempt to evaluate the drought avoidance of leaves as the time required from saturation to 50 percent death by dehydration in a sealed chamber of which the water vapor pressure was controlled, and to evaluate the drought tolerance of leaves as the water saturation deficit at the death.
2. In preliminary, the changes of water saturation deficit and desiccation injury of detached Bartlett leaves were observed at intervals of 30 minutes under four conditions of 8 to 16mm Hg water vapor pressure deficit. When the water saturation deficit of leaves was over 30 percent, a desiccation injury began to occur, and when it reached about 60 percent, about a half of leaf area was injured under any condition of water vapor pressure deficit. The value of water saturation deficit when a desiccation injury occurred in the detached leaves was higher than in the intact leaves reported previously.
3. There were large differences in the ability of drought avoidance of leaves of pears, Japanese pears, apples, peaches and grapes. It was the lowest in pears and the highest in peaches. The ability of drought tolerance was highest in peaches, followed by apples, pears, Japanese pears and grapes in the order, but the differences among them were not so large.
4. Within nine cultivars of pear tested, there was a tendency that the leaves of resistant cultivars to the leaf burn had higher ability of drought avoidance than that of sensitive cultivars, but it was difficult to find out a certain relationship between the ability of drought tolerance of the leaves and the resistance to the leaf burn.
5. The resistance to the leaf burn varied among the trees within a same cultivar. In the Bartlett cultivar which is very sensitive to the leaf burn, the percentage of burnt leaves in a showed significant negative correlation with the ability of drought avoidance of leaves detached from the same tree. On the other hand, there was a very low correlation between the percentage of burnt leaves in a tree and the ability of drought tolerance of leaves detached from the same tree.
6. The abilities of drought avoidance and tolerance of the detached leaves of Bartlett pear, especially the former, showed a tendency to decline with age. This seemed to be closely related to the fact that the occurrence of the leaf burn was localized to the old leaves within a tree.