1981 Volume 50 Issue 3 Pages 297-305
The close relationships between plant water stress and deficient water supply has long been recognized, but no work has been done on the water absorption linked to transpiration in pear trees which often suffer from water stress, for instance ′leaf burn′. In the present study, water balances (transpiration versus absorption) were investigated by employing a technique of auto-irrigator, the method by Impens et al. measuring transpiration and a heat pulse method, using Bartlett and Red Bartlett (Pyrus communis Rehder) on Pyrus serotina rootstocks and Nijisseiki (Pyrus serotina Rehder) on the same rootstock grown in a glass house.
1. The rates of daily water absorption (A) and transpiration (E) were greatest in Red Bartlett among 3 cultivars throughout the season. The seasonal changes in the ratio of (E) to (A) differed little with the cultivars. The value of the ratio exceeded 1 when daily mean VPD (vapor pressure deficit) was above about 9 mmHg or when total solar radiation was over about 400 cal cm-2 day-1. The critical rates of daily transpiration above which it exceeded daily water absorption were about 12g dm-2 day-1 in Bartlett, about 13.5g dm-2 day-1 in Red Bartlett and about 10.5g dm-2 day-1 in Nijisseiki.
2. From the sunrise to the noon, the rates of leaf transpiration exceeded those of water absorption, especially on fine days after the rainy season. From the afternoon to the night, the latter exceeded the former, resulting in the decrease in the extent of water stress. The ratios of the rate of water absorption to that of transpiration were larger late in August than early in July and early in August. The absorption lag in midday was greatest in Bartlett. The diurnal changes in the heat pulse velocities were rather more closely related to those of the transpiration, although they were intermediate between those of the transpiration and water absorption.
3. From these results, it is clear that a severe water stress in midday in pear trees, especially in Bartlett, is caused by the innate, inefficient water absorption, although a causal relation between the water absorption and the transpiration has not yet been clear. Further, some field conditions, especially, soil water deficiency, shallow rooting and root rot will promote the water stress on hot summer days.