[Stomatal opening in the dark at high temperatures]

Transpiration and absorption of water by the roots of Piper betle L. have been measured simultaneously with intact single-leaf plants. The degree of stomatal opening has been calculated from leaf temperature, air humidity and wind speed. The roots were aerated at intervals.In an atmosphere of unchanged composition, the stomata of Piper betle L. remain closed or only slightly open at a leaf temperature of 36-38° C in the dark and at night. However, if the temperature is raised above 40° C, they open rapidly after a short time lag to a degree normally met only under daylight conditions.Opening occurs independently of the length of the preceding dark period and hour of the night. The high degree of opening in the dark does not change considerably for many hours, and is more pronounced at high air humidity. At temperatures above 40° C the degree of opening is little influenced by the change from light to dark or from dark to light.The results give evidence that heat resistance could be strongly affected by stomatal reactions at high temperatures. The maximum temperature during the experiments did not exceed 45° C. To ensure that Piper betle L. had not been damaged by the high temperature the experiments were repeated with the same plants on several consecutive days, and the same results were obtained.Close agreement was found between the highest possible stomatal conductance derived from transpiration and that calculated from scanning electron microscopic analysis.