Abstract
Summary Heat tolerance was found to change markedly during the course of the day in the CAM plant Aeonium haworthii and the C3 plant Aichryson laxum , both Sempervivoideae from the island of Tenerife. The temperature limits of heat tolerance of A. haworthii varied between 39°C and 46.5°C, and of Aichryson laxum between 43°C and 46°C. In a 12 : 12 hours photoperiod, 10°C in dark, 25°C in light, the heat tolerance reached a minimum during the dark phase and a maximum in the light phase. This was also the case when the light-dark sequence was inverted. The heat tolerance cycle appears to be induced by photoperiod and temperature, and maximum levels related to temperature. The diurnal course of heat tolerance is almost the exact inverse of tissue acid accumulation and it appears that high acidity levels may negatively affect heat tolerance. Further investigations into the physiology of the actions of the acidity could provide new insight into the mechanism of heat tolerance.
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