The temperature-related photosynthetic capacity of plants under desert conditions : I. Seasonal changes of the photosynthetic response to temperature.

Temperature dependence of net photosynthesis under conditions of light saturation and maximum air humidity was measured throughout the season in the Central Negev Desert (Israel). Experimental plants were the wild growing Hammada scoparia and Prunus armeniaca cultivated in the runoff farm of Avdat.The optimum temperature for net photosynthesis and the upper temperature compensation point of CO2 exchange showed a characteristic seasonal variation with low values in spring and fall and high values in mid-summer. This shift was exhibited by plants growing under conditions of normal soil-water stress as well as by irrigated plants. There was no general correlation between the changes in temperature dependence of net photosynthesis of the plants, their maximum photosynthetic capacity under the experimental conditions, their daily photosynthesis maximum under natural conditions, and their rate of dark respiration. The seasonal shift of the photosynthetic response to temperature cannot be explained by changes in the temperature sensitivity of the stomata. It may be caused by seasonal changes of biochemical and/or biophysical properties.A number of observations made on other wild plants also showed, in all cases, seasonal shifts of the upper temperature compensation point, with an amplitude of 6.0°C-13.7°C.