The temperature-related photosynthetic capacity of plants under desert conditions : II. Possible controlling mechanisms for the seasonal changes of the photosynthetic response to temperature.

In a previous paper seasonal shifts of the temperature optimum (OP) and of the upper temperature compensation point (CP) of net photosynthesis were described for Hammada scoparia growing wild, and for Prunus armeniaca cultivated in the Negev Desert (Israel). In this paper the relationships between these shifts and the microclimatic conditions, plant-water relations, and plant development are studied.The energy budged of the thin, round photosynthesizing stems of H. scoparia growing in an open desert habitat differes from that of the broad leaves of P. armeniaca within the orchard. This explains the fact that daily maximum temperatures of the apricot increased until August and September, whereas maximum temperatures of H. scoparia reached a peak in May and June and decreased thereafter during the second half of the growing season.For H. scoparia a correspondence was found between the daily maximum tissue temperatures (and also the average temperatures of the warmest periods of the day) and the seasonal changes of the OP and CP values. This may indicate that the shifts in the temperature sensitivity of net photosynthesis of this plant are adaptations to the temperature conditions of the plant. This, however, cannot be the case for P. armeniaca, where during the second part of the growing season a period of rising leaf temperatures coincides with a period of decreasing OP and CP values. Therefore, the seasonal changes of the temperature dependence of net photosynthesis of P. armeniaca could not always be considered an adaptation to the prevailing temperature conditions of the plant. In this case, the changes in temperature sensitivity of photosynthesis could be due to developmental processes such as aging. In both lants the seasonal changes of the OP and CP values correspond to changes of the daily photoperiod and to changes of the daily average light intensity. It appears possible that this correlation indicates a causal relationship.