ALBERT

Notes: Summary Experiments with Prunus armeniaca were carried out under conditions of constant temperature but varying air humidity. Experiments were also contucted with a constant water vapor difference between the evaporating sites in a leaf and the air, but with varying leaf temperature. These served as a basis for predicting the daily course of total diffusion resistance under the natural climatic conditions of a desert. For the simulation, the rsults of the experiments at constant conditions with only one variable factor are fitted with empirical equations which serve as “calibration curves” to predict the change in diffusion resistance caused by a change in humidity and temperature calculated from the meteorological data of a desert day. The simulation shows that for P. armeniaca humidity and temperature are the dominating factors in controlling the daily course of diffusion resistance. For meteorologically very different days the simulation allows the increase in diffusion resistance in the morning to be predicted with an accuracy of 90%–105% as compared to directly observed measurements. In the afternoon, especially after extreme climatic conditions during the morning, the deviation between predicted and observed values of diffusion resistance may be greater, but not more than -20% to -30%. This possibly indicates the existence of an additional factor of significance which was not included in the simulation. The two peaked curves of net photosynthesis and transpiration characteristic of plants living under arid conditions can be explained in this species by the humidity-and temperature-controlled stomatal response. This stomatal regulation leads to a decreasing total daily transpirational water loss on a dry day as compared to a moist one. The significance of this controlling mechanism for the primary production and the water relations of P. armeniaca is discussed.

Notes: Summary 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.

Notes: Summary As described earlier, the native arido-active perennial Hammada scoparia and the cultivated Prunus armeniaca exhibit characteristic seasonal shifts of their temperature optimum of net photosynthesis (OP) under desert conditions in the Negev. In the present paper the OP values were compared with the actual tissue temperatures of the experimental plants. During the growing period from March to September the duration of optimal temperatures for net photosynthesis (OP±3°C) experienced by the plants was 32.2% of the total time at light saturation for P. armeniaca and 27.8% for H. scoparia. For optimal photosynthesis the branchlets of H. scoparia are too cold for 66.1% of the time span and too warm for 6.1% of the time. The respective values for the leaves of the apricot are 28.6% and 39.2%. Simulations at changed tissue temperature show, that for P. armeniaca neither a higher nor a lower temperature regime would lengthen the time span for optimal thermal conditions. For H. scoparia, however, an increase of the general temperature level by 6°C would considerably improve the temperature-related photosynthetic efficiency. The natural temperature responses of the plants were compared with simulations using OP values which are supposed not to shift but to stay constant from March through September at their spring minimum, their summer maximum, or at an intermediate value. For P. armeniaca such constant OP values would result in a shorter duration of optimal temperature conditions. With this plant the natural seasonal shift of the temperature characteristics appears to provide an advantage in respect to its photosynthetic capacity. Contrary to this, for H. scoparia a constant OP value at the low spring level or even at the intermediate level during all the season would result in a substantially prolonged period of favourable temperature conditions for photosynthesis. In this case the seasonal change of optimum temperature for photosynthesis with higher OP values in summer signifies a disadvantage with respect to the temperature-related photosynthetic capacity at the habitat in the central Negev. Apparently this C4 plant is adapted to higher temperatures than were present. It appears that “acclimations” of native plants are not always beneficial.