ALBERT

Notes: Summary The daily course of net photosynthesis and transpiration was measured with temperature and humidity controlled cuvettes at the end of the dry summer season in the Negev Desert. Species studied included: dominant species of the natural vegetation, cultivated plants in the run-off farm Avdat and permanently irrigated plants. An analysis of the influence of single climatic factors on gas exchange was given in part II of this publication. The reactions of the plants to complex changes in all the environmental parameters is the subject of this present study. 1. One-peaked daily courses of net photosynthesis occur in the irrigated species Citrullus colocynthis and Datura metel. After a high rate of net photosynthesis and transpiration before noon CO2 uptake is gradually reduced through stomatal closure even under good soil water conditions. Stomatal closure on C. colocynthis is controlled by the leaf temperatures whereas D. metel closes its stomata due to increasing water stress. 2. Without additional irrigation one-peaked daily courses are only possible with special constitutional adaptations to the extreme climate together with a balanced regulation of water loss. The annual Salsola inermis shows over the whole day no reduction in transpiration. Related to chlorophyll content, CO2 uptake almost attains the rates observed in the irrigated C. colocynthis. It is still unknown what type of water sources are at the disposal of this plant. The perennial chamaephytes Hammada scoparia and Reaumuria negevensis attain a stabilization of net photosynthesis at a lower level through reduction of stomatal water loss and through increased mesophyll resistance to water vapour. This reduces transpiration to a greater degree than CO2 uptake. The stomatal reactions of H. scoparia seem to be mainly controlled by the evaporation conditions in the atmosphere. Related to chlorophyll content in the assimilatory organs, net photosynthesis of both species is higher at noon than in all other chamaephytes. 3. Two-peaked daily courses of net photosynthesis are shown by plants of the run-off farm (Prunus armeniaca and Vitis vinifera). Both have a very high metabolic activity during the morning which is comparable even with that of D. metel and of H. scoparia. At noon, CO2 and H2O exchange is reduced through stomatal closure and falls below the level of all the other plants in the natural vegetation. In apricot, net photosynthesis of vertically oriented leaves does not drop to the compensation point at noon as was found with horizontally oriented leaves which had leaf temperatures some 6–8°C higher. 4. In the natural vegetation, only Noaea mucronata shows gas exchange reactions similar to those of apricot and grapvines. Also N. mucronata has high rates of net photosynthesis in the morning followed by a great reduction of gas exchange through stomatal closure at noon. At a higher water stress this type of an asymmetric two-peaked daily course is changed into a more flat symmetric two peaked curve with low metabolic activity during the morning. In Zygophyllum dumosum and in Artemisia herba-alba the reduction in CO2 uptake at noon is not caused by stomatal closure, but through temperatures above the optimum. The diffusion resistance for water vapour increases steadily during the day. 5. The daily balance of the CO2 exchange is calculated and is compared with the metabolic activity under optimal conditions. The potential photosynthetic capacity of wild plants under optimal conditions is more nearly met under the prevailing desert conditions than in the case with cultivated plants. The effects of the different types of daily courses of gas exchange on the distribution of plants of the Saharo-Arabian region (Reaumuria negevensis, Zygophyllum dumosum) and of plants of the Irano-Turanian vegetation (Hammada scoparia, Artemisia herbaalba) are discussed.