Photosynthetic Pathways in Plants of Coastal and Inland Habitats of Israel and the Sinai )

Abstract

Summary The relationship between photosynthetic pathway and climatic regime has been investigated, for parts of Israel and the Sinai. These regions are characterized by hot, long, dry summer seasons with winter rainfall. The annual rainfall is high in the northern, Mediterranean parts of Israel, and extremely low in the Negev desert and the Sinai. Distinction between C3 photosynthesis, C4, photosynthesis, and Crassulacean Acid Metabolism (CAM), was made on the basis of anatomy, δ13C value and/or malate levels at dawn and dusk of the photosynthetic organs of the plants. The malate levels have already been reported (W inter et al. 1976a). These techniques were applied to 105 species of different coastal and inland habitats of Israel and the Sinai, some of which were highly saline. Seventy nine species had C3 photosynthesis, 22 species C4 photosynthesis and 4 species had CAM. C4 photosynthesis occurred mainly in the halophytic members of the Chenopodiaceae. A feature of the anatomy was the occurrence in most specimens of a special form of Kranz anatomy. The more or less centric leaves or branches are usually composed of a central water storage tissue and of two distinct peripheral concentric rings of green cells. Vascular bundles adjoin the inner band of green cells only at intervals. CAM was found in three annual, halophytic species of the Aizoaceae and in the small perennial, stem-succulent Caralluma negevensis (Asclepiadaceae), which occurs only sporadically in the Negev. The rarity of perennial CAM succulents in the Negev and the Sinai is evidently caused by the extreme dryness of these regions. Analysis of the geographical distribution of the C4 pathway indicated that there was a large proportion in the inland compared to the coastal plant communities. The highest proportion of C4 species is observed in the highly saline desert Dead Sea area. The abundance of C4 species in this region might be due mainly to relatively high minimum temperatures during the humid winter. In the central Negev, where the winters are cooler, C4 species are less common than in the Dead Sea area. It was also possible from the results to indicate the relative importance of photosynthetic pathway compared with other plant characteristics which may be considered important in adaptation of these plants to dry environments. The biomass of C3 species was more dominant than that of C4 species on all study sites, and this includes the arido-active 2) species at the desert habitats. In the Negev desert it was evident that C4 photosynthesis and especially CAM were of secondary importance in arido-active species to such adaptive features as morphological and anatomical characteristics, reduction of the transpiring and carbon assimilating organs during the dry season, and stomatal control of CO2 and H2O gas exchange in relation to ambient conditions.