The relation between photosynthetic pathway and habitat of the grass species recorded in the desert regions of Sinai, Negev, and Judea was investigated. The climatic conditions and micro-environments in the study area vary considerably, and the distribution of the various species is found to conform to specific patterns which reveal the adaptive advantages of the different photosynthetic pathways. There is also a distinct correlation between the phytogeographic origin of the grass species and the photosynthetic pathways that they utilize.
The survey shows that the majority of the grass species in the region are of the C3 type and all except one of these species belong to the Holarctic domain. This is in accordance with the fact that the region forms part of the Mediterranean winter rainfall regime and that C3 species have an adaptive advantage where minimum temperatures are low during the winter growing season.
The occurence of C4 species increases with decreasing rainfall and they dominate in those districts where temperatures are high throughout the year. These C4 grasses are of both Holarctic and Palaeotropic origin according to the classification adopted here, but they are essentially all elements of the Saharo-Arabian, Irano-Turanian, Sudanian, or Tropical phytogeographic regions and are not typical of the Mediterranean or Euro-Siberian floras. The plants with multi-regional distributions that occur in Mediterranean communities may well be intrusive.
Analysis of the three subtypes of the C4 species suggests that the malate-forming NADP-me grasses grow where water stress is not a dominating factor, while the aspartateforming NAD-me grasses are more successful under xeric conditions. The PEP-ck species are not abundant and form an intermediate group between the NADP-me and NAD-me subtypes.