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Arid regions of Saudi Arabia occupy most of the area of the Arabian Peninsula. These areas are at the meeting position of plants from Mediterranean, Irano-Turanian, Saharo-Arabian, and Sudanian phyto-gegraphical regions. Geomorphology of the area reveals a wide diversity of landforms including coastal lines, desert plains, and high mountains. Grasses are well represented in the flora of Saudi Arabia and form an appropriate group for studying the relation of grass distribution, chorology, and photosynthetic pathways. In this paper, geographical distribution of C-3 and C-4 grasses was studied in an area extending between latitude 17 degrees N and latitude 31 degrees N. Two regions were recognized in the study area, namely; a (relatively) cold region north of latitude 24 degrees N with ample winter rainfall, and a hot region south of latitude 24 degrees N with scarce summer rainfall. Work involved field observations and collection of grass species in the study area. Work also depended on published carbon discrimination values of grasses and biochemical analysis of C-4 species subtypes. Climatic conditions in the study area vary considerably, and the distribution of grass species was found to follow patterns that reveal adaptive advantages of different photosynthetic pathways. Grass species in the cold northern region with ample winter rainfall are generally C-3 grasses belonging mainly to Mediterranean/lrano-Turanean chorotypes. C-3 grass species found in the southern hot region were recorded at high altitudes of southern mountains characterized by low temperatures. Grass species recorded at low altitudes in the south hot region with scarce summer rainfall were mainly C-4 grasses belonging to Tropical and Saharo-Arabian-Sudanean chorotypes. Pronounced spatial variations of temperature profoundly control the geographical distribution of C-3 and C-4 grasses. Low temperatures in the northern cold region and at high altitudes of the southern hot region limit the occurrence of C-4 grasses and shift the ecological balance in favor of C-3 grasses. Results are discussed in terms of heat sensitivity of the CO2 carboxylating enzyme of C-3 grasses and high temperature optima for CO2 assimilation of C-4 grasses. Results are also discussed in comparison with geographical distribution of grasses in other parts of the world. (C) 2012 Elsevier GmbH. All rights reserved.