GREEN plants may be divided into two groups, C3 and C 4 species, depending on whether the first product of photosynthetic CO2 fixation is phosphoglycerate (C3) or oxaloacetate (C4). In all species studied to date the C4 metabolism is associated with either 'Kranz' leaf anatomy, low CO2 compensation points or low 13C/12C ratios. Included in the C 4 category are many of the highest yielding and rapidly growing crops, most of which are believed to have originated in tropical and subtropical regions. C4 species so far studied, which are mostly of tropical origin, attain their highest photosynthetic rates at leaf temperatures of about 30 °C and above1,2 so that selective advantages derived from photosynthetic rate will only be evident in tropical and subtropical regions1,2. A recent compilation of the reported rates of photosynthesis and productivity of C3 and C4 crops 3 showed that differences in maximum annual dry matter yields could be explained by differences in lengths of growing season. It thus remains to be shown that there is a selective advantage which is a direct consequence of photosynthetic capacity of C4 species or that there is an inherent physiological barrier to the adaptation of C4 species to a similar range of latitudes to C3 species.
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