C₄ photosynthesis in plants from cool temperate regions, with particular reference to Spartina townsendii

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 above^1,2 so that selective advantages derived from photosynthetic rate will only be evident in tropical and subtropical regions^1,2. 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.