Relationship between leaf nitrogen and photosynthetic rate for three NAD-ME and three NADP-ME C-4 grasses

Theoretical considerations have suggested that there may he differences in photosynthetic nitrogen use efficiency (PNUE) among plants that use differe nt biochemical variants of C-4 photosynthesis. To test this hypothesis we e xamined the leaf nitrogen content and photosynthetic rates of six grass spe cies (three of C-4 subtype NAD-ME and three of C-4 subtype NADP-ME) grown o ver a wide range of nitrogen supply. While there were significant differenc es among the species in various traits, there were no consistent difference s between the C-4 subtypes in either leaf nitrogen content at a given level of nitrogen supply or in the leaf nitrogen-photosynthesis relationship. We suggest that species-level variation in photosynthetic nitrogen use effici ency among C-4 species is large enough to mask any differences that may be due to C-4 subtype.