LEAF PHOTOSYNTHETIC NITROGEN-USE EFFICIENCY OF C-3 AND C-4 CYPERUS SPECIES

A comparison of photosynthetic nitrogen (N) use efficiency between C-3 and C-4 species within the genus Cyperus was made at a range of avail able nitrate levels. Net photosynthetic rates (P-N) Of both types incr eased with N content but CO2 assimilation on a leaf area basis was con sistently higher in C-4 species in comparison to the C-3 species. C-4 plants had a higher photosynthetic N use efficiency than C-3 plants, a nd achieved higher P-N at lower N and protein levels. The higher leaf photosynthetic N use efficiency (PNUE) in C-4 plants is due to their h igh P-N rather than low N and protein contents. Because C-4 leaves con tain less N than C-3 leaves for a given P-N, PNUE is significantly hig her in C-4 than C-3 plants. Nitrogen supply had highly significant eff ect on the leaf N concentration, but had no significant influence on t he CO2 compensation concentration. Carboxylation efficiency was signif icantly higher in C-4 leaves than C-3 leaves of Cyperus species, due t o the CO2 concentrating mechanism of C-4 photosynthesis.