The Nitrogen Use Efficiency of C3 and C4 Plants

The relationships between leaf nitrogen content per unit area (N(a)) and (a) the initial slope of the photosynthetic CO(2) response curve, (b) activity and amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC), and (c) chlorophyll content were studied in the ecologically similar weeds Chenopodium album (C(3)) and Amaranthus retroflexus (C(4)). In both species, all parameters were linearly dependent upon leaf N(a). The dependence of the initial slope of the CO(2) response of photosynthesis on N(a) was four times greater in A. retroflexus than in C. album. At equivalent leaf N(a) contents, C. album had 1.5 to 2.6 times more CO(2) saturated Rubisco activity than A. retroflexus. At equal assimilation capacities, C. album had four times the Rubisco activity as A. retroflexus. In A. retroflexus, a one to one ratio between Rubisco activity and photosynthesis was observed, whereas in C. album, the CO(2) saturated Rubisco activity was three to four times the corresponding photosynthetic rate. The ratio of PEPC to Rubisco activity in A. retroflexus ranged from four at low N(a) to seven at high N(a). The fraction of organic N invested in carboxylation enzymes increased with increased N(a) in both species. The fraction of N invested in Rubisco ranged from 10 to 27% in C. album. In A. retroflexus, the fraction of N(a) invested in Rubisco ranged from 5 to 9% and the fraction invested in PEPC ranged from 2 to 5%.