The Nitrogen Use Efficiency of C3 and C4 Plants

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Abstract
The relationships between leaf nitrogen content per unit area (Na) and (a) the initial slope of the photosynthetic CO2 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 (C3) and Amaranthus retroflexus (C4). In both species, all parameters were linearly dependent upon leaf Na. The dependence of the initial slope of the CO2 response of photosynthesis on Na was four times greater in A. retroflexus than in C. album. At equivalent leaf Na contents, C. album had 1.5 to 2.6 times more CO2 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 CO2 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 Na to seven at high Na. The fraction of organic N invested in carboxylation enzymes increased with increased Na in both species. The fraction of N invested in Rubisco ranged from 10 to 27% in C. album. In A. retroflexus, the fraction of Na invested in Rubisco ranged from 5 to 9% and the fraction invested in PEPC ranged from 2 to 5%.