The Nitrogen Use Efficiency of C3 and C4 Plants

The effect of leaf nitrogen (N) on the photosynthetic capacity and the light and temperature response of photosynthesis was studied in the ecologically similar annuals Chenopodium album (C(3)) and Amaranthus retroflexus (C(4)). Photosynthesis was linearly dependent on leaf N per unit area (N(a)) in both species. A. retroflexus exhibited a greater dependence of photosynthesis on N(a) than C. album and at any given N(a), it had a greater light saturated photosynthesis rate than C. album. The difference between the species became larger as N(a) increased. These results demonstrate a greater photosynthetic N use efficiency in A. retroflexus than C. album. However, at a given applied N level, C. album allocated more N to a unit of leaf area so that photosynthetic rates were similar in the two species. Leaf conductance to water vapor increased linearly with N(a) in both species, but at a given photosynthetic rate, leaf conductance was higher in C. album. Thus, A. retroflexus had a greater water use efficiency than C. album. Water use efficiency was independent of leaf N in C. album, but declined with decreasing N in A. retroflexus.