The Nitrogen Use Efficiency of C₃ and C₄ Plants

Abstract

ABSTRACIThe effect of applied nitrogen (N) on the growth, laf expansion rate, biomass padtioning and laf N levels of Cheaopodium album (C3) and Amaranthus rtroftexus (C4) were investigted. At a given applied N leveL C. album had 50% greater leaf N per unit area (N.) than A. retrojkxw. Nitrate accumulated at lower N. in A. retrojiexus thn C. album. A. rctroJiexus was more productive than C. album at high N, but C. album was more productive at low N. At high appled N, nitrogen use efficiency (NUE) expressed either as net assimilation rate (NAR) per unit N or relative growth rate per unit N, was greater in A. retroflexus than C. album. However, at low applied N, C. album had a greater NUE on both an NAR and growth basis than A. retroflexus. The leaf area partitioning coefficient was similar in the species at high N, but was greater in A. retrofjexus than C. album at low N. At low N, greater leaf area partitioning apparently lowered leaf N in A. retroflexus to levels at which necrosis occurred. In C. album by contrast, leaf area partitioning declined to a greater degree with declining N than it did in A. retroflxus, so that leaf N did not decline as much. Consequently, low N C. album plants did not lose leaf area to necrosis and had a greater NAR and NUE at low applied N than A. retrojfxus.Photosynthetic rate per unit leaf nitrogen, which has been termed photosynthetic nitrogen use efficiency (PNUE2), has been shown to be greater in C4 plants than C3 plants (3, 4, 18, 24, 26, 28). However, the effect of PNUE on the growth and reproductive yield of C4 relative to C3 plants is unclear. Under limiting N, a $reater PNUE may enable C4 plants to have greater photosynthetic rates than C3 species. Alternatively, the higher PNUE of C4 species may allow them to allocate a greater fraction of available N to new leaf, root, or stem production. Increasing N availability increases the leafexpansion rate (8,14,26,27), which is well correlated with growth rate (21). Moreover, reductions in yield following N deficiency are often due more to reduced leaf expansion rates than to a decline in photosynthesis rate per unit area (8,10,27 To assess the importance of PNUE differences between C3 and C4 species, we have examined the effect of applied N on the growth, leaf expansion rate and allocation of N to leaves in the ecologically similar C3 dicot, Chenopodium album, and C4 dicot Amaranthus retroflexus. Both are important agricultural weeds with similar growth forms, periods of activity, and microsite requirements. In addition, both have high photosynthetic capacities when grown at high N (20). Photosynthetic characteristics as a function of leaf N were also determined and are described in an accompanying report (23). MATERIALS AND METHODSLeaf Nitrogen Content. C. album and A. retroflexus plants were grown in a growth chamber at a PFD of 600 ,mol m-2 s-', 27/230C day/night temperatures and a 16 h photopenod. Plants were grown in equal volume mixtures of sand, perlite, and vermiculite. Treatments consisted of daily applications of either...

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