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BIOENERGETICS AND PHOTOSYNTHESIS

Faster Rubisco Is the Key to Superior Nitrogen-Use Efficiency in NADP-Malic Enzyme Relative to NAD-Malic Enzyme C₄ Grasses¹

Molecular Plant Physiology Group (O.G., T.J.A., S.v.C.), Environmental Biology Group (J.R.E.), and Photobioenergetics Group (W.S.C.), Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia; and Centre for Horticulture and Plant Sciences, University of Western Sydney, Penrith South DC, New South Wales 1797, Australia (J.P.C.)

In 27 C₄ grasses grown under adequate or deficient nitrogen (N) supplies, N-use efficiency at the photosynthetic (assimilation rate per unit leaf N) and whole-plant (dry mass per total leaf N) level was greater in NADP-malic enzyme (ME) than NAD-ME species. This was due to lower N content in NADP-ME than NAD-ME leaves because neither assimilation rates nor plant dry mass differed significantly between the two C₄ subtypes. Relative to NAD-ME, NADP-ME leaves had greater in vivo (assimilation rate per Rubisco catalytic sites) and in vitro Rubisco turnover rates (k_cat; 3.8 versus 5.7 s^–1 at 25°C). The two parameters were linearly related. In 2 NAD-ME (Panicum miliaceum and Panicum coloratum) and 2 NADP-ME (Sorghum bicolor and Cenchrus ciliaris) grasses, 30% of leaf N was allocated to thylakoids and 5% to 9% to amino acids and nitrate. Soluble protein represented a smaller fraction of leaf N in NADP-ME (41%) than in NAD-ME (53%) leaves, of which Rubisco accounted for one-seventh. Soluble protein averaged 7 and 10 g (mmol chlorophyll)^–1 in NADP-ME and NAD-ME leaves, respectively. The majority (65%) of leaf N and chlorophyll was found in the mesophyll of NADP-ME and bundle sheath of NAD-ME leaves. The mesophyll-bundle sheath distribution of functional thylakoid complexes (photosystems I and II and cytochrome f) varied among species, with a tendency to be mostly located in the mesophyll. In conclusion, superior N-use efficiency of NADP-ME relative to NAD-ME grasses was achieved with less leaf N, soluble protein, and Rubisco having a faster k_cat.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.054759.

^* Corresponding author; e-mail ghannoum{at}rsbs.anu.edu.au; fax 61–2–6125–5075.

Received October 12, 2004; returned for revision November 25, 2004; accepted November 29, 2004.

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