Is crop N demand more closely related to dry matter accumulation or leaf area expansion during vegetative growth?

Unité d’Ecophysiologie des Plantes Fourragères, INRA, 86600 Lusignan, France; Agricultural Production Systems Research Unit (APSRU), University of Queensland, School of Land and Food Sciences, Brisbane, Qld 4072, Australia; International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines; UMR Agronomie INRA-INAPG, 78150 Thiverval-Grignon, France; Epagri, 89620-000 Campos Novos, SC, Brazil; UMR INRA, UCBN 950 Ecophysiologie Végétales, Agronomie et Nutritions N, C et S, Université de Caen Basse Normandie, 14032 Caen Cedex, France
Field Crops Research 01/2007; DOI: 10.1016/j.fcr.2006.05.009

ABSTRACT The critical crop nitrogen uptake is defined as the minimum nitrogen uptake necessary to achieve maximum biomass accumulation (W). Across a range of crops, the critical N uptake is related to W by a power function with a coefficient less than unity that suggests crop N uptake is co-regulated by both soil N supply and biomass accumulation. However, crop N demand is also often linearly related to the expansion of the leaf area index (LAI) during the vegetative growth period. This suggests that crop N demand could be also linked with LAI extension. In this paper, we develop theory to combine these two concepts within a common framework. The aim of this paper is to determine whether generic relationships between N uptake, biomass accumulation, and LAI expansion could be identified that would be robust across both species and environment types. To that end, we used the framework to analyze data on a range of species, including C3 and C4 ones and mono- and di-cotyledonous crops. All crops were grown in either temperate or tropical and subtropical environments without limitations on N supply. The relationship between N uptake and biomass was more robust, across environment types, than the relationship of LAI with biomass. In general, C3 species had a higher N uptake per unit biomass than C4 species, whereas dicotyledonous species tended to have higher LAI per unit biomass than monocotyledonous ones. Species differences in N uptake per unit biomass were partly associated with differences in LAI and N-partitioning. Consequently the critical leaf-N uptake per unit LAI (specific leaf nitrogen, SLN) was relatively constant across species at 1.8–2.0 g m−2, a value that was close to published data on the critical SLN of new leaves at the top of the canopy. Our results indicate that critical N uptake curves as a function of biomass accumulation may provide a robust platform for simulating N uptake of a species. However, if crop simulation models are to capture the genotypic and environmental control of crop N dynamics in a physiologically functional manner, plant growth has to be considered as the sum of a metabolic (e.g. leaves) and a structural (e.g. stems) compartment, each with its own demand for metabolic and structural N.

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