696 www.newphytologist.org Research Blackwell Publishing Ltd Modelling postsilking nitrogen fluxes in maize (Zea mays) using 15 N-labelling field experiments André Gallais 1,2 , Marie Coque 1 , Isabelle Quilléré 3 , Jean-Louis Prioul 4 and Bertrand Hirel 3 1 Station de Génétique Végétale, INRA-UPS-INAPG-CNRS, Ferme du Moulon, 91190 Gif/Yvette, France; 2 INAPG, 16 rue Claude Bernard, 75231 Paris Cedex 05, France; 3 Unité de Nutrition Azotée des Plantes, INRA route de St Cyr, 78026 Versailles Cedex, France; 4 Institut de Biotechnologie des Plantes, Université de Paris-Sud, 91405 Orsay Cedex, France Summary • In maize (Zea mays), nitrogen (N) remobilization and postflowering N uptake are two processes that provide amino acids for grain protein synthesis. • To study the way in which N is allocated to the grain and to the stover, two different 15 N-labelling techniques were developed. 15 NO 3 - was provided to the soil either at the beginning of stem elongation or after silking. The distribution of 15 N in the stover and in the grain was monitored by calculating relative 15 N-specific allocation (RSA). • A nearly linear relationship between the RSA of the kernels and the RSA of the stover was found as a result of two simultaneous N fluxes: N remobilization from the stover to the grain, and N allocation to the stover and to the grain originating from N uptake. • By modelling the 15 N fluxes, it was possible to demonstrate that, as a consequence of protein turnover, a large proportion of the amino acids synthesized from the N taken up after silking were integrated into the proteins of the stover, and these proteins were further hydrolysed to provide N to the grain. Key words: maize (Zea mays), modelling, 15 N labelling, nitrogen remobilization, nitrogen uptake, nitrogen use efficiency, protein turnover. New Phytologist (2006) 172: 696–707 © The Authors (2006). Journal compilation © New Phytologist (2006) doi: 10.1111/j.1469-8137.2006.01890.x Author for correspondence: André Gallais Tel: +33 169332331 Fax: +33 169332340 Email: gallais@moulon.inra.fr Received: 30 June 2006 Accepted: 7 August 2006 Introduction Nitrogen (N) is one of the main limiting factors for plant growth and ultimately for the production of harvestable plant material used for animal and human food. In most plant species examined so far, the plant life cycle with regard to the management of N can be roughly divided into two main phases occurring successively. During the first phase, i.e. the vegetative phase, young developing roots and leaves behave as sink organs for the assimilation of inorganic N and the synthesis of amino acids. These amino acids are further used for the synthesis of enzymes and proteins mainly involved in building up plant architecture and the different components of the photosynthetic machinery. Notably, the enzyme Rubisco can alone account for up to 50% of the total soluble leaf protein content (Mae et al., 1983). Later, at a certain stage of plant development generally starting after anthesis, the remobilization of N takes place. At this stage, shoots and/or roots start to behave as sources of N by providing amino acids released from protein hydrolysis, which are subsequently exported to reproductive and storage organs represented, for example, by seeds, bulbs or trunks (Masclaux et al., 2001). In maize, 45 – 65% of the grain N is provided from pre-existing N in the stover before silking, a process that is strongly dependent upon the environmental conditions and/or the genotype (Weiland & Ta, 1992; Gallais & Coque, 2005). The remaining 35 – 55% of the grain N originates from postsilking N uptake (Ta & Weiland, 1992; Bertin & Gallais, 2000; Gallais & Coque, 2005). Therefore, two distinct N fluxes must be considered during the grain-filling period: N translocation from the stover (stalks + leaves + sheaths + cob) and N uptake (Crawford et al., 1982; Rendig & Crawford, 1985; Ta & Weiland, 1992).