Modelling N nutrition impact on plant functioning and root architecture in various genotypes of Arabidopsis thaliana Céline Richard-Molard 1 , François Brun 1 , Anne Laperche 1 , Michaël Chelle 1 , Loïc Pagès 2 and Bertrand Ney 1 1 INRA, AgroParisTech, UMR1091 Environnement et Grandes Cultures, F-78850 Thiverval- Grignon, France. 2 INRA, UR1115 Plantes et Systèmes Horticoles, Site Agroparc, F-84900 Avignon, France richardm@grignon.inra.fr Keywords: assimilate partitioning, root morphogenesis, genetic variability, nitrogen uptake efficiency, whole-plant modelling Introduction Breeding new varieties adapted to low-input agricultural practices, mainly for nitrogen and pesticides, is of particular interest when considering economic and environmental concerns. However, screening pertinent traits for crop adaptation to low N supply remains difficult because plant response to N availability is a set of closely interacting processes, and because this response displays a wide genetic and environmental variability. Thus, Loudet et al. (2003) showed that most of the variables involved in N use efficiency exhibit wide variations in response to varying N supplies in a population of Arabidopsis thaliana recombinant inbred line (RILs). Therefore, a quantitative modelling of whole-plant C/N functioning should be a suitable tool to identify the key-parameters determining plant efficiency under low or high N nutrition, that will be pertinent for phenotype screening. Such an approach has been successfully employed to identify QTLs (Quilot et al., 2004; Yin et al., 2000) and to simulate phenotypes with allelic composition (Reymond et al., 2004). A whole-plant structure-function model was developed for that purpose on one genotype (WS) of A. thaliana. The model is based on interactions between N and C fluxes and offers an explicit and dynamic description of root system and leaf area growths (Fig. 1). The model was then simplified (root system output was merely considered as biomass) to interpret the behaviour of several RILs of Arabidopsis and wheat, and of one Arabidopsis mutant impaired on high affinity nitrate uptake. Fig. 1: Schematic diagram of the model. The model was constituted of a shoot and a root compartment, exchanging C and N fluxes. Priority was given to shoots for C and to roots for N. Leaf expansion and root growth were limited by relative maximum growth rates obtained in our culture conditions. Total internal N quantity resulted from root uptake (1) and reserve remobilization (2) and determined the increment of total leaf area (4), after satisfaction of root N demand (3). Total internal C quantity was produced by photosynthetic leaf area (6), obtained from total leaf area using the Beer-Lambert’s law (5). Effective root growth resulted from the growth allowed by internal N quantity (8) and by the C quantity remaining after satisfaction of shoot C demand (7). Root dry mass was then partitioned in root length (9). N and C storage pools emerged (10, 11) when N and C internal quantities were not fully depleted by growth. Leaf Area (LA) Photosynthetic Leaf Area (PLA) C for roots (QCR) Total Root Lenght (RLg) Total internal Nitrogen (QN) Nitrogen Uptake Efficiency (SNU) N Remobilization Rate NO 3 - Storage NO 3 - Storage C Storage Relative Part of Photosynthetic Leaf Area (PLA/LA) N for shoots (QNS) Carbon Assimilation Efficiency (SCA) Total internal Carbon (QC) LA max RDW max Conversion Efficiency of N into leaf area (LA/QNS) Root Dry Weight (RDW) Specific Root Lenght (SRL) N for roots (QNR) (3) (4) (1) C for shoots (QCS) (2) (5) (6) (7) (8) (9) (11) (10) 17-1