Dynamics of Nitrogen Uptake and Mobilization in Field-grown Winter Oilseed Rape (Brassica napus) from Stem Extension to Harvest I. Global N Flows between Vegetative and Reproductive Tissues in Relation to Leaf Fall and their Residual N

International audienceBackground and Aims Despite its high capacity to take up nitrate from the soil, winter oilseed rape (Brassica napus) is characterized by a very low N recovery in the reproductive tissues under field conditions. A significant part of the N taken up is lost to the soil in dead leaves during the growth cycle. An accurate description of N dynamics at the whole plant level in each compartment under field conditions should lead to a better understanding of N allocation in B. napus and improvements in the nitrogen harvest index. Methods An experiment was conducted in field conditions using sequential weekly 15 N labelling to follow N uptake, partitioning and mobilization. Nitrogen labelling (2Á5 kg N ha À1 ; 10 % excess) was analysed weekly (from stem extension to harvest) to distinguish between uptake of new N (labelled) and mobilized N (unlabelled) in the different plant components. Key Results and Conclusions N requirements for seed filling were satisfied mainly by N mobilized from vegetative parts (about 73 % of the total N in pods). Determination of the endogenous N flow showed that there was net transfer of N to the pods by leaves (36 %), stem (34 %), inflorescences (22 %) and taproot (8 %). Precise study of N flow from leaves at different nodes revealed the existence of two main groups of leaves in terms of their apparent capacity to mobilize N; 30–60 % and 70–80 % of peak N content occurring during flowering and pod filling, respectively. Moreover, the latter group was found to be the main source of endogenous N from leaves. The mobilization of endogenous N from these leaves was prolonged and concomitant with N accumulation in the pods. A complex pattern of N mobilization from the leaves, to vegetative or reproductive tissues, was revealed. These results will be used to model N partitioning during the growth cycle