Nitrogen storage and remobilization in Brassica napus L. during the growth cycle

Oilseed rape (Brassica napus L.) is commonly grown for oil or bio-fuel production, while the seed residues can be used for animal feed. It can also be grown as a catch crop because of its efficiency in extracting mineral N from the soil profile. However, the N harvest index is usually low, due in part to a low ability to remobilize N from leaves and to the fall of N-rich leaves which allows a significant amount of N to return to the environment. In order to understand how N filling of pods occurs, experiments were undertaken to quantify N flows within the plant by 15N labelling. N uptake capacity decreased at flowering to a non significant level during pod filling. However, large amounts of endogenous N were transferred from the leaves to the stems and to taproots which acted as a buffering storage compartment later used to supply the reproductive tissues. About 15% of the total N cycling through the plant was lost through leaf fall and 48%, nearly all of which had been remobilized from vegetative tissues, was finally recovered in the mature pods. A 23 k Da polypeptide, accumulated in the taproots during flowering and later fully hydrolyzed to sustain grain filling, has been characterized as a vegetative storage protein (VSP). A mechanistic N uptake model based on the functioning of NO3- transport system has also been proposed. The overall results are discussed in relation to plant strategies which optimize N cycling to reproductive sinks by means of buffering vegetative tissues such as stems and taproots