Nitrogen storage and remobilization in Brassica napus L. during the growthcycle: nitrogen fluxes within the plant and changes in soluble protein patterns

Oilseed rape (Brassica napus L.) is commonly grown for oil or bio-fuel prod uction, 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 fr om the soil profile. However, the N harvest index is usually low, due in pa rt to a low ability to remobilize N from leaves and to the fall of N-rich l eaves which allows a significant amount of N to return to the environment. In order to understand how N filling of pods occurs, experiments were under taken to quantify N flows within the plant by N-15 labelling and to follow the changes in soluble protein profiles of tissues presumed to store and su bsequently to remobilize N. Whereas N uptake increased as a function of gro wth, N uptake capacity decreased at flowering to a non-significant level du ring pod filling. However, large amounts of endogenous N were transferred f rom the leaves to the stems and to taproots which acted as a buffering stor age compartment later used to supply the reproductive tissue. About 15% of the total N cycling through the plant were lost through leaf fall and 48%, nearly all of which had been remobilized from vegetative tissues, were fina lly recovered in the mature pods. SIDS-PAGE analysis revealed that large am ounts of a 23 kDa polypeptide accumulated in the taproots during flowering and was later fully hydrolysed. Its putative function of storage protein is further supported by the fact that when plants were grown at lower tempera ture, both flowering, its accumulation and further mobilization were delaye d. The overall results are discussed in relation to plant strategies which optimize N cycling to reproductive sinks by means of buffering vegetative t issues such as stems and taproots.