Do initial S reserves and mineral S availability alter leaf S-N mobilization and leaf senescence in oilseed rape?

Winter oilseed rape is sensitive to S limitation, however few studies have clearly assessed the impact of initial S reserves on the remobilization of leaf N-S compounds and senescence dynamics within the leaves in S limited plants. As a consequence, the impacts of high or low initial S reserves on these parameters, further cross-combined with either high or low S availabilities, were examined using a ¹⁵N and ³⁴S double-labelling method associated with a study of gene expression of relevant tonoplastic sulphate transporters (BnSultr4;1 and BnSultr4;2) and a molecular indicator of leaf senescence (BnSAG12/BnCab). Plants with high initial S status and S limitation showed an optimal growth comparable to control plants. Moreover, in response to S limitation, leaf soluble protein content, total S, recently assimilated S (i.e., ³⁴S) and the sulphate content in the oldest leaves declined, and the expression of genes encoding tonoplastic sulphate transporters were up-regulated. However, compared to control plants, S limitation delayed leaf senescence. These data suggested that in response to S limitation, plants with high initial S were able to sustain optimized leaf growth by increasing endogenous N and S remobilization independently of the leaf senescence process. In contrast, if these low S plants had no initial S reserves, leaf N-S remobilization was not sufficient to allow optimal growth. As a conclusion, our study supports a model where oilseed rape is able to compensate transiently for S limitation through a fine management of leaf N-S remobilization and a delayed leaf senescence dynamics.