Effect of sulphur deprivation on osmotic potential components and nitrogen metabolism in oilseed rape leaves: Identifcation of a new early indicator

Abstract

Identifcation of early sulphur (S) defciency indicators is important for species such as Brassica napus, an S-demanding crop in which yield and the nutritional quality of seeds are negatively affected by S defciency. Because S is mostly stored as SO42- in leaf cell vacuoles and can be mobilized during S defciency, this study investigated the impact of S deprivation on leaf osmotic potential in order to identify compensation processes. Plants were exposed for 28 days to S or to chlorine deprivation in order to differentiate osmotic and metabolic responses. While chlorine deprivation had no signifcant effects on growth, osmotic potential and nitrogen metabolism, Brassica napus revealed two response periods to S deprivation. The frst one occurred during the frst 13 days during which plant growth was maintained as a result of vacuolar SO42- mobilization. In the meantime, leaf osmotic potential of S-deprived plants remained similar to control plants despite a reduction in the SO42- osmotic contribution, which was fully compensated by an increase in NO3-, PO43- and Cl- accumulation. The second response occurred after 13 days of S deprivation with a signifcant reduction in growth, leaf osmotic potential, NO3- uptake and NO3- reductase activity, whereas amino acids and NO3- were accumulated. This kinetic analysis of S deprivation suggested that a ([Cl-]+[NO3-]+[PO43-]):[SO42-] ratio could provide a relevant indicator of S defciency, modifed nearly as early as the over-expression of genes encoding SO42- tonoplastic or plasmalemmal transporters, with the added advantage that it can be easily quantifed under feld conditions.