Sulfur starvation and restoration affect nitrate uptake and assimilation in rapeseed.

We analyzed the effect of omission of sulfur (S) from the nutrient solution and then restoration of S-source on the uptake and assimilation of nitrate in rapeseed. Incubation in nutrient solution without S for 1-6 days led to decline in uptake of nitrate, activities, and expression levels of nitrate reductase (NR) and glutamine synthetase (GS). The nitrite reductase (NiR) and glutamate synthase (GOGAT) activities were not considerably affected. There was significant enhancement in nitrate content and decline in sulfate content. Evaluation of amino acid profile under S-starvation conditions showed two- to fourfold enhancement in the contents of arginine, asparagine and O-acetyl-L-serine (OAS), whereas the contents of cysteine and methionine were reduced heavily. When the S-starved plants were subjected to restoration of S for 1, 3, 5, and 7 days, activities and expression levels of NR and GS recovered within the fifth and seventh days of restoration, respectively. Exogenous supply of metabolites (arginine, asparagine, cysteine, glutamine, OAS, and methionine) also affected the uptake and assimilation of nitrate, with a maximum for OAS. These results corroborate the tight interconnection of S-nutrition with nitrate assimilation and that OAS plays a major role in this regulation. The study must be helpful in developing a nutrient-management technology for optimization of crop productivity.