Effect of nitrogen and/or sulfur deprivation on the uptake and assimilation steps of nitrate and sulfate in maize seedlings

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Abstract
Seedlings of 2 maize genotypes, differing in grain yield, were deprived for 5 days of either N or S, or both. At the end of each shortage period, prior to the development of any deficiency symptom, uptake rates and internal pools of sulfate and nitrate, extractable activities of ATPS [ATP sulfurylase] OASS [o-acetylserine sulfhydrylase], NR [nitrate reductase], GDH [glutamate dehydrogenase] and GS [glutamine synthetase] were evaluated. The comparison with seedlings supplied with complete solution showed that after S + N deprivation the rate of sulfate uptake was enhanced 46 and 30% in the higher and lower yield genotype, respectively, while that of nitrate was the same of control. In both genotypes GS activity did not change, sulfate and nitrate internal pools were reduced and significant changes in activity concerned NR, which was strongly depressed, GDH, which increased 35%. OASS increased 60 and 30% in the higher and lower yield genotype, respectively. In the case of deprivation of only one of the two elements, each of the metabolic parameters tested, except GS activity, was differently affected by N and S deprivation. SO42- uptake rate, OASS activity and SO42- pools were affected mainly by S; NR and GDH activities by N deprivation, the responses being similar to those given to S + N deprivation. Interactions between S and N metabolism were demonstrated by NO3- pool in roots, which was enhanced mainly by S deficiency, and by ATPS activity, which was depressed mainly by N deficiency. S metabolism appears better adapted than N metabolism to withstand nutrient shortage. A key-role in productivity could be assumed for the SO42- transport system and OASS activity which were the only parameters showing a better efficiency in the higher yield genotype.