Interactive effect of sulfur and nitrogen on growth and nitrogen assimilation of lettuce (Lactuca sativa L.) varieties

Nitrogen (N) is one of the key elements for all living organisms and an indispensable constituent of many important macromolecules.Use of N containing fertilizers has dramatically increased over the years for increasing the crop productivity.However, the use efficiency of applied N is only 30-40%, and the rest is lost into the environment, causing severe environmental and health problems.Green leafy vegetables, especially lettuce, accumulate high nitrate content that adversely affects human health.Therefore, lettuce production with low nitrate content under the N fertilization is vital important, not only for health benefit but also for maintenance of environmental sustainability.Sulfur is the tenth most plentiful element in the universe, the nutrient that the plants overlook.Since, sulfate assimilation has metabolic coupling with nitrate assimilation, supplementation of sulfur (S) along with N fertilization has been recommended in many studies in various crops for improving the use efficient of N. In consideration of the need, a field experiment was carried out with combinations of N and S and split doses of N application to identify the most appropriate fertilizer (N and S) application rates in order to enhance lettuce productivity and to reduce nitrate accumulation.Two varieties of lettuce (Lactuca sativa), 'Pusa Kiran' (V1) and 'Pusa Chaulai' (V2) were chosen for the study.Six fertilizer treatments S0N0 (control), S0N2 (S0N25+20), S0N3 (S0N45+25+20), S1N0 (S40N0), S1N2 (S40N25+20) and S1N3 (S40N45+25+20) were conducted in a randomized block design with three replicates.Growth, nitrate assimilation and leaf nitrate content were assessed and compared at various stages of growth and development in both the varieties.The results suggested that the application of the combined doses of S and N (S1N3) in which N is applied in three split doses (S40N45+25+20) resulted in the maximum growth of the lettuce and accumulation of the least level of nitrate in leaves.Irrespective of the treatments, V1 accumulated more nitrate in leaves than V2 at all the phenological stages.Differential growth and leaf nitrate accumulation of lettuce varieties by the effect of supplementation of the N with S fertilization are evident with the change in the activities of the enzymes of nitrate assimilation.With this knowledge, a more economical and environmentally responsible approach to nitrogen management should be able to be developed for increased lettuce yield with lower nitrate levels.