Effect of nitrogen nutrition stress on grain weight, starch quality in bread wheat

Grain weight in wheat is influenced by genetics and the environment, with nitrogen (N) nutrition playing a crucial role by improving grain filling and dry matter accumulation. Starch constitutes 60 to 70% of wheat grain dry matter, consisting of amylose and amylopectin. Amylose/amylopectin ratio determines the resistance nature of the starch and glycaemic index, which are important for human health. Nitrogen nutrition affects dry matter accumulation in wheat grain by influencing starch biosynthesis and enzymes involved in it. In this study, four wheat genotypes (HB-208, VL-829, chotilerma and HD-2967) with varied grain weights were grown under optimum nitrogen (ON) and low nitrogen (LN) conditions. The effect of nitrogen stress on grain weight; starch, amylose and amylopectin content were studied in these genotypes. Further, the important enzymes linked to amylose and amylopectin biosynthesis activity were also assayed in grains during the active grain-filling stages. HB-208 showed the highest grain weight, followed by VL-829, HD-2967 and chotilerma. Under nitrogen stress, grain weight was significantly reduced in HB (29%) and VL (25%), whereas no reduction was observed in case of HD and CL. Genotypic difference in grain filling was observed, where maximum starch accumulation in HD was observed before 21 days after anthesis (DAA) whereas, after 21 DAA in VL. We observed an increase in grain protein content under nitrogen stress in all three genotypes except HD. Among the two components of starch, amylose content was significantly reduced across the genotype under N stress, whereas amylopectin was unchanged in HB and VL and increased in HD and CL. Reduction in amylose resulted to starch reduction in HB and VL, which also corroborated with grain weight reduction. AGPase enzyme activity at 21 DAA was reduced in HB and VL, resulting in decreased starch content.