Physiological basis of nitrogen use efficiency at variable nitrogen application rates in maize

Nitrogen (N) is one of the major essential elements for crop growth and development and is extensively used in modern agriculture to maximize crop yields. Identification of genotypes with efficient N capture and utilization ability can help reduce production costs and N pollution risks. The study of regulatory mechanisms controlling N uptake, assimilation and recycling is essential to identify marker traits controlling N use efficiency under low and high input systems in maize. The objective of this study was to understand various physiological and biochemical attributes associated with N use efficiency and simultaneously to study the differences in response pattern between maize genotypes (parents and their derived hybrid) at both high and low N levels. Application of N significantly increased the content of various biochemical constituents (total soluble sugars, total soluble proteins, total starch content, activity of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT) and PEP carboxylase enzymes) being maximum at RDN+50% in two sets of maize genotypes (late and early maturing). There was a decreasing pattern in N use efficiency (NUE) values with increasing fertilizer rates in both late and early maturing genotypes. The high NUE and high grain yield for PMH 1(late maturing) and JH 3459 (early maturing) hybrids as compared to their parents (LM 13, LM 14;a and CM 143, CM 144) was due to high photosynthetic capacity associated with increased activity of PEP carboxylase and high N metabolic efficiency which was associated with increased activity of enzymes associated with N metabolism (nitrate reductase, glutamine synthetase and glutamate synthase) and ability to reallocate the surplus of absorbed N to the grain (more N harvest index).