Nitrate Uptake and Assimilation Potential Under Limiting N-Status in Wheat Genotypes (Triticum aestivum L.)

Abstract

The excess use of nitrogen fertilizers causes various problems, including higher costs of crop production, lower nitrogen use efficiency, and environmental damage. Crop breeding for low nitrogen tolerance has become the major focus in solving these issues. Genotypic variation for NUE traits has been reported in cereal crops which include N accumulation, N remobilization and high yield under low N. However, identification of genetic and metabolic control linked to phenotypes NUE is difficult looking to the complexity of N metabolism. The present work with thirteen wheat genotypes was undertaken to understand the basic difference in uptake, assimilation, remobilization of nitrate and other physiological limiting factors with limiting nitrogen (N) supply. The higher leaf nitrate content in limited nitrogen supply in nitrogen-efficient wheat genotypes GW-322, MACS 6222, NIAW-301, NIAW-34, NIAW-3170, NIAW-1994 and HD 281 appears to be low-nitrate tolerant probably indicating that both low and high affinity nitrate transporter are actively operating. Chlorophyll content, leaf nitrate content, in vivo NR, in vitro NR, GS, Fd-GOGAT, NADH-GOGAT and total grain protein were significantly higher in the N-efficient wheat genotypes, even at low dose of applied nitrogen both at pre and post-anthesis stages. The significantly higher in vitro NR activity compared to in vivo NR activity at saturating nitrate concentration (+KNO3) signifies that beside nitrate, NADH appears to be a limiting factor. Glutamine synthetase, Fd-GOGAT and NADH-GOGAT activities were higher at post anthesis stage signifies its importance in remobilization of photorespiratory ammonia. In pre-anthesis, the Nefficient genotypes had higher relative levels of GS2 protein than the N-inefficient genotypes; it was found that GS2 is important in the vegetative growth stage and carbon-nitrogen metabolic balance. In the post-anthesis stage, there were increased relative amounts of the GS1 protein under limiting N condition. The results revealed that the N-efficient genotypes have increased assimilation ability in the pre-anthesis stage and remobilisation ability in the post-anthesis stage. The relative contents of the GS1 and GS2 proteins at pre and post-anthesis could be used as a marker for breeding varieties with high NUE. Study of the activities of nitrate assimilatory enzymes in N-efficient and N-inefficient wheat genotypes showed that nitrate uptake (in vivo) nitrate reductase activity, glutamine synthetase (GS) and glutamate synthase (GOGAT) play important role in N assimilation and remobilization under low-nitrogen conditions.