Differential response of wheat genotypes to applied nitrogen: biochemical and molecular analysis

Nitrogen-efficient and nitrogen-inefficient wheat genotypes were identified on the basis of the differential response of 16 wheat genotypes with low (1 mM) and high (25 mM) nitrogen (N) supply. Growth performance, measured in terms of fresh weight, dry weight and length of root and shoot, was higher in N-efficient than N-inefficient wheat genotypes at low N levels. Interestingly, although the growth of N-efficient genotypes did not show any change with increasing level of N supply, there was a marked increase in the growth of N-inefficient genotypes with the increase in N level. To work out the basis of this differential response of wheat genotypes to low N levels, we investigated the nitrate uptake rate of root and activities of nitrate assimilatory enzymes in the leaves of N-efficient and N-inefficient wheat genotypes. Nitrate uptake kinetics of these genotypes revealed that the uptake of nitrate in N-efficient genotypes was mediated by high-affinity nitrate transporter systems, whereas those of N-inefficient genotypes was mediated by low-affinity nitrate transporter systems. Study of the activities and expression levels of nitrate assimilatory enzymes in N-efficient and N-inefficient wheat genotypes showed that nitrate reductase (NR) and glutamine synthetase (GS) play important roles in N assimilation under low-nitrogen conditions.