Nitrate reductase activity in maize leaf: Influence of nitrogen sources, doses and crop seasons

Influence of the source and level of nitrogen on nitrate reductase activity in maize plants grown during monsoon and winter was assessed. Three different sources of N (diammonium phosphate, urea and ammonium nitrate) @ 0, 80, 120, 160 kg N ha⁻¹ were applied to the crop during the monsoon (88 day crop duration) and winter (150 day crop duration). Physiological parameters were compared at seven growth stages based on phenology. In vivo nitrate reductase (NR) activity in the last fully expanded leaf (LFEL) during vegetative growth was significantly higher during the monsoon compared to the winter season, but during grain filling period (GFP), the NR activity was higher in winter. High positive correlations between NR activity, N concentration in LFEL and N accumulation in plant between growth stages were observed in both seasons. The results indicate increase in NR activity and nitrogen accumulation between growth stages with each increment in supplemental nitrogen dose. Plants treated with ammonium nitrate had higher NR activity compared to those treated with urea or DAP. The NR activity had high positive correlations with grain protein concentration and grain yield plan⁻¹. The NR activity during GFP appeared to determine total N content in plants irrespective of NR activity during the vegetative period, and the higher NR activity during the early growth phase had positive correlation with grain protein concentration. Leaf NR activity could be useful as a marker enzyme for N content and concentration in plants, grain protein concentration and grain yields under low or high N regimes in a given environment but not across the seasons. However, it can be a marker for maize growth rates across the seasons.