Physiological studies on the leaf blades of wheat (Triticum aestivum L.) in relation to nitrate assimilation and grain protein accumulation

Abstract

Two wheat (Triticum aestivum L.) cultivars, Shera and Kalyansona, which differ in grain protein percentage and in its enhancement by fertilizer application were selected for the present study. The analysis was confined to the leaf blades of the main shoot of plants grown at three rates of application of urea giving 0 (N₀), 90 (N₉₀) and 180 (N₁₈₀) kgN/ha. Cv. Shera produced ten leaf blades on its main shoot while cv. Kalyansona had 11. The total dry weight duration of the leaf blades of cv. Kalyansona was higher than that of cv. Shera, particularly at N₉₀ and N₁₈₀. This was due to a great increase in dry weight of the leaf blades in response to fertilizer nitrogen and to an additional leaf blade. In both the cultivars the in vivo nitrate reductase (NR) activity was highest in the first formed leaf blades and showed a gradual decline in the successively formed ones, at all three nitrogen levels. Cv. Shera showed higher NR activity than cv. Kalyansona in most of the weekly samplings (weighted mean values) through the season. Seasonal mean NR activity was higher in cv. Shera at all three rates of nitrogen application. The difference was, however, significant only at N₉₀. The total amount of NO₃^- reduced by the leaf blades, which is a function of their NR activity and dry weight duration, was higher in cv. Shera than in cv. Kalyansona at N₀. However, at N₉₀ and N₁₈₀ the differences decreased because of the higher dry weight duration of the leaf blades of cv. Kalyansona. The estimated amount of NO₃^- assimilated was less than the actual reduced N at harvest at N₀ but at N₉₀ and N₁₈₀ there was an overestimation. The grains of cv. Kalyansona showed significantly lower protein percentage than cv. Shera at all three nitrogen levels. While low nitrogen per spike was the main cause of lower protein percentage in cv. Kalyansona at N₀ as compared with cv. Shera, distribution of almost the same amount of nitrogen to a larger quantity of grains was largely responsible for the lower protein percentage in this cultivar at N₉₀ and N₁₈₀. The results are discussed in relation to selection criteria used in breeding programmes for the improvement of grain protein content of wheat cultivars.