Mustard (Brassica juncea L.) is characterized by high nitrogen (N) requirement for improved productivity and a large number of broad leaves in lower layers. The N applied to the soil is not fully utilized by the crop due to various constraints and the leaves in the lower layers remain below light compensation photosynthetic point. Earlier researches have shown that an early removal of 50% lower leaves enhances photosynthesis and growth of mustard. The present paper evidences that the activities of N assimilation enzymes, nitrate reductase, nitrite reductase and glutamine synthetase increase in defoliation plants, increasing photosynthesis, growth and yield. Application of 150 kg N ha^-1 at sowing time and defoliation at pre-flowering i.e., 40 d after sowing increased N assimilation, photosynthesis and growth maximally than no defoliation and 150 kg N ha^-1. The other N rates (0, 60 or 100) and defoliation treatment proved less effective. The redistribution of photoassimilates towards developing sink in plants subjected to defoliation and given 150 kg N ha^-1 enhanced seed yield substantially. The results suggest that plants assimilated N more efficiently after defoliation, increasing photosynthesis, growth and yield. This management practice could be adopted in mustard cultivation for increasing seed yield and decreasing N wastage.

Mustard (Brassica juncea L. Czern & Coss.) is an important crop of the tropical and sub tropical regions of the world. The crop is characterized by high nitrogen (N) requirement but only 50% of the applied fertilized N is available due to significant loss of N to the environment (Schjoerring et al., 1995). Moreover, the plant bears large and broad oblong shaped leaves on the lower layers of the plant (Weiss, 1983) which do not receive sufficient light and remain below light compensation point. They contribute to the development of supra optimal leaf area indices with accompanying self shading and by other leaves within the plant axis. Therefore, photosynthetic potential of lower leaves is less compared to the upper leaves. The efficient use of N by plant is influenced by the leaf photosynthetic capacity which can be enhanced by reducing the competition between assimilating organs (leaves). Reducing the competition between assimilatory leaves by the removal of shaded leaves allows functional and young leaves to grow efficiently. Earlier researches have shown that removal of 50% of the leaves on lower regions at pre-flowering i.e., 40 d after sowing (DAS) stage improved photosynthetic efficiency of rest of the leaves, increased the dry mass yield of mustard (Khan, 2002; and 2003). However, defoliation at post-flowering stage, i.e., 60 DAS, was less effective (Khan and Lone, 2005). It was postulated that defoliation of leaves in lower layers brings faster growth of new emerging leaves which use N more efficiently with enhanced photosynthetic capacity. The objective of the reported research is to study the N assimilation following defoliation and its subsequent effect on photosynthesis, growth and yield of mustard.

The Effect of Defoliation of Lower Leaves and Nitrogen Application Rate on N Assimilation, Photosynthesis, Growth and Yield of Mustard (Brassica juncea L.), defoliation, photosynthesis, assimilation, characterized, compensation, Mustard, nitrogen, preflowering, Brassica, tropical, enzymes, fertilized, glutamine, influenced, cultivation, nitrate, nitrite, oblong, productivity, research, Schjoerring, synthetase, enhances