The effects of rate and timing of N fertilizer on growth, photosynthesis, N accumulation and yield of mustard (Brassica juncea) subjected to defoliation

Mustard (Brassica juncea L.) is characterized by large number of broad oblong shaped leaves in the lower layers. Our earlier studies have shown that removal of these shaded lower leaves on mustard plant axis enhanced growth, photosynthetic capacity and yield of the crop. We now present evidence that soil-applied nitrogen (N) at pre- or post-flowering stage following defoliation of lower leaves influences plant growth, photosynthesis and assimilation balance. Following defoliation at pre-flowering, i.e. 40 d after sowing (DAS) and N applied at the rate of 100 kg ha−1 at the time of sowing and 50 kg ha−1 at post-flowering (60 DAS) enhanced the characteristics maximally. The defoliation treatment together with N combinations and the time of its application, N at 150 kg ha−1 applied as single dose at the time of sowing or N applied in split; 100 kg ha−1 at the time of sowing and 50 kg ha−1 at 40 DAS or 75 kg ha−1 at the time of sowing or 75 kg ha−1 at pre- or post-flowering time proved less effective. The plants which were not defoliated and received 75 kg N ha−1 at the time of sowing and 75 kg ha−1 at 60 DAS showed lowest values. Furthermore, N assimilation was more efficient in plants following defoliation at 40 DAS. The results suggest that split N application (100 kg ha−1 at sowing and 50 kg ha−1 at post-flowering) enhances substantially growth, photosynthesis, N assimilation and yield of mustard following defoliation. This management practice could be adopted in mustard culture for increasing seed yield together with minimizing N loss.