Effects of early and late season defoliation on photosynthesis, growth and yield of mustard (Brassica juncea L.)

Abstract

Mustard (Brassica juncea L.) is characterized by a large number of leaves in the lower layers. These leaves are poorly illuminated, remain below the light compensation point and abscise at maturity. The effects of removing 50 % of all leaves taken from the lower half of the plant at 40 (pre-flowering) or 60 (post-flowering) days after sowing (DAS) were studied on photosynthesis and growth at 80 DAS and on yield at harvest. A comparison of stomatal and mesophyll effects on photosynthesis upon removal of such leaves was also made. Defoliation at 40 DAS resulted in a higher rate of photosynthesis (P N), growth and yield compared to defoliation at 60 DAS or the no defoliation control. The non-significant variation in stomatal limitation (I S), increased activity of carbonic anhydrase (CA) and constant Ci/Ca ratio in defoliated plants suggest that the main effect on P N was through enhanced mesophyll rather than stomatal effects. Defoliation at 40 DAS caused the greatest emergence of new leaves with rapid growth, which contributed to maximum leaf area as well as leaf and plant dry masses. The favourable effects of defoliation at 40 DAS on photosynthesis and growth were reflected by seed yield and attributing parameters.