CO2 EXCHANGE, PRIMARY PHOTOCHEMICAL-REACTIONS AND CERTAIN ENZYMES OF PHOTOSYNTHETIC CARBON-REDUCTION CYCLE OF BRASSICA PODS UNDER REDUCED IRRADIANCE

Brassica campestris L. plants grown under net house conditions were su bjected to reduced irradiance (50 %) at three stages of pod developmen t, viz. 15 (stage I), 30 (stage II) and 45 (stage III) d after anthesi s (DAA). Rate of net photosynthesis (P-N) under reduced irradiance dec reased by 50-70 %, whereas rate of transpiration (E) and stomatal cond uctance (g(s)) decreased by 20-40 %. Internal CO2 concentration (c(i)) , however, remained unchanged. Amongst the enzymes studied here, only ribulose-5-P kinase and 3-phosphoglycerate kinase were affected more s everely (by 30-50 %). The effects of treatment were more pronounced at stage I, which was the active phase of pod photosynthesis. Similarly, the whole chain electron transport decreased by 56 % when the plants were subjected to the above treatment at stage I. However, the activit y remained unaffected at stages II and III. Photosystem 2 activities a nd fluorescence spectrum of pod chloroplasts remained unchanged irresp ective of the stage of treatment. Among the yield parameters, the numb er of pods per plant and the number of seeds per pod remained more or less the same as in the control, but the seed mass per pod, total pod mass and one thousand seed mass decreased by about 20 %, indicating th at it was the seed size which was affected adversely under low irradia nce.