Changes in structure and function of photosynthetic apparatus of Brassica pods during seed maturation

Rate of photosynthesis; primary photochemical reactions, viz. whole chain electron transport, PSI and PSII activities; polypeptide profiles and lipid composition of thylakoid membranes were determined at different stages of Brassica pod development. Net CO2 exchange rate increased till 20 days after anthesis (DAA), remained constant till 30 DAA and decreased thereafter. While stomatal conductance followed a similar pattern, transpiration rate and internal CO2 concentration were least affected by age. Chlorophyll content decreased from 10 DAA till maturity, However, Chl a:b ratio increased during the period of active photosynthesis and was maximum at 30 DAA. Among the photochemical activities, PSII activity was reduced to 30% in mature pods. Qualitatively, the polypeptide composition of PSI, PSII and their light harvesting complexes were not affected adversely by aging. Total lipids, phospholipids, glycolipids and total free fatty acids decreased with maturity. However, the decrease in glycolipid content was more severe particularly during maturity phase. It seems that decrease in glycolipid content of thylakoids and chlorophyll content of chloroplasts are responsible for the decrease in photochemical activity and hence a reduction in CO2 assimilation at later stages of pod development.