Photosynthetic carbon reduction cycle metabolites and enzymes of sucrose and starch biosynthesis in developing Brassica pods

Abstract

Rate of net CO₂ exchange and activities of the key enzymes of fru-2,6-P2, sucrose and starch synthesis and levels of certain intermediates of Calvin cycle were determined in Brassica pods at different stages of their development. The rate of net CO2 exchange, activities of FBPase, UDPG-pyrophosphorylase and SPS, and the contents of 3-PGA, DHAP, RuBP and UDPG increased up to day 21 after anthesis followed by a continuous decrease thereafter. However the content of fru-6-P started decreasing only after 28 days of anthesis. Changes in the levels of fru-2,6-P2 were closely associated with the changes in F6P 2-kinase activity rather than with F2,6-P2ase activity. Similarly, activities of ADPG-pyrophosphorylase and ADPG-starch synthetase closely followed the pattern of starch accumulation in pod tissues. These observations suggest that during the early phase of pod development (up to 21 days after anthesis), which is also the active phase for pod photosynthesis, carbon is mainly utilised for sucrose synthesis and that during the later phase of pod development (from day 21 to 42 after anthesis), there is shift in metabolic path of carbon from sucrose to starch.