Photosynthetic carbon fixation characteristics of fruiting structures of Brassica campestris L.

Abstract

Activites of key enzymes of the Calvin cycle and C₄ metabolism, rates of CO₂ fixation, and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in the podwall, seed coat (fruiting structures), and the subtending leaf (leaf below a receme) of Brassica campestris L. cv 'Toria.' Compared to activities of ribulose-1,5-bisphosphate carboxylase and other Calvin cycle enzymes, e.g. NADP-glyceraldehyde-3-phosphate-dehydrogenase and ribulose-5-phosphate kinase, the activities of phosphoenol pyruvate carboxylase and other enzymes of C₄ metabolism, viz. NADP-malate dehydrogenase, NADP-malic enzyme, glutamate pyruvate transaminase, and glutamate oxaloacetate transaminase, were generally much higher in seed than in podwall and leaf. Podwall and leaf were comparable to each other. Pulse-chase experiments showed that in seed the major product of ¹⁴CO₂ assimilation was malate (in short time), whereas in podwall and leaf, the label initially appeared in 3-PGA. With time, the label moved to sucrose. In contrast to legumes, Brassica pods were able to fix net CO₂ during light. However, respiratory losses were very high during the dark period.