POD PHOTOSYNTHESIS AND SEED DARK CO2 FIXATION SUPPORT OIL SYNTHESIS IN DEVELOPING BRASSICA SEEDS

Rate of photosynthesis and activities of photosynthetic carbon reducti on cycle enzymes were determined in pods (siliqua), whereas rate of da rk CO2 fixation, oil content and activities of enzymes involved in dar k CO2 metabolism were measured in seeds of Brassica campestris L. cv. Toria at different stages of pod/seed development. The period between 14 and 35 days after anthesis corresponded to active phase of seed dev elopment during which period, seed dry weight and oil content increase d sharply. Rate of pod photosynthesis and activities of photosynthetic carbon reduction cycle enzymes were maximum in younger pods but suffi ciently high levels were retained up to 40 days after anthesis. The ra te of dark (CO2)-C-14 fixation in seeds increased up to 21 days after anthesis and declined thereafter but maintaining sufficiently high rat es till 35 days after anthesis. Similarly various enzymes viz., phosph oenolpyruvate carboxylase, NAD(+)-malate dehydrogenase and NADP(+)-mal ic enzyme, involved in dark CO2 metabolism retained sufficient activit ies during the above period. These enzyme activities were more than ad equate to maintain the desired supply of malate which mainly arises fr om dark CO2 fixation in seeds and further translocated to leucoplasts for onward synthesis of fatty acids. Enzyme localization experiments r evealed phosphoenolpyruvate carboxylase and enzymes of sucrose metabol ism to be present only in cytosol, whereas enzymes of glycolysis were present both in cytosolic and leucoplastic fractions. These results in dicated that oil synthesis in developing Brassica seeds is supported b y pod photosynthesis and dark CO2 fixation in seeds as the former serv es as the source of sucrose and the latter as a source of malate.