CHANGES IN ENZYME-ACTIVITIES INVOLVED IN MALATE METABOLISM IN OAK LEAVES DURING RHYTHMIC GROWTH

Amide content, ATP level and activities of enzymes linked to malate me tabolism were determined in leaves of three successive flushes of comm on oak during the development of the third flush. In the expanding lea ves, all studied enzymes showed a maximum activity around the 7th day after budbreak. Phosphoenolpyruvate carboxylase (PEPc), NAD-malate deh ydrogenase (MDH) and NADP-malic enzyme (ME) maintained high activity u p to full leaf expansion. In contrast, fumarase (FUM), pyruvate kinase (PK) and NADP-MDH activities sharply decreased to reach, on the 10th day after budbreak, the same low activity levels as those measured in mature leaves. Two patterns were observed in oak leaves during growth. Firstly (7th-10th day after budbreak), PK, FUM and NADP-MDH could con tribute to the supply of ATP through glycolysis and Krebs cycle; the A TP profile corroborated those results. Secondly (after the 10th day), the maintenance of an active PEPc pathway led to a respiratory CO2 ref ixation and provided carbon skeletons for amino acid synthesis, Furthe rmore, nitrate reductase (NR) activity was high in young oak leaves, S light changes in activities of NR as well as NAD(P)-ME, NAD(P)-MDH can be noted on days 7 and 10 after budbreak in the mature leaves. These changes could be necessary in supplying the third flush with amino aci ds. These data suggest that MDH, ME, PK and PEPc have important functi ons in the young leaves which are not directly linked to C-3 photosynt hesis but rather to nitrate assimilation and malate provision to mitoc hondria.