EVIDENCE FOR A MANNITOL CYCLE IN OROBANCHE-RAMOSA AND OROBANCHE-CRENATA

The activities of various enzymes associated with mannitol metabolism in Orobanche ramosa L. and Orobanche crenata Forsk. were measured both in the crude extracts of plant tissue and callus. The detection of D-mannose-6-phosphate ketol isomerase (EC 5.3.1.8), D-mannitol-1-phosphate:NADP oxidoreductase (EC 1.1.1.nn) and D-mannitol-1-phosphate phosphohydrolase (EC 3.1.3.22) activities revealed a similar pathway as postulated by Rumpho et al. (1983) and Loescher et al. (1992) for celery and privet, while the appearance of D-mannitol-1-phosphate: NAD oxidoreductase (EC 1.1.1.17) activity in O. callus suggests the possibility of an alternative pathway. To exclude artefacts, the above mentioned enzymes were partially purfied and further characterized. The occurrence of a low affinity D-mannitol: NAD oxidoreductase (EC 1.1.1.67) gives evidence for a mannitol cycle in the two Orobanche species.