Mannitol metabolism in darkness in the leaves of the hemiparasitic angiosperm, Thesium humile

Thesium humile and wheat were submitted to a prolonged dark period following photosynthesis. Measurements of leaf respiratory rates indicated that the plants were still in a good physiological state at the 100th h of darkness and that the parasitic weed exhibited a rapid decrease in respiratory metabolism during the first 6 h of the treatment. After 100 h in darkness, T. humile showed a much stronger decrease in mannitol content than in sucrose and starch. Mannitol utilization as a respiratory substrate in darkness was demonstrated by supplying leaves with [14C]mannitol. The highest rate of hexitol metabolism occurred in the young leaves, but this metabolism was not totally restricted to growing organs. In the young leaves, [14C]mannitol utilization led to 14CO2 release and 14C-incorporation into sucrose, hexose, soluble anionic and cationic substances and insoluble material. Moreover, the sucrose pool exhibited a rapid turnover. Enzymes responsible for these carbon fluxes were then sought. Mannitol dehydrogenase, sucrose-phosphate synthase, sucrose synthase and neutral invertase activities were detected in the young leaves. Carbon metabolism and respiratory rate are clearly controlled by the mannitol oxidizing enzyme. These results could be used in order to define a chemical control method against the mannitol-producing parasitic weeds.