Thallus samples ofEvernia prunastri, rehydrated in the light, accumulate both mannitol and arabitol as products of the conversion of photosynthates which have been translocated from the photobiont to the mycobiont. The concentration of accumulated polyols, measured as milligrams mannitol per gram dry thallus, is about 600 times higher than that of glucose. By supplying urea to the thallus, a urease is induced to produce carbon dioxide and ammonia from urea. However, the increase in CO2availability does not lead to an increase in the concentration of photosynthates, since the ratio of mannitol to glucose decreases to a third of its initial value. This decrease does not result from the loss of polyols to the incubation media but from an inhibition of the conversion of glucose to polyols.
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