The daily variation of sorbitol, starch, sucrose, glucose, and fructose were estimated in the leaf of the apple,Malus pumilaMill., at 2-h intervals over 24-h periods in each of the months June to October, inclusive, 1968. Marked daily fluctuation in sorbitol concentration was observed in all months, accompanied by more moderate variations in the other carbohydrates. The patterns of concentration variability in sorbitol and starch were relatively similar compared to the more irregular patterns of the three sugars, sucrose, glucose, and fructose. In conjunction with its invariably very high leaf concentration relative to the other carbohydrates, the general daily pattern of sorbitol content, namely, a marked nocturnal decrease followed by a great increase during daylight, indicated that sorbitol is the major photosynthetically derived carbohydrate and is further evidence that sorbitol is a reserve carbohydrate in apple leaves.
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