14C-Studies on Apple Trees. VIII. The Seasonal Variation and Nature of Reserves

The nature, seasonal variation and mobilization of reserves in Malus × domestica have been studied by means of ¹⁴C, carbohydrate analyses and extractions of xylem sap. Following exposure to ¹⁴CO₂ in the autumn, the majority of the ¹⁴C absorbed is found in the root. During the winter and in particular the spring the amounts of ¹⁴C in the top and root are reduced to approximately 40 per cent of the autumn values; in the root the amount of dry matter was also considerably reduced. In the tops, most of the ¹⁴C absorbed was found as methanol (80 %)-soluble ¹⁴C which also showed the greatest seasonal reduction; sorbitol, sucrose or glucose in particular are responsible for the decrease in concentration within this fraction. Maximum values for methanol-insoluble ¹⁴C were found in March. In the root, the highest values for absolute changes were found for methanolinsoluble ¹⁴C. Hydrolysis of this fraction showed considerable activity in glucose. In the root there was also considerable activity in a precipitated fraction of the methanol extract. Eluates of xylem sap from apple branches contained primarily sorbitol, the highest concentration of which was found at the beginning of March. For a tree with a dry matter weight of about 300 g, the utilization of reserves from the tree in the spring was calculated to be at least 13 g of dry matter. However, only a minor part (less than 25 per cent) of the latter appears to serve as building material for new growth.