Accumulation and Translocation of Sorbitol in Apple Phloem

Abstract

In intact apple plants, sorbitol, rather than sucrose, is the main carbohydrate involved in phloem transport. The behaviour of excised phloem, either freshly excised (fresh) or washed for 20 hr after excising (aged), towards sucrose, glucose, and sorbitol was studied. All three carbohydrates were accumulated rapidly, rates being higher when more concentrated solutions were supplied and when aged tissues were used. Both effects were more marked for sorbitol than for sucrose or glucose. When sucrose or glucose was accumulated by fresh or aged phloem, sucrose was the main product and no sorbitol was formed. When sorbitol was accumulated it was the main product, though in aged phloem sucrose was also formed. In comparison with sucrose sorbitol is readily accumulated from the more concentrated solutions, but is only slowly metabolized by phloem tissues. It is suggested that, in the intact plant, sorbitol, which is present in the leaf in high concentration, is preferentially accumu· lated into the phloem, but once there is metabolically rather inert and so is not altered until it reaches its destination. The tissue there, like aged phloem, can readily utilize sorbitol. Thus, sorbitol is well suited to translocation in apple.