14 C‐Studies on Apple Trees. V. Translocation of Labelled Compounds from Leaves to Fruit and their Conversion within the Fruit

Following application of 14 CO 2 to fruit spur leaves, the majority of the 14 C absorbed is transfered to the fruit on the same spur, and the total content of 14 C within the leaf‐fruit system as a whole remains virtually constant with time. The considerable reduction in activity in the leaves is accounted for mainly by a decrease in the amount of 14 C‐sorbitol, although relatively speaking the decrease in 14 C‐sucrose is also considerable. The major part of the activity of the sugar fraction in the conducting tissues between blade and fruit (petiol, spur) is found in sorbitol. Immediately following uptake of 14 C yia the leaves a large part of the activity of the sugar fraction in the fruit is found in sorbitol; but this activity is rapidly reduced, accompanied by an increase in sucrose activity, and over longer periods of time increases in particular in glucose and fruclose activity, and in that of methanol insoluble compounds. The changes in activity distribution in the fruit vary with the variety of fruit and the dates within the growing season. By injecting labelled sorbitol directly into the fruit sorbitol is converted into sucrose, glucose and fructose, while injection of labelled sucrose, glucose and fructose has yielded proof of interconversions between these compounds but no measurable amounts of surbitol. After application of 14 CO 2 directly to the outer skin of the fruit considerably less of the activity is found in sorbitol than is the case in leaves following exposure to 14 CO 2 . A minor, but significant, translocation of 14 C away from the fruit was found to take place following the application of labelled 14 C compounds to the fruit. The smallness of the respiratory loss of 14 C in the leaf‐fruit system is discussed. It is concluded that in apple trees considerable translocation occurs in the form of sorbitol which in the fruits rapidly converted into other compounds.