14C-Studies on Apple Trees. II. Distribution of Photosynthates from Top and Base Leaves from Extension Shoots

The photosynthates from leaves from extension shoots may be used either for new growth of the shoot itself, or for growth in other parts of the tree. An attempt has been made to elucidate this problem by determining the content of ¹⁴C in long shoots to which ¹⁴CO₂ was applied either through fully developed leaves at the base, or through very young leaves at the apex. In the case of ¹⁴C application at the base, 80 per cent or more of the ¹⁴C initially taken up disappears from the shoot, and only a very minor part is translocated upwards in the shoot. When the leaves at the apex are exposed, on the other hand, 80 per cent of the ¹⁴C absorbed is retained in the leaves and shoot components treated as long as there is still considerable terminal growth taking place, although a small percentage is deposited in the lower parts of the shoot. At the same time, a much higher proportion is incorporated into methanol-insoluble components. As terminal growth decreases, a larger proportion of the ¹⁴C activity of the apex leaves also disappears from the shoot. The distribution of activity between the sorbitol, sucrose, glucose and fructose fractions was not significantly different in young and in fully developed leaves. The ¹⁴C labelling in the sugar fraction was highest for sorbitol, then sucrose, but decreases with time compared to glucose and fructose.