Seasonal Changes in the Distribution of Photo-assimilated 14C in Young Pine Plants

Pinus strobus L. plants in their third year of growth were permitted to photoassimilate (14)CO(2) for about 1 hour at monthly intervals between April and October, and the subsequent distribution of (14)C in these plants was determined 8 hours, 1 month, 2 months or 4 months after photo-assimilation. In this way, the fate of (14)CO(2) photo-assimilated during different months of the growing season was observed.In the spring, old needles played a significant role in photo-assimilating (14)CO(2) and exporting current photosynthate to the developing new shoots and roots. By July, the new shoot had replaced the old shoot both as the primary photo-assimilating part of the plant and as an exporter, particularly to the root.The root received current photosynthate from the shoot throughout the entire growing season, although plant analysis only 8 hours after photo-assimilation did not always reveal this. Translocation of recent photosynthate from shoot to root was particularly high in August, September, and October.The amounts of photo-assimilated (14)C lost from the plants over a 4 month interval, principally through respiration and photorespiration, were about one-half of that absorbed during photo-assimilation, with the greatest loss occurring within the first month.