Involvement of exported photosynthetic products in the CO2 exchange of the skeletal shoots of pine

CO2 exchange in the leafy and skeletal parts of attached shoots of Pinus sylvestris L. was measured with an infrared gas-analyzer in an open differential system during daylight hours. The (CO2)-C-14 assimilation rates in the leafy parts of shoots and (CO2)-C-14 evolution from current photosynthetic products in the lower skeletal part of shoots were measured in afternoons. Chlorophyll content was measured in the needles of the same shoot. The carbon of exported assimilates contributed only about 4% to CO2 exchange in the heterotrophic tree tissues. Only this component of CO2 evolution from the surface of the skeletal part of the tree was related to the losses of the net primary photosynthetic production (NPP) in the aboveground part of the pine stand during the current growth period.