Seasonal Changes in Translocation of 14 C from Different Age‐Classes of Needles on 20‐Year‐Old Scots Pine Trees ( Pinus silvestris )

Needles of 20‐year‐old Scots pine trees ( Pinus silvestris L.) were permitted to photoassimilate 14 CO 2 for 1 h on different dates during the growing season. The loss of radioactivity from current, 1‐year‐old, and 2‐year‐old needles was followed, and the translocation of photoassimilated 14 CO 2 from older needle age‐classes to the elongating new needles studied. The effects of good mineral and water supply on translocation were also considered. In the spring, 1‐year‐old and 2‐year‐old needles accumulated 14 C. These reserves, together with current photosynthate, were utilized when the trees started growing. The 1‐year‐old needles exported 14 C to the current needles during the first weeks of elongation of the later, while no such translocation occurred from the 2‐year‐old needles. Removal of the 1‐year‐old needles resulted in translocation of assimilates from the 2‐year‐old needles to the current needles. The general pattern of translocation observed in the control trees was not changed when the trees were fertilized and irrigated. The new needles started to export assimilates in the middle of July when the photosynthetic rate per needle was comparable with that of the older age‐classes. This occurred about 4 weeks after positive net photosynthesis was first measured for the current shoot. The current needles of trees with good nutrient and water supply seemed to become self‐sufficient in photoassimilates earlier than the current needles of the control trees.