Seasonal changes in photosynthate transport within elongating shoots of Populus grandidentata

Abstract

Individual leaves on 5-year-old suckers of bigtooth aspen (Populus grandidentata Michx.) were exposed to 14CO2 to detect seasonal changes in photosynthate transport. As new leaves developed at the base of currently elongating shoots, they initially acted as translocation sinks and utilized stored carbohydrates. Basal (first-formed) leaves began to translocate around the first of June. Initially, these leaves transported mainly acropetally to the shoot tip, but within 2 weeks they were transporting photosynthate basipetally toward the main stem. Translocation patterns from leaves midway on the shoot were similar, but these leaves continued to export mainly to the stem tip until early July. Tip (last-formed) leaves did not begin to translocate significant quantities of photosynthates until late July.Basal, middle, and tip leaves transported photosynthates into axillary buds; base leaves mainly in June, middle leaves mainly in July, and tip leaves mainly in August. The terminal bud imported photosynthates from all leaves on the shoot and accumulated several times more 14C label than did axillary buds at the base of the shoot.