Yearly changes in the use of stored carbon to develop new aboveground organs of young and mature apple trees

Temperate deciduous trees depend on stored carbon for their growth and metabolic processes in early spring. Despite their importance, annual changes in the contribution of stored carbon to the development of new organs are not adequately understood. We conducted 13C labelling experiments on apple (Malus domestica) saplings and a mature tree grown under normal conditions, and investigated 13C concentration in new aboveground organs in early spring for several years after labelling. In the saplings, assimilated 13C during the growing season was detected in the terminal buds around the bud break from the first to the fourth year after labelling. Similar results were found in the flower buds, leaves, and annual shoots of mature trees pink bud stage, in which the concentration of assimilated 13C was detected from the first to the third year after labelling. The concentration of assimilated 13C in the new organs of saplings and mature trees decreased exponentially over time after labelling. The reduction patterns of the assimilated 13C in the new organs of the saplings and mature trees fit better with the two-pool than the one-pool exponential models, indicating that while stored carbon in fast and slow pools was included in the new organs, almost all stored carbon used was from the fast pool. Our findings also suggested that the use of stored carbon in the development of new organs in early spring would not differ between saplings and mature trees.