Growth Responses from One-year-old Apple Branches to Heading as a Factor Governing Terminal Shoot Length in Cultivars with Different Branching Habits

A series of one-season experiments were carried out with 'Fuji' and 'Jonathan' trees of different ages and with young 'Hokuto' trees. Twigs (1-year-old branches) on trees that were trained to the central leader or open center systems were headed to elucidate the twig-to-twig relatonships on terminal shoot growth. Terminal shoots were almost the same length between pruned and unpruned twigs in 'Fuji' and 'Hokuto' trees, whereas those of 'Jonathan' grew longer when headed. Unpruned twigs of 'Fuji' sent out a few shoots but the number of shoots and total shoot length that developed from headed twigs were almost the same as those from unpruned counterparts. Unpruned twigs of 'Jonathan' tended to develop a large number of shoots, whereas heading cuts caused a significant reduction in the number of shoots and total shoot length. The foregoing cultivar-specific responses to pruning also occured in experiments with water sprouts. The above observations were integrated into the following assumptions: (1) The strength of shoot growth on twigs is determined to a considerable extent by inherent twig conditions and the circumstances under which they developed. Twig location on the crown and distance from the tree base as well as length and thickness of the twigs seem to be important factors related to the shoot growth. (2) Terminal shoot length is a product of conditions determined by the individual twig and the mutual interaction between terminal and lateral shoots. (3) In cultivars with infrequent branching habits, such as 'Fuji', in which the terminal shoots have a strong dominance over lateral shoots, heading cuts do not encourage terminal shoot growth because growth relations between terminal and lateral shoots are little changed by pruning. (4) In cultivars having frequent branching habits, i.e., 'Jonathan', in which the terminal shoot is weakly dominant, heading induces the remaining terminal shoots to grow longer by reducing the number of active buds on the twig; it favors the early stage of terminal shoot growth and reduces subsequent inhibition of terminal shoots by lateral ones.