Résumé
This paper presents the results of a study to identify the first developmental expression of dwarfing rootstock effects during the initial growth of young apple trees. In the first annual growth cycle of the cultivar ‘Royal Gala’ scion bud, no differences in stem growth, leaf emergence rate, total node number or internode length were found among dwarf and non-dwarf rootstocks or their reciprocal dwarf/non-dwarf and non-dwarf/dwarf interstock combinations. Using M.9 as a dwarfing rootstock and MM.106 as a non-dwarfing rootstock, axillary bud growth in the following spring indicated that dwarfing-associated effects were expressed as an increase of the number of floral axillary nodes per tree as well as the positional distribution of floral node production along the trunk axis. Hidden Semi-Markov Chain (HSMC) models were used to quantify rootstock/interstock and positional effects on flower distribution along the trunk. The modelling approach identified a succession of three zones (we named as top zone, floral zone and vegetative zone) down the trunk, with each zone characterised by certain probabilities of floral or non-floral types of axillary node production. Model outputs showed dwarfing rootstock effects characteristically increased both the size (number of nodes) of the floral zone and the probability of axillary buds within the zone being floral. However, not all trees on all rootstocks produced floral buds. An increasing proportion of trees expressed axillary bud flowering as the degree of dwarfing influence increased among treatments in the rootstock/interstock combinations