Differentiation and growth traits associated with acrotony in the apple tree (Malus xdomestica, Rosaceae)

Branching is a key factor in the evolutionary diversification of plants and is a main criterion for plant architecture analysis. Among descriptive features, acrotony is defined as increased vigor of the vegetative proleptic branches (from dormant buds), from the proximal to the distal part of the parent growth unit. I hypothesized that acrotony could be extended to other, usually poorly described, architectural traits. The study was conducted on two architecturally contrasting apple (Malus ×domestica) cultivars, 'Pitchounette' and 'Chantecler.' The proportion and size of various offspring entities were assessed according to their position along the shoot for 2 years after parent shoot growth. Acrotony was characterized by two inverse phenomena: an acropetal decrease in the proportion of latent buds and of laterals that aborted, and an acropetal increase in the proportion of reproductive laterals among growing laterals. Distally located reproductive laterals had more spur leaves and flowers and higher fruit set and flowered earlier than reproductive laterals lower on the parent annual shoot. The results suggest that the length-based criterion used for acrotony should be integrated into a general conceptual framework in which the organogenetic potential of the axillary meristem increases from the proximal to the distal part of the annual shoot, leading to greater branching density, larger offspring, and a greater propensity for flower bud formation over consecutive years.