Air-Pruning Containers Modify Root and Scion Growth and Alter Resource Allocation of Bench-Grafted Apple Plants

The profitability of modern apple orchard plantings depends largely on how rapidly the costs of planting are returned. Tree establishment and growth in the formative years are often can be limited by transplant shock associated with bare-root trees. In this experiment, we examined the effect of two planting systems, air-pruning containers, and field-grown liners, on above- and below-ground growth and development during the first year in the nursery. M9 ‘Nic 29’ bench grafts for three apple cultivars of different vigor classes, ‘Fuji’ (high vigor), ‘Gala’ (moderate vigor), and ‘Honeycrisp’ (low vigor). We hypothesized that air root-pruning containers would alter rooting distribution compared to field-grown liners by increasing fine root production, and this, in turn, would result in improved resource allocation and greater biomass partitioned to above-ground organs. Bench grafts were divided evenly between a field-grown liner bed (for bare root production) and an air-pruning container system. Air-pruning containers produced trees with significantly more root tips and greater total root length per tree than field-grown liners. Importantly, air-pruning containers resulted in a marked and significant increase in roots < 0.4 mm in diameter; however, field-grown liners produced trees had significantly more roots with diameters > 1.5 mm and, thus, significantly greater root dry matter content (DMC). Above-ground parameters (scion length, DMC of wood and leaves, and individual leaf area and DMC) were significantly increased for plants in air-pruning containers. Generally, the growth benefits from air-pruning containers were less pronounced in ‘Gala’ compared to ‘Honeycrisp’ or ‘Fuji’. The percentage of total DMC partitioned to the canopies of air-pruning containerized trees was significantly greater than field-grown liners.