Effects of rootstock vigour and in-row spacing on stem and root growth, conformation and dry-matter distribution of young apple trees.

The effects of M.9 and MM.106 rootstocks and in-row spacing (0.4, 0.7, and 1 m) on the vegetative behaviour of three-year-old apple (Malus domestica Borkh) trees were studied. Analysis of digital images was used to estimate canopy and root growth and conformation (shape and size), whereas dry-matter distribution was used to determine partitioning to various plant parts. Trees on less vigorous M.9 rootstocks showed reduced stem and root growth as they produced fewer and/or shorter shoots and fewer and/or shorter and thinner roots. Canopy spread area (CSA) and volume were similar for trees on both rootstocks and differences in stem length density (stem length per unit canopy volume; SLD) compensated for the difference in stem growth. For M.9 roots, the decrease in growth was reflected in the reduction in root spread area (RSA), which was not compensated for by the increase in root length density (root length per unit RSA; RLD). Trees on M.9 allocated more dry matter to above-ground structures, whereas trees on MM.106 partitioned more to roots. Leaf to root surface area ratio (L:R) was also greater for trees on M.9. Stems of trees grown 0.4 m apart weighed less because they were thinner, whereas the same trees produced fewer and/or shorter, but thicker roots, which resulted in similar root dry weights for trees at all spacings. Canopy height increased with spacing for trees on M.9, whereas it was negatively related to spacing for MM.106. CSA was rounder at the wider spacings and exceeded the allotted soil surface at the closest one. For three year old apple trees, optimum in-row spacing (no shoot overlap and full coverage of allotted space) was about 0.7 m for M.9 and 0.8 m for MM.106. RSA was also rounder at the larger spacings, but never filled the allotted space. Dry-matter distribution was significantly influenced by spacing, and root to shoot ratio (R:S) was highest at the widest spacing. The vegetative behaviour of young apple trees was influenced by both rootstock vigour and tree spacing. Also, trees on the M.9 rootstock reflected greater ability to compete for available resources and adapt to space confinement and, therefore, may represent a better option for cultivation of apple in the settings and at the planting densities considered. [ABSTRACT FROM AUTHOR]