The effects of root growth control using root restricting bags on the growth, fruitfulness, and fruit calcium concentrations and contents of apple (Malus x domestica Borkh. cv. 'Fuji')

Abstract

A field trial was planted in the spring of 1989 to investigate the effects of root growth control using root restricting bags on the growth, fruitfulness, and fruit calcium concentrations and contents of apple (Malus x domestica Borkh. cv. 'Fuji').

The trial was planted as a 2 x 5 factorial randomised complete block design. Trees were conventionally planted or planted in in-ground fabric containers of 15, 34, 65 or 139 litre volume on MM106 or MM115 rootstocks. The fabric container used was the Root Control Bag℠, (Root Control Inc., Oklahoma City, OK). These containers are cylindrical, with one end closed with clear plastic to form the bottom of the bag and the wall material is porous which allows new root growth in the form of fine roots around the periphery of the container.

Trees were planted in two rows at a spacing of 3.0 x 5.0 metres. Throughout the experiment, trees were untrained, un pruned , and allowed to develop natural crown form and size. Fruit thinning was not carried out in any year to avoid obscuring treatment effects. The within-row and inter-row area was maintained weed free using an overall herbicide spray programme. Irrigation water was applied through drippers with all trees receiving the same frequency of irrigation and amount of water. In other respects, normal orchard management practices were followed.

Root restriction reduced tree trunk cross-sectional area (TCSA) in the years 1991 to 1994, and canopy volume and tree height in 1995 when compared with the unconfined treatment. Throughout the experimental period, TCSA decreased linearly as bag volume was reduced. However, growth differences in TCSA within bag sizes were not reflected in canopy volumes.

Trees grown on MM106 rootstock compared with MM115 had smaller TCSA and canopy volumes. However, root restriction reduced the invigorating effect of MM115 as the difference in TCSA or canopy volume (the absolute difference or expressed as a percentage) between the two rootstocks was considerably greater in the unconfined than in the bagged treatments.

The smaller trees resulting from root restriction had lower yields when compared with the unconfined treatment. Throughout the experiment, there were no differences in yields per tree within bag volumes or between rootstocks.

However, the cumulative yields per unit of TCSA were higher where roots were restricted, indicating that these trees had a higher productive efficiency. The effect of root restriction across all bag volumes was to increase cumulative yield efficiency by between 215 to 1589 grams per cm² of TCSA at the 95% confidence interval.

Treatment differences in mean fruit weight in 1992 to 1994 and fruit size distribution in 1994 were attributable to their effect on crop density. Crop densities (fruit number/TCSA) and yield efficiencies (yield per tree/TCSA) between the bagged and unconfined treatments varied between years indicating the trees experienced an 'over charge' of fruiting. Confirming this view, all trees showed a biennial bearing trend although deviations in yield from constant cropping were not strong. The propensity towards biennial bearing did not differ between treatments.

Root restriction increased apple fruit calcium concentrations and contents. This effect was independent of fruit size and was not adequately explained by differences in crop density between treatments.