Vigour control and its physiological causes in an ultra high density planting system for apple
Abstract
Five year old 'Braeburn' and 'Oregon Spur Delicious' apple (Malus domestica Borkh) trees grafted onto MM106 rootstocks and growing in an ultra high density planting system (20,000 trees per hectare) were selected to investigate the effect of carbohydrate reserves (TCHO) on fruit yield and vegetative growth. From the correlations produced in this trial, a strong biennial pattern among yield efficiencies (g yield cm⁻² trunk area) 1993-1995 was observed. Strong correlations also existed between root and shoot TCHO concentrations and yield efficiency. Shoot TCHO correlated positively with the previous years yield efficiency whereas root TCHO correlated negatively with the previous years yield efficiency. An explanatory model was developed which was based on total carbohydrate (TCHO) concentrations in various tissues of the tree. The data resulted from a two year biennial cycle of apple production and they showed that carbohydrates reserves are correlated with yield efficiency and biennial yield pattern of apple trees growing in an ultra high density planting system.
A second experiment, begun in September 1994, investigated the effect of root pruning on vegetative, reproductive, CHO concentration and total yield. The data set was used to test the model under modified conditions. Six different apple cultivars (Braeburn, Royal Gala, Oregon Red Delicious, Splendour, Granny Smith and Fuji) on MM106 rootstock growing in a ultra high density planting system (20,000 trees per hectare) were used. Root pruning treatments (20, 30cm) were effective in achieving reductions in vegetative growth. Tree diameters of root pruned trees were less than the control in the two seasons after treatment.
Differences in reproductive growth following root pruning were generally greater in the second season and usually reached significant levels. First season differences were similar but of lesser degree suggesting effects increased with time. Root pruning increased floriferousness compared to control. There was an increase in flowering spurs, in the second season following root pruning, on the main stem (93.5%) and on measured shoots (58.3%). Flowers per spur were slightly greater in the first season but reduced in the second season. Fruit set per spur decreased in both seasons relative to control. Total yield and individual fruit weight decreased significantly in both seasons.
There was close agreement between the absolute amount of storage carbohydrates held in the root system in the preceding winter and yield for the following season. Total reserves in season 1 for control, 20 and 30cm root pruned trees were 54, 47 and 40g TCHO per tree and yields were 3.3, 1.8 and 1.2 kg/tree, respectively. There was also agreement between soluble carbohydrates concentrations in shoots and flower and fruit numbers. In 1995/96 the flowering spurs per shoot on control trees increased to 2.41 spurs per shoot from approximately 2 in 1994/95. The proceeding winter TCHO concentrations rose from 8.7% to 10.7%.
Apples harvested from trees in the root pruning experiment in 1995 and 1996 were evaluated for quality at harvest and following 40 and 60 days of storage at 10 °C. Fruit from root pruned trees had decreased harvest fresh fruit weight, increased °Brix and fruit flesh firmness. Fruit seed number and flesh colour was higher in fruit from root pruned trees. Fruit tissue samples were taken after 40 days of storage from Braeburn and Royal Gala and analysed for Ca, and Mg and K content during for second season only. Significant increase in Ca, and Mg but not for K level was found.
