Effect of girdling and bridge grafting on apple tree growth, yield, apple quality and biochemical composition for apple trees growing under moderate or ultra high density conditions

Abstract

Two experimental trials were conducted on apple trees between 1994 and 1996 in order to assess the effect of different girdling and bridge grafting treatments on vegetative and reproductive growth and physiology of apple trees and the apples they produced. The treatments were chosen to assess the possible utility of bridge grafting and girdling as methods to control apple tree vigour while maintaining production and quality of the apples. Physiological measurements were also taken to see whether organ carbohydrate levels could give at least a partial explanation of the effects induced.

In the first trial, four apple cultivars ('Granny Smith', 'Cox's Orange Pippin', 'Oregon Red Delicious' and 'Gala') on three rootstocks, (MM106, M793 and Northern Spy) were used. The trees were interstock bridge grafted with segments of either M9 dwarfing rootstocks or with cuttings from the same variety or were left alone as controls. The trees were in a 12 year old apple orchard, planted at moderate density. In the second trial three apple cultivars ('Braeburn', 'Royal Gala' and 'Fuji') growing under an ultra high density planting system on rootstock MM106 (ranging from 5000 to 40,000 tree/ha) were used. The trees were 6 years old at the start of the trial. The trees were treated with three different girdling techniques (Ring, Guillotine or none) in order to investigate the effects of girdling relative to uncut controls on vegetative growth, reproductive growth, organ carbohydrate levels, fruit quality parameters and fruit storage potentials.

In general the bridge grafting treatments reduced vegetative growth, but increased flower number, fruit set and total yield. They also changed the carbohydrate levels above and below the cut. The treatment reduced average shoot length by 20% while increasing flower density (143%) flowers per spur (17%), fruit set (70%), and total yield (31.7%) the year after the treatment. Similar results were found in the second trial where trees were girdled without bridges grafted in. The greater effect was found in the second year, with the more severe ring girdling treatment having a greater effect. In this second trial the different varieties and populations produced substantial variation in all parameters but few significant interactions with the girdling treatments. This leads to the conclusion that girdling has a consistent main effect as described. In general the girdling treatments reduced trunk diameter, branches per tree and shoot length by 7.2%, 10.2% and 10.9% respectively. At the same time girdling increased flower number (11.1 %), fruit set (25%) and total yield (62%) in the second growing season.

Fruit were picked as specified by the New Zealand Apple and Pear Marketing Board. They were then evaluated at harvest and during storage at 10°C after periods of 40 and 60 days.

In both trials the treatments reduced mean fruit weight and fruit LID ratio, while there were significant increases in flesh firmness, brix value, and seed number. There were more severe effects from M9 bridge grafting and ring girdling treatments in both growing seasons. In both trials storage for 40 and 60 days at 10°C significantly enhanced weight loss, flesh colour and brix value while there was a highly significant reduction in flesh firmness with increasing storage time, for both growing season.

Fruit tissue samples were taken from both trials after 40 days and analysed 'for calcium, potassium and magnesium content for the second season only. There was a highly significant increase in calcium level associated with the girdling and grafting treatments while potassium and magnesium levels were lower in both trials. As with other parameters, in the first trial the bridge grafting M9 treatment had the greater effect while in the second trial the ring girdling treatment showed greater differences compared to the control. These data and evidence from reports of other workers indicate that girdling is effective in controlling the vegetative growth of apple trees. The observed increase in reproductive growth was greater in the second year after treatment. There was a reduction in fruit size with girdling but this might have been due to crop load rather than reduced vigour. The reduction in shoot growth may be determined by hormonal signals rather than CRO (Carbohydrate) concentration a possibility not examined here. However, the hormonal signals may themselves be affected by CRO transport within the tree. Reduced transport to the roots of CRO may in turn reduce hormone signals to promote vegetative growth and consequently result in the correlations demonstrated between CRO levels and treatments reported on here. The data generally supported the model of organ carbohydrate level effects on growth, yield and quality of apple trees proposed by Khan et ai., (1998), with the added benefit of indicating both bridge grafting and girdling can reduce tree vigour while maintaining yields and apple quality.