Effect of rest-breaking and fruit thinning treatments on reproductive development in apple

[摘要] Lack of winter chilling is a major problem in producing temperate-zone fruit in warm climates.Delayed foliation and protracted bud burst and flowering are the main problems necessitatingartificial means to break dormancy. In South Africa (SA), most apple production areas receiveinsufficient winter chilling, and an annual application of rest breaking (RB) agents is included asstandard practice. The most used RB agent in SA was dinitro-o-cresol (DNOC) but its use wasdiscontinued. Hydrogen cyanamide (HC) became the replacement. It has been effective in apple,but variable effects on fruit set, blossom, yield and fruit quality have been reported. Thidiazuron(TDZ) has also shown the ability to break dormancy in apples. Another important practice in appleproduction is chemical thinning (CT). However, results are highly influenced by the type ofchemical, weather conditions, cultivar and blossom pattern.With the increasing efficacy of RB and by identifying its effects on vegetative and reproductivedevelopment, it will be possible to determine more effective chemical thinning treatments. Theobjective of this study was to determine appropriate RB treatments for apple trees in a warm winterclimate, identifying their effect on vegetative and reproductive development and the influence onCT efficacy. The research was performed in the Elgin area (34°S, 300 m) SA, over a period ofthree years, on 'Golden Delicious' and 'Royal Gala'.In evaluating the effect of different HC concentrations and oil, no synergistic or antagonistic effectswere observed on budburst and yield. Mineral oil at 4% plus 1 to 2% Dormex® combined weresufficient to break dormancy. Dormex® at 4% (2.08% HC) reduced fruit set and yield. In general,the rest breaking treatments (DNOC, HC and TDZ) enhanced the final vegetative bud burstcompared to the control, while reproductive bud burst in 2002 and 2003 was not significantlyinfluenced. The treatments compressed and advanced flowering periods, but this effect was notalways evident when the spring was warm. The treatments synchronised flowering on the tree andbetween the two cultivars. The mixture of 0.245% HC and 4% oil was less effective in terms ofincreasing bud burst in 'Royal Gala' compared to other rest-breaking treatments. The mixture of0.49% HC and 4% oil effectively compressed and synchronised flowering in 'Golden Delicious'.TDZ-oil used at the lower rates also increased bud burst and concentrated flowering. However, it appears that after a cooler winter, higher rates could result in an exacerbated bud burst effect withexcessive vegetative growth.The rate and timing of TDZ-oil application influenced the reproductive development of apples andtherefore fruit quality. In 'Golden Delicious' increased fruit set, number of seeds, and reduced fruitrusseting appear as beneficial results of TDZ-oil, whereas fruit set and russeting was not affected in'Granny Smith'. TDZ-oil, when applied late and at increasing rates, led to an increase in themalformation of calyx cavities, especially when chemical thinning was performed using thecytokinin-like compound benzyladenine. The effect seemed to be cultivar specific, with 'GoldenDelicious' being the most severely affected. Increased return bloom in response to late TDZapplication in 'Golden Delicious' and 'Royal Gala' appeared to be beneficial.

[发布机构] Stellenbosch University