Flower thinning method affects mineral composition of 'Braeburn' and 'Fiesta' apple fruit

Fruit mineral concentrations measured at harvest can have major effects on apple fruit quality on the tree or during storage. Orchard practices must t herefore seek to optimize fruit mineral composition. The purpose of this st udy was to describe and elucidate the effects of hand thinning on whole tre es and individual spurs on apple fruit mineral composition. Two methods of flower and fruitlet thinning were compared with no thinning on 'Braeburn' a nd 'Fiesta apple trees. Alternate whole flower/fruitlet clusters or all but one flower/fruitlet within every cluster were removed at full bloom or 14- 21 d after full bloom. Alternate-cluster thinning reduced final fruit numbe rs per tree and fruit Ca concentrations by up to 22%, while increasing fina l fruit size by up to 21%, compared with no thinning. These effects on frui t Ca concentrations were also measured across a range of fruit size classes . Within-cluster thinning at full bloom or up to 21 d after full bloom also reduced fruit numbers per tree but increased fruit size substantially, by up to 65% compared with no thinning. this effect being less for later thinn ing. However, fruit mineral concentrations were not influenced by this trea tment. Some fruiting spurs were singled to one fruit 14 d after full bloom on alternately flower cluster thinned trees and on trees that had not been thinned at bloom, and compared with unthinned spurs on the same trees. Frui t Ca concentrations, primary spur leaf areas and primary spur leaf areas pe r fruit were greater for spurs bearing a single fruit (achieved by thinning manually or through natural abscission) than for multi-fruited spurs on th e same trees. Spurs bearing one fruit on unthinned trees had greater fruit Ca concentrations, primary spur leaf areas and primary spur leaf areas per fruit, but lower fruit weight than the same spurs on alternate-cluster thin ned trees. However. spurs on unthinned and alternate-cluster thinned trees with the same primary leaf areas per fruit had similar final Ca concentrati ons. Fruit size and crop loads were found not to be important in explaining fruit Ca concentration differences between thinning methods. However our r esults suggest that thinning method may affect Ca accumulation in apple fru it by altering the relationship between fruit numbers and leaf areas on ind ividual spurs.