Influence of Altered Light Levels on Growth and Fruiting of Mature ‘Delicious’ Apple Trees1

Abstract

Available light in mature apple trees ( Malus domestica Borkh.) of an unknown red strain of ‘Delicious’ on Mailing (M) 9 rootstocks was altered throughout the 1978 growing season by the use of reflectors, overhead lights, and shade material. Undertree reflectors increased light within the tree canopy about 20% at anthesis, 10-30% through mid-season, and 35% at harvest. Light was reduced within the canopy 20% at bloom and 10-20% the remainder of the season by over-tree shade. Shading reduced fruit set by 62% and reflectors had no effect. Red fruit color was reduced by shading and increased by reflector treatments, particularly in the lower half of the tree. Total yield and soluble solids were decreased by shading, but no significant differences were obtained by attempts to increase light. Reflectors and overhead light increased and shading reduced specific leaf weight. Shading increased levels of P, K, B and Zn and reduced Ca and Mg. None of the treatments altered total nonstructural carbohydrate (CHO) levels in several tissues at bloom. However, shading reduced percent CHO in both spur and shoot leaves in June and July, but increased CHO in these tissues at harvest.