Root growth, yield and fruit quality of 'Red Fuji' apple trees in relation to planting depth of dwarfing interstock on the Loess Plateau

This study investigated how the planting depth of dwarfing interstock influences vertical root distribution, root hormone contents, yield, and fruit quality of 'Red Fuji' apple (Malus domestica Borkh.) trees in a semi-arid region on China's Loess Plateau. Three-year-old plants of the apple cultivar 'Changfu 2' were grafted on M26 dwarfing interstock at different planting depths, i.e., 5, 10, 15, 20, and 30 cm. When dwarfing interstock was planted at 15 cm depth, fine roots (<2 mm diameter) were mainly distributed in the soil layers above 40 cm, 13.5-41.6% more than those in the other treatments. Total number and dry weight of roots were respectively 18.3-33.2% and 11.2-68.4% greater in the 15-cm treatment, corresponding to the higher fruit yield, greater single fruit weight, and improved fruit quality in terms of soluble solids content, firmness and color. Among different treatments, planting dwarfing interstock at 15 cm depth resulted in the highest contents of growth-promoting hormones (indole-3-acetic acid, IAA; zeatin riboside, ZR; and gibberellic acid, GA(3)) but less growth-inhibiting abscisic acid (ABA) and lower (IAA + GA(3) + ZR) / ABA ratio in roots, all of which facilitated the stimulation of root growth. The results indicated that planting dwarfing interstock at 15 cm depth effectively promoted root growth and increased fine roots, thereby providing more nutrients for plant growth and development, ultimately improving fruit yield and quality of apple trees.