Effect of planting density oil root growth in 'Starking Delicious' apple trees grafted on dwarfing and semi-dwarfing rootstocks

The effect or planting density oil root growth was studied in 11 to 13-year-old 'Starking Delicious' apple trees (Malus domestica Borkh.) grafted oil M.9, M.26, M.7 and MM.106 without pruning. At the 330 trees/ ha plot the root system was composed of major lateral roots with few vertical sinkers, but at the 3178 trees/ ha plot it was mainly of lateral roots. The root spread decreased as planting density increased, but the degree of intermingling of adjacent root systems increased. At the 3178 trees/ha plot the adhesion phenomenon between adjacent root systems was observed. The root/canopy spread ratio decreased as planting density increased, indicating that root spread was more strongly inhibited than canopy spread by an increase of plant density. The distribution ratio of root weight with depth was not affected by planting density, but the root dry weight per tree ill each depth decreased as planting density increased, particularly in the major (> 15 min) root of the 0-30 cm layer. On all rootstocks, the root city weight per tree (R) decreased as planting density (rho) increased. The relationship between R and rho could be represented by the reciprocal equation: l/ R = A(R rho) + B-R-(1), where A(R) and B-R are the coefficients depending oil the tree age and rootstock. The relationship between root dry weight per tree (R) and trunk cross-sectional area (0) could be represented by the relative growth equation with h > 1: R = H theta(h)-(3), where H is the coefficient depending oil rootstock. The root dry weight per hectare in each depth increased as planting density increased, and the increase was markedly in the fine (<1 mm) and small (1 -5 mm) roots. On all rootstocks, the root dry weight per hectare (<(R)over bar>) increased as planting density (p) increased. The relationship between R and p well fitted the theoretical equation obtained from R = H theta(h) (equation 3), 1 /theta = A rho + B (equation 4) and (R) over bar = R rho (equation 5): (R) over bar = H rho/(A rho + B)(h)-(6), where A and B are the coefficients depending oil the tree age and rootstock. The planting density (rho(R) over bar (pk)) that (R) over bar becomes the maximum is given by rho(R) over bar (pk) = B/A(h - 1)-(7). These results indicated that root dry weight per hectare increases as planting density increases, while it starts to decrease at the rho(R) over bar (pk).