INTERACTIONS OF ROOT PRUNING AND DEBLOSSOMING ON GROWTH, DEVELOPMENT AND YIELD OF GOLDEN DELICIOUS APPLE

Mature 'Golden Delicious'/M.9 apple trees were: 1) mechanically root pruned on two sides 50 cm from the trunk to a depth of 40 cm; 2) deblossomed by hand; 3) both root pruned and deblossomed; or 4) untreated. The treatments were applied annually for four years at full bloom. Root pruning reduced trunk cross-sectional area, shoot length, shoot number to spur number ratio, shoot leaf size, and shoot leaf area to spur leaf area ratio in cropping apple trees. The effects were much less on the vegetative growth of deblossomed trees. Root regeneration was abundant in root pruned plus deblossomed trees, but low in cropping trees. Compared with unpruned control trees, root pruned trees had more negative leaf water potential, reduced stomatal conductance, and reduced transpiration, while deblossomed trees had less negative leaf water potential, lower net photosynthesis, stomatal conductance and transpiration. Leaves of root pruned plus deblossomed trees had intermediate rates of net photosynthesis. Mid-terminal shoot leaves from deblossomed trees had a higher specific leaf weight, lower chlorophyll content and higher starch concentration than those from untreated trees, whereas leaves from root pruned trees had similar specific leaf weight, leaf chlorophyll content and starch concentration to controls. Those from root pruned plus deblossomed trees had an intermediate starch concentration. Transmission electron microscopy revealed major accumulation of starch within the chloroplasts of leaves on deblossomed trees but only minor accumulation in those that had been both root pruned and deblossomed. Root pruning had no effect on leaf mineral nutrient content, while deblossoming led to reduced foliar concentrations of calcium and increased foliar concentrations of potassium when values were considered on a unit leaf area basis to compensate for changes in unit leaf weight because of high starch accumulation in the leaves of deblossomed trees. Root pruning advanced the time of floral development, while deblossoming delayed it. Root pruning had no effect on return bloom, fruit set or yield, but it did reduced fruit size. Deblossoming increased return bloom and stimulated a second flush of bloom in two successive years, from flowers that were formed in the previous season. The yield and growth responses to root pruning obtained in this study, where trees were grown on a dwarfing rootstock, were very similar to those obtained previously with more vigorous rootstocks.