Effects of root restriction on the physiology of apple tree growth

The effects on the growth, flowering, fruiting and water relations of apple (cv. 'Queen Cox') of planting trees within root-restricting nylon membranes were assessed. Trees on M.9 and MM.106 rootstocks were compared both with and without trickle irrigation. The aims of the experiment were to explore the practical benefits of this novel approach to growth control, to highlight possible negative effects, particularly those influencing fruit quality, and to improve understanding of the way in which root growth controls shoot development. Root restriction severely reduced shoot extension growth in the absence of irrigation. Supplementary irrigation increased the amount of shoot growth, but it did not restore growth to the levels of the unrestricted treatment. Yield efficiencies were increased by root restriction, but fruit size was reduced. Although much of this latter effect was alleviated by irrigation, sizes achieved were still just below those desired by the markets. Measurements of plant water relations suggested that water was the most likely factor limiting shoot extension growth under the most severe treatment (i.e., root restriction in the absence of irrigation). The cause of the decline in fruit size on irrigated trees with restricted roots did not, however, appear to be the lack of water. The results and implications of these experiments are discussed.