Some Aspects of the Physiology of Apple Rootstock Varieties under Reduced Illumination

Six different varieties of apple rootstock were grown under conditions of stress imposed by reduction in the light intensity. The varieties tended to maintain their relative contents of resources at a steady level in all treatments but stress reduced the proportion of dry-matter increment deposited in the root system. Some varieties grew better than others in shade but differences in the partition of resources or total dry-matter increment did not seem to correspond with the predicted effect of the rootstock varieties on the cropping performance of trees grafted on them. It is postulated that differences in the relative proportion of leafy shoot and root growth, such as were observed, may be brought about by the way in which root systems metabolize the carbohydrates and other resources which are surplus to the amount needed to maintain a basic or standard level in existing cells.