Growth Rates in relation to Assimilate Supply and Demand

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Abstract
It is generally accepted that the translocation of carbohydrates is determined by the concentration gradient from leaves having an exportable surplus to utilizing regions along a conducting system whose efficiency declines as the season advances. The experiments showed that this mechanism alone could account for a good deal of the characteristic seasonal pattern of distribution of the vegetative increment, but that four additional mechanisms were necessary to account for the major features of the results. These were (1) limitation of the quantity of growing leaf on a shoot, (2) cessation of leaf production in early autumn, (3) correlation between extension and radial growth, (4) stimulation of root growth in the autumn. The experiments, on potted cuttings of an apple rootstock, involved removing all but ten leaves on particular parts of the shoot, or reducing the extent of the cambial surface (by bark stripping), or reducing the size of the root system, and then determining the increase in weight of leaves, stem, and roots over periods of several weeks. Following defoliation and continual removal of new young leaves, the reduced increment was utilized more in new stem and less in the roots. The summed dry-matter production of leaves in the upper, middle, and lower parts of the stem was greater than the production by intact plants during July-August, but no different during June and the autumn. Removal of a fifth of the bark surface of the old stem was followed in July by sufficient regeneration of the cambium at the vertical edges of the wound to give the same growth in all regions of the plant as the controls, and in September by regeneration from the wound surface to give 84 per cent of the control growth, the deficiency occurring in the root and old stem. Removing half of the root system, using a split-root technique, in July reduced growth by 30 per cent but in September had no effect. The relative growth rate of the remaining root was increased. Removing nothing but fine root by a special technique in July reduced growth by 14 per cent, all parts being equally affected; the September treatment was invalidated by thickened roots being removed in addition to the fine when the treatment had the same effect as root pruning.