In trees, nutrient reserves built up in the previous year are of primary importance for early spring growth. Despite the relatively great importance of roots for nutrient storage, the root system should not be regarded as a special storage organ. Quantitatively, carbohydrates predominate in these reserves, but qualitatively N and other minerals are of more than minor significance. In roots carbohydrates are usually stored in insoluble form, mainly as starch; sorbitol is the predominant soluble compound in apple and peach. For nitrogen reserves, the soluble form predominates in roots, especially arginine in apple and peach, followed by asparagine. The level of reserves usually becomes maximal early in the winter. During leafing-out the reserves are drawn on until, later in the season, the supply of newly produced or absorbed nutrients exceeds the demand and replenishment occurs. The initial carbohydrate reserves do not determine the amount of new growth, whereas reserve nitrogen is of decisive importance for shoot growth vigour. Environmental factors such as light intensity and temperature affect the level of carbohydrates in roots; the concentration can be reduced by defoliation and summer pruning and increased by ample supply of nitrogen fertilizer in the autumn. The main cultural factors that influence nitrogen reserves are the amount and the time of nitrogen fertilization.
