The Influence of Root Temperature on Apple Trees. IV. The Effect on the Mineral Nutrition of the Tree

SummarySeveral ungrafted one-year-old apple rootstock clones and grafted scion cultivars were grown at constant root temperatures, between 20°C and 35°C, either in sand cultures for 8-11 weeks or a clay soil for 11 to 14 months. Root temperatures exceeding 25°C with several rootstock clones and 30°C with others decreased the K and Zn content of all parts of the plants. This decrease was accompanied by increases in the Na, Ca and Mg contents of the roots, and at 35°C in the Na and Mg of the leaves also. The content of K+Ca+Mg+Na reached a maximum in the roots at 36°C and in the tops at 29°C. A root temperature of 35°C lowered the total N content of the leaves. Supplying K salts by fertilization or foliar spray to plants at a root temperature of 29°C increased the K content of the roots and leaves, but no such effect was found at 36°C. The spray treatment was particularly effective in raising the root K content at 29°C. All treatments with K caused a shift in the distribution of the Ca in the plant, reducing the relative Ca content of the roots and increasing that of the tops.