INFLUENCE OF ROOT TEMPERATURE ON APPLE-TREES .1. GROWTH RESPONSES RELATED TO APPLICATION OF POTASSIUM FERTILIZER

One year old apple plants were grown with controlled root temperature, either in sand culture or in a sandy loam or clay soil. Experiments lasted 10 wk in sand culture and 11 mo. in the 2 soil types, starting in late summer but without applying the controlled temperature treatments in winter and spring. A temperature of 29.degree. C reduced root and shoot growth, when applied for a relatively long period, compared with 22.degree. C or 25.degree. C. At 35.degree. C shoot growth was affected even when the experiment lasted only 10 wk. The root: shoot ratio also decreased with rising temperature. The reduced shoot growth was due to a decrease in the number of nodes. Total leaf weight of the plants was reduced by supraoptimal root temperature in short-term experiments, but in long-term ones this was largely compensated for by the formation of many spur-type branchlets bearing a large number of small leaves. The proportion of dry matter in the roots decreased at supraoptimal root temperatures. The deleterious effect of supraoptimal root temperature in summer was not noted in the following spring, when root temperatures were lower and no heating was applied. The application of K fertilizer to the soil or spraying the plants with potassium nitrate, reduced the deleterious effect of a root temperature of 29.degree. C, at 36.degree. C such treatments only resulted in increased growth in spring, when the root temperature was considerably lower. The spray treatment was particularly effective, but in clay soil at 29.degree. C the application of K fertilizer to the soil was equally effective. Without the application of K the deleterious effect of supraoptimal root temperature was more pronounced in the sandy loam than in the clay soil.