Two experiments were conducted on the effect of high temperature on the thickening of radish hypocotyls.
In the first experiment, the effect of high temperature (30°C/25°C, day/night) on growth and endogenous substances (cytokinin, auxin and sugar) in comparison with optimum temperature (22°C/18°C, day/night) was studied. An inhibition of thickening growth, accompanied by abnormal elongation was observed under high temperatures, indicating a significantly higher T/R ratio than at the optimum temperature. Under optimum temperature conditions, cytokinin, auxin and sugar contents increased rapidly in the hypocotyl in parallel with the thickening growth.
In the second experiment, the effect of direct application of growth regulators (Kinetin and NAA) by injection method on the thickening growth of radish hypocotyl at high temperature (constant 30°C) was examined. When treated with kinetin, the hypocotyl thickened normally, and elongation did not occur. Treatment with NAA resulted in a slight increase in hypocotyl diameter but browning of the tissue and formation of callus at the treated part were observed.
The results of these two experiments suggest that a low cytokinin content is one of the main factors in inhibition of the thickening growth of radish hypocotyls under high temperature condition. However, this inhibition can be overcome by the application of cytokinin.