Abstract
Under 33 °C high air temperature conditions in deep hydroponic culture, the effect of nutrient solution temperature on root activity was studied. Spinach plants (Spinacia oleracea L. cultivar 'Okame') were raised and their growth, soluble sugar content, and ion and water uptake were measured and analyzed. In the first experiment, the solution temperature of all treatments was initially set at 20 °C, then raised to 20, 26, 30, and 34 °C each for 3 days. When solution temperature was above 26 °C, root growth was inhibited and sugar content increased. Thus for high nutrient solution temperature, there is a disturbance in the carbohydrate metabolism of the roots, which is a major factor in growth inhibition. In the second experiment, plants were grown in four different solution temperature treatments (20/20, 26/20, 30/20, and 34/20 °C in light/dark periods) each for 12 days. The plants in the 30/20 °C treatment had good ion uptake and root growth. The soluble sugar content also was the lowest of all of the treatments. This indicated that even if the solution temperature was raised to 30 °C, the carbohydrate metabolism recovered due to the cooling of the solution during the dark period. The carbohydrate metabolism of the roots exposed to the 34 °C treatment could not recover.
