Root-zone cooling at high air temperatures enhances physiological activities and internal structures of roots in young tomato plants

Low-cost technology is needed to alleviate high-temperature injury for high-yield greenhouse tomato production. To acquire information about the physiological and morphological effects of root-zone cooling, we grew young tomato plants for 2 weeks in nutrient solution held at about 25°C, considered to be the optimum temperature for tomato plants. We investigated plant growth, nutrient uptake, root activity (xylem exudation and root respiration rate), root indole-3-acetic acid (IAA) concentration, and internal root structure. The root-zone temperature was maintained at 24.7°C by cooling, while the air temperature and control temperatures were higher than optimum (30.8 and 33.7°C, respectively). Root-zone cooling increased the relative growth rate (RGR) of roots compared with the control, followed by shoot RGR. Root IAA was positively correlated with root RGR. Root-zone cooling increased Ca and Mg uptake as well as root xylem exudation and respiration. It also advanced the development of the internal structure of the xylem near the root tip. Thus, possibly by increasing root activity and root IAA, root-zone cooling promoted root growth and nutrient uptake mediated by the development of the root xylem, and thus shoot growth. These results suggest a physiological and morphological mechanism of growth enhancement by root-zone cooling under high air temperature conditions.