Effect of root-zone heating on root growth and activity, nutrient uptake, and fruit yield of tomato at low air temperatures

Low-cost heating is needed to reduce chilling injuries, heating costs, and CO₂ emission during greenhouse tomato production. To acquire information about the physiological and morphological effects of root-zone heating, an economical option at low air temperatures, we grew tomato plants on a nutrient film technique hydroponic system in a heated nutrient solution. We investigated the effects of short-term root-zone heating after transplanting and long-term heating until harvest. We measured short-term plant growth, nutrient uptake, root activity (xylem exudation and root respiration rates), root indole-3-acetic acid (IAA) concentration, internal root structure, and long-term fruit weight and dry matter distribution. The minimum root-zone temperature was maintained at 16.6°C, while the minimum air temperature (5.9°C) and the minimum root-zone temperature in the control (5.8°C) were lower than optimal. After 7 days of root-zone heating, root dry weight and relative growth rate increased compared with those of the control, accompanied by increased mineral nutrient uptake and xylem exudation. These changes may explain the increased shoot growth after 21 days of heating. In roots, development of the epidermis and stele, including the xylem, was promoted by heating, in contrast to previous research on root-zone cooling at high air temperature, which promoted xylem-specific development. Although the proportion of dry matter distributed to the fruit was not changed by root-zone heating, individual fruit size and total yield were higher than in the control due to a higher total dry weight in the heating treatment. Our results suggest that root-zone heating is an effective lowcost heating technology at low air temperature because of its effects on root activity, growth, and fruit yield, but that the mechanisms may differ from those in root-zone cooling at high air temperature.