The Effects of Root-Zone Temperature Regulation on the Growth and Quality of Hydroponic Lettuce in Summer

High-air temperature stress inhibits the growth of hydroponic lettuce. The practical application of conventional air cooling is constrained by high cost and moderate efficacy. However, root-zone cooling represents a more promising temperature regulation strategy for vegetable production, offering advantages such as ease of integration and lower cost. This study used lettuce (Spanish Green) as the plant material under four RZT treatments: T0 (control: 24.65~31.65 °C), T1 (24.5 °C), T2 (20.5 °C), and T3 (16.5 °C). Growth parameters and nutritional quality indicators under each treatment were systematically monitored, and a comprehensive evaluation was performed using the fuzzy membership function method. All cooling treatments (T1–T3) enhanced lettuce plant height, leaf area, and shoot dry weight. According to the fuzzy membership function analysis, the T1 treatment was found to exhibit the highest overall nutritional value. Although the T0 control group displayed the poorest growth performance, with a shoot dry weight 47.24% lower than that of T1, it accumulated significantly higher levels of P, Ca, and Zn. These findings demonstrate that regulating RZT to approximately 24.5 °C synergistically enhances both biomass and quality in lettuce, providing theoretical and practical support for optimizing hydroponic production in summer conditions.