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

In the Nutrient Film Technique (NFT) system, air-temperature stress presents a severe challenge to the cultivation of hydroponic lettuce, often limiting its yield and quality during warm seasons. Although previous studies have confirmed that regulating air temperature can alleviate the stress on crops, challenges such as uneven temperature distribution and high equipment costs hinder effective air temperature management in greenhouse environments. In contrast, independently regulating root-zone temperature (RZT) under the same air-temperature stress conditions offers a more practical approach to enhancing crop growth, which can significantly improve crop outcomes without incurring substantial additional costs. This study used 'Spanish Green' lettuce as the test material and established four RZT treatments: T0 (control: 24.65–31.65℃), T1 (24.5℃), T2 (20.5℃), and T3 (16.5℃). Over a 38-day cultivation period, we systematically monitored the effects of different RZT treatments on lettuce growth parameters (such as plant height, leaf area, and shoot dry weight) and nutritional quality indicators (including vitamin C, nitrate, and mineral element content). The fuzzy membership function method was employed for a comprehensive evaluation of lettuce quality.The results showed that all cooling treatments (T1, T2, T3) promoted increases in plant height, leaf area, and shoot dry weight. Based on calculations from the fuzzy membership function, lettuce under treatment T1 achieved the best balance between growth performance and nutritional quality. Lettuce in treatment T0 exhibited the poorest growth; compared to T0, shoot dry weight increased by 47.24%, 16.24%, and 12.21% for treatments T1, T2, and T3, respectively. However, mineral element contents such as P, Ca, and Zn were significantly higher in treatment T0 than in the other three treatments. The overall quality of lettuce in treatment T2 was superior to that in T3; although T2 promoted growth relative to T0, its growth performance was significantly lower than that of T1, and both T2 and T3 exhibited a decline in overall growth quality. Nutrient solution consumption was highest for treatment T1 and lowest for treatment T0.This study demonstrates that independently regulating RZT to approximately 24.5℃ can achieve a synergistic enhancement of biomass and quality in lettuce, providing a theoretical basis and practical guidelines for optimizing NFT system production during summer conditions.