A Comprehensive Evaluation of High Air Temperature and Low Light Based on Tomato Development and Water Use

This study investigated the combined effects of high air temperature and low light intensity on the growth, quality, yield, and water use efficiency (WUE) of greenhouse tomato. A full factorial design was employed to simulate the dynamic air temperature and light intensity of a greenhouse in the controlled environment chambers. Three air temperature levels (control: 25/15 °C, moderately high: 30/20 °C, and high: 33/23 °C, day/night) and three light levels (low: 400, medium-low: 600, and normal: 800 μmol·m−2·s−1) were established. A comprehensive assessment approach that integrated linear weighting, TOPSIS, and GRA was employed. A multiple regression model was developed to quantify the temperature–light combined effect. Elevated air temperatures accelerated the flowering, fruit-setting, and veraison periods, and improved fruit brightness and chroma, but severely reduced yield by 13.9% for each 1 °C increase, while increasing water consumption. Yield and WUE declined by 5.0 and 3.5%, respectively, for every 50 μmol·m−2·s−1 decrease in light. Combined effects were observed: moderately high temperature and low light intensity (30/20 °C, 400 μmol·m−2·s−1) promoted lycopene accumulation; moderately high temperature and normal light (30/20 °C, 800 μmol·m−2·s−1) maximized the sugar–acid ratio and vitamin C (VC) content; and high temperature and low light (33/23 °C, 400 μmol·m−2·s−1) optimized fruit brightness and chroma. Furthermore, each simultaneous 1 °C temperature increase and 50 μmol·m−2·s−1 light decrease resulted in a 14.4% yield reduction and 15.0% WUE decline. Quantitative analysis results indicate that air temperature exerts the most influence on tomato growth; however, the combined effect of high air temperature and low light intensity is less than the individual effects of each factor. These findings provide a basis for environmental regulation in protected tomato cultivation.