Exploring optimal soil moisture for seedling tomatoes using thermal infrared imaging and chlorophyll fluorescence techniques

• Effects of various soil moisture contents was investigated on tomato seedlings. • Dynamically characterize and visualize the changes in plant canopy temperature when leaves suffer from water stress. • Characterized the morphological and physiological changes of tomato seedlings. • Provided a theoretical basis for mitigating heat-induced damage. Understanding the distribution of plant moisture during the seedling stage of greenhouse crops is challenge in developing scientific irrigation strategies and proposing effective cooling methods. This study investigated the effects of different soil moisture contents [W1: 25–35 % (severe drought), W2: 35–45 % (mild drought), W3: 45–55 % (suitable moisture), and W4: 55–65 % (excess moisture)] on tomato seedlings under summer greenhouse thermic extremes. Furthermore, thermal infrared imaging and chlorophyll fluorescence multi-dimensional digital image sensors were used to determine differences in tomato seedling morphology and plant physiology. The increase in canopy area under W1 and W4 soil moisture content was smaller than that of W2 and W3, and the canopy area of the W1 group decreased as the high temperature condition continued. The average canopy temperature of each treatment generally increased first, and then plateaued. From high to low, average temperatures were 28.15℃ in W1, 27.73℃ in W4, 26.67℃ in W2, and 25.72℃ in W3. The canopy temperature gradually decreased from the middle to the edge of the leaf (stem temperature > leaf base temperature > leaf vein temperature > leaf edge temperature). The F v /F m ratio in the chlorophyll fluorescence index qualitatively expresses the degree of water stress. φPSII, non-photochemical quenching (NPQ), and qP values were used as indicators to quantitatively analyze stress in leaves of different maturity. A preliminary mathematical relationship between the canopy and NPQ was established. This study quantitatively characterized the morphological and physiological changes of tomato seedlings in the greenhouse during summer, visualized the process of canopy temperature changes, and provided a theoretical basis for mitigating heat-induced damage.