Effects of micro-spray in greenhouses on the leaf interception water types and the evaporative cooling

This study aims to examine how intermittent micro-spray cooling regulates the greenhouse thermal environment via leaf-surface water interception at high temperature in summer, with emphasis on the environmental thresholds, evaporation behavior, cooling performance, and physiological effects of the interception morphologies on tomato leaves. A greenhouse experiment was conducted from January to June 2025. A Venlo-type glass greenhouse was taken in Zhenjiang, Jiangsu Province, China. The indeterminate tomato cultivar, Hezuo 908, was selected in north-south soil troughs under drip irrigation. Micro-spray cooling was activated at indoor air temperatures over 30 ℃ and then operated for 1 min at hourly intervals from 10:00 to 15:00 during flowering, fruit setting, and fruit maturation. Canopy air temperature and relative humidity were recorded every 60 s, while the leaf interception morphology was monitored by time-lapse imaging, and leaf temperature with the actual photochemical quantum yield of photosystem Ⅱ was measured using a chlorophyll fluorescence monitoring. The results showed that there were two leaf-surface interception morphologies after spray deposition. The first was leaf interception in a water droplet (LI-WD), which was characterized by spherical droplets distributed over the leaf surface. Another was leaf interception in water film (LI-WF), which was characterized by a continuous thin water layer covering most of the lamina. Furthermore, their initial coverage varied, averaging 31.56%±1.59% for LI-WD and 99.92%±0.02% for LI-WF. The occurrence of the two forms was strongly controlled by pre-spray canopy conditions. Among them, the LI-WD occurred when canopy air temperature ranged from 30-42 ℃, and vapor pressure deficit ranged from 3.0-5.0 kPa, whereas the LI-WF occurred when canopy air temperature ranged from 42-46 ℃ and vapor pressure deficit ranged from 5.0-7.5 kPa, indicating a transition threshold 42 ℃ and 5.0 kPa. The evaporation also differed markedly between them. The LI-WD showed a linear decline trend in coverage, indicating stable evaporation, whereas the LI-WF followed an inverse S-shaped decline trend, indicating a nonlinear and short-lived evaporation. These differences caused the different cooling responses. The canopy air temperature decreased by (1.72±0.76) ℃ on average and the cooling effect lasted about 25 min under LI-WD, whereas under LI-WF, the average canopy cooling was (0.74±0.21) ℃ and lasted about 12 min. Leaf temperature under both forms showed three consecutive stages, namely decline, stable low-temperature, and recovery. The decline stage lasted 3 min and the stable period extended from 3-25 min under LI-WD, whereas the decline stage lasted 2 min and the stable period lasted from 2-13 min under LI-WF. Leaf temperature also responded earlier than canopy air temperature. Photosystem Ⅱ activity varied synchronously with leaf temperature. The mean increase in actual photochemical quantum yield of photosystem Ⅱ was 0.120±0.056 under LI-WD and 0.080±0.048 under LI-WF, with the effective durations of about 30 and 18 min, respectively. Overall, intermittent micro-spray cooling generated two interception pathways with different environmental thresholds, evaporation dynamics, and cooling efficiencies. Compared with LI-WF, the LI-WD provided greater canopy cooling performance, longer leaf temperature reduction, and stronger recovery of photosystem Ⅱ function. Therefore, leaf interception morphology is a key intermediate process. Leaf temperature can serve as a rapid and reliable indicator of physiological effectiveness in greenhouse micro-spray cooling.