Daily-Scale Chlorophyll Fluorescence Reveals the Mitigation Effects of Micro-Sprinkling on Greenhouse High-Temperature Stress in Tomato

Micro-sprinkler irrigation is commonly used to optimize plant growing environments and prevent growth inhibition and yield losses caused by high air temperatures (Ta) in greenhouses. Nevertheless, instantaneous photosynthetic rate measurements suffer from time lag, and the temporally dynamic microclimate alterations induced by micro-sprinkling make it difficult to reproduce the real ambient conditions for crop growth. Therefore, two treatments, namely micro-sprinkling combined with drip irrigation (MSDI) and conventional drip irrigation control (DI), were established in a Venlo-type greenhouse. Continuous chlorophyll fluorescence (ChlF) monitoring combined with rapid light curves under fixed photosynthetically active radiation was adopted to investigate the diurnal alleviation effects of micro-sprinkling on tomatoes under high-temperature stress. This study found that ΦPSII was more sensitive than Fv/Fm in detecting changes in PSII photochemical performance under high-temperature stress. Micro-sprinkling showed greater mitigation effects under moderate heat stress, with the highest enhancement in ΦPSII (approximately 0.12) observed when leaf temperature (Tl) was around 34.5 °C. However, the improvement effect decreased under extreme heat conditions, and ΦPSII increased by only 0.017 when Ta exceeded 38 °C. The slope of the fitted line between ΦPSII and PAR on sunny days increased with increasing heat stress, indicating the enhanced sensitivity of PSII photochemical regulation to thermal stress. Compared with DI, MSDI increased tomato yield by 31.2% and 47.6% in 2021 and 2022, respectively, while improving fruit quality by increasing single fruit weight, fruit shape index, and soluble sugar content. In addition, MSDI increased SPAD, Fv/Fm, ΦPSII, and ETR by 9.6–15.6%, 8.8–14.8%, 10.3–13.3%, and 10.3–19.6%, respectively, indicating improved PSII photochemical performance under high-temperature conditions. In conclusion, micro-sprinkling mitigated part of the negative effects of high-temperature stress and significantly improved tomato yields and fruit quality, which could be used in agricultural production.