Silicon Enhances Water Use Efficiency by Improving Thermal Responses in Solanum lycopersicum L.

Increasing pressure on water resources highlights the need for irrigation strategies that improve water use efficiency (WUE) while maintaining tomato (Solanum lycopersicum L.) performance. This study evaluated the effects of silicon (Si) application in the form of silicon dioxide (SiO2) on thermal responses assessed using infrared thermography, gas exchange, yield, and WUE under four irrigation levels (40, 60, 80, and 100% crop evapotranspiration, ETc) over two growing seasons. Water deficit progressively impaired physiological performance, with photosynthesis decreasing by up to 20.5% and stomatal conductance by 33.3% under 40% ETc compared with full irrigation. Leaf temperature increased by up to 3.75 °C, while the Crop Water Stress Index (CWSI) reached 0.87. Across all irrigation regimes, the main effect of Si application significantly improved overall plant physiological performance, increasing mean photosynthetic rate by 5.4%, transpiration by up to 16.9%, and leaf relative water content by 3.1%, while decreasing canopy temperature by 0.69–0.84 °C and crop water stress index (CWSI) from 0.60 to 0.62 to 0.44–0.49 across seasons. Si improved WUE, with the highest value (79.07 kg m−3) recorded under 40% ETc with Si in the second season, representing a 22.8% increase over the corresponding treatment without Si. Effect-size analysis indicated the largest Si response under moderate water deficit (60% ETc; Hedges’ g = 1.958). Agronomically, 80% ETc optimized the yield–WUE trade-off, cutting water use by 20% and boosting WUE by 14.7% with only an 8.9% yield loss compared to 100% ETc. Furthermore, canopy temperature and CWSI closely reflected plant physiological status across water regimes. These findings demonstrate that thermal imaging can characterize irrigation-related stress responses and that Si application can improve WUE under deficit irrigation, although its benefits depend on water availability and growing conditions.