Comparative Physiological and Biochemical Responses of Lettuce (Lactuca sativa L. var. romana) to Foliar Methyl Jasmonate, Nano-Kaolin, and Melatonin Under a Progressive High-Temperature Regime

High temperature is an important environmental constraint affecting lettuce growth and physiological performance. This study comparatively evaluated the responses of lettuce (Lactuca sativa L. var. romana) to individually applied foliar methyl jasmonate (MeJA; 0.5 and 1.0 mM), nano-kaolin (0.5 and 1.0%), and melatonin (0.1 mM) under a common progressive high-temperature regime. Plants were grown in two contrasting growing media, soil and peat–perlite, and were exposed to temperatures progressively increased from 20 to 40 °C. Growth characteristics, visual damage, leaf temperature, membrane leakage, relative water content (RWC), leaf color, total soluble solids, pH, DPPH and ABTS radical-scavenging capacity, and total phenolic content were evaluated. Treatment responses differed substantially between growing media. In peat–perlite, nano-kaolin treatments produced the highest fresh biomass and relatively low visual damage, whereas responses in soil were more variable among treatments. RWC was significantly affected by treatment in both growing media, although the response pattern differed between substrates. Membrane leakage did not differ significantly among treatments in either growing medium. Nano-kaolin treatments produced comparatively high DPPH and ABTS radical-scavenging capacity and total phenolic content in both growing media. Overall, the findings demonstrate that the physiological and biochemical responses of lettuce to the tested foliar treatments were strongly influenced by the growing medium under the imposed high-temperature regime.