Synergistic Effects of Humic and Salicylic Acids on Water Productivity, Physiology, and Antioxidant Defense in Salt-Stressed Lettuce

Salinity stress severely limits global crop production by disrupting essential physiological and biochemical processes, highlighting the need for innovative approaches to increase plant resilience. Salicylic acid (SA) and humic acid (HA), both established plant growth regulators, are known to improve plant tolerance to salinity. However, their potential synergistic interaction in enhancing the salt tolerance of lettuce remains largely unexplored. This study evaluated the single and together effects of soil-applied HA (500 mL L− 1) and foliar-applied SA (1.0 mM) on water productivity, yield, photosynthetic efficiency, nutrient balance, and antioxidant responses in lettuce (Lactuca sativa L.) grown under control (0.18 dS m⁻¹) and saline (5 dS m⁻¹) irrigation. Salinity markedly reduced yield, evapotranspiration (ET), and water use efficiency (WUE), while increasing oxidative stress and Na+ accumulation. Both HA and SA alone mitigated these effects, but their combination produced the most pronounced improvements. Co-supplementation of HA + SA enhanced yield by 50.3%, WUE by 28.1%, and chlorophyll a + b content by 56.7% under control conditions, while reducing yield loss under salinity to only 6.7% relative to non-sprayed plants. Under salinity, HA + SA increased membrane stability by 15.2%, stomatal conductance by 80.8%, and carotenoid content by 67.5%, alongside significant rises in catalase, superoxide dismutase, and peroxidase activities (162.2%, 128.5%, and 289.8%, respectively) and a 15.9% decline in malondialdehyde. Besides, the conjoint use of SA and HA improved nutrient homeostasis by elevating K+/Na+ and Ca2+/Na+ ratios by 59.2% and 46.4%, respectively, relative to the non-sprayed salt-stressed plants. Principal component analysis confirmed strong positive associations between HA + SA treatment and enhanced photosynthetic performance, ion regulation, and antioxidant defense. The synergistic action of HA and SA effectively alleviated salt-induced stress, improved water utilization and photosynthetic efficiency, and strengthened cellular stability—demonstrating a promising strategy for sustainable lettuce production under saline conditions.