Foliar Application of Bamboo-Derived Nano-Biochar Enhances Morphological and Biochemical Responses of Lettuce (Lactuca sativa L.) Under Salt Stress

Nano-biochar (n-BC) is an emerging eco-friendly material with potential to improve crop performance under salt stress. This study aimed to evaluate the effects of foliar applications of bamboo-derived n-BC on the morphological and biochemical responses of lettuce plants under salt stress (40 mM NaCl). n-BC solutions (1.0, 3.0, and 5.0% <i>w</i>/<i>v</i>) were foliar-applied every five days until harvest. Salt stress markedly increased hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) by 264.54% and 14.02%, disrupted Na⁺/K⁺ homeostasis, and reduced biomass. Foliar n-BC mitigated these effects by reducing Na⁺ accumulation by 22.24-25.11% and enhancing K⁺, Ca²⁺, and Mg²⁺ uptake. The treatments also improved photosynthetic pigments and increased proline, soluble proteins, and soluble sugars. Oxidative damage was alleviated, as reflected by reductions in H₂O₂ and MDA together with enhanced ascorbate peroxidase, catalase, and superoxide dismutase activities. Total phenolics, flavonoids, and ABTS and DPPH scavenging activities also increased under n-BC application. Among all the concentrations, 3.0% (<i>w</i>/<i>v</i>) n-BC consistently produced the greatest improvements in growth, ionic balance, and antioxidant responses. These findings demonstrate that bamboo-derived n-BC is a promising foliar biostimulant for enhancing lettuce performance under saline conditions.