Lignin and chitin-based biostimulants from agro-industrial residues improve growth in lettuce plants under salt stress

Facing global environmental challenges, such as high soil salinity and increasing demand in crop production in terms of both quantity and quality, waste valorization for plant biostimulant development represents a viable strategy. In this study, chitin and lignin were extracted from shrimp and nut shells, respectively, and chemically modified through deacetylation and phosphorylation to improve their solubility and biodisponibility. All biopolymers were characterized and evaluated as biostimulants applied via foliar weekly in lettuce grown under salinity stress (100 mM and EC 4 dS/m) at concentrations of 0, 10, 50, and 100 ppm, resulting in fifteen treatments with ten replications each. Plant sampling was conducted eleven weeks after sowing, and growth parameters, chlorophylls, and proline were assessed. Chitin application at 10 ppm increased root dry weight by 150.9% and leaf fresh weight by 19.25%, while reducing proline content by 37%. In contrast, the 50-ppm treatment decreased leaf fresh weight by 22.5%. At 100 ppm, chitin increased leaf fresh weight by 35.5% and proline content by 35%. Chitosan at 10 ppm enhanced root fresh weight by 145% and reduced proline by 52%, whereas 100 ppm increased leaf fresh weight by 16.9%. Lignin at 10 ppm increased proline content by 70%, while 50 ppm improved root fresh weight by 66.2% and leaf dry weight by 57.6%. Phospholignin at 10 ppm increased leaf area by 76.6%. Overall, most of the biopolymers induced effects on growth and physiological responses under salinity stress, supporting their potential as value-added agricultural inputs derived from agro-industrial residues.