Mitigation of adverse effect of cadmium toxicity in lettuce (Lactuca sativa L.) through foliar application of chitosan and spermidine

Cadmium (Cd) stress is considered among the most harmful abiotic stresses because of its toxicity and ability to alter the ultrastructure of plants. Lettuce (Lactuca sativa L.) can readily accumulate Cd from the soil, but its elevated level posed negative effect on their development and nutritional quality. In this study, efficacy of chitosan and spermidine synergistic application was evaluated to improve Cd metal tolerance or its exclusion in lettuce. A pot experiment was conducted in a four-way completely randomized design (CRD) with 3 replicates, using two L. sativa varieties (VRIL-0205 and Green Check). Following treatments, Cd stress (10 ppm CdCl₂), chitosan (200 ppm) and spermidine (145 ppm) were applied along with their respective controls. The negative effects of Cd stress on the morphological, physiological, and biochemical attributes of both L. sativa varieties were evaluated along with counter effect of chitosan and spermidine alone and synergistic application. Cd stress resulted in significant accumulation of Cd²⁺ ions in the shoot of both varieties (0.038 mg kg^{- 1} in VRIL-0205 and 0.041 mg kg^{- 1} in Green Check). It also impaired growth, biomass, gas exchange, water relation, antioxidant activities and nutrient uptake in both varieties. Foliar application of both chitosan and spermidine improved growth, biomass, chlorophyll content, photosynthesis rate, stomatal conductance, water content, antioxidant activities and nutrient uptake in both control and stressed plants. Their combined treatment reduced stress indicators including relative membrane permeability (VRIL-0205; 19% and Green Check; 22%), H₂O₂ (VRIL-0205; 27% and Green Check; 26%) and malondialdehyde content (VRIL-0205; 6% and Green Check; 7%) in stressed plants, compared with stress only plants. These findings showed that chitosan and spermidine synergistic application effectively mitigated the Cd toxicity in both L. sativa varieties and improved their growth under stress condition. This study provides insight into the potential use of chitosan and spermidine foliar spray as sustainable tools for improving Cd resilience in crop plants.