Alteration in Photosynthetic and Yield Parameters, Content of Metabolites, and Antioxidant Activity of Pepper (Capsicum annuum): Effect of Bio-Organic Substrate and Depolymerized Chitosan

Peppers are of substantial economic importance and hold a prominent position among vegetables rich in health-promoting compounds, which drives continuous efforts to develop improved cultivation strategies. The study aimed to determine the effects of substrate type and depolymerized chitosan on the physiological parameters, the chemical composition of leaves and fruits, and the yield of two bell pepper cultivars: 'Marta Polka' and 'Oda'. The plants were grown in a 100% peat substrate and in a mixture of peat, wood fiber (<i>Pinus sylvestris</i>), and green compost (2:1:1 <i>v</i>/<i>v</i>/<i>v</i>), with or without drenching with a solution of depolymerized chitosan. Results indicated that the growing medium, chitosan application, cultivar type, and their interactions altered several physiological, morphological, and biochemical traits. The highest total fruit weight fresh (471.23 g plant^-1) was obtained for the 'Marta Polka' cultivar grown in peat drenched with chitosan, whereas the lowest (192.02 g plant^-1) was recorded for 'Oda' grown in a substrate mix without the biostimulant. Net CO₂ assimilation rate, stomatal conductance, fresh weight of fruit, and antioxidant activity (ABTS and FRAP assays) were improved in the 'Oda' cultivar grown in the substrate mix and treated with depolymerized chitosan compared with plants grown in 100% peat without chitosan. The 'Marta Polka' plants grown in the substrate mix and treated with chitosan had a higher net CO₂ assimilation rate, photosynthetic water-use efficiency, total free amino acid content, and antioxidant activity (FRAP assay) than those grown in peat alone and not treated with the biostimulant. The results demonstrate that both substrate composition and the response to depolymerized chitosan are cultivar-specific, and that wood fiber and compost can serve as ecological alternatives to peat, enhancing overall pepper fruit quality.