Comparative Effects of High-Molecular-Weight Chitosan and Chitosan Oligosaccharide Lactate on Metabolic Adjustment and Salinity Tolerance in Cucumis sativus L.

Salinity is one of the most harmful abiotic factors limiting crop productivity. Chitosan is a natural polysaccharide biostimulant able to alleviate salt stress, but its efficiency depends on its physicochemical properties, which remain insufficiently understood. The present study aimed to determine whether two chemically distinct forms of chitosan, i.e., high-molecular-weight chitosan (ChT) and chitosan oligosaccharide lactate (ChL), differentially mitigate the adverse effects of NaCl-induced salinity in cucumber (Cucumis sativus L.). Plants were grown hydroponically under control conditions or in the presence of 50 mmol L-1 NaCl, with or without medium-applied ChT or ChL (10 mg L-1). Salinity reduced growth and the accumulation of photosynthetic pigments, whereas chlorophyll fluorescence parameters remained largely unaffected, indicating limited impairment of PSII photochemistry. Chitosan, particularly the ChT form, partially alleviated NaCl-induced growth inhibition; ChT increased shoot and root fresh weight and leaf area under salt stress, whereas the effects of ChL were not statistically significant. Chitosan also enhanced free proline accumulation and was associated with changes in selected tricarboxylic acid (TCA) cycle intermediates, particularly malate and α-ketoglutarate, indicating an association with altered central carbon metabolism under salt stress. In contrast, neither chitosan form substantially improved the ionic status of cucumber, as excessive Na accumulation and reductions in K concentration and the K/Na ratio persisted despite treatment. These findings suggest that the beneficial effects of chitosan, particularly ChT, in salt-stressed cucumber were associated more closely with metabolic and osmotic responses than with restoration of ion homeostasis or direct protection of the photosynthetic apparatus, with the two chitosan forms showing different response patterns under the experimental conditions used.