Acid-modified biochar improves the adaptability of sugar beet seedlings to saline-alkali soil by enhancing their physiology.

Biochar is widely recognized for enhancing plant stress tolerance, and using acid modified-biochar may further enhance its suitability as a soil amendment for plant growth under saline-alkali conditions. However, the underlying mechanisms by which modified biochar regulate sugar beet (Beta vulgaris L.) growth and physiology remain unclear. Here, a pot experiment was conducted to evaluate the effects of acid-modified biochar on the physiology of sugar beet seedlings grown in saline-alkali soil. The unfavourable saline-alkali conditions decreased photosynthetic efficiency and inhibited leaf development, whereas all acid-modified biochar treatments restored sugar beet physiological performance, with sulfuric acid-modified biochar (BS) delivering the strongest favourable effects. BS increased the total foliar chlorophyll content by two folds, enhanced the net photosynthetic rate by 86.22%, and improved the photosynthetic performance index (PIABS) and maximum photochemical efficiency (Fv/Fm) by 1.5 folds, and 4.13%, respectively. BS also increased the chlorophyll fluorescence energy flux parameters (RE0/CSm, ET0/CSm, and ET0/CS0). In addition, BS lowered leaf relative electrical conductivity by 6.79%, decreased superoxide dismutase (SOD) activity by 23.53%, and increased catalase (CAT) activity by 22.43%; suggesting reduced membrane damage and potentially a more balanced antioxidant response. As a result, leaf area and aboveground biomass increased by 15.61% and 139.44%, respectively. Taken together, these findings demonstrated that sulfuric acid-modified biochar amendment mitigated the saline-alkali stress in sugar beet by stabilizing foliar PSII activity, optimizing energy allocation, and maintaining membrane integrity, and providing the physiological basis for restoring sugar beet growth in unfavourable saline-alkali soils.