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 (PI ABS ) and maximum photochemical efficiency (F v /F m ) by 1.5 folds, and 4.13%, respectively. BS also increased the chlorophyll fluorescence energy flux parameters (RE 0 /CS m , ET 0 /CS m , and ET 0 /CS 0 ). 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.