Objectives We studied the effect of exogenous alginic acid on alleviating the NaCl-induced salt stress from cotton seedlings growth and the key physiological responses.

Methods A two-factor completely randomized pot experiment was conducted. Three NaCl were added at rate of 0.57, 2.57, and 4.57 g/kg soil and constructed none-salt (S0), mild- (S1), moderate-salinity soil (S2).. Four alginic acid application rates were set: 0 (H0), 3 (H1), 6 (H2), and 12 (H3) mL/kg. At 55 days after seedling emergence, soil pH and electrical conductivity (EC), root morphology, plant biomass were measured,. The third leaves from the apex were sampled for the determination of photosynthetic pigments, gas-exchange parameters, antioxidant enzyme activities, malondialdehyde (MDA), and osmotic adjustment substances were measured. Principal component analysis (PCA) and partial least squares path modeling (PLS-PM) were used to analyze the physiological pathway of alginic acid-mediated salt-stress mitigation.

Results With increasing salt stress, root establishment and dry matter accumulation of cotton seedlings were markedly inhibited. Under H0 conditions, total root length, total root surface area, and total dry weight under S2 decreased by 48.5%, 49.1%, and 47.6%, respectively, compared with S0. Net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) decreased by 61.2%, 40.2%, and 37.3%, respectively, whereas intercellular CO₂ concentration (Ci) increased by 30.9%, suggesting that photosynthetic decline under moderate salt stress may not be solely attributable to stomatal limitation, but may also be associated with impaired mesophyll CO₂ assimilation. The S2 treatment also increased MDA content by approximately 73% and decreased soluble protein content by approximately 52.7%, suggesting aggravated membrane lipid peroxidation and disrupted protein metabolic homeostasis. Exogenous alginic acid reduced soil EC at harvest and alleviated the inhibitory effects of salt stress on cotton seedling growth and physiological processes, with the H2 treatment showing the best overall performance. Under S2 conditions, H2 increased root volume, total root length, total root surface area, and total dry weight by 104.0%, 87.6%, 115.5%, and 26.1%, respectively, compared with H0. Meanwhile, H2 increased SOD, POD, and CAT activities by approximately 140%, 81%, and 130%, respectively, reduced MDA content by approximately 28%, increased total chlorophyll, chlorophyll b, and carotenoid contents by 24.3%, 77.5%, and 40.7%, respectively, and improved Pn by 147.4%. Compared with H2, H3 decreased total dry weight, Pn, total chlorophyll, and soluble protein content by 12.4%, 41.2%, 13.3%, and 20.3%, respectively, and increased MDA content by 16.9%, indicating an attenuated effect at the high application rate. PCA and PLS-PM results showed that alginic acid promoted dry matter recovery under salt stress mainly by enhancing antioxidant–osmotic defense, reducing membrane lipid peroxidation, and maintaining photosynthetic function.

Conclusions Salt stress aggravated oxidative damage and osmotic imbalance in leaves and reduced mesophyll photosynthetic capacity, thereby suppressing root expansion and plant growth of cotton seedlings. Appropriate application of exogenous alginic acid reduced root-zone EC, enhanced the antioxidant and osmotic adjustment capacities of leaves, maintained the stability of photosynthetic pigments and protein metabolism, and alleviated the non-stomatal limitations dominated by the decline in mesophyll photosynthetic capacity, thereby promoting root growth and dry matter accumulation. Under the conditions and dosage range of this experiment, the 6 mL·kg^-1 alginic acid treatment exhibited the best overall alleviating effect.