Objectives We studied the promotion effects of protein hydrolysates (PHs) priming on cotton seed germination and seedling growth from the responses of seedling photosynthetic efficiencies and physiological activities under salt stress.

Methods A germination test was conducted first under NaCl stress condition of 150 mg/kg sand. Seven PHs concentrations (0, 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 g/L) under priming time of 8 h, and no priming CK, and four priming time treatments (0, 8, 16, and 24 h) under PHs 0.4 g/L were set up, respectively. The germination rate and hypocotyl length were investigated for consecutively 7 days. Then, a completely randomized two-factor pot experiment was conducted. The three NaCl salt stress levels, 0.52 g/kg (S1), 2.52 g/kg (S2), and 4.52 g/kg (S3), and five PHs concentrations, no priming (CK), and 0 (P0), 0.2 (P1), 0.5 (P2), and 0.8 g/L (P3) under priming 8 h, composed a total of 15 treatments. At 55 days since cotton emergence, the growth indicators of cotton plants, leaf chlorophyll content, photosynthetic parameters, antioxidant enzyme activity, and osmoregulatory substance content were analyzed.

Results Under salt stress, the highest germination rate of cotton at the day 7 was recorded at PHs 0.4 g/L, and priming treatment for 8 hours. As the salt stress degree increased, the plant height, stem diameter, and biomass of cotton significantly decreased. The contents of chlorophyll a and b initially rised and then declined, while the net photosynthetic rate, stomatal conductance, and transpiration rate continuously decreased. Conversely, the concentrations of carotenoids, intercellular CO₂, antioxidant enzyme activities, and malondialdehyde (MDA) content steadily increased. Under S2, S3 salt stress, compared to the P0 treatment, the cotton plant height in the PHs priming treatments (P1, P2, P3) did not show significant changes. However, the biomass of leaves, roots, and total biomass significantly increased, with the increase order being P2>P3>P1, among them, the P2 treatment also significantly increased the stem diameter and stem biomass. The contents of chlorophyll a, b, a+b, and carotenoids in cotton leaves were significantly enhanced, with the improvement effect following P2>P1>P3. The net photosynthetic rate, stomatal conductance, and transpiration rate of leaves also significantly increased, showing P2>P3>P1. The intercellular CO₂ concentration notably decreasesd, with the decrease order being P2<P3<P1. Under the same degree of salt stress, PHs priming significantly boosted the activities of superoxide dismutase, peroxidase, catalase and ascorbate peroxidase (SOD, POD, CAT, and APX) in cotton, while reducing the MDA content. The enhancement of antioxidant enzyme activities followed P2>P3>P1, and the reduction in MDA content showed P2>P3>P1. Compared to the non-salt stress condition (S1), the free proline and soluble sugar contents in cotton under S3 significantly increased, while the soluble protein content significantly decreased under S2 and S3. Under S2, S3 salt stress level, P2 and P3 priming significantly promoted the free proline, soluble sugars, and soluble protein content in cotton, with the overall performance being P2>P3.

Conclusions Under salt stress conditions, treating cotton seeds with appropriate concentration of protein hydrolysates through priming can effectively improve seed germination rate and hypocotyl elongation, enhance the synthesis of photosynthetic pigments and photosynthetic rate in cotton seedlings, promote the accumulation of cotton biomass, increase antioxidant enzyme activities and the synthesis of osmolytes, and enhance the resistance of cotton seedlings to salt stress. The optimal concentration for protein hydrolysate priming is 0.5 g/L, with a duration of 8 hours.