Effects of different compounded exogenous substances on the growth, physiological characteristics, and yield quality of tomato under low temperature

In this study, we investigated the effects of exogenous sodium alginate oligosaccharide (AOS), melatonin (MT), nano-SiO₂ (NPs), and their combinations on the growth and physiological characteristics of tomato plants, along with fruit yield and quality under conditions of low-temperature stress. As experimental plants, we used the tomato variety ‘Zhonza 9’, which were subjected to low-temperature treatments at 15°C/6°C (day / night). Five treatments were established as follows: foliar spraying with distilled water (CK), 75 mg·L⁻¹ AOS+50 mg·L⁻¹ MT (T1), 75 mg·L⁻¹ AOS+50 mg·L⁻¹ NPs (T2); 50 mg·L⁻¹ MT+50 mg·L⁻¹ NPs (T3), and 75 mg⁻¹ AOS+50 mg·L⁻¹ MT+50 mg·L⁻¹ NPs (T4). The results revealed significant increases in the fresh weight of tomato seedlings sprayed with different combinations of exogenous substances compared with the CK seedlings, among which, the aboveground fresh weight of tomato seedlings subjected to the T3 treatment was significantly increased by 37.61% (P<0.05), and that of the belowground fresh weight of tomato seedlings subjected to the T4 treatment was significantly increased by 57.56% (P<0.05). Compared with the CK treatment, both T2 and T4 treatments promoted significant increases in the total root surface area of plants, with the T2 treatment promoting a significant increase in the total root surface area of 72.37% (P<0.05). Furthermore, compared with the CK treatment, the T1 and T2 treatments contributed to significant reductions in the contents of superoxide (O₂^−.) and hydrogen peroxide (H₂O₂) in the old leaves, new leaves, and the root systems of tomato plants, and the T1, T2, T3, and T4 treatments induced significant reductions in malondialdehyde (MDA) contents. In addition, we detected significant changes in the contents of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in old and new leaves, and the root systems of tomato seedlings subjected to treatment T1 after spraying with different combinations of the exogenous substances. Compared with the CK treatment, the T1 treatment promoted significant increases of 18.89% and 71.80% in the contents of SOD in old leaves and root systems, respectively (P<0.05), a significant increase of 42.27% in the contents of POD in new leaves (P<0.05), and significant reductions of 47.33% and 47.22% in the contents of CAT in old leaves and root systems, respectively (P<0.05). Furthermore, compared with the CK plants, there were significantly increases of 51.22%, 124.42%, 40.69%, 157.69%, and 172.96% in the fruit weight, contents of soluble proteins, lycopene, and soluble sugars, and sugar-to-acid ratio of tomato fruits, respectively, in plants subjected to the T3 treatment (P<0.05), and in those plants receiving the T4 treatment, the corresponding values were increased by 70.55%, 112.08%, 84.41%, and 37.47% (P<0.05). Principal component analysis of tomato plant resistance, growth, and quality indices revealed that, overall, the T2 treatment had the best mitigation effect against low-temperature stress. In summary, foliar spraying of tomato plants with AOS, MT, and NPs under conditions of low-temperature stress can enhance the antioxidant capacity of these plants, reduce the accumulation of reactive oxygen species, promote root system growth, alleviate the low-temperature stress, and enhance the yield and quality of tomato fruits, among which, the AOS+NPs treatment was established to have the best effect in alleviating low-temperature stress in tomato.