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

In this study, we investigated the effects of exogenous alginate oligosaccharides (AOS), melatonin (MT), and nano-silicon dioxide (NPs) 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 increases in the fresh weight of tomato seedlings sprayed with different combinations of exogenous substances compared with the CK treatment, among which, the aboveground fresh weight of tomato seedlings under the T3 treatment was significantly increased by 37.61% (P<0.05), and the belowground fresh weight of tomato seedlings under the T4 treatment was significantly increased by 57.56% (P<0.05). Compared with the CK treatment, the total root surface area of tomato seedlings under T2 and T4 treatments significantly increased by 32.09% and 53.87% (P<0.05), respectively. Furthermore, compared with the CK treatment, hydrogen peroxide (H₂O₂) contents in the root system of tomato seedlings under the T1, T2, and T3 treatments significantly reduced by 90.57%, 92.36%, and 92.47% (P<0.05), while changes in contents of H₂O₂ and superoxide (O₂⁻·) in the tomato seedlings were relatively stable under T4 treatment; the T1, T2, T3, and T4 treatments induced significant reductions in malondialdehyde (MDA) contents in new leaves and root system, and T1 and T3 treatments significantly decreased MAD contents in old leaves. In addition, we detected significant changes in the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in old and new leaves, and the root systems of tomato seedlings under the T1 treatment. Compared with the CK treatment, the T1 treatment promoted significant increases of 18.89% and 71.80% (P<0.05) in the activities of SOD in old leaves and root systems, respectively; significant increases of 33.51% and 42.27% (P<0.05) in the activities of POD in old and new leaves, respectively; significant reductions of 47.33%, 17.91%, and 47.22% (P<0.05) in the activities of CAT in old and new leaves, and root systems, respectively. Furthermore, compared with the CK treatment, there were significant increases of 51.22%, 124.42%, 40.69%, 157.69%, and 172.96% (P<0.05) in the fruit weight per plant, contents of soluble proteins, lycopene, and soluble sugars, and sugar-to-acid ratio of tomato, respectively, under the T3 treatment; the tomato yield, and contents of soluble proteins, lycopene, and free amoni acid were significantly increased by 70.55%, 112.08%, 84.41%, and 37.47% (P<0.05), respectively, under the T4 treatment. 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 could enhance the antioxidant capacity of tomato 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, foliar spraying with AOS and NPs treatment was established to have the best effect in alleviating low-temperature stress in tomato.