Effects of silicon and phototrophic bacteria on the growth of tomato seedlings and the physicochemical properties of the cultivation substrate under low light

To clarify the effects of silicon fertilizer and photosynthetic bacteria on tomato growth in greenhouses under low light environment, the 'Zhongza 9' tomato variety was used as the experimental material. A substrate cultivation model was adopted, and a shade net was used to simulate a weak light environment. Exogenous applications of 1.5 mmol·L⁻¹ sodium silicate and photosynthetic bacteria at a 75-fold dilution were applied singly and in combination. Five experimental treatments were set up: Normal light (CK), low light (LL), low light + silicon (LS), low light + photosynthetic bacteria (LP) and low light + silicon + photosynthetic bacteria (LPS), in order to provide theoretical basis for the application of silicon fertilizer and photosynthetic bacteria to alleviate the low light stress in facility-grown tomatoes. The results showed as follows: 1) Under LL treatment, the pH of the substrate and the activities of sucrase, urease, (neutral, alkaline) phosphatase, and catalase significantly decreased by 3.54% to 70.19% compared to CK, while the EC and the contents of available phosphorus, available potassium, organic matter, total N, and total P significantly increased by 12.21% to 227.32%. Compared with LL, LS, LP and LPS can increase the activities of sucrase, neutral and alkaline phosphatase and catalase, among which LPS has the best effect, with enzyme activities increasing significantly by more than 30%. 2) Under the LL treatment, the accumulation of dry matter in both the aboveground and underground parts of the plants significantly decreased; LS, LP, and LPS all significantly increased the dry matter accumulation of tomatoes, significantly improving the robust seedling index by more than 40%, and balanced the growth of the aboveground and underground parts. 3) Under the LL treatment, the content of photosynthetic pigments in tomato plants significantly increased, the chlorophyll a/b ratio significantly decreased, and the net photosynthetic rate, intercellular CO₂ concentration, stomatal conductance and transpiration rate significantly decreased. Compared to the LL treatment, the LS, LP, and LPS treatments increased the content of chlorophyll b in tomato plants, and the net photosynthetic rate also significantly increased by more than 40%. 4) Under the LL treatment, the absorption of N, Ca, and Mg elements in tomatoes significantly increased compared to CK, while the absorption of P, K and trace elements, as well as the accumulation of all elements were significantly inhibited. Compared with LL, tomato plants treated with LS, LP and LPS had certain promotion effects on the absorption of macronutrients and trace elements in tomato plants, with the LPS treatment showing the best promoting effect. 5) The results of RDA analysis showed that the EC value, alkaline nitrogen, total phosphorus, and total nitrogen content of the substrate under low light significantly affected root morphogenesis, and the aboveground morphogenesis of plants was significantly influenced by root architecture. Principal component analysis of 19 indexes including tomato morphological parameters and mineral elements, and the results showed that the comprehensive ranking of each treatment was CK> LPS> LP> LS> LL. In summary, the application of a mixture of 1.5 mmol·L⁻¹ sodium silicate and photosynthetic bacteria at a 75-fold dilution resulted in varying degrees of increased substrate enzyme activity, significantly improved growth conditions of both the aboveground and underground parts of the tomato plants, and simultaneously enhanced the absorption and utilization of mineral elements by tomatoes.