Essential macronutrient nitrogen (N) is crucial for plant growth and yield, but excessive chemical N fertilizer not only increases unnecessary production costs but also causes environmental pollution. Therefore, reducing N fertilizer use by increasing organic fertilizer use is crucial for sustainable agriculture. In this study, we investigated the effects of three nitrogen levels—the recommended rate (N), a 20.0% reduced rate (0.8N), and a 40.0% reduced rate (0.6N)—and two levels of organic fertilizer—a normal dose (M) and a four-times the normal dose (4M)—combined with root application of the beneficial element silicon (Si) on the photosynthetic characteristics, yield, and fruit quality of the tomato cultivar ‘Tianxi No. 5’. Compared with M + N treatment, the longitudinal diameter, transverse diameter, fruit weight, and fruit yield of tomato fruit in 4M + 0.6N treatment significantly increased by 12.4%, 14.6%, 14.5%, and 12.8%, respectively, while the yield was further improved with Si application. In addition, a reduction in N fertilizer and an increase in organic fertilizer, combined with Si application, improved fruit quality parameters such as concentrations of vitamin C, lycopene, phenols, flavonoids, sucrose, fructose, etc., and promoted sugar metabolism-related enzyme activity (sucrose synthase, invertase, and sucrose phosphate synthase) and the accumulation of N in the fruit. The principal component analysis and three-factor analysis of variance (ANOVA) of the fruit quality and yield indices showed that nitrogen fertilizer, organic fertilizer, silicon fertilizer, and the interaction of the three had significant effects on the quality and yield of tomato fruits, and that the 4M + 0.6N + Si treatment had the best combined effect on the yield and quality of the tomatoes. Thus, a moderate reduction in chemical N fertilizer, combined with increased organic fertilizer and Si, could be an effective agronomic practice for improving the yield and quality of tomatoes.

Environmental stress, notably the exposure to low temperatures during the early developmental stages of seedlings, has been identified as a critical determinant impacting the yield and quality of tomato crops cultivated in greenhouses. Silicon (Si), recognized as a beneficial element, is posited to mitigate the adverse effects of such stress on plant physiology. This study explores whether exogenous Si fertilizer can effectively alleviate the stress of low temperature and cold damage on tomato plant growth, fruit yield, and quality. Tomato plants were placed under low temperature conditions (6 °C at night, daily average temperature 15 °C), with normal temperature conditions as the control (below 16 °C at night, daily average temperature 28 °C), and two different concentrations of nano Si and ionic Si (50 mg·L⁻¹ and 200 mg·L⁻¹) were sprayed on the leaves, with an equivalent amount of deionized water as the control, for a total of 10 treatments. Relevant indexes were measured to investigate the effects of exogenous Si on tomato resistance, yield, and quality under low-temperature stress. The results show that compared with the control treatment, the plant height, stem diameter, and fresh weight of above-ground and underground parts of tomato seedlings decreased significantly by 46.52%, 42.53%, 28.81%, and 28.97%, respectively, after 15 days of low-temperature stress (p < 0.05), and in order to resist low temperature, the activity of antioxidant enzymes and the content of osmotic adjustment substances were up-regulated in seedlings. Ultimately, low-temperature stress inhibited the morphological growth, nutritional quality, and yield of fruits. Both concentrations of Si application can promote the growth and biomass accumulation of tomato plants under low temperature conditions. Moreover, it significantly ameliorated the osmotic adjustment and antioxidant capacity of the plants, thereby alleviating the low-temperature stress. Under low-temperature stress, 50 mg L⁻¹ ionic Si was the most effective for increasing tomato yield per plant, which was significantly increased by 22.44% compared with the control treatment (p < 0.05). Consequently, the study advocates for the application of 50 mg·L⁻¹ ionic Si fertilizer as a strategy to mitigate the impact of low-temperature stress on tomato plants. Furthermore, the use of nano Si fertilizer has been demonstrated to exert a significant influence on enhancing both the yield and quality of tomatoes, with a 50 mg·L⁻¹ concentration of nano Si fertilizer leading to a notable increase in yield by 20.15% under normal temperature conditions (p < 0.05). These findings are intended to furnish a theoretical foundation and practical direction for advancing research aimed at combating the detrimental effects of low-temperature stress in the context of protected vegetable cultivation.

Secondary salinization caused by the overaccumulation of calcium nitrate [Ca(NO₃)₂] in soils due to excessive fertilization has become one of the major handicaps of protected vegetable production. Brassinolide, a bioactive plant steroid hormone, plays an important role in improving abiotic stress tolerance in plants. However, whether and how brassinolide (BR) can alleviate Ca(NO₃)₂ stress remains elusive. Here, we investigated the effects of exogenous BR on hydroponically grown tomato (Solanum lycopersicum L.) plants under Ca(NO₃)₂ stress through proteomics combined with physiological studies. Proteomics analysis revealed that Ca(NO₃)₂ stress affected the accumulation of proteins involved in photosynthesis, stress responses, and antioxidant defense, however, exogenous BR increased the accumulation of proteins involved in chlorophyll metabolism and altered the osmotic stress responses in tomatoes under Ca(NO₃)₂ stress. Further physiological studies supported the results of proteomics and showed that the exogenous BR-induced alleviation of Ca(NO₃)₂ stress was associated with the improvement of photosynthetic efficiency, levels of soluble sugars and proteins, chlorophyll contents, and antioxidant enzyme activities, leading to the reduction in the levels of reactive oxygen species and membrane lipid peroxidation, and promotion of the recovery of photosynthetic performance, energy metabolism, and plant growth under Ca(NO₃)₂ stress. These results show the importance of applying BR in protected agriculture as a means for the effective management of secondary salinization.