Autotoxicity is a key factor that leads to obstacles in continuous cropping systems. Although Si is known to improve plant resistance to biotic and abiotic stresses, little is known about its role in regulating leaf water status, mineral nutrients, nitrogen metabolism, and root morphology of cucumber under autotoxicity stress. Here, we used cucumber seeds (Cucumis sativus L. cv. “Xinchun No. 4”) to evaluate how exogenous Si (1 mmol L−1) affected the leaf water status, mineral nutrient uptake, N metabolism-related enzyme activities, root morphology, and shoot growth of cucumber seedlings under 0.8 mmol L−1 CA-induced autotoxicity stress. We found that CA-induced autotoxicity significantly reduced the relative water content and water potential of leaves and increase their cell sap concentration. CA-induced stress also inhibited the absorption of major (N, P, K, Ca, Mg) and trace elements (Fe, Mn, Zn). However, exogenous Si significantly improved the leaf water status (relative water content and water potential) of cucumber leaves under CA-induced stress. Exogenous Si also promoted the absorption of mineral elements by seedlings under CA-induced stress and alleviated the CA-induced inhibition of N metabolism-related enzyme activities (including nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase). Moreover, exogenous Si improved N uptake and utilization, promoted root morphogenesis, and increased the growth indexes of cucumber seedlings under CA-induced stress. Our findings have far-reaching implications for overcoming the obstacles to continuous cropping in cucumber cultivation.

(1) Background: Cinnamic acid (CA) is a harmful substance secreted by the roots of continuous-cropping crops. (2) Methods: This study aimed to investigate how exogenous Si affects chlorophyll content and carbon metabolism in cucumber seedlings under CA-induced stress. (3) Results: The levels of chlorophyll a, chlorophyll b, chlorophyll a+b, and carotenoids were significantly reduced due to CA-induced stress. The addition of exogenous Si significantly alleviated this reduction. Under CA-induced stress, exogenous Si significantly increased the activities of ribulose-1,5-bisphosphate carboxylase, glyceraldehyde-3-phosphate dehydrogenase, fructose-1,6-bisphosphatase, fructose-1,6-bisphosphate aldolase, and transketolase. CA-induced stress significantly increased the fructose, glucose, and sucrose contents and reduced the starch content in the leaves and roots of seedlings. Similarly, the sucrose phosphate synthase, sucrose synthase, acid invertase, and neutral invertase activities were significantly reduced in plants under CA-induced stress. Overall, exogenous Si significantly reduced the soluble sugar content, increased the starch content, and promoted sucrose metabolism-related enzymatic activity in seedlings. (4) Conclusion: Exogenous Si can effectively increase the content of photosynthetic pigments in leaves of seedlings and maintain the balance of osmotic potential in the plant by reducing the accumulation of carbon assimilation products, which ultimately promotes tolerance to CA-induced autotoxicity stress.

To address the low Ca−induced growth inhibition of tomato plants, the mitigation effect of exogenous Si on tomato seedlings under low−Ca stress was investigated using different application methods. We specifically analyzed the effects of root application or foliar spraying of 1 mM Si on growth conditions, leaf photosynthetic properties, stomatal status, chlorophyll content, chlorophyll fluorescence, ATP activity and content, Calvin cycle−related enzymatic activity, and gene expression in tomato seedlings under low vs. adequate calcium conditions. We found that the low−Ca environment significantly affected (reduced) these parameters, resulting in growth limitation. Surprisingly, the application of 1 mM Si significantly increased plant height, stem diameter, and biomass accumulation, protected photosynthetic pigments, improved gas exchange, promoted ATP production, enhanced the activity of Calvin cycle key enzymes and expression of related genes, and ensured efficient photosynthesis to occur in plants under low−Ca conditions. Interestingly, when the same amount of Si was applied, the beneficial effects of Si were more pronounced under low−Ca conditions that under adequate Ca. We speculate that Si might promote the absorption and transport of calcium in plants. The effects of Si also differed depending on the application method; foliar spraying was better in alleviating photosynthetic inhibition in plants under low−Ca stress, whereas root application of Si significantly promoted root growth and development. Enhancing the photosynthetic capacity by foliar Si application is an effective strategy for ameliorating the growth inhibition of plants under low−Ca stress.