Silicon Supplementation Improves Growth, Physiology, and Biochemical Traits in Hydroponic Lettuce: Implications for Controlled-environment Agriculture

Silicon (Si) uptake and deposition in plant cell walls is associated with enhanced strength, water-use efficiency (WUE), and pest/disease resistance, leading to enhanced plant stress resilience. However, the optimal Si concentration and application method, and species-specific responses in a hydroponic system under controlled-environment agriculture, are still not fully understood. We investigated the potential of monosilicic acid, a soluble form of Si, to enhance growth, WUE, and antioxidant capacity of hydroponically cultivated lettuce ( Lactuca sativa L.), even under nonstressful conditions. We investigated two Si application methods [foliar spray (FS) and root application (RA)] and three Si concentrations (C0 = 0 mM, C1 = 0.07 mM, and C2 = 0.14 mM) in two lettuce cultivars: Green Forest and Rouxai. Plants were grown for 4 weeks after transplanting into deep-water culture (DWC) hydroponic systems, with nutrient solutions renewed weekly. Silicon was applied during each renewal using identical concentrations for the FS and RA treatments. RA at C1 produced the most favorable outcomes, increasing shoot fresh weight by 56.2% in ‘Green Forest’ and 36.1% in ‘Rouxai’, while also enhancing root growth, chlorophyll content, and antioxidant compounds such as total phenolic and flavonoid contents, particularly in ‘Green Forest’. In ‘Rouxai’, FS promoted root development and certain antioxidant responses, suggesting it as a viable alternative method. In contrast, the high concentration rate (C2) offered no additional advantage and, in some cases, reduced growth. Weekly growth analysis showed RA at C1 gave the greatest relative growth rates in both cultivars, whereas excessive foliar Si suppressed growth in ‘Rouxai’. Silicon also improved WUE through greater biomass accumulation and reduced evapotranspiration, while enhancing both enzymatic (superoxide dismutase and glutathione reductase) and nonenzymatic antioxidant activities, along with chlorophyll and carotenoid levels, even under stress-free conditions. Our results demonstrate that Si supplementation can improve growth, WUE, and biochemical responses significantly in hydroponic lettuce, highlighting its potential to improve crop performance in controlled-environment agriculture.