Impact of Nitrogen-Supplemented Plasma Activated Water on the Germination and Early Growth of Curly Lettuce (Lactuca sativa var. crispa)

Plasma-activated water (PAW) has garnered significant attention in agriculture for its ability to stimulate seed germination and plant growth. However, its low concentration of reactive nitrogen species may not fully meet plant nitrogen demands, especially during rapid growth phase. Therefore, in this study, the impact of nitrogen-enriched PAW on the germination and early growth of curly lettuce (Lactuca sativa var. crispa) was investigated. Seed germination percentage, vegetative organ development, and photosynthetic efficiency were assessed over a 7-day period by soaking seeds and irrigating plants with PAW supplemented with various NH4NO3 concentrations. Results revealed that while a 60–80 ppm NH4NO3 solution was needed for optimal growth, 20 ppm NH4NO3 supplementation in PAW (PAW20) effectively stimulated germination and early vegetative development. Specifically, the highest germination percentage was observed in PAW20 samples at 90.56 ± 0.96%, significantly outperforming NH4NO3 solution treatments (76.67 ± 1.67%). Nitrogen-enriched PAW also promoted healthier root development, characterized by thicker primary root and more root hairs. The combination of low-dose nitrogen and PAW effectively boosted above-ground growth, including a 10–20% improvement in stem height and leaf expansion compared to plants treated with PAW or NH4NO3 solutions. Furthermore, significantly higher chlorophyll concentrations and dry biomass ratios (10–40%) were recorded with PAW20 irrigation compared to other conditions. These findings suggest that actively adjusting nitrogen content in PAW via NH4NO3 supplementation greatly enhances its growth-promoting efficiency and can substantially reduce chemical fertilizer use, thereby advancing sustainable agriculture.