Optimized Blue Laser Priming Improves Lettuce Seed Germination and Early Seedling Establishment Under Reduced Water Availability

Continuous-wave (CW) 450 nm blue diode-laser irradiation was evaluated as a physical seed-priming treatment for improving lettuce ( Lactuca sativa L.) germination and early seedling establishment under reduced water availability. Dry seeds were irradiated at 50–250 mW for 2 min and at 100 mW for 0.5–10 min; each condition included three independent Petri-dish replicates with 100 seeds per replicate, and the replicates were monitored for 72 h. Laser treatment produced a dose-dependent biological response. The optimized condition, 100 mW for 2 min, increased final germination from approximately 65–70% to 90–95%, increased the germination speed index, and promoted root elongation more strongly than shoot elongation. Longer exposures reduced germination and seedling growth. Under reduced water availability (0.5–6 ml per Petri dish), laser-treated seeds germinated earlier, maintained final germination of 83–93% compared with 57–72% in controls, and produced better-developed seedlings. The relative benefit increased as water availability decreased, indicating that optimized blue laser priming partially compensated for low water supply during germination and early establishment. These results identify CW blue laser priming as a contactless, chemical-free approach for improving lettuce seed performance and early seedling vigor under limited water availability.