Optimal Air Temperature for Producing High-Quality Grafted Watermelon Seedlings in a Plant Factory System

This study aimed to determine the optimal air temperature for producing high-quality grafted watermelon (Citrullus lanatus) seedlings in a plant factory-based seedling production system.Grafted seedlings, with 'DongjangGun' as the rootstock and 'SantaKkul' as the scion, were cultivated for 21 days under three daytime air temperature treatments of 23C, 28C, and 33C with a constant nighttime temperature of 18C.Growth traits including plant height, root length, leaf number, leaf area, stem diameter, and biomass, as well as seedling quality indices such as top-to-root ratio, compactness, and crop growth rate (CGR), were evaluated at 7, 14, and 21 days after grafting.Seedling growth responses were strongly influenced by temperature.Higher temperatures promoted stem elongation, with seedlings at 33C showing the tallest height but also exhibiting excessive elongation and reduced stem thickening, which are indicative of lower seedling quality.In contrast, seedlings grown at 28C exhibited greater leaf area, thicker stems, and a more balanced allocation of shoot and root biomass.Seedling quality indices also confirmed that compactness, T/R ratio, and CGR were most favorable under 28C by 21 days.Collectively, these findings demonstrate that while both 28C and 33C promoted vigorous growth, only 28C supported sturdy morphology and balanced biomass allocation.Therefore, 28C can be considered the optimal daytime air temperature for grafted watermelon seedling production in plant factory systems, providing a basis for stable year-round seedling production and improved environmental control strategies.