Response of Watermelon Scions to Dynamically Adjusted High and Low Blue/Red LED Light Ratios

Watermelon is an important vegetable crop cultivated worldwide with significant nutritional and economic value. Due to its susceptibility to soilborne pathogens, grafted seedlings are highly preferred. Robust scions are a prerequisite for the quality of the grafted seedling. Seedling production in a plant factory with artificial lighting offers advantages in space-use efficiency and control over growth parameters. In these systems, dynamic lighting, i.e., changing lighting conditions over a given period, can improve energy-use efficiency or enhance the physiological performance of growing plants. Hence, this study aimed to test whether dynamically adjusted blue/red ratios during the production period influence the physiology, growth, quality, phytochemical composition, and antioxidant capacity of scion watermelon seedlings produced in a PFAL. Two blue/red (high and low) ratios were tested over a 10-day growth period under constant illumination (H and L) or with a change in the middle of cultivation (HL and LH). The results showed that a high blue/red light ratio during the growth of young watermelon scions, especially during the final days of production (average of H and LH), enhanced biochemical responses (+29.5% phenolic compound content and +26–158% antioxidant capacity compared to L). Conversely, a shift to a low blue/red light ratio (HL) improved overall seedling quality (+35% Dickson’s quality index, +33% sturdiness quotient, and +18% total leaf chlorophyll content compared to L).