Growth and Fruit Quality of Watermelon Affected by Different Supplemental Light Sources in a Greenhouse

This study evaluated the effects of various LED spectra—white (W), red and blue (RB), W plus far-red (FR), and RB plus FR—on the growth, fruit quality, and phytochemical accumulation of greenhouse-grown hydroponic watermelon. Watermelons were cultivated with controlled temperature and humidity and subjected to four LED treatments at an equivalent PPFD of 200 ± 3 µmol·m−2·s−1 and a 15 h light period for 43 days, with sunlight as a control. The photosynthetic rate and stomatal conductance were significantly higher in the RB LEDs than in all other treatments. Fv/Fm and PIABS exhibited time-dependent differences among treatments after 13:00, with all LED treatments showing higher values than the control, except for the Fv/Fm of RB LEDs. SPAD, chlorophyll, and carotenoid contents were the highest in the RB LEDs, and 40%, 30%, and 19% higher than those in the control group, respectively. Growth characteristics, such as plant height and node and leaf number, were highest in the control group and were significantly higher than the RB LEDs. Petiole length tended to increase in LEDs treated with FR. Sweetness was the highest in W LEDs. Therefore, supplemental LED lighting can potentially improve the production and fruit quality of greenhouse watermelons.