Evaluation of the quality of watermelon and squash seedlings produced indoors

Watermelon is an economically important species which is almost entirely propagated through grafting onto squash rootstocks. Controlled environment agriculture (CEA) in closed plant cultivation systems (greenhouses and growth chambers) has been introduced as a mean for optimal crop production unaffected by external environmental conditions and seasons. Therefore, nowadays artificial light sources such as fluorescent (FL) lamps and light-emitting diodes (LEDs) are extensively used in CEA. The objective of the present study was to test the growth response of watermelon scions and squash rootstocks grown in a growth chamber under FL or LED. In the growth chamber temperature was set at 22/20°C (day/night), photoperiod was set at 16 h and PPFD was maintained at 85±5 μmol m^‑2 s^‑1. Light was provided by FL lamps or an LED lamp commercially labelled as L20AP67 (11% blue, 26% green, 49% red, 14% far-red; Valoya Oy, Helsinki, Finland). Characteristics determined for both species were height, stem diameter, leaf and cotyledon area, chlorophyll content, fresh and dry weight of shoots and roots, root-to-shoot ratio, and cotyledon colour. In the case of watermelon, L20AP67 enhanced the height (+11%), leaf (+300%) and cotyledon (+59%) area, as well as shoot biomass (+88%) development compared to FL. Squash seedlings grown under L20AP67 developed more expanded leaf (+138%) and cotyledon (+24%) area, as well as greater shoot biomass (+36%) compared to FL. For both species, FL led to seedlings with inferior values in almost all tested parameters, except for the root system which is completely removed during grafting procedure in both scion and rootstock seedlings. The study demonstrates that LEDs employed in growth chambers can enhance the growth of watermelon scions and squash rootstocks compared to traditionally used FL lamps, adding an efficient option for fully controlled, high-quality seedling production.