LED lighting can potentially reduce the growth cycle of watermelon seedlings to-be-grafted

Watermelon is a worldwide cultivated crop and most of the cultivated area use grafted seedlings. The first stage of grafted watermelon seedling production is the growth of seedlings to-be-grafted which can be achieved with controlled environment agriculture (greenhouses and growth chambers). These seedlings are essential to be of certain status in order for the grafting to be successful and the grafted seedlings to be of high quality. Quality categories for watermelon seedlings to-be-grafted have already been established. Controlled environment agriculture is practiced as a way of crop production which is unaffected by seasons and external environmental conditions. The use of artificial light sources (fluorescent-FL lamps and light-emitting diodes-LEDs) is absolutely necessary in controlled environment agriculture. The aim of the study was to test the potential of broad spectra LEDs to reduce the growth cycle of watermelon seedlings to-be-grafted. Temperature in the growth chamber was set at 22/20 degrees C (day/night), photoperiod was 16 h and PPFD was 85 +/- 5 mu mol m(-2) s(-1). Light treatments included FL lamps (control) or two LED light sources with broad spectra (L20AP67 with 11% blue, 26% green, 49% red, 14% far-red; AP673L with 12% blue, 19% green, 61% red, 8% far-red; Valoya Oy, Helsinki, Finland). Seedlings were grouped in categories namely "not acceptable" and "acceptable" depending on their potential to develop into high quality final products. After 11 days in the growth chamber, all seedlings of the three light treatments were characterized as "not acceptable". After 12 and 13 days, all seedlings of FL and AP673L were also "not acceptable", while L20AP67 developed 13 and 38% "acceptable" seedlings, respectively. The results indicate the ability of white LEDs to efficiently replace traditionally used FL lamps and potentially reduce the growth cycle of watermelon seedlings to-be-grafted.