Growth and nutritional properties of lettuce affected by mixed irradiation of white and supplemental light provided by light-emitting diode

• Intentionally designed white LED panels and white plus another color LED panels which were intensity-adjustable were applied in the study.
• Blue, green, yellow, red and far-red lights were respectively added to the basal white light and effects of the supplemental lights on growth and qualities of hydroponically cultured ‘Green Oak Leaf’ lettuce were investigated.
• The results in our research showed that supplemental light quality could be strategically used to enhance growth and nutritional properties of ‘Green Oak Leaf’ lettuce grown under white light.

‘Green Oak Leaf’ lettuce were hydroponically cultured for 35 days under mixed light qualities of white light and a different supplemental light such as blue (B), green (G), yellow (Y), red (R), and far-red (Fr) provided by light-emitting diodes (LEDs). Effects of supplemental light qualities on the growth and nutritional properties of lettuce plants were investigated. The pure white LED (W) was used as control with a photosynthetic photon flux (PPF) value of approximately 135 μmol m−2 s−1, and the basal white light in each treatment was 105 μmol m−2 s−1 while the photon flux added by supplemental LED was approximately 30 μmol m−2 s−1. The results indicated that supplemental lights led to obvious morphological changes, plants with WR appeared compact and vigorous while those with WY and WFr looked sparse and twisted, dwarfed plants with large leaves were detected under WB. Compared with those grown in the white light control, the fresh weight of shoots increased by 63.2% and 21.7% with supplemental R and B respectively, while decreased by 35.9% with supplemental Fr. Chlorophyll and carotenoid contents were significantly higher with supplemental R and B than other treatments. Supplemental B and G resulted in decrease of nitrate content. Supplemental G significantly promoted soluble sugar accumulation. Supplemental Fr increased S/R ratio and ascorbic acid accumulation but resulted in lower biomass and pigment contents. However, no positive impact of supplemental Y light was observed in the research.