Enhancing Growth and Nutritional Quality in Greenhouse-grown ‘Little Gem’ Lettuce using LED Supplemental Lighting

Lettuce is regarded as one of the most important leafy vegetable crops. Despite lettuce being mostly grown in open-field, its greenhouse production is widely increasing. Lettuce growth evolves two distinct stages known as the seedling and head stages. The effect of supplemental lighting (SL) on the lettuce growth was well studied but the lighting requirement during the early stage of lettuce growth and the head development remained unknown. To shed light on the subject, the lettuce’s nutritional quality and growth in response to SL were studied in separate stages of seedling and head development. The experiment was carried out using SL with different daily light integral (DLI) provided by light-emitting diodes (LEDs). Light treatments included supplemental DLI of 8.64, 11.52, 12.96, and 17.28 mol m−2 d−1. The results revealed that the light period was more effective on lettuce biomass increase than the light intensity. Although the SL increased the photosynthetic pigment content of lettuce, its impacts on the two growth stages were not the same. In a way that the chlorophyll a, total chlorophyll, and carotenoid contents decreased under the SL (DLI of 17.28 mol m−2 d−1; light intensity of 300 μmol m−2 s−1 for 16 h) conditions. Increasing DLI caused a significant increase in the nutritional quality of lettuce but the trends of the antioxidants accumulation were not consistent for the seedlings and the mature plants. These findings confirmed that SL improves the lettuce growth and quality, but the optimal lighting requirements may vary depending on the growth stage.