Quality changes in hydroponic lettuce grown under pre-harvest short-duration continuous light of different intensities

SummaryThe effect of pre-harvest light intensity on the quality of hydroponically-grown lettuce (Lactuca sativa var. capitata L.) was studied by growing lettuce under 48 h of continuous illumination delivered by red- or blue-light-emitting diodes (LEDs) with a red:blue ratio of 4.0. Four light intensity treatments (50, 100, 150, or 200 µmol m–2 s–1) were applied. The results showed that the nitrate concentrations in lettuce shoots decreased significantly after treatment with 48 h of continuous light, while the contents of soluble sugars and vitamin C increased substantially. It was observed that the effect of pre-harvest short-duration continuous light (PSCL) on improving lettuce quality was significantly influenced by the intensity of the light. The decrease in nitrate concentration and the increases in soluble sugars and vitamin C contents were relatively low at a light intensity of 50 µmol m–2 s–1, but increased gradually as the light intensity increased from 50 µmol m–2 s–1 to 200 µmol m–2 s–1. However, the marginal benefit of increased light intensity in lowering nitrate concentration and increaseing vitamin C content declined rapidly when the light intensity increased beyond 100 µmol m–2 s–1. In conclusion, PSCL offers an effective method by which to improve the quality of lettuce, and a light intensity of between 100 µmol m–2 s–1 – 150 µmol m–2 s–1 is the economic optimum.