THE EFFECT OF SUPPLEMENTARY LED LIGHTING ON THE ANTIOXIDANT AND NUTRITIONAL PROPERTIES OF LETTUCE

We report on the application of supplementary solid-state lighting within an industrial greenhouse for the cultivation of various baby leaf lettuces (Lactuca sativa L.) varieties grown under natural solar irradiation and high pressure sodium lamps. Red leaf ‘Multired 4’, green leaf ‘Multigreen 3’ and light green leaf ‘Multiblond 2’ lettuce were grown to harvest time within a greenhouse under daylight with supplementary lighting provided by standard high-pressure sodium lamps (HPS) (16-h). Supplementary lighting from blue 455 and 470 nm and green 505 and 530 nm light-emitting diodes (LEDs) was applied within a 16-h photoperiod. The generated photosynthetic photon flux density of each type of solid-state lamps was 30 μmol•m^-2•s^-1 and the photosynthetic photon flux density of HPS lamps was 170 μmol•m^-2•s^-1. The day/night temperature was 17-22/14-17°C and relative air humidity was 50-60%. Supplementary LED treatment was found to change antioxidant and nutritional properties of lettuce due to the increased activity of the metabolic system for the protection from a mild photo-oxidative stress. The reduction of nitrates was accompanied by an increased concentration of nutritionally valuable carbohydrates, what is also in line with stimulation of expression of nitrate reductase by photosynthetic metabolites. Another indicator of nutritional quality, the content of vitamin C, exhibited some variation that was not directly correlated with the nitrate reduction rate. However, the effect of supplementary blue and green light was found to be variety dependent. The sensitivity to the light conditions was determined by the natural level of total anthocyanin concentration in red, green or light green lettuce.