End-of-Production Supplemental Lighting from Red and Blue Light-emitting Diodes Increases Leaf Pigments of Lactuca sativa L. 'Cherokee' and 'Vulcan' in the Greenhouse

Under low-light greenhouse conditions, such as those found in northern latitudes, the foliage of red-leaf lettuce (Lactuca sativa L.) varieties is often green and not aesthetically appealing to consumers. Our objective was to quantify the effect of end-of-production supplemental lighting (SL) of different intensities and sources on foliage color of two red leaf lettuce cultivars, ‘Cherokee’ and ‘Vulcan’. Plants were grown for 5 weeks at 20 °C and under a daily light integral (DLI) of 8.3 mol·m^–2·d^–1. Two weeks prior to being marketable, plants were placed at 18 °C and under an ambient DLI of 6 mol·m^–2·d^–1 and provided with a 16-h photoperiod from low-intensity LED lamps providing 4.5 μmol·m^–2·s^–1 (control), or 16-h of SL from high-pressure sodium (HPS) lamps providing 70 μmol·m^–2·s^–1, or high-intensity LED arrays providing 25, 50, or 100 μmol·m^–2·s^–1 blue light, 100 μmol·m^–2·s^–1 red light, or 100 μmol·m^–2·s^–1 50:50 (%) red:blue light. Chromametric and relative chlorophyll content (SPAD) values of ‘Cherokee’ and ‘Vulcan’ foliage were significantly different among all treatments after 3, 5, 7, and 14 days of SL. For example, after 14 days ‘Cherokee’ and ‘Vulcan’ plants grown under 100 μmol·m^–2·s^–1 red:blue light were the darkest in color with an L* (lightness) value of 31.01 and 33.97, respectively. Chromametric a* (change from green to red) and b* (change from yellow to blue) values increased with 100 μmol·m^–2·s^–1 red, blue, or combination of red:blue light. Chromametric a* and b* values of ‘Cherokee’ were 4.61 and -1.63 and 3.42 and -1.33 when plants grown under 100 μmol·m^–2·s^–1 red:blue and blue light, respectively. Under the same lighting treatments, chromametric a* and b* values of ‘Vulcan’ were 4.46 and 11.11 and 5.15 and 12.08, respectively. Our data suggests that as little as 7 days of end-of-production SL of 100 μmol·m^–2·s^–1 red:blue or blue LED light promotes enhanced red pigmentation of L. sativa ‘Cherokee’ and ‘Vulcan’ foliage when the crop is grown under a low greenhouse DLI.