Light supplementation and growing season affect the quality and antioxidant activity of lettuce

The light spectrum plays a major role in regulating plant growth and development, influencing photosynthesis and photo-morphogenesis. The objective was to evaluate the weight and antioxidant activity of ‘Lavinia’ lettuce (Lactuca sativa L. var. capitata L.) plants under LED lights with different red:blue ratios (R:B) used as a supplement to ambient light in a greenhouse at different seasons. Five LED light supplemented treatments were used, white (W, as control, ratio blue:green:red:far-red (B:G:R:Fr) = 30:45:20:5; 0.7:1.0), blue (B = 50:20:20:10; R:B = 0.4:1.0), white-r (Wr = 25:30:40:5; 1.6:1.0), white-R (WR = 15:15:63:7; 4.2:1.0) and red (R = 10:10:75:5; 7.5:1.0) were applied in early autumn, late autumn and winter. Immediately after transplant into the hydroponic system, lettuces plants with 3-4 true leaves (5-6 cm root length) were treated with light treatments for 14 d and harvested. In early autumn, late autumn, and winter, the PAR range applied in ambient light and supplemental lights was between 361 to 495, 222 to 304, and 297 to 407 µmol m-2 s-1, respectively. Lettuce showed the highest fresh leaf mass in early autumn compared to late autumn and winter due to the highest leaf number. In early autumn, only lettuce under the highest R:B (7.5:1.0) had a lower weight among the light treatments. Overall, lettuces supplemented with R:B between 1.6 (Wr) to 4.2 (WR) had significantly higher DM than the control (0.7:1.0). Total phenolic contents (TPC) and antioxidant capacity (AC) were highest in early autumn, followed by winter and late autumn. Light with the highest R:B (7.5:1.0) diminished TPC and AC in all growing season. This study showed that the quality of ‘Lavinia’ lettuce plants grown in early autumn under R:B between 1.6 and 4.2 improved DM, and higher red component decreased the antioxidant activity.