Alternation of temporally overlapped red and blue light under continuous irradiation affected yield, antioxidant capacity and nutritional quality of purple-leaf lettuce

The responses of yield, nutritional quality and antioxidant capacity of hydroponic purple-lettuce were investigated in a plant factory to different alternations of temporally overlapped red (R) and blue (B) light under continuous irradiation in which spectral photon flux density distribution changed with various combinations of R and B LEDs on a 24-h cycle. Two constant simultaneous rend and bleu (R-B) light treatments including one normal photoperiod (RB, 16/8 h, 225 μmol·m−2·s−1) and one continuous light (RB’, 24/0 h, 150 μmol·m−2·s−1) as well as three alternating temporally overlapped light treatments (B to R-B and then R, each part lasts the same time) with alternating frequencies of one time (T1), two times (T2) and four times (T4) respectively in 24 h were set up with the same daily light integral (DLI). The lettuce plants were exposed to the irradiation patterns for 14 days. In all treatments, plants received the same quantum number of red or blue light and the same daily light integral (12.96 μmol·m−2). In comparison with normal photoperiod (RB), the results showed that lettuce plants gained greater shoot fresh and dry weight under RB’ and T1 both on the 7 and 14 days after treatment, which depended on leaf area and biomass allocation. Among alternating light treatments, the shoot fresh weight of lettuce decreased with the increase of alternating frequencies. Lettuce plants under alternating temporally overlapped treatments had significantly higher MDA, H2O2 and O2− contents as well as higher SOD and CAT activities than that under RB and RB’. Alternating light treatments also induced phenols accumulation, and particularly lettuce under T2 had significantly higher anthocyanin, flavonoid and total phenol contents than RB. However, prolongation of light period under the same DLI condition might promote nitrate accumulation in lettuce while had no positive effects on starch and soluble sugar contents. Therefore, alternating temporally overlapped red and blue light under continuous irradiation could improve the antioxidant capacity of lettuce to a certain extent, and the responses of lettuce antioxidant system to alternating light exposure might depend on the duration and alternating frequency of two monochromatic light qualities.