Alternating red-blue light alleviated physiological injury by reducing oxidative stress under both high light and continuous light from red-blue LEDs

CL (continuous light)-sensitive lettuce cultivar were treated under high light (HL, 480 µmol·m^− 2·s^− 1, 20/4 h) and CL (400 µmol·m^− 2·s^− 1, 24/0 h) provided by red-blue LEDs (red-blue ratio 3:1) without or with R/B alternation (HL-A and CL-A, alternating duration ratio 3:1) at the same daily light integral. On the 16th day after light treatment, lettuce was diurnally sampled four times at equal 6 h intervals, thus the effects of HL-A and CL-A on growth, leafy injury, yield, and carbohydrate accumulation, oxidative stress and their circadian rhythms of lettuce were investigated and evaluated. The results showed that the lettuce plants under HL-A and CL-A had greater canopy size, leafy area and shoot fresh biomass than those under HL and CL correspondingly. However, the specific leaf weight, SPAD value of new leaves and leaf injury grade were decreased. The contents of starch and sucrose, and DPPH free radical scavenging rate were significantly increased under HL-A and CL-A compared with HL and CL, while the contents of hydrogen peroxide, superoxide anion and MDA were decreased. Compared with HL and HL-A, the above physiological indices of lettuce under CL and CL-A were higher respectively. HL-A and CL-A did not change the circadian rhythm of lettuce. In conclusion, HL-A and CL-A improved the yield and external quality of CL-sensitive lettuce. Importantly, they alleviated HL- and CL-induced physiological injuries of lettuces by reducing the accumulation of ROS only, rather than decreasing carbohydrate accumulation or circadian rhythm disorder.