Regulation of accumulation and metabolism circadian rhythms of starch and sucrose in two leaf-color lettuces by red:blue ratios of LED continuous light

There are interspecies differences in lettuce CL tolerance to LED red and blue light. It is important to clarify the response mechanism of tolerant and sensitive lettuce to CL to achieve efficient application of CL in artificial light plant factories. In the experiment, red-blue LEDs were used to provide a conventional light (360μmol·m−2·s−1, red:blue ratio 4:1 and 18/6 h photoperiod) and three continuous light (CL) treatments with various red:blue ratios (270μmol·m−2·s−1, red:blue ratio 4:1, 2:1 and 1:1) for CL-sensitive and tolerant lettuce cultivars (Yidali and Zishan) with the same daily light integral. The effects of CL quality on the growth, diurnal starch and sucrose accumulation and metabolism of two lettuces were studied by sampling at 6:00, 12:00, 18:00 and 24:00 on the 12 days after light treatment. The results showed that red-blue CL up-regulated shoot biomass of two cultivars, which might be attributed to root and leafy morphology, and chlorophyll content responses. CL with the 80% red light produced the greatest yield, but minor and medium leafy injury was found only on Yidali. CL improved starch and sucrose contents diurnally in two lettuce cultivars, and they totally decreased with blue light proportion. Sucrose synthase (SS), sucrose phosphate synthase (SPS), acid invertase (AI), neutral invertase (NI), adenosine diphosphate glucose pyrophosphorylas (AGP), α-amylase and β-amylase activities under CL were improved in two lettuces compared with the conventional light. Starch and sucrose contents and SS, SPS and AGP activities basically decreased with blue light proportion for two cultivars, while AI, NI, α-amylase and β-amylase activities increased inversely totally. CL irrespectively of its quality disordered the circadian rhythms of carbohydrate accumulation and metabolic enzyme activities in two lettuces. Starch content related closely positively with AGP, α-amylase and β-amylase activities for Zishan, but positively AGP and β-amylase activities for Yidali. Sucrose content correlated negatively with AI and NI activities for Zishan, while positively SS and SPS activities for Yidali. To sum up, red-blue CL with low-dose blue light could improve Yidali and Zishan lettuce yield, starch and sucrose accumulation, and metabolic enzyme activity, but CL disordered the circadian rhythms of carbohydrate accumulation and metabolic enzyme activity irrespectively of spectral quality. Yidali and Zishan lettuces are sensitive and tolerant cultivars to red-blue CL, which is not up to their abilities to maintain circadian rhythms of carbohydrate accumulation and metabolism. Furthermore, starch accumulation in two cultivars was controlled jointly by synthesis and decomposition enzymes, but sucrose accumulation in Yidali lettuce depended on decomposition enzymes rather than synthesis enzyme for purple lettuce.