Alternating red and blue irradiation affects carbohydrate accumulation and sucrose metabolism in butterhead lettuce

Sucrose metabolism and carbohydrate accumulation were evaluated in butterhead lettuce cultured under alternating red light (R) and blue light (B) irradiance. The R/B treatments were respectively R/B(5 m) (i.e. R 5 min/B 5 min), R/B(10 m), R/B(15 m), R/B(30 m) and R/B(60 m). Meanwhile, concurrent R and B (RB) as well as monochromatic R (R) and monochromatic B (B) were conducted. Compared with concurrent RB, all the alternating R/B treatments increased the fresh weight (FW), dry weight (DW) and the content of pigment and soluble sugar in lettuce on the basis of the same daily photon amount and electric energy consumption. Lettuce exposed to R/B(30 m) required the least number of photons (1.82 μmol) and electricity (1.04 MJ) per gram DW produced among all the treatments, thus R/B(30 m) was the recommended lighting mode in terms of the use efficiency of photons and electricity. Relatively higher soluble sugar content and the total sweetness index (TSI), as well as lower crude fiber content were detected in lettuce subjected to R/B(60 m), which was regarded as optimal irradiation mode in terms of lettuce taste. The content of sucrose, glucose and fructose, and the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS) in the synthesis direction, as well as the gene expression level of SPS in lettuce were all enhanced by the alternating irradiation of R/B (15 m), R/B (30 m) and R/B (60 m) in comparison to the concurrent RB treatment. Moreover, higher enzyme activities in the synthetic direction of sucrose were found to be accompanied with higher soluble sugar content in lettuce subjected to R/B (30 m), R/B (60 m) and the solely R, indicating that the enzyme activities in sucrose synthetic direction seemed more related to the soluble sugar accumulation in lettuce. The results also showed that SS in lettuce leaves mainly acted on sucrose decomposition rather than synthesis. The hexose (glucose and fructose) content of lettuce subjected to R/B(60 m) basically kept the highest level among all the treatments through the whole processing period, which might be due to that lettuce exposed to R/B (60 m) owned the highest gene expression level of SS and invertase, as well as the highest enzyme activities of SS (cleavage) and invertase among all the treatments. Solely B inhibited the gene expression of sucrose metabolism-related enzymes in lettuce, and the photon use efficiency to R by lettuce was higher than to B from the angle of biomass and soluble sugar accumulation. A low intensity of B (1/9 of the R level) alternating with R improved the photon and electric use efficiency of lettuce in this study.