Regulation of Ascorbate Accumulation and Metabolism in Lettuce by the Red:Blue Ratio of Continuous Light Using LEDs

Abstract

Ascorbate (AsA), an antioxidant that cannot be synthesized and stored by the human body, plays an essential role in the proper functioning of both plants and humans. With the goal of increasing the AsA level in lettuce, the effects of different ratios of red (R) to blue (B) light (75R:25B, 50R:50B, and 25R:75B) on AsA pool sizes as well as the transcript levels and activities of key enzymes involved in AsA metabolism were constantly monitored for 12 days under continuous light (200 μmol⋅m^-2⋅s^-1) from LEDs. The results showed that lettuce biomass was positively correlated with the ratio of red light, while the AsA pool size had a positive correlation with the ratio of blue light during the whole experiment. The 25R:75B treatment increased the expression of genes involved in AsA biosynthesis (GMP, GME, GGP, GPP, GLDH) and regeneration (APX, MDHAR, DHAR, and GR) on day 3 but only significantly elevated the activities of enzymes involved in AsA regeneration (APX, MDHAR, DHAR, and GR) subsequently. AsA regeneration enzymes (MDHAR, DHAR and GR) had greater correlations with the AsA level than the AsA synthesis enzyme (GLDH). Thus, it is concluded that a high ratio of blue light elevated the AsA level mainly by promoting AsA regeneration rather than biosynthesis. Taken together, altering the red:blue ratio of continuous light from high to low before harvest is recommended for lettuce cultivation to achieve both high yield and high quality.

Keywords: ascorbic acid; enzyme activity; expression level; light quality; regulatory mechanism.

FIGURE 1

Light spectra of light treatments.

FIGURE 2

Effects of the red:blue light ratio on the total ascorbate (T-AsA), ascorbate (AsA) and dehydroascorbate (DHA) contents as well as the AsA/T-AsA ratio in lettuce leaves under continuous light (200 μmol⋅m^{– 2}⋅s^{– 1}). Values and bars represent the means of four replicates ± SD. Different letters in the same column indicate significant differences at the p < 0.05 level according to Tukey test. Data in brackets were the increasing rate of the maximum value to the minimum value on the same day.

FIGURE 3

Effects of the red:blue light ratio on the transcript level of enzymes involved in the AsA biosynthesis pathway and activity of L-galactono-1,4-lactone dehydrogenase (GalLDH) in lettuce leaves under continuous light (200 200 μmol⋅m^{– 2}⋅s^{– 1}). Transcript level data are the mean values of three replications. Enzyme activity data are the mean values ± SD of four replications. Different letters indicate significant differences between different red:blue light ratio treatments at p < 0.05 according to Tukey test. Data in brackets were the increasing rate of the maximum value to the minimum value on the same day.

FIGURE 4

Effects of the red:blue light ratio on the transcript level and activity of APX (A), GR (B), MDHAR (C), DHAR (D) in lettuce leaves under continuous light (200 μmol⋅m^{– 2}⋅s^{– 1}). Transcript level data are the mean values of three replications. Enzyme activity data are the mean values ± SD of four replications. Different letters indicate significant differences between different red:blue light ratio treatments at p < 0.05 according to Tukey test. Data in brackets were the increasing rate of the maximum value to the minimum value on the same day.

FIGURE 5

Effects of the red:blue light ratio on the H₂O₂ and MDA contents in lettuce leaves under continuous light (200 μmol⋅m^{– 2}⋅s^{– 1}) (200 μmol⋅m^{– 2}⋅s^{– 1}). Data are the mean values ± SD of four replications. Different letters indicate significant differences between different red:blue light ratio treatments at p < 0.05 according to Tukey test. Data in brackets were the increasing rate of the maximum value to the minimum value on the same day.

FIGURE 6

Principal component analysis of AsA pool sizes and enzyme activities in lettuce leaves under continuous light of different red:blue ratios (yellow symbol-25R:75B; blue symbol-50R:50B; red symbol-75R:25B) on different days (△–day 3; □–day 6, ♢–day 9,○–day 12). Biplot of the first two principal components (PC1,2) of the measured network topological properties.