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.
Highlights
Ascorbate, the reduced form of vitamin C, is well recognized as one of the most pivotal antioxidants in plants (Gallie, 2013; Ntagkas et al, 2018)
The shoot dry weights (DW) and leaf area showed a positive correlation with the red:blue light ratio, the differences in them between 50R:50B and 75R:25B treatments were greater than those between 25R:75B and 50R:50B, and there was no significant difference in these parameters among the treatments
This is because red light has higher relative quantum efficiency than blue light (McCree, 1972; Stutte, 2009), and photosynthetic capacity will be decreased when the proportion of blue light exceeds 50%, even though moderate blue light (e.g. 7%) is essential for preventing dysfunctional photosynthesis under pure red light (Hogewoning et al, 2010)
Summary
The reduced form of vitamin C, is well recognized as one of the most pivotal antioxidants in plants (Gallie, 2013; Ntagkas et al, 2018). The metabolic pathway of AsA in plants has been well established since Wheeler et al (1998) proposed the main AsA biosynthetic pathway: the L-galactose pathway (D-mannose pathway) In this pathway, D-glucose is catalyzed to AsA by a series of enzymes, including several key enzymes: Abbreviations: APX, ascorbate peroxidase; AsA, ascorbate; DHA, dehydroascorbate; DHAR dehydroascorbate reductase; GGP, GDP-L-galactose phosphorylase; GLDH, L-galactono-1,4-lactone dehydrogenase; GME, GDP-D-mannose 3 ,5 epimerase; GMP, GDP-D-mannose pyrophosphorylase; GPP, L-galactose-1-P phosphatase; GR, glutathione reductase; MDHAR, monodehydroascorbate reductase; T-AsA, total ascorbate. The electron donors of these two enzymes are NADPH and glutathione, respectively, and the latter electron donor is recovered from oxidized glutathione by glutathione reductase (GR) (Ntagkas et al, 2018)
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