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.

LED Light Quality of Continuous Light before Harvest Affects Growth and AsA Metabolism of Hydroponic Lettuce Grown under Increasing Doses of Nitrogen

“…However, the fresh weight of ACL (4R: 1B) was significantly higher than that of ACL (2R:1B) only at 12 mmol·L −1 , nitrogen level. Zha et al [ 35 ] found that continuous light with high ratio of red to blue light quality could promote the growth of lettuce, which was consistent with the results of this experiment. It is because red light is the most absorbed light by green plants [ 39 ].…”

“…Continuous light is a convenient and efficient means to control the light environment in plant factory. Although most studies have shown that continuous light can cause damage to plants, reasonable setting of the light environment parameters of continuous light could effectively promote the growth and quality of hydroponic leafy vegetables [ 35 ]. Continuous light could promote the yield of sweet pepper [ 36 ], the growth of young tea plants [ 37 ] and the germination rate of alfalfa seeds [ 9 ].…”

The Proportion of Blue Light from Light-emitting Diodes Alters Microgreen Phytochemical Profiles in a Species-specific Manner

“…Therefore, the accumulations of reduced and total ascorbate concentrations in the majority of microgreens could be due, in part, to the positive effect of blue light on photosynthesis; however, this physiological parameter was not assessed during this study. Regarding our study, the possibility remains that an increase in the proportion of blue light promotes increased gene expression and activity of enzymes operating in the ascorbate recycling pathway (e.g., ascorbate peroxidase) because these phenomena have been described for hydroponically grown lettuce seedlings under LEDs supplying 75% blue light relative to 25% red light (Zha et al, 2020). Interestingly, the aforementioned lettuce seedling study determined that an increase in the proportion of blue light does not alter the dehydroascorbate concentration.…”

“…microgreens (Samuolien_ e et al, 2017). Similarly, the proportion of blue light has no effect on ascorbate levels in rapeseed (Brassica napus L.) plantlets (Li et al, 2013); however, ascorbate tends to be present at higher levels in lettuce (Lactuca sativa L.) cultivated under LEDs supplying 75% blue:25% red light relative to those delivering 25% blue:75% red and 50% blue:50% red light (Zha et al, 2020). The impact of blue light on ascorbate metabolism has not been widely explored in microgreens.…”

Challenges and Opportunities of Light-Emitting Diode (LED) as Key to Modulate Antioxidant Compounds in Plants. A Review

“…In lettuce plants under continuous light for 12 days, the ratio 25RL/75BL increased the Vit C level mainly by promoting Vit C regeneration rather than its biosynthesis. In this case, only the expression of regeneration genes ( APX, MDHAR, DHAR, and GR ) was significantly increased [ 77 ]. The concentration of Vit C was greater in non-heading Chinese cabbage ( Brassica campestris L) seedlings under BL plus RL, followed by those grown under BL, and fluorescent lamps [ 78 ].…”