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• This is the first study focused on the dynamic responses of ascorbate metabolism to long-term continuous light. • The increase of lettuce shoot biomass under continuous light dependent on specific leaf weight rather than leaf area. • Increased AsA and DHA level under continuous light was mainly attributed to greater GalLDH and APX activity respectively. • Ascorbate-glutathione cycle pays little contribution to ascorbate accumulation under continuous light. To understand the dynamic changes of lettuce growth, ascorbate pool and metabolism under long-term red and blue continuous light (CL, 24/0 h, 200 μmol·m−2·s−1), growth parameters, ascorbate pool size, as well as activities and gene expression levels of key enzymes involved in the ascorbate metabolism in lettuce leaves were investigated every 3 days during 15 days. In comparison with normal photoperiod (Control, 16/8 h, 200 μmol·m−2·s−1), lettuce plants gained greater shoot fresh weight and dry matter under CL, which depended more on specific leaf weight than leaf area. Both reduced ascorbate (AsA) and dehydroascorbate (DHA) contents were remarkably enhanced by CL and keep a relatively stable level during the experiment. The consistent greater contents of AsA and DHA were respectively attributed to the up-regulation of L-galactono-1,4-lactone dehydrogenase and ascorbate peroxidase activity rather than their gene expression. Although glutathione contents and glutathione reductase activities were elevated by CL, their contribution to AsA regeneration was limited on account of similar activity and expression level of dehydroascorbate reductase with control. In addition, H 2 O 2 content was also raised obviously under CL compared with control, but it was maintained at a relative safety and stable level from day 6. Fluorescence imaging results also showed that CL did not induce oxidative damage on lettuce leaves compared with control. It is concluded that lettuce plants grown under red and blue continuous light for 15 days obtained greater shoot biomass and ascorbate pool size without leaf injury, and 9 days was the most cost-effective duration for yield improvement from an energy standpoint. Ascorbate accumulation under continuous light was primarily and rapidly regulated by the enzymes involved in AsA biosynthesis and oxidation at enzymatic activity level rather than gene expression level. [ABSTRACT FROM AUTHOR]

• EOP high light irradiation increased the ascorbate content of lettuce. • EOP high light intensity regulated the expression of genes and the activity of enzymes involved in ascorbate synthesis and cycling. • EOP high light intensity induced the production of ROS, and the content of ascorbate, as an important antioxidant, increased accordingly. • EOP high light intensity increased the carbohydrate content of lettuce. Increasing the L-ascorbate (AsA) content of vegetables is very important for improving plant quality in greenhouses and plant factories because AsA is an antioxidant that is essential for the proper function of plants and the human body. End-of-production (EOP) high light irradiation is an economical preharvest practice for improving product quality. Here, we studied the dynamic responses of the AsA pool and AsA metabolism in lettuce exposed to EOP high light irradiation (300, 400, and 500 μmol m−2·s−1) from red and blue LEDs over a 16-h time course. Compared to the control (200 μmol m−2·s−1), the content of AsA and total AsA (T-AsA) in lettuce leaves significantly increased after 8 h of high light irradiation, and the contents of AsA and T-AsA increased by 42.4 % and 36.9 %, respectively, under 500 μmol m−2·s−1 irradiation. Transcript levels of the AsA metabolism-related genes GME , GGP , GPP , GLDH , APX , MDHAR1 , MDHAR2 , DHAR1 , DHAR2 , and GR were upregulated under higher light. Likewise, the activities of AsA-related enzymes (GLDH, APX, DHAR, and GR) increased under 500 μmol m−2·s−1. The expression of genes (GGP , GPP , GLDH , MDHAR1 , MDHAR2 , DHAR1 , DHAR2 , and GR) was more sensitive to high light irradiation in AsA metabolism. Both high light intensity and long irradiation time increased the soluble sugar content. Moreover, high light intensity (500 μmol m−2·s−1) increased the content of H 2 O 2 , O 2 *−, and MDA in lettuce leaves, especially at 16 h. In conclusion, EOP high light increased AsA levels by upregulating the expression of genes and the activity of enzymes in AsA metabolism. The increase in soluble sugar availability and the signaling of reactive oxygen species also contributed to AsA accumulation. We therefore recommend 8 h of high light (500 μmol m−2·s−1) irradiation before harvest to increase the AsA content of lettuce. [ABSTRACT FROM AUTHOR]