Regulation of ascorbate accumulation and metabolism in lettuce by end-of-production high light irradiation provided by red and blue LEDs

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 mu mol m- 2s- 1) from red and blue LEDs over a 16-h time course. Compared to the control (200 mu mol m- 2s- 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 mu mol m- 2s- 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 mu mol m- 2s- 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 mu mol m- 2s- 1) increased the content of H2O2, O2*-, 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 mu mol m- 2s- 1) irradiation before harvest to increase the AsA content of lettuce.