Effects of Ultraviolet-B Radiation on the Regulation of Ascorbic Acid Accumulation and Metabolism in Lettuce
Shujuan Liu | Lei Yu | Lipan Liu | Aihong Yang | Xingyu Huang | Anfan Zhu | Hua Zhou
To understand the effect of ultraviolet (UV)-B irradiation on the antioxidant capacity and growth of lettuce (<i>Lactuca sativa</i>), we subjected lettuce plants to UV-B irradiation (15.55 kJ m<sup>−2</sup> d<sup>−1</sup>) for 7 days and measured yield, photosynthetic performance, hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), reduced glutathione (GSH), and ascorbic acid (AsA) contents, and the enzyme activity and expression of genes involving AsA recycling. UV-B exposure did not significantly decrease the fresh/dry weight of the lettuce shoots. The net photosynthesis rate, internal CO<sub>2</sub> concentration, transpiration rate, and stomatal conductance decreased during the first 4 days of irradiation and light but recovered at day 7. In UV-B-treated plants, the levels of AsA, GSH, and H<sub>2</sub>O<sub>2</sub> increased significantly and simultaneously, with a positive correlation found between H<sub>2</sub>O<sub>2</sub> and AsA or GSH levels. UV-B exposure upregulated the expression level of most genes encoding the enzymes involving AsA recycling but downregulated the associated enzymatic activities. The increase of AsA content in UV-B-exposed lettuce might contribute to the AsA–GSH cycle, leading to downregulation of ascorbate oxidase (AO) enzymatic activity and gene expression. UV-B irradiation had a greater impact on metabolite levels than time of UV-B treatment. These results suggest that AsA homeostasis in UV-B-treated lettuce is regulated through a feedback loop between the expression and activity of enzymes associated with AsA recycling. Short-term UV-B supplementation (24 h) could be a promising approach to enhance AsA content in lettuce.
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