# Vitamin C in Plants: From Functions to Biofortification
## 5 Light-Dependent Vitamin C Accumulation in Plants  
Several papers have reported that plant exposure to light significantly increases vitamin C content [139; 174; 175]. Consistently, probably due to a reduction of irradiance levels, plants grown in greenhouses show lower levels of vitamin C compared with plants cultivated in the field [176]. However, the enhancement of vitamin C in plants seems to be dependent on the total amount of incident global radiation, which can be regulated by modulating light intensity or the duration of plant's exposure to light. The use of continuous light for 48 and 72 h after a period of darkness causes a great increase of vitamin C levels in lettuce and _Arabidopsis_, respectively [70; 177]. However, the continuous exposure of lettuce plants to very high irradiation causes a loss of vitamin C, whereas continuous low irradiation improves the content of this metabolite [178]. [PERSON] et al. [179] found that in lettuce, continuous illumination with red-blue light emitting diodes (LEDs) increases the vitamin C content in relation to the exposure time. Interestingly, in the same plants cultivated for 15 days under continuous red-blue LEDs exposure, the maximum peak of vitamin C content was found after nine days from the beginning of light exposure [180].  
The quality of light can also influence the vitamin C pool. The regulatory effects of monochromatic lights of the UV-Vis spectrum on the modulation of vitamin C content have been studied in different species, but the obtained results are often contradictory, suggesting that different species can respond differently to specific wavelengths. High-red/far-red ratios enhanced vitamin C levels in the leaves of _Phaseolus vulgaris_[67]. Lettuce plants illuminated with single or combined blue and red lights showed higher vitamin C contents than plants grown in white light [181]. In Chinese kale, different lights, except for blue, used during sprout growth improved the vitamin C content, and the highest concentration of ASC was found in shoots exposed to white LEDs [182]. The influence of light on the vitamin C content was also evaluated in numerous fruits, such as apple [183], tomato [175; 184], Satsuma mandarin, Valencia orange, and Lisbon lemon [185]. In these three citrus varieties, the enhancement of vitamin C content in the fruits was greater with the increasing intensity of blue LEDs. Furthermore, continuous irradiation with blue LEDs is more effective than pulsed irradiation and is related to increased expression of genes involved in the modulation of the vitamin C pool, suggesting a control at the transcriptional level [185]. On the contrary, accumulation of vitamin C in lettuce grown under continuous light is mainly due to changes in activity, rather than in expression, of the enzymes involved in vitamin C biosynthesis and oxidation [180].  
Light treatments have also been tested in the postharvest, but also in this case, contradictory results have been reported. In cabbages stored for 15 days at low temperatures, the content of vitamin C was higher in the presence of blue light [186], unlike what was observed in asparagus stored at 4 \({}^{\circ}\)C, in which the vitamin C content after six days of blue light did not differ from the control in the dark [187]. In broccoli as well, blue light had no positive effects on the vitamin C content, whereas green, red, and yellow lights, probably stimulating metabolic and physiological activity, permitted de novo vitamin C synthesis [188].  
In addition to visible light radiation, plants are also exposed in nature to UV radiation, which constitutes about 7% of solar radiation [189]. UV was shown to improve vitamin C content in soybean spyoutus [190]. Increased levels of vitamin C in cucumber plants illuminated with UV-B have been linked to a significant increase in the activity of MIOX, GLDH, and enzymes of the ASC-GSH cycle [191].