Quantification of DNA damage by the comet assay in radish sprouts exposed to excess light irradiation

The comet assay was adapted for quantifying the degree of photo-induced DNA damage by using radish sprouts exposed to varied light conditions. An index, IND, was defined to express the DNA intactness, based on image-analyzing of nuclei in protoplasts prepared from the plant leaves. The IND value gradually decreased with increasing light intensity (22–430 W m−2) and exposure time (0–6 h), and ultimately fell to 21% at 6 h under a light intensity of 430 W m−2, as compared to a reference level in the plants virgin of the exposure. Furthermore, the DNA damage was found to be restored to an appreciable extent when the plants were fed with antioxidants such as ascorbic acid and green tea extract, suggesting that DNA damage from strong light can be caused by photo-oxidative stress generated by the excess energy over a scavenging capacity of antioxidative defense mechanisms in the plant cells.