Effects of UV-B Radiation Timing on Growth and Antioxidants in Edible Flower Pansy

Abstract

The present study aimed to investigate the growth response and changes in antioxidant profiles of edible pansy (Viola comuta ‘Purple’ and ‘Blue’) following UV-B treatment during the day (UV-D) and at night (UV-N). The pansies were grown under white LED illumination (120 μmol·m-2s-1) for 16 hours per day and UV-B was applied for 20 minutes in daytime lighting (11:50 - 12:10) or at nighttime in dark conditions (23:50 - 00:10). For plant growth, no significance was observed between the control and UV-D or UV-N, for either fresh or dry weight values of flowers and leaves; however, the number of flowers was significantly decreased in plants exposed to UV-N. In addition, the chlorophyl content of the leaves was unchanged after the treatments, except for the ‘Purple’ v ariety a fter t he 3 rd w eek of U V treatments, when Fv/Fm or non-photochemical quenching (NPQ) were less extensively differentiated. No significant levels of reactive oxygen (superoxide, O2 -) were observed microscopically following staining with nitroblue tetrazolium (NBT). Total flavonoids showed that ‘Purple’ had a higher response to UV-B treatment than the control up to the third week, whereas for ‘Blue’, UV-D and UV-N responses were higher than that in the control. The anthocyanin content was the highest after UV-D treatment for ‘Purple’, but no significant change was observed for ‘Blue’. Therefore, UV-B treatment for 20 minutes, during the day or at night, had little effect on pansy plant growth or stress, though flavonoids increased in ‘Purple’.