Coordinated induction of the sunscreen pigments and antioxidant enzymes is indispensable in watermelon (Citrullus lanatus) plants for the tolerance of UV-B radiation, the scary face of the Sun

This study aimed to explore the effect of the interaction between photosynthetic and antioxidative responses on UV-B tolerance (UV-B1: 15 min; UV-B2: 30 min) in two cultivars of watermelon [Citrullus lanatus (Thunb.) Mansfeld]: (1) Celebration; and (2) Faroe. The photosynthetic pigment content was not affected by the UV-B2 radiation, probably due to the protective impact of anthocyanin accumulation in the leaves of the watermelon cultivars. The induction of the accumulation of the inactive reaction centres, the reduction of the utilisation of the absorbed light energy photochemically, and the induction of the non-photochemical quenching of the light energy were more significantly increased in cv. Faroe due to UV-B2 radiation. The impairment in the photosynthetic electron transport reactions and insufficient activity of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and guaiacol peroxidase in cv. Faroe under the UV-B2 stress led to increased oxidative stress and membrane damage, as demonstrated by higher levels of H2O2 and malondialdehyde content. However, in cv. Celebration, the photosynthetic efficiency was less affected by the UV-B2 stress, hence reducing the oxidative stress and membrane damage due to higher antioxidant capacity, which allowed us to conclude that cv. Celebration is more tolerant to UV-B stress than cv. Faroe.