Antioxidant and Photochemical Responses of Watermelon (Citrullus Lanatus L.) Genotypes to Lead Toxicity

In the present study, lead toxicity-dependent physiological changes were comparatively investigated through photosynthetic and antioxidant activity in two watermelon (Citrullus lanatus L.) cultivars (Celebration and Faroe). Plants were grown for 21 days in perlite and then exposed to 0.5- and 1-mM Pb(NO₃)₂ for additional 6 days. Lead toxicity decreased the photosynthetic pigment contents in the leaves of Faroe. The malondialdehyde content in leaves showed that lipid peroxidation in Celebration was lower than in Faroe. H2O2 accumulation was more remarkable in the leaves of Faroe exposed to 1 mM lead. Significantly higher superoxide dismutase activity observed in in the leaves of Celebration indicates a likely higher rate of superoxide radical dismutation. Lower ascorbate peroxidase activities were observed at 1 mM lead in Celebration and at 0.5-and 1-mM lead in Faroe. The glutathione reductase activity in both watermelon genotypes was stimulated by Pb toxicity. Chlorophyll fluorescence measurements demonstrated that Pb toxicity caused the decreased photosynthetic activity in both cultivars. However, lower excitation pressure of photosystem II (1-qp) and higher non-photochemical quenching (NPQ) could indicate some regulatory mechanism(s) in photosynthetic apparatus of Celebration under lead toxicity. As a result, Celebration is more tolerant to lead toxicity while Faroe was sensitive.