The differential antioxidant capacity of watermelon flesh at different maturity stages and its inhibitory effects on seed aging may explain the significance of fruit flesh colors

Fruit can use different skin colors to attract animals for seed dispersal. Interestingly, however, many fruits such as watermelon, which has a green outer rind, also have colored flesh. The potential reasons underlying this phenomenon were investigated here. White (low maturity), pink (medium maturity), and red-fleshed (high maturity) watermelons were collected and their flesh antioxidant capacities were compared by evaluating Fe3+ reducing power and DPPH radical scavenging capacity. Results showed that the antioxidant capacity of fruit flesh at different maturity stages was highest in red flesh, lower in pink flesh, and lowest in white flesh. Moreover, extracts of these flesh samples were obtained and the germination rate and lipid peroxidation of aged seeds, which were pre-treated with these extracts during the artificial aging process, were investigated. This showed that the extracts can significantly increase germination rate, but decrease lipid peroxidation of aged watermelon seeds, with the greatest changes observed with red-flesh extracts and the smallest with white flesh. Dimethyl thiourea, a specific scavenger of free radicals and reactive oxygen species, had a similar effect on the germination rate and lipid peroxidation of aged seeds. Combining previously reported results and our findings, we propose a hypothetical model in which pigment biosynthesis results in the enhancement of antioxidant capacity to illustrate the physiological significance of flesh color on seed aging and germination.