Mehmet Isleyen and colleagues explain that farmers graft watermelon and other fruits onto the roots of gourd plants because it makes the fruit more resistant to diseases. In Turkey, where the group did the study, more than 95 percent of watermelons grow from grafted seedlings. Although the gourds are hardier, previous research has shown they accumulate pesticides called organochlorines. Organochlorines have been widely banned because of concerns about their effects on human health and wildlife. Despite the fact that their remnants can linger in the soil for decades, some organochlorines remain in use. While traditional watermelon plants do not take up these compounds, the researchers wanted to resolve uncertainty about watermelon grown on the roots of plants in the squash family.
The group grew common Turkish watermelon-squash graft seedlings in soil taken from a farming region there. They tested the roots, stems, leaves and fruit of the plants and found that organochlorine levels were as much as 140 times higher in the stems of squash-grafted watermelons than in intact watermelons. However, while still urging caution, the group notes that these levels are 6-12 times lower than accepted limits of the pesticides in produce in the U.S. and Turkey.
More information: Accumulation of Weathered p,p'-DDTs in Grafted Watermelon, J. Agric. Food Chem., Article ASAP. DOI:10.1021/jf204150s
The grafting of melon plants onto cucurbit rootstocks is a common commercial practice in many parts of the world. However, certain cucurbits have been shown to accumulate large quantities of weathered persistent organic pollutants from the soil, and the potential contamination of grafted produce has not been thoroughly evaluated. Large pot and field experiments were conducted to assess the effect of grafting on accumulation of weathered DDX (the sum of p,p'-DDT, p,p'-DDD, and p,p'-DDE) from soils. Intact squash (Cucurbita maxima × moschata) and watermelon (Citrullus lanatus), their homografts, and compatible heterografts were grown in pots containing soil with weathered DDX at 1480–1760 ng/g soil or under field conditions in soil at 150–300 ng/g DDX. Movement of DDX through the soil–plant system was investigated by determining contaminant levels in the bulk soil and in the xylem sap, roots, stems, leaves, and fruit of the grafted and nongrafted plants. In all plants, the highest DDX concentrations were detected in the roots, followed by decreasing amounts in the stems, leaves, and fruit. Dry weight concentrations of DDX in the roots ranged from 7900 ng/g (intact watermelon) to 30100 ng/g (heterografted watermelon) in the pot study and from 650 ng/g (intact watermelon) to 2430 ng/g (homografted squash) in the field experiment. Grafting watermelon onto squash rootstock significantly increased contaminant uptake into the melon shoot system. In the pot and field studies, the highest stem DDX content was measured in heterografted watermelon at 1220 and 244 ng/g, respectively; these values are 140 and 19 times greater than contaminant concentrations in the intact watermelon, respectively. The xylem sap DDX concentrations of pot-grown plants were greatest in the heterografted watermelon (6.10 µg/L). The DDX contents of the leaves and fruit of watermelon heterografts were 3–12 and 0.53–8.25 ng/g, respectively, indicating that although the heterografted watermelon accumulated greater pollutant levels, the resulting contamination is not likely a food safety concern.