Selecting appropriate rootstocks in grafting is an important aspect to increase yield and fruit quality of vegetables. Interspecific hybrids and gourd are the most frequently used rootstocks for watermelon (Citrullus lanatus var. (Thunb.) Matsum. & Nakai) grafting. However, other Citrullus species such as bitter apple (Citrullus colocynthis (L.) Schrad) have not been completely exploited and can be a source of interesting traits. The present study aimed to evaluate bitter apple as potential rootstock for watermelon (cv. Crimson Sweet) in comparison to the commercial rootstock Shintosa (Cucurbita maxima Duch. × Cucurbita moschata Duch.), and non-grafted plants. An experimental design of Randomized Complete Blocks was applied with treatments replicated three times. Grafted and non-grafted watermelon plants were compared in terms of yield, yield components, morphometric and textural characteristics, sweetness, acidity and mineral composition. The effect of Shintosa rootstock on total yield and mean fruit weight resulted in 20.2% and 14.0% increase over the non-grafted and Crimson Sweet/bitter apple combination, respectively. Flesh firmness and fruit mineral composition (Ca, P and Zn) increased by grafting Crimson Sweet on Shintosa over the other grafting combinations. The most valued fruit quality traits for consumer satisfaction: fruit dry matter (DM) and total soluble solids (TSS) were significantly higher with bitter apple rootstock. The fruit DM and TSS contents recorded for Crimson Sweet grafted on bitter apple were significantly higher than those grafted on Shintosa by 11.7% and 8.4%, respectively. Though grafting watermelon on bitter apple slightly decreased yield characteristics than Shintosa, on the other hand it improved flavor compounds with no serious detrimental effects on other quality attributes, thus it could be proposed as potential germplasm for watermelon rootstock breeding.
