Sensitivity to boron stress in growing media during the early seedlings stage could severely inhibit watermelon growth and development. The aim of this study was to test if watermelon (Citrullus lanatus cv. Mahbubi) performance can be improved under different boron concentrations (0, 46.31 and 935.11 µmol·L−1 H3BO3) by grafting watermelon plants onto Cucurbita pepo Tiana F1 hybrid rootstock. Shoot dry weight of grafted watermelon was significantly higher than those of non-grafted plants under boron stress conditions (0 and 935.11 µmol·L−1). Boron concentration of the aerial organs of the grafted plants was higher than in the non-grafted ones when treated with 0 µmol·L−1 H3BO3. However, with the application of boron (935.11 µmol·L−1 H3BO3), the aerial organs of grafted plants accumulated lower boron than non-grafted ones. The Cucurbita pepo rootstock also improved the absorption of several other nutrients and leaf chlorophyll content and the Fv/Fm ratio was also increased at high boron root concentration. Higher activities of several antioxidant enzymes such as catalase (CAT), superoxide dismutase (SOD) and peroxidase (POX) in grafted watermelons were associated with a significant reduction in H2O2 and malondialdehyde (MDA) generation when compared with those of non-grafted plants under boron stress conditions (0 and 935.11 µmol·L−1 H3BO3). Grafted plants reported improved growth, nutrient uptake and several other physiological parameters when compared to non-grafted watermelon plants under boron stress conditions.
