Water Relations, Yield and Fruit Quality of Grafted, Field-Grown Watermelons

Vegetable grafting is becoming popular not only for disease control, but also for managing abiotic stresses such as drought.  The ability of suitable rootstocks to efficiently absorb soil moisture and maintain a favourable plant water status is critical in this role for grafting. In the present study, we assessed whether the physiological responses of grafted plants are consistent with predictions of a high capacity for water uptake by the rootstocks.  Two commercial watermelon (Citrullus lanatus) cultivars (‘Summer Flavor 800’ and ‘Super Seedless 7167’) were grafted on hybrid squash (Cucurbita maxima × C. moschata) rootstocks and grown on a sandy soil with a history of vine decline disease.  Vine development during the vegetative growth stages was faster in grafted plants regardless of rootstock however, this developmental effect disappeared after canopy closure and fruit set. Grafted plants consistently had higher midday leaf water potentials, stomatal conductance and transpiration rates compared to non-grafted plants, consistent with ample water supply from the root systems.  In addition to developing more crown side branches, canopy duration was much longer in grafted plants, allowing total yields to be greater than those of non-grafted plants.  Average fruit soluble solids concentrations of grafted plants were higher than those of control non-grafted plants but this effect was not consistent. The results are consistent with the hypothesis that grafting commercial scions on rootstocks with vigorous root systems can improve tolerance to drought and soil-borne diseases through ample soil moisture uptake and maintenance of a favorable plant water status.