Rootstock Effects on Plant Vigor and Watermelon Fruit Quality

Watermelon plants grafted onto rootstocks from squash or gourds were planted at Lane, Oklahoma. Weight, dimensions, and quality (SSC, firmness, lycopene content) of a seeded (Summer Flavor® 800) and a seedless (Summer Sweet® 5244) cultivar were measured. Watermelons from non-grafted plants weighed more than those from grafted plants. Fruit shape, determined by length, circumference, and their ratio, did not change significantly from fruit harvested from grafted and not grafted plants. Rind thickness was increased in fruits grafted to the gourd rootstock, ‘451’. Lycopene and firmness tended to be lower in non-grafted plants. Grafting decreased SSC in diploid fruit but had varying effect on the triploid fruit. The rootstock affected plant vigor in both triploid and diploid scion. These results show that fruit quality varies with both cultivar and rootstock and more vigorous plants tended to have better quality fruit. Introduction: Grafting of cucurbits to rootstocks of different cucurbit species is a common practice in Asia, parts of Europe, and the Middle East. Use of rootstocks enhances the vigor of the plant through avoidance of soil pathogens, tolerance of low soil temperatures and/or salinity, and vigorous attainment of soil nutrients (Ruiz et al., 1997). The type of rootstock has been shown to affect watermelon plant growth and yields (Yetisir and Sari, 2003). In Cucumis melo, the type of rootstock determined yield and foliar phosphorous and magnesium content in the scions (Ruiz et al., 1997). Watermelon production in the U.S. is increasingly challenged by soil pathogen pressure. Fields have to be rotated on a five to seven year cycle, or even abandoned, if fumigation treatment is unavailable. Grafting of differing cucurbit species to avoid soil pathogen problems is new to many U.S. producers but it promises to be an alternative to fumigation. In fact, some seed companies now offer watermelon transplants that are grafted onto squash or gourd rootstocks. Some literature indicates the rootstock used can alter the scion and resulting yield and quality attributes of the fruit. The purpose of this study was to determine rootstock effects on watermelon fruit quality, including size, sweetness, lycopene content, and firmness under Oklahoma growing conditions. Materials and Methods: Watermelons of diploid and triploid cultivars Summer Flavor 800 (SF800) and Summer Sweet 5244 (SS5244) were obtained as grafted or nongrafted transplants from Abbott & Cobb (McAllen, TX). Plants were controls (no graft), grafted onto cushaw pumpkin (Cucurbita argyrosperma C. Huber) or hybrid squash (C. pepo L.) rootstocks. Plants were arranged in a randomized complete block design and fertility and care provided as outlined in Motes and Cuperus (1995). Watermelons were harvested, weighed, and length and circumference measured from blossom end to stem end. Fruits were cut lengthwise through the ground spot and rind thickness measured to 0.1 mm using calipers at the thinnest point near the blossom end. Color was taken at two locations in the heart and two locations in the locule areas with a Minolta CR200 colorimeter (Ramsey, NJ), using an 8 mm aperture and in CIE L*a*b*, using D65 illuminant. Firmness was measured in the heart and locule areas of both triploids and diploids, taking care to avoid seeds and seed traces, with a hand held gauge (Wagoner, fruit tester no. FDN20). Tissue in the heart and locule areas was measured for soluble solids content (SSC) by squeezing juice onto an Atago PR100 digital