Fruit Quality and Volatile Compounds of Seedless Watermelon As Affected By Grafting

Interest in watermelon grafting is growing in the U.S.; however, limited information is available regarding the changes in flavor-related fruit characteristics, particularly volatile compounds of seedless watermelons as a result of grafting. This study was conducted to explore the influence of grafting with interspecific squash rootstock on fruit quality modification including volatile compounds in seedless watermelon. Triploid watermelon ‘Melody’ (Citrullus lanatus) and interspecific squash ‘Super Shintosa’ (Cucurbita maxima × C. moschata) were used as the scion and rootstock, respectively. Both grafted and non-grafted watermelon plants were grown with two different in-row spacings of 0.76 and 1.68 m and a constant between-row spacing of 2.44 m in a split-plot design with four replications during Fall 2015 in Citra, FL. Fully ripe watermelon fruit from the second harvest were sampled for quality evaluations. Rind and flesh firmness of grafted watermelon fruit were significantly higher than that of non-grafted fruit. The wider in-row spacing resulted in reduced flesh firmness. Neither grafting nor plant spacing exhibited any significant impacts on fruit soluble solids content, titratable acidity, pH, flesh color, or lycopene content. Out of the 50 volatile compounds identified from the watermelon tissue using solid-phase microextraction (SPME) with gas chromatography-mass spectrometry (GC-MS), twenty compounds, including alcohols, aldehydes, esters, and terpenes, were compared among the treatments. Amounts of each volatile compound in fruit relative to non-grafted ‘Melody’ at 0.76 m spacing were determined, and comparisons among different grafting and spacing treatments were performed. (E)-2-nonen-1-ol, (Z)-3-hexenol, and (E, Z)-3,6-nonadien-1-ol were found to be significantly higher in their relative levels at 1.68 m spacing in grafted vs. non-grafted ‘Melody’. Grafting with ‘Super Shintosa’ significantly decreased (E)-2-nonenal content in ‘Melody’ at 1.68 m spacing, while (E)-2-nonenal in non-grafted ‘Melody’ showed a significantly higher level at 1.68 m spacing as compared to 0.76 m spacing. Levels of other alcohols and aldehydes did not differ between grafted and non-grafted ‘Melody’. Hexyl butyrate content was significantly higher in non-grafted vs. grafted ‘Melody’ at 1.68 m spacing, while fruit from grafted ‘Melody’ showed a higher level of limonene than the non-grafted treatment at 0.76 m spacing. Overall, grafting did not show adverse impacts on fruit quality characteristics of seedless watermelon. Future research is warranted to determine how modification of the watermelon volatile profile by the rootstock selected for grafting may affect fruit flavor perceived by consumers.