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Title: The influence of rootstock selection on fruit quality attributes of watermelon

item Bruton, Benny
item Fish, Wayne

Submitted to: Open Food Science Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2008
Publication Date: 4/1/2009
Citation: Bruton, B.D., Fish, W.W., Roberts, W., Popham, T.W. 2009. The influence of rootstock selection on fruit quality attributes of watermelon. Open Food Science Journal. 3:15-34.

Interpretive Summary: As a result of the discontinuation of methyl bromide use for fumigation, the reduced availability of land for crop rotation, and the increased production of seedless fruit, watermelon crops throughout the United States are highly susceptible to the increased incidence of soilborne diseases. For example, approximately 75% of the watermelons grown in the United States are at risk for Fusarium wlt. Grafting watermelon onto other cucurbit rootstock for the control of Fusarium wilt and environmental stresses has been practiced in Europe and Asia for many years. In contrast, this cultural practice is a new concept for farmers in the United States because of the high costs associated with grafting. Watermelon fruit quality does not rely on a single property but depends, rather, upon a cadre of properties of which only a few have been identified and measured. However, as the marketing of watermelon fruit steadily evolves into its sale as fresh cut, crispness of the fruit and its storage stability become increasingly important. Grafting onto gourd rootstocks has sometimes been associated with reduced sugars and off-flavor development in the fruit. The purpose of this study was to evaluate the effects of grafting different watermelon scions onto squash or gourd rootstocks by quantifying grafting’s effects on the resulting fruit quality attributes of flesh firmness and total soluble solids (TSS) as well as on the content of the phytonutrient, lycopene. All rootstocks used in this study were found to be highly resistant or tolerant to the Fusarium and Verticillium wilt pathogens. This study demonstrated that the only fruit quality trait consistently affected by grafting watermelon scion onto various rootstocks was enhanced fruit firmness. We found that grafting had little effect on TSS and lycopene content of the fruit. We did observe that fruit maturity of grafted watermelon was delayed about 5 to 7 days beyond the non-grafted counterpart which could account for reduced TSS and off flavor as noted in some grafting studies. Although some studies have concluded that squash rootstocks may produce inferior watermelon fruit quality, our studies do not support that hypothesis. The enhanced fruit firmness of grafted watermelon fruit may contribute significantly to the fresh-cut industry.

Technical Abstract: Grafting watermelon (Citrullus lanatus) to control Fusarium wilt has been practiced in Europe, the Middle East, and the Far East for decades. Until recently, grafting watermelon has not been practiced in the United States due to labor costs and land availability. There is some disagreement in the literature as to the effects that grafting has on watermelon fruit quality. This study was designed to determine the effects of grafted watermelon on fruit firmness, lycopene content, and total soluble solids (TSS) using five different rootstocks. When using Cucurbita ficifolia or Cucurbita maxima x Cucurbita moschata hybrid as the rootstock, watermelon fruit consistently had higher fruit firmness values. Other C. maxima x C. moschata hybrids or Lagenaria siceraria rootstocks generally produced lower or more varied fruit firmness values. Grafting increased fruit firmness by as much as 25% in some cases, but field and year effects were observed. In addition, grafting had no effect on lycopene content or TSS. Furthermore, no off-flavors were detected in fruit from grafted plants, but there was a 5- to 7-day delay in fruit maturity compared to their non-grafted counterpart. Although environment can have a major influence on fruit quality attributes, rootstock selection may be equally important in achieving the desired outcome.