Research on watermelon rootstock extends shelf life and firmness

Fruit quality is an important attribute of the fresh-cut fruit industry, especially when the fruit has a limited shelf life. But scientists at Lane Agricultural Center near Lane, Okla., have found that grafting watermelon plants onto gourd rootstock not only increases fruit quality and crispness, it also increases shelf life of fresh-cut fruit.

Grafting has been used extensively in Europe and Asia, where land availability is limited and crop rotation considerably more difficult. It hasn’t gained much popularity in the United States, due to the cost, availability of land and greater access to fungicides for disease control. Grafting research has been conducted in the United States, but no studies were extensive and few studies were performed on watermelons. Scientists at Lane Agricultural Center performed larger grafting experiments on watermelon to find better results.

“The whole project was done for two reasons: One was to look at disease control and the (other for) effects of grafting on fruit quality,” said Wayne Fish, USDA-ARS research chemist at the center. “There was a fair amount of (test) results of grafting on fruit quality. We tried to make it a much more extensive study.”

The fresh-cut project is part of a larger watermelon grafting study between scientists at the USDA-ARS South Central Agricultural Research Laboratory and Oklahoma State University’s Wes Watkins Agricultural Research & Extension Center, known jointly as the Lane Agricultural Center. In addition to fruit quality research, researchers there also looked at disease control and lycopene and sugar content in grafted compared to non-grafted watermelons.

The watermelon grafting project has been held at the center for about 10 years, with the fruit quality study taking up about the last four years of the project. Scientists used four watermelon cultivars grafted onto several rootstocks, including gourd, squash and a squash/squash cross, to study the effects on sugar and lycopene content and fruit texture of watermelon on different rootstock. Non-grafted plants of the same cultivars were used as controls. The watermelons were planted in several test plots and on different soils at the center.

Sugar and lycopene contents of the grafted watermelons were usually equal to the non-grafted plants, although fruit from some grafted plants exhibited higher levels of both. The grafted plants produced fruit that had considerably firmer flesh than non-grafted plants.

The scientists used a penetrometer, an instrument used for measuring firmness, mounted on a drill press and lowered into 2-inch fruit cubes. They found that the overall quality of the fruit from grafted plants was considerably higher than the non-grafted plants, and noticed a crisper flesh that kept its firmness and appearance considerably longer than non-grafted fruit. In fresh-cut studies, they studied the longevity of the firmness over a period of about seven days, and found that fruit from grafted plants tended to be firmer at the end of the study than the fruit from non-grafted plants at the beginning.

Grafting increased firmness in some watermelons by as much as 30 percent, depending on the rootstock, and while they lost firmness over time, that firmness was still higher than non-grafted watermelons.

“All rootstock increased firmness to some degree,” said Benny Bruton, USDA-ARS plant pathologist at Lane Agricultural Center. “If firmness increased by 30 percent and you get fruit softening and firmness decreases by 10 percent, it’s still greater than fruit from non-grafted plants on day one.”

Previous research indicated that fruit from grafted plants exhibited an off-flavor, but the scientists believe that may be from harvesting the fruit too early. Fruit from grafted plants takes about five to seven days longer to mature than fruit from non-grafted plants, and early harvesting doesn’t allow for sugar content to fully develop, creating a fruit with less sugar and a considerably poorer taste than mature fruit.

“If (harvest) is too early, it’s going to taste like a cucumber because it won’t have the sugar with the flavor,” Bruton said.

One major drawback of fruit grafting is the cost. Seedless watermelon plants cost about 28 cents each, while grafted plants cost about $1. That would require a premium, which the fresh-cut industry may not be willing to accommodate at this point.

“You can justify (grafting) for disease control in a highly-productive field, but the industry isn’t going to pay more at this point,” said Merritt Taylor, director of the Wes Watkins Agricultural Research & Extension Center.
Public demand for grafted fruit could change those attitudes.

“The fact that the fruit is firm and maintains its firmness is an important attribute for the fresh-cut industry,” Fish said.