New Vegetable Grafting Technique Reduces Labor
Vegetable grafting, which reduces the incidence of disease and contributes to the creation of stronger and healthier plants, has been used extensively in other countries for more than a half century. In the U.S., however, the benefits of grafting have been given a serious look only in the last several years.
Just ask Richard Hassell, a researcher at Clemson University in South Carolina, who led a team that devised a way to successfully graft disease-resistant roots to vegetable-producing tops. In fact, Hassell’s team patented a process designed to eliminate rootstock regrowth and, in the process, reduce labor and increase yield.
Initially, Hassell says U.S. researchers looked at grafting as an alternative to methyl bromide and a way to reduce the cost of soil fumigants. USDA’s Specialty Crop Research Initiative (SCRI) grants have allowed scientists such as Hassell to examine how grafting technology can benefit the industry.
Hassell, who is the leader for the cucurbit portion of the SCRI project, “Development Of Grafting Technology To Improve Sustainability And Competitiveness Of The U.S. Fruiting Vegetable Industry,” was charged with determining ways to make grafting more affordable in the U.S.
‘“Successful’ grafting is an art as well as a science,” he explains. “The art is to continue to have successful grafts. The science part is to figure out how to improve the reason for the graft and determine the shortcuts, such as how to avoid having workers scout the greenhouse or field for rootstock regrowth on grafted plants and then remove the regrowth. Most of the grafting is now done in Asia and Europe where labor is [less expensive] and abundant. In the U.S., the labor situation is the opposite.”
Elimination Of Rootstock Regrowth
Hassell and his graduate student, Shawna Daley, learned that to eliminate rootstock regrowth and avoid additional labor, the meristematic tissue — which is comprised of groups of cells that divide to form new cells — must be eliminated from the rootstock. In most cases, the rootstock used for cucurbit grafting is either interspecific squash or bottle gourd rootstock.
“If regrowth is not stopped, it will cause the abortion of the grafted top part or scion,” he explains.
Hassell, Daley, and the research team looked at several different methods to solve the problem, and came up with a way to chemically destroy the rootstock meristem. The process has been nearly 100% successful in watermelon grafting.
By treating rootstock meristems with a fatty alcohol product such as Fair 85 (Fair Products, Inc.), which is typically used to control sucker growth in tobacco production, the researchers were able to destroy the tissue, which prevents regrowth.
Hassell also says an organic formula of the tobacco product (Kleen-Tac, Fair Products) also is available.
Move To Automate
According to Hassell, the next step was to automate the grafting process, which has cost-saving potential. Of the two methods commonly used for cucurbit grafting — the one-cotyledon and the whole insertion methods — the one-cotyledon method is best suited for automation. Part of the automation process, however, includes eliminating the need for a rootstock cotyledon, he says.
“The demands on the robot to precisely cut the rootstock to consistently eliminate the meristem tissue but keep at least one cotyledon intact requires constant adjustments,” he explains. “If a graft could be successfully performed without requiring a cotyledon, the automated process could become faster and cheaper in the long run.”
Hassell says the importance of the cotyledon leaf is specific to cucurbits.
“You don’t have this problem with tomato grafting,” he explains. “With cucurbit grafting, however, that cotyledon is actually a functioning leaf. So the cotyledon is helping to heal the graft and stimulate new roots to form. Without it, grafting success has not been currently possible. In the process of treating the rootstock with the fatty alcohol, however, we observed that the cotyledon leaf continued to function, and there was an enormous buildup of carbohydrate in the hypocotyl (stem).”
According to Hassell, the carbohydrate buildup occurs because no growing point was available. As a result, the rootstock continued to store this energy, or carbohydrates, in the stem while increasing in size and thickness.
It was this finding that led to another discovery. “We found after a certain period of time after treating the rootstock we no longer needed that cotyledon for grafting, and that there is enough stored energy in the stem or the hypocotyl [allowing us to] harvest the stems and graft them,” he says. “There is enough energy available in the stem to stimulate new roots, heal the graft, and there is a 100% chance you will never get rootstock regrowth.”
The removal of the rootstock cotyledon also impacts the size of the tray cell that can be used, allowing you to use the typical plug size for watermelon and save space in the greenhouse.
“By eliminating the cotyledon and grafting onto the stem, we are back to the original plug size for watermelon production,” Hassell says. “Therefore, we have decreased the space in the greenhouse and reduced the cost of the overall transplant because you are not taking up as much space in the greenhouse itself.”
Grafting Interest Grows
Today, Hassell has observed a growing interest in grafting in the U.S. as well as in Asia and Europe. For example, an Israeli company, Tri-Hishtil, is in the process of completing a large grafting facility in Henderson, NC. Tri-Hishtil has been testing grafted plants throughout the U.S., and the plan is for the plant to be in full production with watermelon grafting by Jan. 1, he says.
In California, Plug Connection also has been doing extensive grafting of cucurbits and tomatoes. Hassell says growers from across the country are now purchasing grafted plants for their operations.
Extensive Research Continues
Researchers continue to look for ways grafting can benefit growers. Hassell says a scientist from North Carolina State University, Penelope Perkins-Veazie, has made three observations about grafted watermelon: the fruit contain more lycopene than non-grafted watermelon, fresh-cut watermelon are firmer than their counterparts, and grafted melons will hold in the field for up to three weeks.
When asked why the grafted watermelon offered those specific benefits, Hassell says researchers are still trying to figure that out. Now, scientists are examining soil fertility and water management with grafted watermelon and they are also looking at spacing requirements for grafted plants — among other things.
“Can we get by with fewer plants per acre? If grafted, we may be able to do so,” Hassell says. “So we are looking at all kinds of ways to save money and increase grafting in the U.S.”