Grafting. The term probably makes you think of roses, grapes, apples and nuts, which have long been grafted to improve disease resistance and productivity. But it won’t be long before grafting is also commonly associated with tomatoes, eggplants, peppers, watermelon and other vegetables that are highly susceptible to crop destroyers, such as bacterial wilt, nematodes and soil-borne diseases.
That’s because, over the last decade, researches experimenting with grafting at many U.S. universities have reported increasingly positive results with several types of vegetables. The three main benefits of grafting being: improved disease resistance, higher yields (and sometimes quality), and increased ability to adapt to harsh environmental conditions like temperature extremes, floods and drought.
While much of the research centers around the needs of commercial growers, new lines of grafted vegetables will be available to home gardeners, too. As an urban gardener with little space to grow edibles, I’m intrigued by the idea of trying grafted veggies, which might save me from the inevitable blight and pestilence my crops are likely to suffer since I can’t rotate them as often as I should. But before I plant, I want to understand more about how grafting works and, specifically, how it might affect the fruiting part of the plant. Here’s what I’ve learned so far.
If you’re unfamiliar with how grafting works, it’s a fairly simple process of joining (by hand, or robots—seriously) a rootstock, the below-ground portion of the plant, to a scion (the above-ground portion). Rootstocks are chosen for their genetic ability to resist or tolerate disease. Scions are chosen for their fruit.
Though grafting has long been practiced in East Asia and other countries around the world as a way to make crop production possible on limited, intensely cultivated land, grafted vegetable seedling production is fairly new to North America. Greenhouse growers like grafting because it allows them to do things like grow tomatoes in soil infected with fusarium without having to use costly sterilizing methods between plantings.
Tomatoes are the focus of much of the current grafting research because they are not only a high-value, disease-prone crop, they are also fairly easy to graft compared to other vegetables. Most of the tomatoes that are commercially grown in Australia, Korea and Japan are grafted. In the United States, an increasing number of growers of organic, hybrid and heirloom tomatoes are looking into grafting.
Got verticillium, fusarium or bacterial wilt? There are rootstocks that can handle those, and the same is true for tobacco mosaic virus, crown rot, corky root rot and many nematodes. Two of the most common rootstocks for tomatoes are ‘Maxifort’ and ‘Beaufort’. Both are F1 (first generation) hybrids from Monsanto (food for thought), and they are listed as being resistant to tobacco mosaic virus, verticillium, two races of fusarium, cladosporium, corky root rot, nematodes and southern blight. Seed for these two rootstocks can be purchased online, as can seedlings and grafting clips to hold the two portions together.
Scions such as ‘Brandywine’, ‘Cherokee Purple’ and ‘German Johnson’ are among the most popular varieties for grafting. Of course, hybrids make good scions, too. The sweet, cherry tomato ‘Sun Gold’ is a favorite among grafters. Territorial Seed even offers a grafted plant with two fruiting tomato varieties on top, ‘Sun Gold’ and ‘Sweet Million’.
Once a rootstock and scion have been joined, humidity plays an important role in the healing of the vascular tissue around the graft. Universities and growers use special healing chambers in which humidity, light and temperature are controlled. Gardeners like us can graft successfully using low-tech tools like fluorescent lights, humidifiers, shade cloth and plastic sheeting. (YouTube is practically bursting with helpful how-to videos on grafting, including the healing process.)
Carol Miles, an associate professor of horticulture at Washington State University, is one of many researchers experimenting with vegetable grafting. Over the last four years, she and some of her students have worked with watermelons, eggplants and tomatoes grown in a certified-organic field. She’s primarily looking to help growers who want to manage verticillium wilt.
After overcoming the hurdle of making successful grafts, Miles found that while grafting did improve verticillium wilt resistance in eggplants and watermelons, it did not make a difference with the ‘Cherokee Purple’ tomatoes they tested. “We chose ‘Cherokee Purple’ because it’s a common heirloom variety and organic growers like to work with heirlooms because that’s what the market wants, she explains. (Read about her work here.)
The thing is, there is more than one type of verticillium wilt and, as it turns out, ‘Cherokee Purple’ must have a natural resistance to the type that is present in Miles’ research fields. “People always say heirlooms are more prone to disease than hybrids, and that’s often the case, but many heirlooms do have resistance to specific diseases,” Miles continues.
Improving the disease resistance of heirlooms is one of the biggest focuses of tomato grafting research. Public desire for heirlooms is growing, and grafting is seen as a sustainable, organic way to produce heirlooms that will be more resilient and vigorous. Grafting passes muster with heirloom lovers and organic growers because the benefits offered by the process don’t alter the genes of the scion, so plants grow true to type. Or do they?
Until very recently, it was thought that there was no DNA transfer across the graft union, and many university websites echo that sentiment. And then I met with Andy Petran, a graduate student in the Applied Plant Sciences Department at the University of Minnesota. He is researching how grafting can be used to improve tomato production in the tropics, particularly the U.S. Virgin Islands. And he pointed me to a March 2012 paper by Victor M. Haroldsen, a scientific analyst at Morrison and Foerster, a law firm in San Francisco that practices both regulatory and intellectual property law. The study was published online through the National Center for Biotechnolgoy. Click here to read it.
I am no scientist, but the gist of what Haroldsen and his colleagues found is that DNA from chloroplasts (not DNA from the nucleus of the cell) has been found to cross the graft union. Though it has been detected adjacent to the graft union, the ability of this DNA to reach throughout the entire scion is not yet known.
Petran explains: “The graft definitely causes an epigenetic response, meaning that although the nuclear structure of the DNA in the scion may not be changed, how the DNA is used in the scion is changed.” If you think of DNA as an instruction manual,” Petran kindly laid out, “an epigenetic response means the cell is interpreting its manual in a new way. This results in changes in the plant without changing the DNA itself.”
At this point, further study is needed to determine what is really going on when rootstock and scion are joined, including whether epigenetic changes could alter future generations of plants. But even if the DNA in these new grafted crops is not changed, Haroldsen points out the likelihood of future discussion over how the crops will be classified from a regulatory standpoint. The choices appear to be: organic, conventional or, possibly, genetically engineered and/or modified.
Obviously, there’s much more to be learned. But as grafted vegetable seedlings, rootstocks and grafting kits become more prevalent online and in catalogs, it would be good to know as much as we can about what we’re planting in our gardens.
[A version of this story appeared in the January/February issue of Northern Gardener magazine.]