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The Science of Pomato Plants and Fruit Salad Trees

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A Photoshopped image of an ideal, but improbable, fruit salad tree (Credit: Adapted from various Wikimedia images by Guety, Fir0002/Flagstaffotos, Tobias and MikeyMoose)

In an episode of Matt Groening's animated science fiction sitcom Futurama, Leela offers her friend Fry an unusual housewarming gift: a bonsai tree sprouting tiny bananas, melons and plums. "It's a miniature fruit salad tree," she explains.

Here's the thing: fruit salad trees are real.

In Australia, James and Kerry West grow and sell four types of fruit salad trees, each of which bears several different kinds of fruit. Stone fruit salad trees grow peaches, plums, nectarines, apricots and peachcots. Citrus salad trees offer a winter and summer orange, mandarins, lemons, limes, grapefruits, tangelos and pomelos. Multi-apple trees boast between two and four different kinds of apples and multi-nashi trees produce between two and four different kinds of Asian pears.

In an online video, Kerry West explains how her husband James created their first fruit salad trees more than twenty years ago by learning the craft of grafting. "He started putting the different fruits but of the same family and seeing what would happen if you grafted them all onto the one tree," Kerry says in the video. "We were amazed at the results."

Grafting unites the tissues of two or more plants so that they grow and function as a single plant. One plant in the graft is called the rootstock, selected for its healthy or hardy root system. The other plant or plants, chosen for their fruit, flowers or leaves, are known as scions. You can join a scion to a rootstock in many different ways. In one of the most common techniques, you remove a branch from a plant whose fruit you want to reproduce and cut the broken end of the branch into a V-shape not unlike the reed for a woodwind. Shaving the scion in this way exposes its vascular cambium—a ring of plant tissue full of dividing cells that increase the branch's girth. Once the scion is ready, you slice lengthwise into a branch on the rootstock—exposing its vascular cambium—and wedge the scion into the cleft. Successful grafting requires placing the vascular cambia of both the rootstock and scion in close contact. Another grafting method involves cutting small pockets between the rootstock's bark and cambium and slipping scions into those pouches. To seal the deal, you bind the scion and rootstock with a rubber band, tape, staples, string or wax.

Click to enlarge (Credit: Ferris Jabr; adapted from photo by J.smith, via Wikimedia Commons)

Over the next few weeks, the scion and rootstock fuse their internal tissues and grow thickened scar tissue around the graft. First, both plants kill and wall off damaged cells. Meanwhile, callus cells in the vascular cambia proliferate and cement themselves together with sticky proteins, forming a living link between scion and rootstock known as the "callus bridge." If you were to look from above at a cross-section of a cleft graft undergoing this process, it would look something like an intact slice of kiwi—the scion—jammed between the separated halves of another kiwi slice—the rootstock—with the living threads of the callus bridge merging the middle kiwi slice with the semicircles on its flanks. Callus cells also provide temporary links between the primary vascular tissues in the scion and rootstock—the xylem, which transports water, and the phloem, which carries sugars. Eventually, the vascular cambia builds brand new xylem and phloem that unite scion and rootstock into a single functional organism (Reference: Plant Propagation: Principles and Practices).

Farmers have been grafting fruit trees and other crops for thousands of years, but most grafts involve only two plants. The basic idea is to attach whatever kind of plant you want to grow onto a root system that is well adapted to the local soil. As John McPhee explains in his book Oranges:

"In Florida, most orange trees have lemon roots. In California, nearly all lemon trees are grown on orange roots. This sort of thing is not unique with citrus. With the stone fruits, there is a certain latitude. Plums can be grown on cherry trees and apricots on peach trees, but a one-to-one relationship like that is only the beginning with citrus. A single citrus tree can be turned into a carnival, with lemons, limes, grapefruit, tangerines, kumquats and oranges all ripening on its branches at the same time…Most of the trees on the Ridge [a mountain range in Florida renowned for its orange tree orchards] are growing on Rough Lemon…As a rootstock, it forages with exceptional vigor and, in comparison with others, puts more fruit on the tree."

In addition to increasing yield, grafting can improve resistance to bacteria, viruses and fungi, attract a more diverse group of pollinators and provide a sturdy trunk for delicate ornamental plants. In many cases, grafting is the most reliable way to propagate fruit trees because apples, citrus fruits and many others are not "true to seed"—if you plant the seeds from such fruit, the new generation will not necessarily produce the same fruit as their parents. Instead, you might get something completely different—a grapefruit from a lime, a lemon from an orange—or a genetic surprise.

The main advantage of multi-graft plants like fruit salad trees is convenience. Many people do not have room for several large fruit trees in their backyard, but would ideally like to harvest more than one kind of fruit. Different multi-graft techniques work best for different combinations of species. In general, the more closely related the plants, the more successful the graft. Getting a single tree to bear apples, oranges and bananas is probably too problematic a goal to come to fruition. That's why the Wests' fruit salad trees are fusions of related stone fruits or related citrus fruits, but not a mixture of fruits from different botanical families. Other nurseries offer similar multi-graft trees. Ison's nursery in Georgia sells a multi-graft stone fruit tree that produces peaches, plums, nectarines and apricots. Yamagami's nursery in California has multi-graft apple and cherry trees. And Stark Bro's in Missouri sell a 2-N-1 pear tree. Nurseries and hobbyists also graft different types of cacti. Some farmers and gardeners have created pomato plants, which grow potatoes underground and tomatoes above ground. Potatoes and tomatoes might seem very different based on appearances, but they both belong to the genus Solanum (genus is the taxonomic order just above species).

Although grafting woody plants, like fruit trees, is an ancient horticultural technique, grafting soft-stemmed vegetables is a much more recent agricultural practice. Perhaps nurseries will soon start selling mixed vegetable shrubs alongside fruit salad trees. Brassica oleracea seems like a particularly good candidate for such an experiment. This one species includes cabbage, broccoli, cauliflower, Brussels sprouts and kale. Yes, all these plants are cultivars of the exact same species—their appearances and characteristics have been altered through artificial selection over the generations, in the same way people have created so many different dog breeds. A broccauliflower sprouts plant sounds particularly delicious. Maybe it's time for a family reunion.

The views expressed are those of the author and are not necessarily those of Scientific American.

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