The Science and Craft (and a Little Magic)
By Guy K. Ames, NCAT Horticulture Specialist
People practiced the craft of grafting well before they understood the science. Grafting dates back to at least 1,000 B.C. The ancient Chinese, Greeks, and Romans all knew about grafting as witnessed by their written records. But much of the underlying science that explains the efficacy of grafting was not at all or incompletely understood.
They well understood the reason to graft, however! When humans began to domesticate certain crops, beginning with annuals like grains and most vegetables, they could reliably reproduce the desired type and traits of a species through seed propagation. They could further improve (for their purposes) such a species by collecting seed from superior plants. Like begat like. However, when these ancients tried to reproduce a particular perennial crop, like apple or pear, from seed, the result was often a confounding array of fruits. Very few of them closely resembled the fruit from which the seed was taken! Who first took the mental steps that led to the joining of woody shoots from one plant to another is lost in time. It must have seemed magical then as it still somewhat does today.
The Science of Grafting
Scientists—in this case, geneticists and plant breeders—today describe apples, pears, and many other perennial fruiting plants as “extreme heterozygotes.” This label explains little by itself. But I hope it will be enough to say for this blog that modern plant breeders understand enough of the genetics to confirm the ancients’ conclusion: seed won’t suffice. We have to graft (or otherwise clone) apples and pears in order to maintain the desired traits of a particular tree’s fruit.
The ancients figured out, probably quickly, that you could preserve the traits of a particular plant by grafting a piece of it (the scion) onto another plant (the rootstock or stock). However, there were precise limits to what that other plant could be. We know now that grafting, with rare exception, only works within a botanical genus; i.e., the scion and rootstock have to be closely related.
In fact, scientists have used such graft compatibility or incompatibility to demonstrate relatedness among species. It has even prompted the re-categorization of some species. For instance, the Asian cudrang (also known as the Chinese mulberry), was once dubbed Cudrania tricuspidata. However, its Latin binomial was changed to Maclura tricuspidata when it demonstrated graft compatibility with Maclura pomifera, the common American bois d’arc tree. Likewise, the muscadine grape’s former Latin binomial was Vitis rotundifolia. It was recently changed to Muscadinia rotundifolia partly (there were other botanical reasons) because grafting between muscadines and bunch grapes (Vitis labrusca, V. vinifera, et al.) is virtually impossible.
So, we must graft apples onto apples and pears onto pears. But without grafting at all there would be only one ‘Honeycrisp’ apple tree, and wouldn’t that be a shame?
A Side Note On “Stooling”
For millennia, people grafted scionwood from a particularly valued apple variety onto apple seedlings or onto another, already-established apple tree. It wouldn’t matter where the seed came from as long as it was in the apple genus Malus. The seedlings could even be from crab apples. In the 20th century, English researchers started exploiting the diversity in an apple seedling population for size and other genetic traits. They discovered that using certain individual apple seedlings as rootstocks produced fairly small apple trees (dwarf). The trees had other desirable traits like coming into bearing early in their lives (precocious). The researchers then learned how to clone these rootstocks by “stooling,” which is NOT grafting, so I digress. Still, clients and customers often ask me about “clonal rootstocks” like dwarfing and semi-dwarfing rootstocks, so I wanted to say something! Perhaps this is a topic for another blog.
The Craft (and More Science) of Grafting
The craft of grafting relies on the scientific truth that the cambium of the scion needs to be in close, tight contact with the cambium of the rootstock. In order to maximize that contact, it is advantageous that the cut surfaces of both scion and stock are smooth. Ragged surfaces would reduce the contact between the two.
When the grafting knife wounds the scion and stock—and if the temperatures are warm enough to prompt some plant activity and the graft union doesn’t desiccate (that’s why you tape up the union)—both scion and stock respond to the wounding by producing callus. The callus is a mass of undifferentiated cells. In this case “undifferentiated” means that such cells have no programming for a special purpose like transporting water (xylem) or transporting nutrients (phloem) in the plant’s cambial tissues. It might help the reader to think of the callus as analogous to a scab that your skin produces when it is wounded.
So, the callus from both scion and stock proliferate and they begin to knit together. Once knit together and given a little time, the callus cells begin to RE-differentiate into xylem and phloem. Much the same thing happens with a scab on your skin. After some time the tissues begin to redifferentiate into epidermis and subdermis and it regains the character of your skin!
A Little Magic
It is this last-described step that sometimes begs me to say “magic!” Well, depending on what one means by magic, I would argue that it is not. Still, it is this last step that sometimes fills me with awe if I think about it and bother to ask the big question, “Why?” “How” is a pretty awesome question here, too! How does the plant know to turn this cell into xylem and this one into phloem so that the vascular cambium provides the all-important, life-giving functions of the plant? Well, it’s not magic, but it fills me with awe as if it were!
Learn More About Grafting
Find the details—including photographs—of the craft of grafting in ATTRA’s budding and grafting fruit trees webinar here: https://attra.ncat.org/budding-and-grafting-fruit-varieties-for-organic-production/. Also, Guy and his son Dagen will conduct a grafting workshop at the Southern Sustainable Agriculture Working Group’s (SSAWG) annual conference in Little Rock, AR, January 23, 2020. Find more information about the grafting workshop here: https://www.ssawg.org/2020-mini-courses#MC1.