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KEN MUDGE: Now let's consider grafting as one of the propagation techniques that's commonly used today. You recall, this is a process of fusion rather than the regeneration of new organs.
Now a couple of years ago, Swissair ran this ad in Time magazine. The guy there sitting in first class is musing, I wonder how they grow seedless grapes. You've probably figured it out because this is the section on grafting. The answer is grafting.
Grafting is an amazing art. It goes way, way back. There are many, many different kinds of grafting you can see here on your right. In fact, Liberty Hyde Bailey, the father of horticulture at Cornell, back in 1928 in his cyclopedia of horticulture, he put it very nicely.
"The ways or fashions of grafting are legion. There are as many different ways as there are ways of whittling. The operator may fashion the union of the stock and scion to suit himself, if only he apply cambium to cambium, make a close joint, and properly protect the wood."
I particularly like this quote, and I use it in class because he touches on three of the four essential criteria for successful grafting and formation. And they are alignment of the vascular cambium of the stock and the scion so the wood will grow together properly. Make a close joint refers to the pressure and alignment that's necessary so that the cell division will begin leading to callus formation and eventually xylum wood formation. Properly protect the wood refers to the environmental management that is so critical after the graft has been made.
As I mentioned, grafting is an ancient art. I was surprised a couple of years ago to see in Madagascar grafting being performed as it probably was hundreds of years ago. Here's a shot, on your right, of a grafted citrus tree, probably a lemon. And instead of wrapping the grafted area, the graft union between stock and scion, with special waterproof grafting tape or similar materials that we use today, once again, they used a ball of mud and dung to wrap off the union.
Now the question is, why graft rather than root from cuttings. Let's consider the reasons for grafting.
First of all, clonal propagation of plants that are very difficult to root from cuttings is a very important reason. You may or may not know that the shade trees in your neighborhood, many of those are grafted. Oak trees-- not so much oak-- but maples, ash, honeylocust, and so forth are all very difficult to root from cuttings. And to the extent that there are selected clonal cultivars available, those have to be grafted to a seedling root system. You simply can't root them from cuttings.
Another important reason for grafting is to create special growth forms. For example, you may have seen weeping cherries and tree roses. Take tree roses as an example. A rose is-- most varieties grow low as a shrub. But in order to get that shrub up off the ground and behave like a tree, what they do is they grow a special rootstock variety, train it so it's tall and straight, and then graft the bush growing rows up on top of that stem at about four or five feet. And then you get the bushy rose developing right on top of that long, straight underside.
Now another example of using grafting to create specialized growth forms is the creation of tree sculptures or arboric sculptures. This is a really remarkable, rather bizarre use of grafting. It involves creating special sculptures by use of grafting. You can see some examples on your right. Richard Reames, for example, who does this for a living in California, creates tables and chairs that are basically grafted together. Rather remarkable sculptures are sometimes made.
The third reason for grafting that I'd like to cover today-- of course, there are many others, but the third one for today-- is grafting to take advantage of specific rootstock effects. In other words, independent optimization of the characteristics, the genetic characteristics of the upper and lower part of the plant.
This allows modern horticulturists to breed apples and other fruits, for example, for the best characteristics of fruit taste shape, color, and so forth and not have to worry about breeding in the ideal root stock characteristics. But instead, other genotypes intended for use as root stocks are bred for characteristics that make them optimal for performance in different soil types.
In most cases, for example, those seedless grapes, when we asked, why do they grow seedless grapes, and I said the answer was grafting-- you could also, of course, root those grapes, seedless grapes from cuttings. As I indicated earlier, that's how grapes became domesticated.
But the reason for grafting them is that there are certain root stocks, American rootstocks, that are resistant to a pest called phylloxera. Vinifera, or French grapes, on the other hand, that are used to produce wine grapes and so forth, are highly susceptible to phylloxera. So if they were on their own roots as rooted cuttings, they would be attacked by the phylloxera pest. But when those vinifera grapes are grafted onto American rootstocks, species Lambrusca, they are resistant to the pest.
Now we want to talk about the use of grafting to achieve special, specific rootstock effects in the case of apple, because we were talking earlier about apple layering. There are apple rootstocks, clonal apple rootstocks propagated by mound layering that have been selected for various characteristics like tolerance to diseases and pests and cold soils and so forth.
But one of the major selection criteria for rootstock production has been to select rootstocks that can control the rate of growth of the scion, or the upper portion of a grafted plant. Some rootstocks cause extreme dwarfing of the scion so that the final size of the apple tree is only three or four feet tall. Others either can give an intermediate level of dwarfing or size. And still others can result in a tree that's essentially as tall as it would be if it wasn't grafted. That would be, say, 20 foot tall or more in the case of a mature apple.
This diagram shows an example of some of the more commonly used root stocks and the range of size control that they can provide when they're used as rootstocks for varieties like Macintosh, Mutsu, or whatever. M27, for example, causes extreme dwarfing, a tree that's only 25% the size of a tree that was not grafted. And MM111 results in only about 15% dwarfing. It's a rather large tree.
And this is all controlled by the genetic makeup up of the understock, not the genetics of the scion.
Now let's consider the apple again, because it's I think a wonderful example of the use of grafting in modern culture. It began, as we already discussed, with mound layering in order to clonally propagate the rootstock.
And that rootstock is mound layered, and the rooted layers are harvested in the fall, placed in cold storage, and the following spring they're lined out in the field. They're may be six or eight inches tall. They're allowed to grow through that growing season, and by the time late summer to early fall comes along, they're ready for bud grafting. And bud grafting is simply the use of a very small scion piece that has only a single bud.
Now after the plant is bud grafted in the late fall, it's allowed to overwinter in the field. And the following spring, as things just begin to push, the upper portion of the rootstock plant is cut back just above the newly inserted bud. And that forces the new bud graft into growth. It grows on for the rest of that growing season till it's four to six feet tall, even taller on the west coast. It's dug in the fall, placed in cold storage, and shipped out on trucks for use by orchardists.
Now I'd like to demonstrate the bud grafting technique that is used for apples. We have here in the greenhouse a couple of apple trees that have been here since the fall when we brought them in.
And right here is an example of a successful bud graft. This is a T-bud, and you can see the union between the stock and scion. And that little line there demarcating the two is basically callus. Callus refers to cell growth that bridges the stock and the scion, and eventually new wood and phlorem form across that callus.
Now the history of this particular bud graft is rather interesting. At the National Bureau of Standards, they have a tree that is a clone, a descendant, apparently, of the original tree from Sir Isaac Newton, apple hit his head and he discovered gravity. This tree was cloned, apparently, and taken to the National Bureau of Standards.
Recently we got some scion wood from that very same tree, brought it up here, and grafted it onto these apple rootstocks. And so when this thing begins to grow later on, we'll have a clone, genetically identical individual of Sir Isaac Newton's tree.
Now let's pick another one, and I'll demonstrate the process. I think I'll move to this plant right here. We want to find-- let's assume this is the understock of the grafted plant, that clone understock that's been selected for size control and whatever other characteristics. We find a zone between two existing buds, and we begin with a cut from the top down, down into the wood, not very deep into the wood, but slightly below the bark and into the wood.
And then we make a second cut at an angle so we can liberate a piece and create a space for the scion that we're going to take off our selection that's been made for fruit characteristics like Macintosh and so forth.
So I'm going to use this same plant. Let's just assume now that this was a Macintosh tree or something. And we go to a portion now that has a good bud. And I think you can see the bud here. And we basically perform the same operation, cutting down from the top and behind the bud itself. And then we have to cut a piece that is as close as possible to the same size as the piece we took out of the understock.
so There's my bud piece. You can see the bud right there. Now we're ready to place the bud scion that we've just cut from the scion donor plant, like a Macintosh apple tree, for example, place it into the space that we've cut out of the understock. And this fits pretty well, even though I'm kind of out of practice.
Then we take a latex rubber budding band, and we tie it from the bottom up, capturing the bottom end of that latex band and wrapping, overlapping slightly as we go to create a sort of shingle effect, careful not to wrap the bud itself, to leave a space for it, continue to wrap until we get the entire core graft covered. And we can pull out a section like that, slip our free end through, and pull the thing tight.
And here we have created the cambial alignment BY lining up the two pieces very carefully. That's why they have to be the same size. We've also created the necessary pressure. And to some extent, this latex band is impervious to water so that it protects the joint from drying out, which is part of that environmental control that I was talking about.
Now let's consider one more propagation technique, and that is micro, propagation.
In this Room I share with you my fascination with plant reproductive biology and its application to horticulture and related disciplines.
I begin by dispelling the widely held oversimplification that "plants grow from seeds" - indeed many of them do, but quite a few have evolved the capacity for asexual (clonal) reproduction. Even before the origins of agriculture, about 12,000 years ago, mankind has been observing wild plants performing feats of asexual reproduction.
From this increasingly sophisticated understanding of the natural history of cloning, early agriculturists domesticated a number of fruit, nut and other food crops and eventually a host of ornamentals as well. The Room includes hands-on demonstrations of clonal propagation by layering, cuttings, grafting and micropropagation.
This video is part 5 of 7 in the Natural and Human History of Plant Cloning series.
