James and the Giant Corn Genetics: Studying the Source Code of Nature

October 20, 2009


Filed under: agriculture,Plants — Tags: , , , , , , — James @ 1:13 am
Grafted Apple Tree

Grafted Apple Tree

Imagine if all it took to replace a lost leg was to put another leg against the amputation site, tie up the wound and let the two grow together. We can do that with plants! We’ve been doing it for thousands of years and it is an important part of crops production for woody plants (think fruit trees).

The technique is called grafting and it really is almost that simple. A branch or stalk* from one plant is cut and attached to another plant of the same or a related species**, making sure to line up the vascular tissue*** of the cut branch and host plant. That connection is covered with grafting wax or grafting compound which keeps the exposed ends of the cuts from drying out which would stress or kill the cells.

Plants have to know how to regenerate after a wound. Consider all the things that can damage the living layer of a tree (just under the bark). From insect attacks to fire, from branches breaking under the weight of snow to woodpecker nests. Trees will at least attempt to heal all these wounds. That’s what has happened when you see a lump in a tree where a branch broke off.  The living tissue that produces the bark re-grew over the wound. When you see exposed wood, surrounded by bark, the tree either hasn’t had enough time to heal, or wasn’t able to heal completely.

If you’ve done everything right, both the cut branch and the host plant consider their new neighbor to be just another part of the same plant. As they heal the wounds inflicted on them by whoever performed the graft, they will connect their vascular systems. Circulation in many ways makes the organism. The newly grafted branch will grow leaves and photosynthesize, sending sugars down to support the roots, and the roots will in turn supply water and nutrients to the branch. From pieces of two separate living things, a single chimeric**** one is made. A grafted plant will grow, flower, and give fruit quite happily, unconcerned by its macabre origin.

Why go to the trouble? Grafting is a lot of work, doesn’t have a 100% success rate, and if you look at it from the wrong point of view it can seem creepy. (I think it’s awesome, but I think most things we can do with plants are awesome, practical or not). But for people who don’t share my automatic love of weird plant things I’ll give four answers for the price of one.

  1. The Best of Both Worlds. Instead of breeding one strain to be perfect, a plant breeder can specialize his or her breeds. One can be bred for vigorous, cold and disease resistant roots. The other for tasty and delicious fruit. After the breeder has both he or she can graft a cut branch from the tasty tree onto the base of the hardy tree. After the graft takes, the breeder will cut away all the above ground shoots of the host, leaving only the tasty one to grow. The result is a tree with the hardiness and vigor of the host, called a rootstock, and the tastiness of the tree the branch was cut from. Grapes are a great example of using root stocks to provide disease resistance and climate tolerance while maintaining old flavors, (and no grapes are not a tree but a woody vine). Many of the grapes grown around the world today are old breeds of European grapes that produce the various favors of wine western our culture is accustomed to, grafted on to rootstock from a separate species native to North America which provides resistance against phylloxera, an insect that devastated vineyards around the world. (Yes, disease resistance in the rootstock can sometimes provide protection for the entire plant, and no, I have no idea how it works, but I’m sure others do.)
  2. Saving Rarities: Maintain a type of crop that either can’t reproduce (for example seedless orange) or produces offspring very different from the parent (breeds of apple). All seedless navel oranges originated from a single mutant tree in Brazil almost 2o0 years ago. Pieces of that tree were grafted to other orange trees and the grafts also produced seedless fruit. Every seedless orange you’ve ever eaten is a clone, genetically identical to every other. Wikipedia puts the situation in a different but also accurate light and says “all navel oranges can be considered to be the fruit of that single over-a-century-old tree.” Apples have more diversity. In fact they have too much. The offspring of two delicious breeds of apple might be grow into a healthy and resilient tree that produces heavenly fruit, but much more likely it will be a lesser tree than either parent and give fruit of little distinction. Apple growers could plant dozens more trees than they need and then, once the trees are old enough to fruit (2-5 years), select the strong looking trees that also produce the tastiest fruit. But it’s much easier and more resource-efficient to graft a cutting from a tree known to produce tasty fruit onto a rootstock known to be be hardy and resilient. Every red delicious apple is genetically identical to every other red delicious apple, and every cripps pink is identical to every other cripps pink.
  3. Controlling the plant from beneath the ground. Rootstocks that grow bigger or smaller can control the size of the whole plant. Genetically identical macintosh apple trees can be either huge or dwarfed depending on what root system they were grafted onto. Dwarf trees obviously are easier to harvest, and it turns out they often also produce more fruit per acre than bigger trees.
  4. Novelty/Conversation pieces. If a single apple tree giving a diversity of different breeds of apple isn’t exotic enough, consider that the various species of citrus trees can be grafted with one another. A gardener or hobbyist can create a tree that produces lemons, oranges (seedless or otherwise), and grapefruits, each from a different branch. Or if vegetables are more to your interest, perhaps you’d be interested in growing your very own tomato AND potato plant. <– Keep in mind there is no such thing as a free lunch. The grafted plant will produce both potatoes and tomatoes, but less of either than a normal tomato or potato plant would.

So that’s what grafting is and why it is such a valuable tool. The techniques have been around for millennia, yet even in the late 1800s Luther Burbank (creator of the Russet Potato, the Iceberg blackberry and dozens of other awesome plants) ran into resistance against his work with grafting by those who considered it unnatural. To quote another man who lived in that century

“They can improve the apple in that way, but that is only a device of man, and it is wicked to cut up trees that way. The correct method is to select good seeds and plant them in good ground, and only good can improve the apple.”

The man who spoke those word was John Chapman, known to school children across the country as Johnny Appleseed. While John’s name is known to millions across the country, and Luther Burbank is remembered almost solely by those of us who study plants, it was the idea championed by Burbank that is used, without concern, across the globe.

Think of grafting the next time you bite into a crisp, perfect, apple that tastes just like the ones you remember from your childhood, or enjoy the flavor of a seedless orange, or take a sip of a wine made from grapes not grown in Chile or Australia.

*Sometimes even a single bud will be grafted. Each bud of a plant contains a meristem, a lump of plant stem cells that can reproduce and differentiate to product other meristems, stems, leaves, and eventually even flowers.

**For more on the various techniques used for this, check out this great resource from North Carolina State

*** Cells that move water and sugary sap around the plant, like the arteries and veins of our own bodies.

****Chimera means a single living thing containing two genetically distinct types of tissue. So a person who has received an organ transplant can be considered a chimera, as can conjoined twins. But this post is about plant ones. The name comes from a greek myth about a creature with the head of a goat, the body of a lion, and the tail of a snake.

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