I’ve been teaching plant physiology or related courses for a long, long time, and one of the tenets is that woody plants don’t heal. In contrast to animal tissues, when trees and shrubs are wounded the damaged tissues are permanently destroyed. Wounds are compartmentalized and covered with wound wood. Arborists are fond of saying "plants seal, not heal."
That’s all fine and good for woody plant parts, but what about grafts? Since grafting reconnects cambial and phloem tissues, is this "healing?" And what about nonwoody plants, like annual flowers and vegetables?
Oddly, this type of information is sadly lacking in physiology textbooks, but it’s a question that I get routinely from gardeners. And it’s not just an exercise in semantics. People make some poor choices in treating tree wounds, for example, laboring under the false impression that such wounds should be treated with wound paint or bandages so they can "heal."
Excuse me for being a lay person, but I believe that the tenant you start with is not truly correct. I was taught that there are two groups of tissue in woody plants. The living outer layers, (i.e. bark, cambial, phloem , etc.) and the non living structural tissue (i.e. heart wood). A tree will bleed sap, which gums up and causes a scab like structure. If you scratch a soft Bark tree it will scar over and over time will be grown over and disappear. You’re right that heart wood (being mostly plant cell wall)will not heal. using this theory I would think most annuals and veggies would follow course. As for bandages,paints, etc. isn’t it more about the spores, microbes, etc. that potentially get trapped inside and the cosy home they provide for those organisms?
Gil, you may have a point, but as an arborist, I have had the “Trees don’t heal” mantra drilled into me for years. The gist is, there is no immune system, no regeneration of damaged tissue. Trees compartmentalize decay and, if all goes well, they eventually form new green tissue to cover the external wound. Paints not only have the potential to trap bad stuff underneath and incubate them, but also can interfere with the tree’s CODIT system, resulting in what’s called a ram’s horn, which is weaker than a branch that forms the typical donut ring of callus and eventually seals the wound.
As for grafts and non-woody plants, I don’t know the answer, but I’m inclined to say it’s still not healing–still no immune system, still no regeneration. Though I know an arborist who has been advocating for a new world view for years. He says we should be experimenting with things like grafting the two sides of a crotch with included bark by damaging the bark where the two sides touch, in hopes that the callus tissue will fuse together. I have doubts that a.) the grafted crotch will be significantly stronger and b.) that this would be legitimately considered “healing” but it seems to coincide with LCS’s ideas above. Despite my doubts, I am very intrigued and all for such experiments.
Even Though Ms. (Prof.) Chalker-Scott says it’s not a question of semantics, it all comes down to the definition of the word heal doesn’t it. I ask you to check out the following web site: http://www.csrees.usda.gov/newsroom/impact/2008/nri/09161_tree_scabs.html
I would also remind you just because It’s been “taught” for years doesn’t make it true. If it were we might still be living on a flat world.
As for plants having an immune system, again a questionable point try this web site:
http://www.nature.com/nature/journal/v444/n7117/full/nature05286.html
Let’s clarify a few points first. Gil, living tissue in the form of ray parenchyma penetrates the heartwood of trees and is connected to the other living tissues you mentioned. So the heartwood is not “dead,” though the xylem elements in it are. The material that “bleeds and forms a scab like structure” includes callose (a specialized sugar) and various sticky compounds found in resin ducts and elsewhere. Often these include antibiotic chemicals to kill insects or microbes. Scabs that form over animal wounds are chemically dissimilar. And scabs form over the top of the wound, period, while resin, etc. flow and cover whatever surface they meet. They may have some similar properties, but they are not the same thing.
Further, plant physiologists do not consider plants to have an immune system in the same sense that animals do. Plants do not have T cells or B cells or lymph nodes. They do have defensive responses, however, including hypersensitive responses (their reaction to foliar fungi, for example) and systemic acquired resistance.
All that being said, it’s the difference between healing (actual replacement of damaged tissues with the same type of tissue) and sealing (isolating and covering wounds with a different type of tissue) that I’m curious about. Are the tissues that form in a graft the same as those that were damaged? I would assume yes – but I don’t know. Which is why I’m hoping some other GP type familiar with grafting can weigh in here.
Again, it’s more than a semantic argument. The danger is when people think that plants have the same physiological responses to stresses that animals do, they tend to make plant care decisions not necessarily in the best interest of the plant.
What would you think about covering a tree wound with resin from a different tree? I am thinking of covering a japanese maple wound with douglas fir resin. Maples tend to get infected with fungal diseases when pruned
VG, I would advise not covering wounds with anything. Trees generally have their own mechanism for wound closure, and oxygen is important for that process to work. When you prune, make sure you disinfect your pruners and/or saw with a household cleanser like Lysol or Pinesol or rubbing alcohol. This will reduce the likelihood of bacterial transmission.
What about the practice of using white glue over the stubs of pruned roses to deter borers?
Without getting into our previous posts I offer to you the following book passage and Journal articles. Though they don’t mention “grafts” specifically, They talk about xylem and vascular cambium generation after a wound. They might answer your questions or they might just raise more questions, I’ll let you decide.
All of these you should be able to pull up with a google search.
“Physiology of woody Plants” By Stephen G. Pallardy, Theodore Thomas Kozlowski copyright 2008
pages 67-68
“Tree wounds and wound closure” by Dan Neely Journal of Arborculture Vol.5 #6: June 1979
Page 135
“Wound healing in bark of woody plants” by George Hudler Journal of Arborculture vol. 10 #9 Sept 1984
and finally a quote from Alex L. Shigo (who of course agrees with you when it comes to the definition of Healing; and as the Father of Modern Arborculture I guess I’ll defer to him for now);
“Humpty Dumpty said a word means only what he wants it to mean. Socrates, a great philosopher, said, just tell us what you want your word to mean. And Voltaire, another great thinker, said, when we know what you mean by your words, arguments and misunderstandings will seldom happen. So be it. ” (http://www.treedictionary.com/DICT2003/shigo/TEP.html)
This is a great conversation being had here. As a physical therapist I have been getting very interested in plant healing. It interesting to be able to comprehend how we function similarly and differently.
Of course, it would be silly to every say that humans/mammals function the same as plants; but, it is interesting to draw relatable parallels. I say this because in the context of grafting, it would be similar to operations on humans with grafts and transplants. Even in this case, human cells still do not recognise these as original parts, but will be used to help the circumstances. Any healing done in humans will always leave traces where there is less optimal function than if the wound had never been created
Does anybody know of a good reference (website, book, research articles or anything) that goes into the details of how plants (not just woody based) heal?
Yes plants can “heal”. Not all plants “heal” and it depends on what is attacking them. Unlike animals plants do not repair cells in place, they produce new cells in new places and let the damaged ones die. The ability of plants to “heal” is dependent on many factors and in some cases they can’t and wont “heal” because the process they are involved in is fatal. There is no therapy for many diseases. It is hard to speak in these broad generalities because the discussion is unique to each plant/pathogen interaction. Varieties of plants also have varying resistance to disease agents and that is another discussion. Sometimes plant defense can be induced by another organism, a chemical agent or the environment. Sometimes plant defenses are constitutive or built in.
I hope people really focus on the second sentence of Jim’s answer. Plants do NOT form scabs that fall off after the underlying tissue is replaced as in animal healing. That’s why we talk about sealing rather than healing – because the plant does wall off tissues to prevent pathogens from spreading. We avoid the term “heal” because people automatically think it’s the same as in animal tissues.