What is the difference between grafts, air layers, seedlings and tissue cultures? We are often confronted with this question in one form or another at the nursery and plant shows. Seedlings, air-layers and grafted fruit trees, are all propagation techniques that will help a grower produce plants. Understanding these techniques, incorporating their knowledge back into general culture and furthermore learning how to use them in a homegarden or homestead setting will allow you to improve the level of food security and connection with your surroundings.
Grafting, air-layering, sowing seeds and tissue culturing are methods of plant propagation. When a person is looking to grow a certain tree or plant they will either grow it themselves from scratch or source it from a nursery and it will be produced through any of these methods. Before diving into the details of each method we would like to go over some concepts, most of the information in this article is directly related to food/fruit tree propagation but applies equally to other categories of trees and plants.
What is a variety of fruit tree? There are hundreds of varieties of mangos, avocados and bananas, for example. This means that within these groups of plants there are different looking, smelling and tasting fruits. Although plant taxonomy is more complex, for basic understanding, flowering plants could roughly be organized in a hierarchy of families, genera, species and varieties. Varieties of one fruit tree for example, belong to one species making them equal but depending on each variety’s genetic makeup they exhibit different characteristics or traits, which translates into different shapes, colors, flavors aromas and textures.
How is a new variety made or determined? There are a few ways a new variety is selected, some are intentional others accidental. New varieties can be selected from randomly planted seedlings of known origin or they can be selected as a result of a hybridization program were usually both parents are known or lastly new varieties can be named from randomly occurring plants of which there is no planting and parentage record but which exhibit desirable traits.
There are multiple factors that affect the genetic makeup of a seedling, for example which other pollen was brought to the flower by pollinators and effectively fertilized the ovary of the fruit in question. Therefore it is very hard to tell what kind of growth habits, shape and flavor a fruit from a random seedling may have before it sets its own flowers and fruit.
Instead, if at least a portion of the parentage of the seed is known one may be able to have some idea of the traits that will be carried over to the next generation. This is why it is a common cultural practice to plant seeds from good tasting fruits and in such a way favor the selection of those genes that exhibit the traits that make up good flavor. However, and this is a big however, some plants will vary a lot when planted from seeds others will resemble their parent tree more. It is important to understand that without planting seedlings new varieties cannot occur. Most of the mango and avocado varieties we consume today exist because between 30 and 100 years ago people planted seedlings and waited for them to fruit to then evaluate the quality of the fruit. If the fruit fulfilled the expectations the variety was named and propagated further through grafting.
Hence, we have responded to one part of the question, growing fruit trees from seed. When one plants a seed from a really good fruit there is a large chance some of those genes will carry over to that seedling but just like when humans reproduce and make children the offspring is not identical to the parents, essentially meaning that when you plant a seed (of a fruit tree) there is no certainty about the quality and characteristics of the fruit the tree will produce nor the time it will take the plant to reach maturity. On the other hand, the more positive one, when you plant seedlings you may come up with a new variety which exhibits desirable traits. Planting seeds is important to ensure genetic diversity. There are some diseases that affect some varieties more than others. If only certain varieties are propagated through cloning there can be a risk of genetic uniformity. There are also some exceptions: some fruit trees grown from seed will be identical or almost identical to the parent tree, so it makes sense to grow them from seed.
On to the next portion of the question. Once a variety of a fruit tree is selected for XYZ reason one may want to keep this variety going over time. In other words, that fruit tree that became a selected variety will not live for ever, one may want to grow more of that specific fruit and it may want to be spread around the area, country or world. This is when grafting and air-layering come in handy as they allow you to produce an exact copy of a selected variety. Neither one of these methods modifies the genetics of the plants. Each method is used on different kinds of trees as some respond better than others to one of the propagation techniques.
Although grafting and air-layering techniques have other purposes in this article we will focus on three of them regarding fruit trees: #1 they ensure a specific variety of fruit tree is preserved over time; #2 they help reduce the time span for a fruit tree to start flowering and consecutively producing fruit; #3 in regards to grafting, certain environmental pressures can be overcome or balanced out by selecting suitable rootstock.
Air layering basically entails triggering a branch of a tree to produce roots that are detached from the ground. Once these roots have matured the branch is cut off with the root mass and planted in a container to root out. Through this process one obtains an identical clone: a fruit tree of the same maturity and characteristics as the mother plant. An important note to keep in mind is that air layered fruit trees do not have a tap root. A tap root is the main root of a tree that generally grows straight down and counter balances the above ground portion of the tree. Tap roots are important amongst other things for stability, trees without them may more easily fall over which is a hot topic in Florida. Air layering is a good propagation technique for bushy fruit trees or plants and also for trees which have an extensive root system and can overcome not having a taproot e.g. Longan.
Grafting, what a beautiful thing it is. I’m pretty sure you have heard the Frankenstein story: a person made out of pieces. Grafted fruit trees are made out of two parts: the rootstock and the scion which is obtained from budwood, graftwood. Through a procedure, these two pieces are put together. In the same way a surgeon is able to replace a piece of a cut off limb and connect all the important nerves, muscles and blood vessels, a grafter can join two pieces of a plant so that they can grow back together. It is important to remember that this is a cloning procedure, joining two parts of two different plants does not change their genetic makeup. It is also only possible to graft plants that belong to the same family, in most cases grafting is only successful to the species level, inter specific grafting amongst the same genus is not always possible. You cannot graft a lemon and a mango and make a lemang, that is science fiction and if attempted would require genetic modification which cannot happen through grafting.
So, where do the two pieces of a grafted fruit tree come from? The rootstock is a seedling that is purposefully planted to be grafted. In some cases particular types of seeds or varieties of a fruit are chosen to be planted as rootstock for their ability to better cope with certain growing conditions. The scion is a piece of a selected variety mother plant, in some cases it is a tip cutting in others it is a middle piece of a branch and in others it is a chip with a single bud; the most important thing is that the scion is cut out from budwood obtained from the mother plant at the correct time when the buds (cells that will develop into new leaves, branches or flowers) are plump and about to emerge.
And finally, the last portion of the question, what are tissue cultured plants? Simply put this method allows the production of plant clones made from a piece of tissue of a mother plant without the need of its support. In a sterile laboratory setting a small portion of tissue is placed in specific artificial growing medium which allows a clone/new plant to grow from it. Generally plants propagated through tissue culture are non-woody or herbaceous and are easily mass produced in this way. E.g. bananas, pineapples, blackberries, gingers. Another benefit of this propagation technique is the control and elimination of pathogens from the propagation process which otherwise could be transferred to the next generation through other methods of asexual propagation e.g. division, cuttings.