A Closer Look at CLONING
September 2013 | Written by:
Eric Hopper | Garden & Greenhouse Magazine
For years commercial horticulturists have been taking advantage of the multiple advantages offered by asexual propagation. These advantages are now being reaped not only by the commercial growers but by hobbyists as well. One of the largest advantages of asexual propagation is the reduced time it takes for a plant to reach maturity or get to a sellable size. This has obvious advantages for the commercial grower but the advantages extend to the hobbyist as well. The faster the plants can reach maturity, the longer the hobbyist can literally enjoy the fruits of his or her labor. Asexual propagation also gives the gardener the ability to create identical replicas of the most prized plants. This can be especially advantageous for the vegetable grower who wishes to replicate the plants with desirable traits. The best tasting tomatoes or crispest cucumbers can be perpetually grown and cloned in a hobby greenhouse and enjoyed year after year. Aside from flavor and aroma, plants can be cloned to preserve other desirable traits like a heightened resistance to stress or disease. Cloning by taking cuttings is just one of the asexual propagation techniques used by horticulturists. Although cloning by taking cuttings is the most common asexual propagation technique used by the hobbyist, the number of hobbyist gardeners who are taking on the more complex asexual propagation techniques, such as tissue culture cloning or grafting, is steadily growing.
As previously mentioned, cloning by taking cuttings is the most popular asexual propagation technique for hobbyist gardeners. This technique is very effective on a wide variety of plants and is also relatively easy to master. The process of cloning some varieties of plants may only consist of cutting a small branch off the plant and placing the stem in water. After a few days roots will begin to form and the plant can be transplanted into soil or another desired medium. The once part-of-a-plant becomes a plant itself; a genetic duplicate of its donor plant. Spider plants, wandering jew, and coleus are a few plants that clone very easily in straight water and with very little effort.
However, not all plants will root as easily as a coleus. Other plant varieties require a little more attention to obtain a high propagation success rate. These varieties usually root better with the aid of a rooting hormone and more precise control over temperature and humidity. Tomatoes, peppers, cucumbers and many other vegetable varieties clone best in a consistent environment. Geraniums, hibiscus, and some African Violet varieties will root much faster and at a higher percentage within a consistent environment. When cloning these more finicky plants, try to keep the temperature consistently between 72-85 degrees F, especially in the root zone. A seedling heat mat may be necessary to ensure a consistent temperature during the night hours. The optimal humidity for clones during the first stages of propagation is 80-100%. After the plants create their own roots they can be acclimated into the ambient humidity. Humidity domes placed over the propagation trays are a great way to control the higher humidity needs of clones and seedlings without affecting the ambient humidity of the greenhouse. Clones that are kept in a consistent environment will root faster and also have a higher overall success rate.
Tip for the Hobbyist
When taking a cutting off a plant, it is best to do so just above a node site (a site where a branch occurs). A 45 degree angle cut just above a node site is my preferred method for most soft stemmed plant varieties. After cutting, place the clone directly into a rooting compound or water. Not only does this method create healthy clones, it also promotes the donor plant to multiply its shoots.
Tissue Culture Cloning
Tissue culture, in horticulture, refers to the replication of a plant from its cells or tissue which are grown in a nutrient culture medium under sterile conditions. Basically, tissue culture is like growing a plant in a petri dish from just the tiniest slice of tissue. Most plant varieties can be successfully duplicated with tissue collected from the root, stem, leaves, seed, flower, or virtually any other part of the plant. Sterile conditions are crucial for the tissue culture technique because living plant materials are naturally contaminated with microorganisms. Tissue culturists must sterilize the starting plant material in a chemical solution, usually consisting of alcohol and sodium hypochlorite. After sterilization, the starting plant material is placed in a sterilized container which contains the nutrient culture medium. The nutrient culture medium consists of a correct balance of plant hormones (auxin and cytokinin) and a nitrogen source. Too much auxin will result in excess root growth, whereas too much cytokinin will result in excess shoot growth. Once the specimens grow large enough, they can be transplanted into potting soil or other desired medium to grow like normal plants.
Tissue culture offers the horticulturist many different advantages. Like conventional cloning, tissue culture offers the ability to create large numbers of genetic copies of a desirable plant. Unlike conventional cloning, tissue culture can be used as a way to preserve rare or endangered plant species and can also be used to rescue embryos in distantly related cross pollinated species. The biopharmaceutical industry uses massive bioreactors filled with tissue cultures which can produce valuable compounds used as biopharmaceuticals. Another huge advantage of tissue culture is the ability to “clean” plant material of contaminates. In other words, a plant that has developed a disease or virus can still be propagated without bringing the pathogen along with it. This advantage alone provides an invaluable asset of preservation to modern horticulture.
Tissue culture kits are available for hobbyists to use. There are even some recipes online for creating tissue culture kits out of mostly around-the-house items. Because sterilization is so important, hobbyists who want to experiment with tissue culture need to have a dedicated area. Although tissue culture may be a fun experiment, other asexual propagation techniques are more practical for the average greenhouse hobbyist.
Another form of asexual propagation that has been used for many years by commercial growers is grafting. In horticulture, grafting is a technique where the tissues of two plants are joined together. The two plants used are known as the rootstock and the scion. The rootstock, appropriately named, is the root portion of the plant. The scion is the portion of the plant with the desired genes, i.e., flowers, fruit, leaves, stem. For successful grafting to occur the vascular tissues of the rootstock and scion must be in contact with one another and both must be kept alive until the graft has taken. This process can take a couple of weeks or a couple of months depending on the plant variety.
Grafting offers many advantages to the horticulturist. One of the biggest advantages is the ability to produce fruit without completing the juvenile phase. Most fruit trees go through a juvenility period of 5-10 years in which they produce little or no fruit. By grafting mature branches onto young rootstock, many trees can begin fruiting in as little as two years. In fact, many of the fruit trees for sale at local nurseries are grafted to reduce the time until fruiting. Different varieties of the same fruit can be grafted on to a single rootstock which can create some pretty cool fruit trees. Apple trees bearing multiple types of apples demonstrate the amazing benefits of grafting. Dwarfing is another reason grafting is used by the horticulturist. Most of the apple trees found in orchards are scions grafted on dwarf trees to limit size and maximize fruit density per area. Grafting may be the only easy option for propagation of some plant varieties. Many of the plants used as scions are very difficult to propagate by conventional cloning and grafting is the most practical way to produce replicas.
It’s easy to take for granted the impact asexual propagation has had on the horticultural industry as a whole. The next time you are browsing through your local nursery or home improvement center you may realize that the uniform plants of each variety that all look exactly the same are, in fact, exactly the same. They are all genetic duplicates of their donor plant and were created by one of the asexual propagation techniques. When you bite into an apple purchased from your local orchard or grocery store you can think, “I bet this apple came from a grafted apple tree.” And guess what? It probably did. Asexual plant propagation is all around us. Hobbyists can use these techniques to their advantage and reap the same benefits once obtained only by the commercial growers. Hobbyists can preserve the desired specimens of their greenhouses and save money by producing their own perpetual generations of plants. They can share plants with special attributes with their families and friends. Hobbyists can create a network of plant share programs that will allow horticulturists to clone and share their prized plants with one another. By mastering the art of asexual propagation, a greenhouse hobbyist’s plant numbers are unlimited and the garden becomes infinite.