Producing “New” Plants—Or More Old Plants
The amount of genetic (inherited) variation within a given plant population varies. Variation is necessary so plant populations can adapt to changing environments. The variation might produce plants with unusual colors or markings in their foliage or flowers, height or shape differences, or plants with greater ability to withstand environmental stresses. In the never-ending quest for plants with bigger, better, or more of something, breeders often choose to start by selecting unique individuals from a population. This is often the preferred method of “discovering” new landscape-worthy herbaceous plants, particularly in native populations, where it is desirable to maintain the plant’s inherent ability to thrive in its environment.
Selecting the one unusual individual from a population, collecting the seed and growing the offspring; doing the same thing time and again, with each successive generation moving closer to the plant that displays the sought-after characteristic, eventually results in a plant that will produce a significant number of offspring with the “new” or desired characteristic. Before they will release the new plant into the horticultural trade as a named variety, good breeders make sure the characteristic is reasonably stable by testing it in many locations. (Fig. 6) This process of selection and reselection can take years. In fact, eight years passed between the discovery of a single Penstemon digitalis with red foliage and the release of Penstemon digitalis ’Husker Red’ by Dr. Dale Lindgren seen in (Fig. 7). Another penstemon example, Dalea villosa ’Sandhills Satin’, also selected by Dr. Dale Lindgren, took thirteen years to perfect.
Continued seed propagation of “new” plants developed through the selection process still results in plants that do not exhibit the desired characteristic, or do so only weakly. Occasional seedlings of ’Husker Red’ penstemon will produce pale pink flowers, not white ones, or foliage that is more green than burgundy. This reversion can result in less than 100% uniformity in a seedling and may ruin a planting. The result could be a dissatisfied customer if the plant doesn’t look like the picture. What a challenge! Therefore, it is essential that people in the landscape and nursery industries know whether a particular plant can reproduce in a predictable manner so growers, sellers, and buyers are all working with plants that have the same traits. Plants that do not breed true from seed (produce identical offspring), need to be propagated vegetatively.
Many of the most popular herbaceous landscape plants, including the ’Red Rocks’ and ’Pike’s Peak Purple’ penstemons, (Fig. 8a & 8b) are 'new' plants developed through deliberate hybridization. By deliberate hybridization, we mean the process of controlled crossing of plants of one species with plants of a different species. Crossing can also occur between different individuals of the same species that display desirable characteristics. Controlled crosses use plants that would not normally be crossing, due to either space or time constraints that we described earlier. The resultant hybrid has exactly half its genetic makeup from each parent, but may show characteristics more like one of the parents or may have intermediate characteristics between the parents.
The most successful hybrids are typically back-crosses between the original pure hybrid and one or more of the parents. This is because one plant often contains several desirable characteristics except for this one new trait. Backcrossing enables breeders to recover all of those original characteristics, as well as the new trait, and form a new variety that can be released for production. (Refer to the Basic-Backcross Breeding lesson at Library of Crop Technology for a review.
What if the new trait you want to incorporate into your new plant does not exist in any of the plant’s distant relatives, or appears in only a small number of plants in a population, or in a plant where pollination is not successful? For example, breeders might discover a plant that is free of a particular virus, but has poor flowers or foliage. The gene that codes for the virus-free condition can be inserted into other plants that do have outstanding flowers or foliage, eliminating the risk of passing along the undesirable flowers or foliage with the virus-free characteristic. Tools in biotechnology have allowed this challenging barrier to be broken.
The controversy over the use of genetic engineering, for development of new plants with specifically selected, genetically inherited traits, continues to rage. Those against the practice often also argue in favor of using only straight species, and may even be against new plants produced through natural selection because of the resulting change in the gene pool. Those in favor of genetic engineering look at the potential for world-changing plants, with the ability to produce more food, higher protein, better fibers, greater disease resistance or ability to withstand the application of broadleaf herbicides. As work continues in biotechnology, scientists will come closer to an acceptable resolution of this debate, but genetic engineering is unlikely to ever be fully accepted by everyone.
Propagating the New Plants
Regardless of the method used to produce a plant with a new trait, whether through selection, hybridization or genetic engineering, it is successful only when the plant with the trait can be successfully propagated. Some hybrids produce viable seed, and some even come phenotypically “true” from seed, at least in terms of general appearance. However, the most reliable method of producing more old plants from these new ones, is through vegetative reproduction. Techniques used in vegetative propagation are explained in the next section.