Plant Improvement

The arrival of February is a certain sign that we have turned the corner on winter (at least mentally) and now are heading toward spring. Many avid gardeners spend this month poring over the abundant supply of seed and plant catalogs that have arrived over the past few months in anticipation of the upcoming growing season. The first several pages of most of the afore-mentioned catalogs usually feature a plethora of new cultivars (short for "cultivated variety") available for the coming growing season and the question might arise relative to how they were developed. In most cases, the answer lies in the use of one of three different techniques: introduction, selection or hybridization.

Introduction is the most simplistic form of plant improvement and involves introducing a desirable plant or cultivar into an area (country) where it has yet to be grown. Most plant introductions come from distant places and may or may not need further refinement to become useful here in the United States. For example, soybean was introduced to the United States from China as a forage crop. It only was after the value of the bean itself was discovered and efforts made to modify plant stature and improve seed yield that soybean became the important grain crop that it is today. On the other hand, many species of woody ornamentals retain the same genetic makeup that existed when they were first introduced to our country many years ago. Ginko bilobia was introduced as a species from China in 1784 and, except for a few selected cultivars, remains unchanged since its introduction. Not all introductions are "success stories" as evidenced by the fact that species such as Johnson grass and kudzu were brought to our country with good intentions but went astray.

Selection is the second method of plant improvement and involves choosing desirable individuals from a mixed population of plants. If plants are vegetatively propagated, it requires only one plant or part of one plant to form the basis of a new, improved cultivar.

Most often the genetic variability responsible for the desirable trait is the result of spontaneous mutation. For example, in the late 1800s, a worker in a South American orange grove noticed a very unusual orange growing on one branch of a tree. On one end of the orange there appeared a shriveled, malformed structure around which (apparently) another orange had developed. The latter contained no seeds. Recognizing the value of a seedless orange the worker saved and propagated wood from that tree branch and thus began the navel orange industry. Many of our garden perennials are improved in the same matter-selecting what appears in nature because of chance variation and maintaining it through vegetative reproduction.

Alternatively, species such as daylily can crossed to encourage new genetic combinations from which superior individuals can be selected and maintained vegetatively as clones. Crops normally reproduced from seed also can benefit from selection but the process is a bit more complex and must lead to a plant that "comes true" from seed (i.e. produces seeds that contain those gene or genes that made the selection desirable). Self-pollinated plants are a bit easier to improve via selection than are crosspollinated plants since the latter are continually "comingling" their genes with neighboring plants of the same species.

Hybridization is the third and most prevalent form of plant improvement today. By definition, a hybrid is simply the offspring that results from the mating of two individual with dissimilar genetic makeup. In that sense all crosspollinated plants are hybrids and hybridization followed by selection has been practiced to a certain extent to improve them. A more restrictive definition of hybridization is plant improvement designed to exploit the phenomenon of hybrid vigor (or heterosis, as it is scientifically termed) by crossing two inbred parents to form F1 hybrids. In this day-and-age of being able to map the entire genetic makeup (genome) of plants and animals, scientists still are at a loss to explain why hybrid vigor occurs. A bit of genetics is required to explain this phenomenon further.

Most plants genetically are diploids meaning they have two sets of chromosomes-one from their male (pollen) parent and one from their female (egg) parent. Contained on these chromosomes are the genes responsible for the expression the various traits of the plant. When the gene(s) for a trait are the same on both the chromosome inherited from the pollen parent as the egg parent the individual is said to be homozygous for that trait. Inbred plants are homozygous for all genes on their chromosomes. When the gene(s) are different the individual is termed heterozygous. For example, fruit color in tomato is controlled by the action of a single gene. Each tomato has two genes for the fruit color trait, one from each of its parents. Genetically, if we assume‘R’=red fruit, ‘r’=pink fruit and red is dominant over pink then tomatoes with the genetic makeup of ‘RR’ and ‘Rr’ would both have red fruit. The former would be called homozygous for the gene (both genes are the same) for fruit color whereas the latter would be heterozygous (the genes differ). Since the gene for pink fruit can only express itself in the absence of the gene for red, pink-fruited plants genetically would be ‘rr’ and also homozygous.

Most economically important traits (e.g. vigor, yield) are controlled by the action of many genes and some believe F1 hybrids are superior because they contain all of the favorable genes for a trait held by both of their parents. But, if this were true then at least some of their progeny should equal their F1 hybrid parents in performance and this is not the case. Others believe it is the fact that corresponding genes for a trait are in a heterozygous state (differ from each other such as the ‘Rr’ red-fruited tomato) in the F1 hybrid when compared with either parent, but there are faults with this theory as well. Suffice to say whatever the reason, F1 hybrids show greater yield, earlier maturity, greater uniformity and increase substance when compared with their inbred parents making them very desirable. Their development, however, is painstaking and expensive. Many, many crosses must be made and evaluated before the plant breeder is likely to find a combination of parents leading to an improvement for the trait(s) under improvement. Pollination must be strictly controlled and often is still done by hand in naturally self-pollinated species. Once a favorable combination of parents has been identified, the cross must be made each time seed of the F1 hybrid is wanted since hybrid vigor only last one generation. This is why gardeners are advised not to try and save seed from year-to-year.

The next time you open a seed catalog and look at all the cultivars available consider the time and expense that went into developing them. While new isn’t always better (at least for everyone) the fact is that companies that release new cultivars probably would not have done so if they didn’t consider it better than anything currently on the market.

Dave Trinklein
Associate Professor of Horticulture
573-882-9631