A fair number of spring wheat fields appear to be quite variable in plant height this season. Obviously varying degrees of drought stress can create height differences that are, in some instances rather striking ( Photo 1). Differences in height, however, are more interspersed and without clear delineations and/or transitions as is the case in photo 1, it is probably not drought stress per se but one of three things:
- a variety blend
- a variety that is segregating for plant height
- a variety that suffers from a genetics anomaly that results in a chromosome being lost across generations.
The second reason for differences in plant height is much less common but the extreme growing conditions we are experiencing this year can bring some differences in plant height that previously had gone unnoticed. Without going into too much detail, you have to understand that most varieties that are released are actually number of sister lines derived from the same cross that are nearly identical. These lines only differ for a few percentage point of the total number of genes, most of which will go unnoticed. After all, to be recognized the Plant Variety Protection Act requires that the variety meets certain minimum for distinctness, uniformity, and stability or DUS requirements for wheat. Breeders will generally select a number of sister lines that phenotypically are hard to distinguish from another in an environment that shows differences well. Dr. Jim Anderson, for example, will often do this work in the winter nursery in Arizona to detect even the slightest of difference in plant height and/or maturity. The extreme conditions we are encountering this season may, however, trigger some differences that previously had gone unnoticed.
The third reason is related to a problem that is particularly unique to wheat. It can create challenges for the DUS requirements, especially if the enforcement of the PVP is strict as is the case in many European countries. With the introduction of the semi-dwarf genes, the breeders quickly noticed that in certain lines and genetic backgrounds a number of tall plants would appear at a low frequency from one generation to the next generation (Photo 2). Subsequent research showed that in certain wheat varieties up to 6% of the time something goes during the formation of the pollen grains in certain g. This, in turn, leads to some 1% of the next generation to be so-called aneuploids, meaning having an individual progeny with one of more chromosomes missing or extra. Monosomic deletions, i.e. plants missing one chromosome, are most commonly encountered. Most often you will never see a difference, except in the case of semi-dwarf wheat varieties.
The odds that a chromosome gets lost in the shuffle that is called meiosis during the formation of pollen grain appears to be equal for all 48 chromosomes that make up wheat. Consequently, ever so often the one of the two chromosomes that carries the semi dwarf genes goes missing. This in turn results in half the reduction in height compared to the genetically equivalent variety without the semi-dwarf genes (another way to think about this is that only one instead of two doses of a medicine are given to shorten the height of the patience). The frequency of this phenomenon appears to be rather constant in certain background and varieties like Vance were notorious for getting 'dirty' over time.
A nice article that describes this phenomenon in more detail can be found here. An interesting experiment to do is to safe some seed of these tall off-types and grows them out next year. Half the plants derived from these tall off-types should revert back to the original variety, the other half of the plant will be the taller off-type again.