Somatic (body) polymorphism (many forms) is a type of biodiversity. It is the
production of different plant parts, or different plant behaviors, within the same
individual plant. Somatic diversity is ultimately genetic, it is the product of
differential genetic expression due to differential penetrance and expressivity of plant
genes. Unlike plasticity, somatic polymorphism is always expressed in plants that possess
those traits, and is not altered much by the environment conditions it encounters.
Many people find the concept of somatic polymorphism confusing, with good reason. I hope
that by the time you have looked at the examples and explanations below it will clearer to
you. Somatic polymorphism is a very important trait for a weed to possess, it gives an
individual plant species the ability and flexibility to adapt its plant parts to the
changing conditions it encounters in agricultural fields.
Somatic diversity can be found in leaves on the same plant, seed from the same plant, and
the form of the plant at different times of its life cycle:
Somatic Polymorphism in Leaves
Soybeans are an example of leaf somatic diversity that most of us have problem seen before. The first set of leaves are the seed leaves (cotyledons), then the unifoliates emerge, then a continuing series of trifoliates. Below left you can see some soybean seedling with root rot. The cotyledons and unifoliates are visible. The soybean seedlings in the middle are suffering from trifluralin injury. Again the cotyledons and unifoliates are visile. The soybean plant on the right has partially survived a dose of acifluorfen. The damaged cotyledons and unifoliates are visible. Different leaf forms, for different functions, at different times of the life cycle.
Green foxtails, as with most other foxtails, have the ability to tiller. These stem
branches allow it to take advantage of opportunities. Each subsequent tiller has a
different shape and size. Stem length, panicle size, all seem to get smaller as tillering
proceeds.
Somatic Polymorphism in Seed
Foxtail plants shed seed with different germination requirements, an important form of seed somatic polymorphism. Most giant foxtail seed shed in the fall is dormant, but some can germinate immediately. Some try to germinate even earlier as can be seen below with the seed that germinated in the pollenation bags that were placed over the panicles before harvest.
Corn, and not just weeds, possess this type of somatic diversity. In corn, plant breeders
have purposefully bred this trait out to the best of their ability. Still, every once in a
while some corn will germinate before it should:
Common cocklebur seed capsules contain two seeds inside. One can germinate in the first
year following dispersal, the other is more dormant and germinates later. This is an
important form of seed dormancy somatic polymorphism: different seed dormancy in seeds
from the same plant.
Wild oats shed seed with different germination requirements, dormancy, also. Seed from different parts of the panicle (below) possess different levels of dormancy.
Common lambsquarters shed both black and brown seed. The darker seed is also more dormant.
Somatic Polymorphism: Seasonal Dimorphism
Another form of somatic diversity is seasonal dimorphism: two distinct phases of growth of an individual plant, each adapted to a specific season (seasonally dimorphic phenotype). Below are those two growth, or plant, forms in common burdock. On the left is the first year form of this biennial. On the right the taller, more leafy form with seedheads apparent in the second year. The cute little dog in the upper right is my old mutt from grad school. A good pheasant dog if not a bit hyper.
