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Weedy Adaptation in Quackgrass


Quackgrass has many adaptations to survive in agricultural fields. These include rhizome, growth habit and herbicide resistance adaptations.

Rhizome adaptations to agricultural practices

174t.JPG (8236 bytes) Left is a fragment of rhizome, notice the branching that occurs at the nodes. In studies we did in Ontario we discovered that these branch rhizomes would "self-fragment", meaning they would form an abscission zone at the node and the two pieces would naturally separate in the soil. This is exactly what seed-producing plants do, but this is vegetative propagation. New abscissed fragments form new "seeds" or propagules to ensure their survival in the bud bank, and to bother growers. Notice also the remaining roots coming out from the nodes.
173t.JPG (9737 bytes) Another interesting thing about quackgrass rhizomes we observed in Ontario bud banks is pictured left. If you look closely at the junction of these two rhizomes you will notice that the more vertical one is going through the more horizonal one, not around each other. This is definitely a dog's tooth, but it's biting itself. Rhizomes are very aggressive, and they have the ability to exert strong physical force to get where they want, even to puncture a neighbor. Competition among species can be violent, even in plants. Another important implication of this puncture is that it destroys the vascular system of the rhizome, preventing herbicides from translocating and killing buds. This is an important form of herbicide resistance. Several years ago Phil Westra at the University of Minnesota (now Colorado State) discovered some nasty little weevils that bored into rhizomes, also preventing herbicide translocation. Weed biology can be very weird sometimes.
175t.JPG (11935 bytes) This is another picture (left) of a quackgrass rhizome. It and its cohorts litter the surface of this Ontario field. Although the rhizome is dried out, it still has the ability to germinate and grow. Tolerance to drying is an important weedy adaptation in buds of perennial weeds.

Growth habit adaptations

12t.JPG (12069 bytes) Left is a variant, or biotype, of quackgrass. Notice that it is growing close to the ground, a prostrate habit. When competition for light is not a problem, being prostrate allows a plant to exploit more surface area. This allows it to capture more light, and allows it to spread more rhizomes over a larger area of the bud bank. Prostrate habit is also important in terms of herbicide control. Having larger surface area of land also exposes more leaf area to herbicide spray and uptake from overhead applications.
167t.JPG (11577 bytes) This is a closeup of the culm of the prostrate quackgrass biotype (left). Notice that is has a horizontal habit, and progressively angles upward at each node, gradually pointing upward.
13t.JPG (10738 bytes) Another variant of quackgrass is featured left, an upright shoot biotype. The culms of this plant are vertical, and the plant covers less surface area. This upright habit also has important implications for its ability to resist herbicides. The more densely arranged leaves intercept less herbicide spray and therefore get a smaller dose. Research and on-farm experience have shown the upright plant has a better chance of surviving foliar herbicide treatment than the prostrate type. Upright and prostrate growth habits are examples of genotypic polymorphism, an essential ingredient in biodiversity.

Herbicide resistance adaptations

169t.JPG (12673 bytes) Left is the base of a quackgrass shoot, sometimes called the crown region. Above this region the shoots spread upward to intercept sunlight; below the rhizomes and roots spread to intercept moisture and nutrients. Herbicide applications that translocate in quackgrass often fail to kill dormant buds in this region. As a result, regrowth often occurs from these buds sometime later, an important adaptation in quackgrass conferring herbicide resistance.



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