|MadSci Network: Botany|
Plant hormones, or really chemicals that work like plant hormones, can be used to control unwanted plants. Specifically the plant hormone known as auxin (abbreviated as IAA) and chemicals that act like auxin are used in various types of weed killers. Several different types of chemicals that act like auxin are in weed killers you can purchase at a garden center or place where garden supplies are sold. Some of the chemicals that have been used in the past or present include 2,4-D; 2, 4, 5-T (contains dioxin), picloram, and dicamba. There is a lot of work to try and understand how auxins and auxin-like chemicals work to kill plants. It is very clear that these chemicals work much better on dicot plants (plants that have two cotyledons and are sometimes known as broadleaf plants) than on monocot plants (plants with only one cotyledon). The monocots include grasses and these herbicides are often used on lawns and in fields where corn, rice, and wheat are grown. Corn, rice, and wheat are types of grass. So how do these chemicals work? It isn't totally clear, actually. It seems as though these chemicals are taken up by dicot leaves and are transported in the plant much more easily than in monocots. We know that plants that are exposed to auxins and herbicides containing auxin like chemicals have changed growth patterns. Certain parts of the leaf and stem begin to grow more quickly and others don't grow as quickly. The result is twisting of the plant parts. These types of growth patterns often result from a second plant hormone ethylene. Auxin increase the amount of ethylene the plant produces. But there is probably more to the story than just differences in uptake of the chemicals and increased ethylene production. The hypothesis is that the chemicals stop the production of chemicals important for proper plant growth. Source Salisbury and Ross. 1992. Plant Physiology, 4th edition. Wadsworth
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