Plant Tropisms and Photoperiodism for AP Biology
Practice problems for these concepts can be found at: Plants Review Questions for AP Biology
A tropism is growth that occurs in response to an environmental stimulus such as sunlight or gravity. The three tropisms you should familiarize yourself with are gravi tropism, phototropism, and thigmotropism.
- Gravitropism. This is a plant's growth response to gravitational force. Two of the hormones mentioned earlier play a role in this movement: auxin and gibberellins. A plant placed on its side will show gravitropic growth in which the cells on the upward-facing side will not grow as much as those on the downward side. It is believed that the relative concentrations of these hormones in the various areas of the plant are responsible for this imbalanced growth of the plant.
- Phototropism. This is a plant's growth response to light. Auxin is the hormone in charge here. Auxin works its magic in the zone of elongation. While the mechanics of the phototropism process may not be vital to this exam, it is still quite interesting to know. When a plant receives light on all sides, auxin is distributed equally around the zone of elongation and growth is even. When one half of a plant is in the sun, and the other is in the shade, auxin (almost as if it feels bad for the shady portion) focuses on the darker side. This leads to unequal growth of the stem with the side receiving less light growing faster—causing the movement of the plant toward the light source.
- Thigmotropism. This is a plant's growth response to contact. One example involves vines, which wind around objects with which they make contact as they grow.
How in the world did we figure out that auxin played such a large role in phototropism? A series of experiments performed by two scientists proved vital to the understanding of this process. Grass seedlings are surrounded by a protective structure known as the coleoptile. Peter Boysen-Jensen performed an experiment in which a gelatin block permeable to chemical signals was placed in between this coleoptile structure and the body of a grass seedling. When the piece of grass was exposed to light on one side, it grew toward the light. When a barrier impermeable to chemical signals was placed in between the two structures instead, this growth toward light did not occur. Another scientist, F.W. Went, came onto the scene and took Jensen's experiment a step further. Went wanted to show that it was indeed a chemical and not the coleoptile tip itself that was responsible for the phototropic response. He cut off the tip and exposed it to light while the tip was resting on an agar block that would collect any chemicals that diffused out. The block was then placed on the body of a tipless grass seedling sitting in a dark room. Even in the absence of light, a block placed more toward the right side of a seedling caused the seedling to bend to the left. A block placed more toward the left side of a seedling caused the seedling to bend to the right. Because there was no further light stimulation causing the growth, the agar block must indeed have contained a chemical that induced a phototropic response. This chemical was given the name auxin.
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