Lesson 3 - Page 1




Plants require energy from light to produce and utilize their own food. This is known as photosynthesis (with light) and aerobic respiration (in dark). They require the raw materials of water and carbon dioxide and minerals to produce food. Water and carbon dioxide provide oxygen, hydrogen and carbon molecules that are combined to form complex carbon compounds. Plant leaves absorb a spectrum of light waves from sunlight. Chlorophyll containing organelles are contained in chloroplast which possess the photosynthetic pigments and starch storing abilities. Lack of light would directly affect growth in that it would affect the photosynthetic capabilities of leaves.


Charles Darwin in 1880 exposed seedlings to light and charted how they bent toward the light only if the tips were exposed to light. He exposed grass to light and noted that it curved toward the source. He covered the top of the seedlings with light proof caps, no bending occurred. He wasn't quite sure what was going on. Boysen-Jensen's study between 1910-1913 with beheaded seedlings, demonstrated that placing a bit of gelatin on the stump, replacing the top on the gelatin, caused curvature toward light to occur just as with intact seedlings. If the tip is not replaced, curvature does not result. Paal (1918) demonstrated that if a cut tip is replaced off center growth results and it will be curved.

The reason for the bending of plants toward light is that growth hormones called "auxins" on the non-illuminated side will cause the plant to grow more than on the illuminated side. Since the elongated cells on the non-illuminated side will grow faster than on the other side, the plant will bend. It looks as if the plant is moving toward light to make food, but no plant can seek anything. The higher concentration of auxins on the darker side stimulate growth. Growth responses resulting from external stimuli are termed tropisms and since this has to do with light (photo=light) this is called phototropism.

Learning more about how plants grow can help scientists find ways of controlling unwanted weeds. Scientists have found that growth hormones, like the ones that were studied in lab, can be artificially synthesized. A plant hormone (growth regulator) is defined as an organic substance produced by a plant that when acting in very small amounts regulates plant physiological processes. Three hormones have been isolated, auxins, gibberellins, and kinetin. We will only deal with auxins, which are the hormones that were discussed in lab.

Scientists have also found that larger amounts of hormones are toxic to plants and can kill certain plants with no adverse reaction to other plants. Some of the synthetic growth regulators affect certain types of plants more than others. Spraying an area with these growth regulators to affect only certain plants is called selective spraying. This is different from chemical weed killers that usually kill all vegetation because they are toxic to the individual plant cells. Many of these weed killers are generally used because they are cheaper, including ferrous sulfate, sulfuric acid, arsenical preparations, and aromatic fractions of fuel oils. Some plants may have a "tougher" outer skin, thereby not allowing the material to penetrate and hence not killing them, but for those cells that it does penetrate, it kills them by upsetting their photosynthetic process. Hormone herbicides are selective on the other hand and tend to affect broad leaved plants (dicots) more than narrow leaved monocots. Grasses on our lawns tend to be monocots, and the "weeds" tend to be dicots. A product on the market like 2,4 D-dichlorophenoxyacetic acid is a hormone herbicide which can eradicate plants like dandelions and morning glory. A chemical weed killer will kill the tops, but not the roots, so the weed will come back. Yes it is cheaper to buy the weed killer, but the hormone herbicide will do a better job.

There are many other applications for these growth hormones. Some of the synthetic herbicides are effective defoliants which are used to take the leaves from the cotton plants before harvesting so the leaves don't plug up the mechanical cotton harvester.

This practical application of weed control was never thought about by the early scientists who were just interested in learning why a plant bends toward light.



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