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Red White and Blue Tortilla Chips. The Domestication of Maize. Physiology of Plants. Randy Wayne Shoals Marine Lab July, 2009. Life: The Ability to Respond Appropriately to the Environment.
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Physiology of Plants Randy Wayne Shoals Marine Lab July, 2009
Life: The Ability to Respond Appropriately to the Environment Consciousness, as defined as an awareness of the external environment—the first step in responding appropriately to the environment, is a characteristic of life and may have begun with the first cell.
Plants Sense and Respond to the Environment If we were to walk quietly and observantly through a garden at the Cornell Plantations, it would become increasingly clear that it is a normal and ubiquitous property of plants to sense and respond to their environment. http://www.plantations.cornell.edu/
Plants Sense and Respond to the Environment: Phototropism Plants sense the direction oflight and bend or turn toward the light.
Plants Sense and Respond to the Environment: Gravitropism Plants sense the vector ofgravity and the shoots bend up; away from the source of the gravitational force, and the roots bend down; toward the source of the gravitational force.
Plants Sense and Respond to the Environment: Thigmotropism Plants, particularly vines, have a sense of touch. When the tendrils come in contact with a support, they quickly respond to the mechanical force and curl around the support. Thigmo is the Greek word for touch.
Venus’ Fly Traps Can Sense Touch, Capture Their Prey, Secrete Digestive Enzymes and Absorb the Nitrogenous Nutrients
Plants Sense and Respond to the Environment: Photoperiodism Plants sense day length (or really the length of the night) so that they can flower in the spring or fall, when the days are short, or in the summer, when the days are long.
Circadian (about a day) Rhythms Plants sense the duration of time and move their leaves throughout a 24 h cycle.
The Living World is But Mankind in the Making Raoul Francé (1905) wrote, “What grander lesson could the speechless plants give than that which they have taught us: thattheirsenselifeisaprimitiveform, thebeginningofthehumanmind... it tells us that after all the living world is but mankind in the making, and that we are but a part of all.”
Germination of Lettuce Seeds • Seeds have a variety of requirements that ensure that they germinate in the right place and the right time. • Lettuce seeds need a pulse of red light (660 nm) to germinate. • If the red light is immediately followed by a pulse of far-red light (730 nm), the seeds no longer germinate. • The photoreceptor the senses the quality of the light is called phytochrome (plant color).
Germination of Lettuce Seeds • The seeds can be exposed to many rounds of red and far-red pulses, but it is only the last pulse that matters in inducing germination. • Harry Borthwick and Sterling Hendricks concluded that phytochrome acts like a switch. Light causes the interconversion between the red absorbing form (Pr) and the far-red absorbing form (Pfr). Only the far red absorbing form causes germination.
Phytochrome Pr Pfr
Pfr Pr
Phototropism: Discovery of the Site of Perception • Charles and Francis Darwin found that coleoptile bent toward blue light and the apex is required for the phototropic response even though the bending takes place below the tip. • They concluded that the tip of the coleoptile sensed the light and then some “influence” traveled from the tip to the bending zone.
Phototropism: Discovery of the Chemical Nature of the Stimulus In order to test the possibility that a hormone was produced by the tip and transported down to the growing zone, Peter Boysen-Jensen inserted a piece of mica in order to block the transport of the hormone and showed that transport of the hormone toward the base occurs on the dark side of the plant as opposed to the side exposed to the unidirectional light.
Phototropism Paal cut off coleoptile tips in the dark and exposed the tips to light. Then he replaced the tip on the plant but off centered to one side or the other. He found that the coleoptile curved away from the side that was exposed to the tip.
Phototropism Frits Went figured that if the influence was a hormone, he could trap it in a block of agar and then put it asymmetrically on the coleoptile and induce curvature in the dark. It worked! Went named the hormone auxin, which means “to grow.”
Humans Eat Plants, and Human Urine Causes Coleoptiles to Bend • Urine has traditionally been a convenient (and cheap) source for the isolation of chemicals (e.g. guanine and other purines). • F. Kogl and A. J. Haagen-Smit first isolated auxin from human urine in 1931.
Winslow Briggs Showed that blue light causes bending in coleoptiles by causing a redistribution of auxin from the lighter side to the darker side.
The Mechanism of Phototropism • Blue light induces bending by causing a lateral redistribution of the hormone auxin from the lighted side of the tip to the dark side. • The increased auxin on the dark side stimulates elongation, which results in bending toward the light.
Ethylene and Aging Responses • In the 1880s, after the installation of coal gas lamps, people began to notice that leaves dropped off the plants that were exposed to gas leaks. • In 1910, H. H. Cousins noticed that rotten oranges caused the premature ripening of bananas and ethylene produced by the oranges was the cause of ripening. • In 1934, R. Gane classified ethylene (C2H4) as a hormone.
Ethylene Promotes Senescence (Aging) of Many Flowers • Older flowers produce ethylene. • The ethylene promotes the senescence of flowers, which has evolved to rid the plant of older flowers that have most likely been pollinated so that the young not-yet-pollinated flowers have a better chance of being pollinated. • This is good for the plant, but not for people who like flowers in their home.
Ethylene Promotes Ripening (Aging) of Many Fruits Ripening, which has evolved to promote fruit dispersal, involves the synthesis of sugars, the degradation of cell walls to make the fruits softer and the production of chemicals that provide color, flavors and aromas.
Ethylene Antagonists Prevent Aging • 1-MCP (1 methyl cyclopropene; C4H6), an ethylene (C2H4) antagonist prevents flower senescence and further fruit ripening. • 1-MCP binds tightly to the ethylene receptor thereby blocking the effects of ethylene.
Roles of Ethylene and Auxin during Abscission Healthy leaves, flowers and fruits produce auxin, which is transported down toward the stem. The cells in the region of the petiole known as the abscission zone are insensitive to ethylene. In response to age, touch or day length, the amount of auxin produced by the organ decreases. When the level of auxin drops, cells in the abscission zone become sensitive to ethylene. The increase in ethylene activates the enzymes that digest the cell walls in the abscission zone, and the weakened organ falls.
Synthetic Auxins Cause Defoliation • While Art Galston (1943) was looking for a chemical means to make soybeans flower and fruit earlier so that they could thrive in regions with a short growing season, he discovered that a low concentration of a synthetic auxin 2,4,5-triiodoacetic acid) would speed up flowing. • He also noticed that higher concentrations were not good for speeding up flowering even more because higher concentrations caused the leaves to fall off. Cornell Alumnus, The “price was right.”
Defoliation • The army was interested in the ability of 2,4,5-T to defoliate jungle plants in Viet Nam so that the Viet Cong would have nowhere to hide. • The army mixed 2,4,5-T with two other synthetic auxins to make Agent Orange.
Agent Orange • The herbicide mixture was named Agent Orange after the color of the stripes on the 55 gallon drums it came in. • Agent Orange contained dioxin, a byproduct of 2,4,5-T manufacture, which was a well known carcinogen.
Dioxin Increases the Risk for: • D1. Birth Defects (Spina bifida andAnencephaly) • D2. Chloracne • D3. Non-Hodgkin’s Lymphoma • D4. Soft Tissue Sarcomas • D5. Peripheral Neuropathy • D6. Hodgkin’s Disease • D7. Porphyria Cutanea Tarda • D8. Multiple Myeloma • D9. Respiratory Cancers • D10. Prostate Cancer • D11. Spina Bifida • D12. Diabetes • D13. Chronic Lymphocytic Leukemia
Bioethics • In 2003, Dr. Galston reconsidered the arc of his research. • “You know,” he said, “nothing that you do in science is guaranteed to result in benefits for mankind. Any discovery, I believe, is morally neutral and it can be turned either to constructive ends or destructive ends.” • He concluded: “That’s not the fault of science.” (Obituary in NYT June 23, 2008).
Archibald V. Hill “It is true that scientific research has opened up the possibility of unprecedented good, or unlimited harm, for mankind; but the use that is made of it depends in the end on the moral judgments of the whole community of men. It is totally impossible now to reverse the process of discovery: it will certainly go on. To help guide its use aright is not a scientific dilemma, but the honourable and compelling duty of a good citizen.”
Erwin Chargaff • The goal of science has changed from exploration and explanation (knowledge) to exploitation (intellectual property and profit). • “What I have called the change in the moral climate is 1) that the majority [of scientists] seem to agree that what can be done must be done…”
The Reaction of Scientific Progress Upon Human Life and Thought • You are a shareholder in the world and have its future in your hands. • Although it is often stated that demand creates supply, it is often the other way around, supply creates demand. • When science produces something, people will buy it.
What do You Value about being Human? • How do you measure the value of being human? • Are you willing to cede your input on this question to biotechnologists using a business model to do science? • Are you willing to take the time to maintain and sharpen your knowledge of biology in order to make informed, knowledgeable and wise personal, athletic, business and political decisions that reflect your values about the value of being human? http://www.hgalert.org/
