Travis Lick Capstone Overview the Chemical Push for the Sun: Theories of Phototropism the Term Tropism, Describes a Plant's D

1 Travis Lick Capstone Overview The Chemical Push for the Sun: Theories of Phototropism The term tropism, describes a plant’s directional growth in response to an external stimulus. The focus of this capstone is to explain the different types of plant tropisms that exist, focusing on phototropism, while describing the two main theories about the processes involved in these growth responses. First, the Cholodny-Went theory developed in the late 1920’s which has withstood the test of time and scientific scrutiny to remain as the leading candidate for describing phototropic response through a plant’s use of a growth hormone called auxin. Second is the growth-inhibitor hypothesis. This hypothesis was actually created prior to the C-W theory by a man named A.H. Blaauw in 1919 and is still considered a possibility based on his and current research. In addition to the content-based portion of this capstone, a unit plan has also been developed as a way to introduce and through inquiry based lessons, give students an enduring understanding of plant tropisms. The unit plan contains a pre-assessment as well as a post-assessment that will help to determine the level of student understanding. As part of the laboratory experiments in this unit, several data sheets have been developed as an additional form of on-going assessment. A short, internet webquest developed by a Pennsylvania teacher is included that leads students through several online questions. These questions are based on previous understanding as well as a student’s ability to watch and interpret plant motions streamed on online videos. Additional educational resources can also be found and used to supplement classroom lessons and activities. This capstone project was created as a way to show students and teachers the exciting world of plants and what they are capable of. This capstone can be used as a resource that provides a thorough background on the historical thinking of plant tropisms, an explanation of different tropisms, the two leading theories about how these growth movements occur, and lessons that can be used to demonstrate them. 2 Travis Lick The Chemical Push for the Sun: Theories of Phototropism Introduction: The idea that plants sense environmental stimuli has been a subject of debate since plants were first depicted swooning over Apollo, the sun god, in early Roman and Greek mythology. Since the time of Plato and Aristotle, a major classification barrier between plants and animals, was a plant’s perceived inability to respond to the stimuli of the surrounding environment. Early Aristotelian beliefs and the lack of experimental data, supressed the thoughts and ideas of generations of botany savvy scientists. Despite what seemed to be an easily observable phenomena - plants growing or bending towards a light source - without experimentation the idea that plants were insensitive to the world around them lingered (Hangarter & Whippo, 2006). Plants react to much of the same environmental changes that humans and other animals respond to, such as light, pressure, temperature, water, contact and even the change of day into night. The term used to describe directional movements of a plant in response to external stimuli is called tropism. Evidence supports the idea that plants respond to the world around them, but exactly how they “move” has been strongly contested for hundreds of years. In the following pages, I will discuss briefly the types of tropisms that plants exhibit and in detail examine the two conflicting theories about how phototropism is chemically carried out in plants; the Cholodny-Went theory and the A.H. Blaauw and the growth inhibitor hypothesis. Types of Tropisms Gravitropism: In order for a plant to successfully grow from a seed, the roots and the shoots need to differentiate between up and down. The reaction of the pre-emergent stems and roots is the result of gravitropism, or the directional growth caused by the gravitational 1 3 pull on the seed. Just as the shoot and the root have opposite reactions to light, they also have opposite reactions to gravity’s pull. Roots exhibit positive gravitropism and therefore grow in the downward direction with gravity while the shoot has negative gravitropism and grows against gravity. This detection of gravity is undertaken by cells at the tip of a root, in a tissue called the root cap. Inside the cells of the root cap, there are sensors called statocytes that contain starch granules. These statocytes settle on the bottom-most side of cells in the root cap and indicate to the root cells the direction they need to grow. Hydrotropism: In addition to a plant’s initial growth response to light and gravity, it responds to differing levels of moisture in the soil by directing root growth towards these areas. This positive growth reaction is called hydrotropism. “The hydrotrophic response has been shown in lab tests where a root system can respond to a gradient in water potential as small as 0.5 MPa by growing toward the higher water potential.” (Hirasawa, Suge. Takano, Takahashi, 1995) Thigmotropism: Plants also exhibit the ability to change growth direction based on physical contact with a stimulus. This phenomenon is called thigmotropism. Thigmotropism in the root system occurs when the root cap contacts an object that is cannot penetrate, such as a rock layer or other subsurface obstruction. In these instances, the plant utilizes outer epidermal cells of the root cap as touch sensors. When these papillae contact an object, the cells become deformed and growth ceases. This continued process of touch, deformation, touch, deformation, causes the root to grow around the object obstructing it (Vartarian, 1997). When the papillae sensors no longer contact the object, they continue to grow normally. This reaction by the root is called negative thigmotropism. Conversely, the stem system of plants exhibits positive thigmotropism and grows towards 2 4 objects that its papillae contact. This can be seen in plants that are viney like the sweet pea, cucumber, hops and kudzu. Phototropism: Plants are extremely sensitive to light and respond to it in a positive and a negative way. The stems and leaves of a plant have positive tropism which means they grow in the direction of light, while roots exhibit negative tropism in that they grow in the opposite direction of light. These observed changes in the directional growth of plants to different types of environmental stimuli have been observed and discussed for centuries. People from the era of Aristotle and even before have noticed that plants moved in conjunction with the positioning of the light sources around them. These early observers believed this to be a passive response that could not possibly be controlled by the plants themselves. The thought that plants merely existed was a long held belief due to early scientists lack of an informed evaluation of what they were observing. The reason for this was the lack of experimental evidence to support ideas. The scientific method, which involves the testing of hypotheses by experimentation, rather than by deducing ideas based on traditional thought, is largely attributed to Roger Bacon, a 13th century English philosopher. Because of his and others that followed, scientists now carry out in depth procedures based on hypotheses and conclusions drawn from observations rather than intuitive thinking. This scientific method has helped to both create and discredit ideas of how plants are affected by their environment. Part I: The Cholodny-Went Theory The Cholodny-Went theory of plant tropisms has been the foundation of understanding plant movements since the late 1920’s. Went suggests that “unilateral light induces a later redistribution of endogenous auxin near the apex of the organ” (McDonald, 2003). Simply put, the group of hormones, called auxins (more specifically indole-3- 3 5 acetic acid or IAA) are responsible for the elongation of plant cells that cause the overall curvature of the plant in the direction of a light source. The auxin in plants is responsible for the cell elongation, root, stem and leaf growth as well as apical dominance and reproduction. The change in the directional growth of a plant is caused by auxin that resides in the coleoptile of a maturing plant shoot. A coleoptile is “the protective sheath that surrounds the young shoot tip and embryonic leaves of a plant during its passage through the soil to the surface (American Heritage Dictionary, 2000). Figure 1 below shows how this phenomenon occurs. In the figure, you can see the auxin (purple) gathering on the left side of the coloptile. As the auxin moves down the shoot, the redistribution stops when it has evenly distributed itself along the shaded portion. Once this happens, the auxin stimulates cell growth and cell division only in the region opposite the light source. The plant cells react to the high auxin levels by transporting Hydrogen ions into their cell walls and lower the pH. Due to this, the plant elongates more rapidly on the auxin rich side causing the overall result, the extension of the plant in the direction of the light source. At the completion of the experiment, a lateral difference in auxin concentration can be measured in conjunction with the curvature of the plant. Experimental Support: 4 6 Corn seeds were raised for 4 days in a plant-culture vessel, soaked under running water for 2 days and then planted with the embryo facing upward on paper towel. These seeds were then soaked with de-ionized water (to stunt growth) and remained under red light for 2 days. After two days, specific plants were chosen based on straightness and length. They were placed parallel to each other in the dark for an additional day and reselected according to likeness.

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