Travis Lick Biology How plants respond to their environment Plants, with their roots firmly fixed in the earth, seem immobile and vulnerable compared to animals, but this does not prevent them from reacting to stimuli in their environment. 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 this movement of a plant in response to an external stimulus is called tropism. Plants accomplish this my manipulating chemicals inside their stems, leaves and roots. 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. The change in the directional growth of a plant is caused by a chemical called auxin that resides in the coleoptile of a maturing plant shoot. See the picture below for an example and explanation of how this phenomenon occurs. Figure 1: Phototropism
Auxin is a chemical that promotes the rapid elongation of growth cells in the shoots of a plant. Auxin also helps the plant to remember where it has branched off in the past and also in which direction it needs to grow. In the figure, you can see the auxin gathering on the left side of the coloptile, which is the covering of a plant shoot that enables it to grow. As 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 raising 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. 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 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 reaction to the force of gravity is undertaken in the nodule at the tip of a root called the root cup. Inside the root cap, there are sensors called statocytes that contain starch sediments. These sediments settle on the bottom most point in the root cap and indicate to the root cells the direction they need to grow. Figure 2: Gravitropism In the figure above, there are two normally growing corn kernels that were turned ninety degrees horizontally. The plants were then given six hours to grow and the results are pictured here. The top kernel labeled C, has
an intact root cap, while the bottom kernel labeled NC, had the root cap removed. Both kernels grew at the same rate, however, the NC kernel was unable to correctly adjust it s growth to the change in gravitational pull, while the C kernel, containing the statocytes and starch sediments was able to correctly change it s directional growth. In order for a plant to survive, it must be able to extend its roots towards a source of water and nutrients. This movement, called hydrotropism, is carried out, once again, in the root cap at the tip of the root. This root cap, in addition to statocytes, contains other sensors that are able to identify water pressure and in turn, stimulate growth using auxin concentration in the root. At times, the gravitational pressure and the water availability may be in opposing directions, when this is the case, the hydrotropic response overrides the gravitropic response. Figure 3: Hydrotropism In the figure above, the root had been growing downward by following the gravitropic response of the root cap until the water gradient was changed. The hydrotropic sensors inside the root cap respond to this by diffusing auxin to the bottom of the root cap causing the elongation of the lower level root cells in the direction of the water concentration. In addition to the plants ability to react to differences in light, pressure and water location, they also have the ability to change growth direction based on physical contact with a stimulus called thigmotropism. Thigmotropism in the root system occurs when the root cap contacts an object that is cannot penetrate such as a rock layer.
Figure 4: Thigmotropism (root system) These carrot roots were part of an experiment in which they purposely obstructed the root growth to see them effect. The outer epidermal cells of the root cap have tactile papillae that act as touch sensors, when these papillae contact an object the cells become deformed and therefore cannot grow. This continued process of touch, deformation, touch, deformation, causes the root to grow around the object obstructing it. When the papillae sensors no longer contact the object, then 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 objects that its papillae contact. This can be seen in plants that are viney like the sweet pea, cucumber, hops and kudzu. Figure 5: Thigmotropism (stem system) The tiny hair-like papillae on this plant have contacted the metal wire growing near it, this causes the positive response of auxin, causing elongation of cells on the side opposite when the papillae touched. This positive
response causes cell growth in a circular pattern, encompassing the wire and giving the plant extra support. At first glance, it may seem that plants have very little control over the environment in which they are living. However, there are numerous processes working inside plants prior to ever growing a single leaf or root follicle and they continue throughout the life cycle of all plants. The development of these complex processes known as tropisms have given plants the ability to not only inhabit, but thrive is almost every corner of the earth.
References: Figure 1: Phototropism: http://resources.ed.gov.hk/biology/english/images/environme nt/coleoptile.jpg Figure 2: Gravitropism: http://images.google.com/imgres?imgurl=http://www.bio.psu.e du/people/faculty/gilroy/ali/graviweb/images/lastphoto.jpg& imgrefurl=http://www.bio.psu.edu/people/faculty/gilroy/ali/ graviweb/toc.htm&h=329&w=325&sz=19&hl=en&start=16&tbnid=wfi tk2x_farvpm:&tbnh=119&tbnw=118&prev=/images%3fq%3dgravitrop ism%26svnum%3d10%26hl%3den%26lr%3d%26sa%3dn Figure 3: Hydrotropism: http://images.google.com/imgres?imgurl=http://www.learn.co. za/04/g12/bio/ang/pla/images/hydrotropism.gif&imgrefurl=htt p://www.learn.co.za/04/g12/bio/ang/pla/page4.html&h=138&w=5 90&sz=20&hl=en&start=4&tbnid=DcbI2Ru9fm2j7M:&tbnh=30&tbnw=1 32&prev=/images%3Fq%3Dhydrotropism%26svnum%3D10%26hl%3Den%2 6lr%3D%26sa%3DG Figure 4: Thigmotropism (root system): http://images.google.com/imgres?imgurl=http://www.kidsgarde ning.com/growingideas/projects/july04/tendril.jpg&imgrefurl =http://www.kidsgardening.com/growingideas/projects/july04/ pg2.html&h=188&w=250&sz=13&hl=en&start=1&tbnid=lrjgdcoxl7xk am:&tbnh=83&tbnw=111&prev=/images%3fq%3dthigmatropism%26svn um%3d10%26hl%3den%26lr%3d%26sa%3dg Figure 5: Thigmotropism (stem system): http://images.google.com/imgres?imgurl=http://botit.botany. wisc.edu/courses/img/botany_130/physiology/thigmotropism.jp g&imgrefurl=http://botit.botany.wisc.edu/courses/botany_130 /Physiology/thigmotropism.html&h=309&w=400&sz=22&hl=en&star t=1&tbnid=4bvhbwxjymokom:&tbnh=96&tbnw=124&prev=/images%3fq %3Dthigmotropism%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DG Vartarian, Steffan (1997). Thigmotropism in Tendrils. Retrieved August 1, 2006, Web site: http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm Takahashi, Nobuyuki (2003, April). Plant Physiology. Retrieved August 3, 2006, from American Society of Plant Biologists Web site: http://www.plantphysiol.org/cgi/content/abstract/132/2/805
The Gilroy Lab. Retrieved August 2, 2006, from Root Gravitropism Web site: www.bio.psu.edu/.../ gilroy/ali/graviweb/toc.htm Me!kauskas A., Moore D., Novak Frazier L. (1999). Mathematical modelling of morphogenesis in fungi. 2. A key role for curvature compensation ('autotropism') in the local curvature distribution model. New Phytologist, 143, 387-399. National Gardening Association. Retrieved August 7, 2006, from Growing ideas, classroom connections Web site: http://www.kidsgardening.com/growingideas/projects/july04/p g2.html