Understanding Plant Tropisms for Optimal Growth

1. Unit Plant Science

2. Problem Area Managing Plant Growth

3. Lesson Homeostatic Responses of Seedling Plants: The Tropisms

4. Student Learning Objectives • 1. Explain plant tropisms. • 2. Identify the different tropisms affecting plant growth.

5. Terms • Negative tropism • Photropism • Positive tropism • Riboflavin • Statocytes • Thermotropism • Thigmotropism • Traumatropism • Tropism • Skototropism • Aerotropism • Amyloplasts • Auxins • Chemotropism • Coleoptile • Electrotropism • Geomagnetotropism • Geotropism • Hydrotropism

6. What are plant tropisms? • Plants demonstrate sensitivity to stimulation. Environmental conditions and external stimuli cause plant growth to be altered. Plant responses to the external stimuli are known as tropisms. • A. A positive tropismis movement or growth toward the stimulus • B. A negative tropismis movement or growth away from a stimulus

7. C. Under ideal environmental conditions, the external stimuli of light and gravity will equally affect the shoot and root development allowing the plant to grow vertically. • With equal stimuli, growth regulators are produced in equal proportions allowing for the cell elongation of the plant to occur at an equal rate. A tropic response results in an imbalance of growth regulators produced due to external stimuli.

9. How do tropisms affect plant growth? • Tropism—a growth response (toward or away from) an external stimulus that determines the direction of growth. • A. Phototropism—growth toward a light source due to unequal cell elongation. Plants have at least three different photo-receptor mechanisms involving three separate groups of pigments. These include phytochrome, which controls the plants developmental sequence. Plant response to lights depends on the exposure time and intensity of light.

10. Auxins, a plant hormone that promotes cell elongation produced naturally by plants or synthetically, will accumulate on the side of the stem in shade. Light decreases the auxin sensitivity of cells on the lighted side. Photoreceptors allow a growth response toward light. Using monocotoleydons, research has shown the coleoptile tip to contain auxin. • Riboflavin, a photoreceptor and yellow pigment, is also found in the tip of the coleoptile, a protective sheath surrounding the emerging shoot of grass seedlings. Riboflavin is the catalyst which aides in the transportation of the auxin away from the lighted side of the stem. With the light source driving auxin to the shaded side, the abundance of auxins will cause the cells to elongate. Due to this elongation, the stem will bend toward the light source.

11. B. Geotropism(Gravitropism)—plant growth response to gravity. Plants demonstrate positive geotropism through their root development and negative geotropism in shoot development. • Statocytes, specialized cells that function to control the direction of growth by perceiving gravitational forces and containing amyloplasts, are found in the root cap and in the plant’s stem. • Amyloplasts, colorless subcellular particles containing starch grains that influence the direction of plant growth, due to gravity position themselves on the lower side of the statocytes. When the root cap is removed, response to gravitational forces is negated.

12. When centrifugal forces are applied, horizontal plant growth is demonstrated. Stem nodes can also exhibit differential growth to regain vertical position of the plant stem when plants are knocked down by external forces. • As soon as the root or shoot of a plant detects that it is not in a vertical position, growth-regulating processes restore it to a vertical course. In vertically positioned shoots, auxin is equally distributed to the sub-apical cells. In horizontal positions, auxin is found in greater concentrations on the lower side of the shoot, thus creating faster growth on the lower side and a return to vertical growth.

13. C. Thigmotropism—plant response to touch. Diverting the direction of a plant, mechanical stimuli can cause plant growth patterns to be altered. Coiling in vine plants around an external object can also occur due to thigmotropism. Example: Cucumber plant tendrils will wrap around an object. • D. Hydrotropism—plant response to water. Roots will exhibit a positive tropic response in relation to higher concentrations of water. While plant roots have been shown to apparently seek out and grow toward water, some plant physiologists today doubt that responses to water are true tropisms. The response of roots to water sources is not clearly understood.

14. E. Thermotropism—plant response to temperature. Weeds will demonstrate horizontal growth in lower temperatures and vertical growth with an increase in temperature. • F. Electrotropismis a response to electricity. • G. Chemotropismis a response to chemicals • H. Traumatropisma response to wounding • I. Aerotropismis a response to oxygen • J. Skototropismis a response to dark • K. Geomagnetotropismis a response to magnetic fields

17. Review/Summary • What are plant tropisms? • How do tropisms affect plant growth?