Plant Reproduction
What Are Plant Responses to Gravity and Touch?
Plant Responses to Gravity
Plants can also sense and respond to gravity. Gravitropismglossary term (opens in a new window) (also called geotropism) describes the way in which a plant grows in response to gravity. Different plant structures demonstrate gravitropism in various ways. The stems of plants grow upward, called negative gravitropism, and roots grow downward, called positive gravitropism.
Both types of gravitropism occur in response to auxinglossary term (opens in a new window). Roots and stems show very different sensitivities to auxin. Stem shoots sense gravitational pull and produce increased levels of auxin. These high levels of auxin cause shoot cells to expand and grow upward. Root tips also sense gravity and produce auxin, but instead of promoting cell expansion, auxin inhibits root cell expansion. In roots, auxin is distributed unevenly, accumulating on the lower side of a horizontal root. This inhibits growth on that side of the root, causing the root to curve downward.
Plant Responses to Touch
Thigmotropismglossary term (opens in a new window) refers to a plant’s sensitivity and response to physical touch. Thigmotropism results in uneven plant growth in response to physical contact with solid objects in the environment. Imagine an ivy plant growing against the bricks of an old building, or a vine wrapped around a tree. How does the plant direct its growth around an object? The answer again involves auxin. In plants exhibiting thigmotropism, the cells in physical contact with an object produce auxin and transfer it to untouched cells. This causes the untouched cells to grow faster, which results in bent plant growth around the object.
Ethylene may also cause thigmotropism. Natural production of this gas by plants causes the stem to grow thicker and more slowly. This results in horizontal growth, which allows the plant to navigate a physical object.
Teacher Note: Practice
In this item, students analyze data in order to make valid and reliable scientific claims about which directional responses of plants (tropisms) are due to various environmental stimuli. After completing this item, further discuss how the amount and location of the plant hormone auxin affects the directional responses to gravity, light, and touch. Extend thinking by having students consider and discuss the limitations of data analysis, such as measurement error, sample size, and how scientists might control variables when comparing plants grown in space to those grown on Earth.
