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Biology LibreTexts

30.7A: Plant Responses to Light

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  • Plants respond to light stimuli by growing, differentiating, tracking the time of day and seasons, and moving toward or away from the light.


    Compare the ways plants respond to light


    Key Points

    • Plants grow and differentiate to optimize their space, using light in a process known as photomorphogenesis.
    • Plants grow and move toward or away from light depending on their needs; this process is known as phototropism.
    • Photoperiodism is illustrated by how plants flower and grow at certain times of the day or year through the use of photoreceptors that sense the wavelengths of sunlight available during the day (versus night) and throughout the seasons.
    • The various wavelengths of light, red/far-red or blue regions of the visible light spectrum, trigger structural responses in plants suited for responding to those wavelengths.

    Key Terms

    • photoreceptor: a specialized protein that is able to detect and react to light
    • photoperiodism: the growth, development and other responses of plants and animals according to the length of day and/or night
    • photomorphogenesis: the regulatory effect of light on the growth, development and differentiation of plant cells, tissues and organs
    • phototropism: the movement of a plant toward or away from light

    Plant Responses to Light

    Plants have a number of sophisticated uses for light that go far beyond their ability to perform photosynthesis. Plants can differentiate and develop in response to light (known as photomorphogenesis), which allows plants to optimize their use of light and space. Plants use light to track time, which is known as photoperiodism. They can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Light can also elicit a directional response in plants that allows them to grow toward, or even away from, light; this is known as phototropism.


    Phototropism of an orchid plant: This orchid plant placed next to a window grows toward the sunlight through the window. This is an example of positive phototropism.

    The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors: a protein covalently-bonded to a light-absorbing pigment called a chromophore; together, called a chromoprotein. The chromophore of the photoreceptor absorbs light of specific wavelengths, causing structural changes in the photoreceptor protein. The structural changes then elicit a cascade of signaling throughout the plant.

    The red, far-red, and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the far-red end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants.