Which wavelengths of light are the most effective in photosynthesis?

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How does light quality affect plant growth?


Light quality refers to the spectral distribution of light or the relative number of photons of blue, green, red and far red & other portions of the light spectrum emitted in a light source.

Photosynthesis requires light between 400 and 700 NM.

Spectroradiometer measures light quality.

Blue light promotes vegetative growth. If it is the only light, the plant will be shorter, hard and dark in color.

R:FR promotes stem elongation. If it is the only light, the plant will be soft and tall.

Red and blue light combined promotes flowering.

R:FR is the proportion of red light relative to the amount of far red light.

Plants that grow best under high light (full sun) are more responsive to changes in R:FR light than plants that tolerate low light (shade).

Light quality, quantity and duration affect plant growth.

The distribution of light and R:FR varies between light sources.

The large amount of far red light emitted from incandescent lamps and their low lamp efficiency illustrates some of the undesirable attributes of these lamps.

Plants grow to position their stems and leaves to receive the most amount of light possible.

There are peaks in the red and blue wavelengths where photosynthetic activity is higher.

Phytochrome is a type of receptor in the plant that is sensitive to R:FR and controls flowering in plants that are sensitive to day length.

4 thoughts on “Which wavelengths of light are the most effective in photosynthesis?

  1. Unfortunately everyone is just copying this image, but it is incorrect.
    If blue is the only light in the spectrum it causes elongation resulting in taller plants instead of more compact plants. If there is relatively much blue light in the spectrum in relation to the red light and there is no FR, then the plants remain compact.

  2. Do you have a research source for the claims made for plants growing under only blue light or under only red light? (I clicked on the research link and found nothing relevant):
    Thanks in advance.

    1. Hi Andrew, thanks for the question.
      Urban Ag News works with and sources information from 3 primary sources in the United States.
      Professor Bruce Bugbee from Utah State University – https://caas.usu.edu/directory/bugbee-bruce
      Professor Erik Runkle from Michigan State University – https://www.canr.msu.edu/people/dr_erik_runkle
      Professor Ricardo Hernandez from North Carolina State University – https://cals.ncsu.edu/horticultural-science/people/rhernan4/
      Each of the links provided will take you to their university pages where you can find their published work.

      This piece is a combination of work from each of them plus other research at land grant universities.

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