Understanding How Plants "See" Light

Did you know that plants can see light?

Plants can differentiate light intensity and color based on electromagnetic wavelengths, in their own way, plants can use this ability to "see" without actually having eyes. Light that is not visible to humans, like ultraviolet (UV), is visible to plants. This ability also lets plants sense the time of day since the wavelengths of light change from sunrise to sunset.
Plants can also sense the brightness and intensity of light, light exposure length, and even the location of a light source. While plants lack a nervous system to translate light into pictures like humans, they are very much able to “see” in a way that is as complex as human sight.

Do Plants Have Eyes?

Light is absolutely vital to plants. Since it is so integral to food production and survival, plants in the wild must be able to compete for it. In order to do this, they have an advanced sight-like system where phytochrome pigment molecules in the leaves act like the photoreceptors of the eye. The tip of the plant acts like the eye itself, and the shoot of the plant moves signals like the brain.

How to Tell if Your Plant is Not Getting Enough Light

Providing greenhouse crops with high-quality lighting with proper intensity, distance, temperature, and exposure time is critical to crop success. If your plants are not growing the way you expect them to, you could have insufficient lighting. If sunlight is too scarce for plants, they may need grow lights to ensure they get enough light to keep growing.

  1. Leaves losing dark green color and becoming lighter (chlorophyll loss)
  2. Overall plant size will be stunted
  3. Elongation of the stem between internodes
  4. Growth of larger leaves (diverts energy away from productive growth)
  5. Plants may start to lean towards the nearest light source in a desperate attempt to reach it

Plant Response to Light Color

Both plants and humans have light receptors called "cryptochromes" which absorb blue light. While blue light does not play a role in plant growth, it does give cues to the plant’s internal clock. Blue-green light like that found during the day does not affect flowering in plants.
Red light specifically (which plants only respond to in darkness) will trigger flowering in plants. Far-red light, the light at the extreme red end of the visible spectrum between red and infra-red light in the region between 710 and 850 nm wavelength, will turn off flowering. "Phytochrome" is the plant molecule that causes this. It is first activated by red light, which primes it to receive far-red light afterward.
Since far-red light is the last color in the spectrum that a plant sees at night, it is a signal for the plant to "turn off" for the evening. It is possible to manipulate light quality in a greenhouse thanks to recent technological evolutions such as the development of "photo-selective" films. Manipulating the red light within a greenhouse can improve crop yield and quality, but it could be difficult or expensive to find photo-selective greenhouse plastics.

Plant Response to Length of Light Exposure

Plants have the ability to sense day length, and in short-day plants, this means that flowering is induced when a shorter length of the day is sensed. This phenomenon is scientifically known as photoperiodism
A 12 hour day length will trigger flowering in short-day plants, longer light exposures of 18-24 hours will keep it in a vegetative growth stage. Light interruptions during the dark period can lead to major problems, so 100% light proofing is a must in a light deprivation greenhouse for short-day plants!
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