Looking at Light
Looking at Light
Light and Solar Radiation can be broken down as follows;
HUMAN EYE PERCEPTION
Foot Candle or Lux meters measure light similar to how the human eye perceives brightness: strongest in the 500-600nm range. Though this is desirable for photography and interior design, plants react to light differently.
The light that drives photosynthesis in plants is Photosynthetically Active Radiation, or PAR light. This is also referred to as Quantum light, because it is measured in units of moles striking an area over time. Though PAR light ranges from 400 to 700nm, the region brightest to human eyes is the area of least effect on plants. Measuring quantum light can tell you if your plants are getting a sufficient amount of usable light.
SOLAR RADIATION
The sun radiates a broad range of light from 300-1100nm. In agriculture, this total radiation is needed primarily to calculate Evapotranspiration (ET). ET is the amount of moisture leaving the ground through evaporation (from the ground) and transpiration (from the leaves), and is dependent upon light, wind speed, temperature, and relative humidity.
UV (200-400nm)
Plants can suffer from sunburn; exposure to radiation in the mid-ultraviolet part of the electromagnetic spectrum (UV-B) triggers stress responses, inhibition of photosynthesis and DNA damage. As an initial defense, plants produce and accumulate UV-B-absorbing chemical sunscreens, such as flavonoids and sinapate esters, to block low-dosage UV-B.
A UV meter can help detect if your plants are being exposed to high levels of detrimental UV radiation, and to check the effectiveness of UV fi ltering materials. In general, a UV meter will measure the quantity of light in the 250-400nm range. Specific meters are also available for individual UV ranges.
UV-C light (200-280nm) can kill your plants. Fortunately, it is absorbed by ozone in the stratosphere.
UV-B light (280-315nm) is harmful as well and can cause plant color to fade.
UV-A light can be subdivided into two bands. The 315-380nm band has no effect on plant growth, while the 380-400nm band begins the range for photosynthesis.
RED/FAR-RED (660-720nm)
Plants absorb red light (660–680nm) and refl ect far-red light (720–740nm). Plants contain phytochromes, photoreceptors that control physiological and developmental reactions to fluctuations levels of red and far-red light. Some responses that are regulated by phytochromes include germination, stem elongation, flowering, gene expression as well as leaf and chloroplast development.
Plant leaves filter light allowing more far-red light to pass through than red light. This changes the red to far-red ratio below the canopy. Similarly, a low red to far-red ratio is created when plants are close together.
Knowing your red to far-red ratio can help you determine plant spacing and decide when to apply plant growth regulators. Greenhouses with high canopy closure or canopy density may need more applications of plant growth regulators to keep the plants a marketable size.
This brochure is a useful and authoritative resource with plenty of explanation about how various parts of the light spectrum affect plant health and growth. It illustrates clearly which Spectrum light meter and sensor is appropriate for the different measurements.
It is a PDF format so you will need Adobe reader or a plugin to view it. Right click on it to save as or just lick on it to open it in your browser.
It is copyright Spectrum Technologies.
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