Photosynthetically Active Radiation (PAR) describes the light within a 400-700nm(nanometer) wave band that photosynthetic organisms use in the process of photosynthesis. PAR is not quantitative so it does not distinguish between different quantities of certain wavelengths. PAR also assumes that wavelengths outside this range have no effect on photosynthesis but as we know both infrared and UV have effects on photosynthesis. We will discuss the effects of the wavelengths outside of PAR further on.

Plants and other photosynthetic organisms absorb and react differently to different wavelengths within PAR. Different wavelengths are more beneficial to photosynthesis than others during different stages of the plant growth cycle. Plants use more blue wavelengths during the vegetative stage and more red wavelengths during the fruiting stage, but it gets more complicated than that. 

How is PAR measured?

PAR is quantified by the measurement of PPFD (Photosynthetic Photon Flux Density) which is the number of photons within the PAR wave band that fall on a square metre of a given surface every second. It is measured as the unit micromoles per square per second.

Adequate PPFD values for plants are species dependent and can range from 0 to 3,000 micromoles per square metre. At night, PAR is zero and at mid-day during summer, PAR often reaches 2,000 to 3,000 micromoles per square metre.

Absorption Spectrum Vs Action SpectrumAbsorption Spectrum Vs Action Spectrum

When it comes to light and photosynthesis there are two occurrences to take in account. Firstly, the amount of absorption of light energy and secondly the rate of photosynthesis occurring due to the absorbed light. The two occurrences can be represented in an absorption spectrum graph and action spectrum graph.

The Absorption Spectrum Graph defines the percentage of each wavelength being absorbed by pigments within the chloroplast. The most abundant photosynthetic pigment in plants is chlorophyll a and chlorophyll b. The graph below shows the most absorption done by chlorophyll a and b is with the violet-blue (400nm-525nm) light and secondly with Red-orange light (625nm-700nm). The least absorption is with green-yellow light (525 nm-625 nm). However, there are other pigments that are able to absorb light that chlorophyll cannot such as carotene.

According to “The Function of Carotenoid Pigments in Photosynthesis and their Possible Involvement in the Evolution of Higher Plants” by Thomas A. Moore, Devens Gust, Ana L. Moore, Carotenoids have two important roles in photosynthetic organisms. First, they act as accessory light-harvesting pigments, effectivelyextending the range of light absorbed by the photosynthetic apparatus. In other words carotenoids act as an auxiliary antenna absorbing the light the chlorophyll cannot and transferring that energy to the chlorophyll. Secondly, they perform an essential photoprotective role by quenching triplet state chlorophyll molecules and scavenging singlet oxygen and other toxic oxygen species formed within the chloroplast. This indicates that green light is still necessary for photosynthesis for the plant as a whole.

The Action Spectrum is a graph that defines the rate of photosynthesis for the different wavelengths being absorbed. Unlike the absorption spectrum, which is plotted in “percentage of absorption”, the action spectrum is plotted in either "volume of O2 produced" or "volume of CO2 taken up" or "mass of sugar produced" ultimately meaning the rate of photosynthesis. Please see the graph below that shows the action spectrum.

The graph of the action spectrum shows a good rate of photosynthesis with Red-orange light (625nm-700nm) however the best rate of photosynthesis is seen with violet-blue light (400nm-525nm). As very little light is absorbed by chlorophyll at wavelengths of green-yellow light (525nm- 625nm), the rate of photosynthesis will be the least within this range although it is still evident that there is photosynthetic activity occurring at these wavelengths where the chlorophyll cannot absorb.

From the two different graphs we can see there is a close relationship between the absorption spectrum and action spectrum for photosynthesis. Chlorophyll a and b are the photosynthetic pigments that most abundantly absorb light therefore the rate of photosynthesis is greatest at the two peak wavelengths that they are absorbed (violet-blue light: 400nm-525nm and Red-orange light: 625nm-700nm).

Food For Thought

The main choice of spectrum for many LED grow light suppliers has been the pink light consisting of solely red and blue diodes. This is a clear indication of their lack of knowledge about the effects of light on plant photosynthesis. They have totally ignored the role that carotenoids (which absorb blue and green light: 400nm-525nm and 525nm- 625nm) have on photosynthesis and transferring energy to the plant.

In conclusion, for optimal whole plant growth more than just blue and red light is necessary. The role that green light plays in conjunction with the blue and red light is necessary for the optimization of photosynthesis.

Grow Candy’s LED grow lights offer an enhanced spectrum unlike any in the market. The reason we differ from our competitors is because we have designed our technology with the backing of science. Our customized spectrum was designed to maximize photosynthesis in order to produce whole healthy and fruitful plants.