Esther de Beer, who is the manager of the plant specialist team at Signify horticulture LED solutions, spends her days trying to make LED grow lights more effective. In this article she explains facts and fiction with regard to white light and how research has shown which light spectrum helps crops grow better.
There are several thoughts surrounding the specific light spectra used for growing crops and, more specifically, the use of white, green, or broad-spectrum light.
How efficient does a plant use green light for growth?
In the first interview, published in the July issue of The Commercial Greenhouse Grower, it was explained that blue, red and green light are absorbed equally by a plant canopy and only far-red light is absorbed significantly less. But are all these colours used equally for growth?
In the early 1970’s, McCree measured the photosynthetic efficiency as a function of wavelength for a large number of plants. This data shows a large commonality between the plants and has been averaged to what is now known as the “McCree curve”, see figure 1 below.
Figure 1 McCree curve: the photosynthetic efficiency of light as a function of wavelength
This shows that green light is used for photosynthesis, but at a lower efficiency when compared to red light. Since this early work, many researchers have found similar results. [Hogewoning 2012, Paradiso 2011]
For overall plant growth, not only the process of photosynthesis is important, but also other processes which, for instance, influence the shape of the crop. Therefore, for practical applications it is relevant to evaluate the total crop growth rather than only zooming in on photosynthesis.
So, what is the effect of green light on the crop?
It was found that several crops have significantly higher fresh weight when actually grown without green light. For other crops, however, the amount of green light has no effect on the fresh weight. Also, to steer the shape of the crop, blue and far-red light are far more effective than green.
A trial at the Philips GrowWise Center showed eight RijkZwaan Salanova lettuce varieties were grown under light spectra with both 0 and 20% green light, but with the same photon flux and percentage of blue light. The graph below shows the relative fresh weight of these crops, comparing the growth under both spectra with 0% green and 20% green light.
Figure 2 Significantly higher fresh weight for lettuce varieties when grown without green light
As can be seen in this graph, not all lettuce varieties react the same. There are two varieties (RZ1 and RZ2) that grow slightly better under the spectrum with 20% green. However, most varieties have significantly higher fresh weight (even up to 20% more fresh weight for RZ8) when grown without green light.
The small effect which green light has on the growth of a crop is confirmed by an extensive academic study by Snowden, who compared the growth of 7 diverse plant species under 8 different spectral compositions: “In contrast to the significant effects of blue light, increasing green light in increments from 0 to 30% had a relatively small effect on growth, leaf area and net assimilation at either low or high PPF”. [Snowden 2016]
A second research example is related to medicinal cannabis. In this trial, two different cultivars were grown under three different light spectra with 0%, 6% and 36% green at the same supplemental light level (600 µmol/m2/s). Here, in addition to looking at the flower weight, also looked at was the quality of the crops.
The graph on the right in Figure 3 below shows the dry flower weight for two different cultivars, whereas the graph on the left shows the percentage of active compounds, which are the key factor in determining the product quality for medicinal cannabis.
Figure 3 Higher active compounds of medicinal cannabis with lower percentages of green light.
The graph on the right shows that the dry weight remains the same for all three spectra, again confirming that the amount of green light has little effect on growth. However, the active compounds reduce substantially when the green content increases. Since these crops are grown specifically for their medicinal compounds, this leads to a preference of light spectra which contain little or no green.
To summarise: the studies found that different crops require different light spectra for optimal growth. However, in most cases there is no benefit in adding more than a few percent of green; both for the yield and for the quality of the crop.
So, if green light has so little benefits, why use it?
That needed some further clarification; all of the above results describe the use of light by the crop, comparing yields at the same photon flux. However, they do not take into account how much electrical energy is needed to create this light. Since there are big differences in efficacy (mmol/Joule), this of course has a huge impact on the total energy usage.
Red LEDs give far more photons per electrical Watt (μmol/W) compared to blue and green LEDs.
The research shows that a spectrum with approximately 6% green light is sufficient for good colour recognition and is 30% more energy efficient when compared to a ‘sun-like’ spectrum, which contains approximately 40% green light.
Green light is much less efficient than blue and red LEDs. Only a limited amount of green can be considered for most lighting applications as only a small amount of green light is required for good colour recognition and because crops do not need a high amount of green light to grow well.