One of the main advantages of LEDs over other sources is the ability to create custom spectra. Depending on the spectrum of the application, the plant morphology, uniformity, yield quality, flowering time and other parameters can be better controlled. The use of spectroscopy to control plant growth is even more important when using LED grow light in a closed environment, ie as the sole source of light.
When considering the development of LED technology, it has recently become apparent that plants will react differently when grown under different light sources. Monochromatic light (eg, LEDs consisting only of red and blue chips) is often lacking in plant response when compared to continuous (full/wide) spectra that overlap in different color wavelengths. In addition to the rich spectrum of various spectral colors, we must also consider different color ratios as factors, such as the red to far red (R:Fr) ratio and the blue to green (B:G) ratio.
Red to red ratio (R: FR)
Most studies on plant response have been conducted on the ratio of red to far red. The leaf surface of plants contains red and far red light absorbing photoreceptors called plant pigment-B (phyB). PhyB switches between biologically inactive (Pr, λ max, 660 nm) and active (Pfr; λ max, 730 nm) forms.
These forms directly affect the photomorphogenesis of plants, such as extension, germination and flowering (Franklin and Whitelam 2005). This is a good example of not only the important percentages of far red and red in the continuous LED spectrum, but also their relationship to the photon loading produced by the spectrum (Croser at al.2016).
LED spectral ratio
There are different definitions of the colors referred to by a certain wavelength. According to Sellaro et al. people. (2010) The red light is between 620-680 nm and the far red light is between 700-750 nm. An older definition of Smith (1982) determined that red is between 700-750 nanometers and far red is between 720-740 nanometers.
If we analyze the scientific literature, we can note that the ratio of red to far red is described as relatively low (2-3), similar to sunlight, with a R:Fr ratio of about 0.6-1,3(0,6) Morning and evening and noon 1, 0-1, 3). R: The higher the Fr ratio, the more compact the plant and vice versa.
Plants also perceive the increase in far red in the spectrum and interpret the high amount as being in the shadow. This leads to the so-called “shade syndrome” which leads to stem stretching and early flowering.
Arabidopsis Thaliana R:Fr ratio
The plants shown above were 27-day-old Arabidopsis seedlings grown under different light treatments of 40 μmol/ m-2 / s-1.
- A) Fluorescent tube: R: FR 5.87 B: G 2.7 FR
- B) Continuous LED 1: R: FR 5.5 B: G 1.8 FR 8%
- C) Continuous LED 2: R: FR 3.3 B: G 1.2 FR 17%
- D) Continuous LED 3: R: FR 3.1 B: G 25.9 FR 25%
Blue ratio (B: G)
The blue color in the spectrum is critical to the photosynthesis process and therefore forms the basis of a good growth spectrum with red. However, the effect of blue on blue is significantly affected by the ratio of blue to green (Möglich et al., 2009). In the recent academic literature, the green wavelength is more emphasized.
It is now known to penetrate the leaf layer of the leaves deeper than the blue and red wavelengths. In addition, we now know that it can penetrate the upper leaves to the lower part of the plant and drive photosynthesis with limited supply of other wavelengths (Smith et al., 2017).
The direction of future research may be to study the ratio of green to red, because green also affects the red light responses also.
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