LED Colors optimal for PAR
A mix of red (660nm) and far-red (740nm) light triggers the so-called Emerson enhancement effect and photosynthetic rates increase as much as 30%.
Robert Emerson discovered several surprising things about photosynthesis. Rajni Govindjee, one of Emerson's students, describes two so-called "Emerson effects". She indicates that her colleague Eugene Rabinowitch named the Emerson Enhancement Effect as the "second Emerson effect", saying that it"is the enhancement of the quantum yield of a photochemical process produced, in plant cells or extracts, by far red light, by simultaneous illumination with light of shorter wavelengths." (Rajni Govindjee, R., & Rabinowitch, E., 1961)
To put it a little more simply, the results of photosynthesis are greater if there is a combination of red and far-red light than if there was an equal amount of light that was onlyred or only far-red. For example, there are five units of red and five units of far-red, the results are greater than they would have been with ten units of red or ten units of far-red. The Emerson Enhancement Effect is an example of the whole being greater than the sum of its parts.
Why is this surprising? Until Emerson made his observations, it was generally believed that all the light that a plant's leaves absorbed was funneled through to one specific kind of chlorophyll molecule: chlorophyll a. (Dictionary of Botany, 2002) Chlorophylla can only process light at approximately 700nm, however. Therefore, Emerson disproved the prevailing belief. His discovery "suggested that for photosynthesis to take place with full yield, at least one accessory pigment must be excited simultaneously withchlorophyll a; failure to do so appeared as the cause of the decrease in the quantum yield of photosynthesis in the red region (referred to as the red drop)." (Rajni Govindjee, R., & Rabinowitch, E., 1961)
The first Emerson effect is also connected to lighting, but it is less important to the making of lighting decisions for horticultural use. Plants give off carbon dioxide in a short burst, after light starts to shine on them following a dark period. (Rajni Govindjee, R., & Rabinowitch, E.,1961)
Rajni Govindjee, Govindjee - Emerson Enhancement Effect in Chloroplast Reactions
The "red : far red ratio" is a comparison of the amount of red light relative to the amount of far red light that is shining on a plant's leaves. For optimum photosynthesis, both red and far red light must be available to the plant. However, too much far red light will cause problems.
According to Chelle et al, "The red:far-red ratio (R:FR) is a key variable in many biological processes from basic ones such as the response of phytochrome to more integrated ones, such as tillering or weed competition." (Chelle, M., Evers, J. B., Fournier, C., Combes, D., Vos, J., & Andrieu, B., n.d)
Experiments with seedlings have indicated that the ideal red: far-red ratio is very close to 1:1. The plants tend to be sturdy, straight, and firm. Also these plants tend to be able to produce good root systems, presuming that they receive appropriate quantities of water. If there is not enough red light, the seedling will tend to be taller but spindly and less resistant to drought.(Anjah, G.M., Fochod, D.A., Annih, M.G., & Kum, C.K., 2003)
Studies by NASA confirm that when green is balanced in with the red and blue wavelengths in the proper ratios growth time can be reduced up to 30% with a 30% enhancement in yield! The most familiar green for LED's is 525nm.
Using only red and blue lighting makes plants appear purplish gray therefor making visual assessment of any problems difficult. The addition of green light would make the plant leave appear green and normal similar to a natural setting.
Green supplemental lighting could also offer benefits, since green light can better penetrate the plant canopy and potentially increase plant growth by increasing photosynthesis from the leaves in the lower canopy.
More Plant Biomass under RGB lightning
In the NASA study, four light sources were tested: 1) red and blue LEDs (RB), 2) red and blue LEDs with green fluorescent lamps (RGB), 3) green fluorescent lamps (GF), and 4) cool-white fluorescent lamps (CWF), that provided 0%, 24%, 86%, and 51% of the total PPF in the green region of the spectrum, respectively. The addition of 24% green light (500 to 600 nm) to red and blue LEDs (RGB treatment) enhanced plant growth. The RGB treatment plants produced more biomass than the plants grown under the cool-white fluorescent lamps (CWF treatment), a commonly tested light source used as a broad-spectrum control.
Kim, Hyeon-Hye et al. Green-light supplementation for enhanced lettuce growth under red- and blue-light-emitting diodes
Blue-Violet Light, and its effect on plant-growth
The blue-violet region of light has a range of approximately 400-520nm. Experimental evidence is clear that blue-violet light is required for photosynthesis. (Dashur, R.H., & Mehta, R.J.,1935) If there is sufficient blue-violet light, the size of the leaves will be good. It also appears to be a factor in the development of the chloroplasts. (Zheng, J., Hu, M.J., & Guo, Y.P., 2008) Chloroplasts are the parts of the leaf's cells which contain the chlorophyll and conduct photosynthesis, so they are important.
UV Light and effects on plant-growth
Ultra-violet light is divided into three ranges: UV-A, UV-B, and UV-C. UV-A is the closest to visible light, with a range of 315-400nm. UV-C is the farthest from visible light, at 200-290nm. Very little UV-C radiation makes it past the outer atmosphere, so it is not
generally seen as important for horticulture. UV-Bholds the middle ground between them, at 290-315nm. It tends to be filtered out by the ozone layer, where that layer is sufficiently thick. (Vass, I., Szilard, A., & Sicora, C., 2005)
In general UV light causes problems for plants, much as it does for humans. The beta-carotene in some kinds of algae and higher plants provides some protection from UV-B (White, A.L., & Jahnke, L.S., n.d.), but generally speaking, all UV light will affect photosynthesis badly, and cause health problems for the plant in other ways. If UV light can be minimized, this is a good thing.
Too much Ultraviolet light can slow down the photosynthesis process(Zheng, J., Hu, M.J., & Guo, Y.P., 2008) and it can damage DNA molecules, much as it can in humans. (Strid, A., Chow, W.S., & Anderson, J.M., 1994) Chloroplasts are particularly hard-hit by over-exposure to UV-B(Strid, A., Chow, W.S., & Anderson, J.M., 1994).
Nevertheless for Cannabis growing with small amounts of UV-B light influence the plant to produce higher percentages of THC as the plant senses these wavelengths as dangerous to it's flowers. As in nature our indoor plants will develop increased amounts of resin, oil and the resin glands trichome production in an attempt to ward off these potentially damaging wavelengths.
A grow lamp that emits even a small percentage of photons in the UV-B range will trigger the plants defensive reaction to the onset of these wavelengths with increased trichome production. The head of the trichome is designed to capture light from all angles and then focus that light within the trichome resin head which is where the plants later stage THC will be found.
In total I count 7 wavelengths that are essential for optimal cannabis growth. 4 stimulate photosynthesis, 1 stimulates the Emerson Enhancement Effect, 1 fulfills the quantum efficiency requirements for faster growth rates and bigger yields and the last stresses the plant to produce more resin.
The Goldilocks zone are those 6-wavelengths you really want to look for:
525nm, green (faster grow, bigger, yields)
320-290nm, UV-B light (stress)
