the peak wavelenghts are for clorophyll A - 430nm ; 662nm
for clorophyll B - 453nm ; 642nm
Those are lab measurements of chorophill absorbances. Whole leafs absorbance is quite different, and ive found many people researching on that forget it.
Leaf's action spectrum peaks for chorophills are 5-10nm higher, and other pigments apart of chorophills have its own photosyntetic action spectrum, wich must be take into account.
I upload some pics that show this.
First one is the Inada photosyntetic action spectrum (1976) average of 26 herb plants species and of 7 arboreus species. It computes the total O2 release for a equal irradiation (in watts, not in micromols of photons=mE) in each wavelenght. The other existing accurate curve of photosyntetic action is from McCree (1972) and its quite similar. Take note of max near 675nm, and the sharp fall off after that. Because of that, the higher experimental quamtum yield achieved was with a LD peaked at 667nm. For leds, it must be a little lower, due to wider band emission.
The blue part have a weak photosyntetic action, peaked a little over 440nm.
Second pic is geranium's quantum yield. Take note how flat it can be, and there are species with a flatter one.
Third one shows the blue light's phototropic action spectrum. Blue light are neccesary to plant's healthy growth.The higher effect on phototropism are between 450-460nm, and the same is valid for photomorphogenesis effect, although all the blue range are valid, and the cyan, too (wich is targeted by HPS blue enhanced lamps).
Plants react most to photons, not to energy radiated. As we obtain more photons per watt radiated as the longer the wavelenght, using the longer wavelenght over the valid range is the best to optimize efficiency.
Taking this into consideration, i choosed the royal blue units for the blue range. The lower binning of this color are peaked below 450nm. The problem of 430nm led is they are GaN dies, wich are far less efficients than modern InGan dies. Analyzing mcd ratings of 430nm peaked (GaN) leds is a bit misleading due to its wide spectrum power distribution (SPD), wich enter the cyan and green range, wich apport most of the photometric value, giving the impression they are as efficients as InGaN.
For red range, any led peaked over 635nm will work fine, but as longer the wavelenght without emmiting over 685nm, the best. 660nm leds are prefered, but most of them uses AlGaAsP dies, valid basically in low currents devices, although Chip On Board tech can pack them very tight. The problem is the manufacturer's new research is being done with AlInGaP dies, wich works better with shorter wavelenghts. It can be obtained easily in peaks over 640nm, wich work well, targeting Chl B peak absortion. I believe a mix of both will work very well.
In the other hand, is proved experimentaly that a bit of 700nm light improve the whole red photosyntetic action. The problem of 700nm leds is they have a very, very wide bandwith emission (from 600 to 800nm).
The far red is required in many plants species to induce flowering, as well as health root development. 730nm is the best, the peak of far red phitochromes action.
A main consideration in plants grow led systems is stomata reaction to light quality.
Leds dont emit noticiable infrared (IR) light, so the water consuption is largely reduced. This lead to stomata closing. Aditionally, its proved the effect of red light (peak at 660nm) inducing stamatas closure, as well as the inverse effect of blue light (all the range). Excesive stomata closure lead to a disminished photosyntesis, due to CO2 lacking, in C3 plants (most of commercially ones, except of maize, C4). So, any action must be taken. Either lower humidity (but not excesive, so it can close stomatas, too), more blue light (reducing the average photosyntetic efficiency), or CO2 supplementing.
To finish this large post, i suggest read the articles in this page:
International Lighting in Controlled Environments Workshop, specially the photosyntesis related, although all are very interesting.
k
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