Avoid Misconceptions When Teaching About Plants

spurr said:

Is green light needed? No. Is blue light needed? No. Is red light needed? No. Plants can grow under only blue light and under only red light, and under only green light too. Does a plant grow better when given the whole PAR range? Yes, for a few reasons, not only about Pn (e.g., quantum yield), but also about greater irradiance intracaonpy, etc.

The main 'negative' effects from green light can be reduced growth rate and reduced stomatal conductance if green light comprises too great a percent of the total PAR irradiance.

This thread is not based on conjecture, it's based on well proven science that plants use green light for photosynthesis, as well as other photo-reactions. The main reason I mented green light in the first post was to point out the myth that plants do not use green light for photosynthesis.



I have known about that figure for a long time, and the web-page whence it came has some incorrect claims about photosynthesis. That is a flawed figure to use, in terms of Chl A/B, I pointed this out in the first post. The Chl A/B absorption spectra does not accurately show what plants use (wavelengths) for photosynthesis via chloroplasts in vivo. That is the whole point of my first post, i.e., only using the Chl A/B absorption spectra is highly flawed. Also, in that figure there only only three cartonoids listed. That chart is a poor representation of what is going on in a leaf, in vivo.


Accessory pigments do absorb green light (ex., some cartonoids), and there is only a slight dip in quantum yield (ex., K.McCree's Quantum Yield Curve) over green wavelengths compared to blue and red. The difference between quantum yield from blue to green to red is not large, read the work of K.McCree and many others for more info.

Only ~50-60% of green light is reflected, that is why we see leaves as being green; to assume nearly all green light is reflected is baseless.

It's a myth that there is a big difference between quantum yield of blue, green and red wavelengths in most higher terrestrial plants.

It's much better think about photosynthesis by chloroplasts than by chlorophyll:

"Chlorophyll alone is insufficient for plant photosynthesis. Many other enzymes and organic compounds are required. “Chloroplasts” is a better requirement." (cite)
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FWIW, cannabis like most higher terrestrial plants, are phytosystem I and II plants.


That has been covered in this thread ad nauseum. However, as noted in this thread, green light (photons) do not act upon lower chloroplasts until blue and/or red photons reduce the 'screening' effect of upper section of a leaf upon green photons (put simply).

Honestly it does not, at least to me. It seems you have fallen victim to the Chl A/B absorption spectra myth. What you wrote, the parts that were correct, have been covered in this thread already, except I didn't mention the energy of each photon at various wavelengths. I.e., blue light = more energy per photon (as much as twice that of red photons) but fewer photons per second at the same power as red light (power = energy/second); and red light = lower energy per photons but more photons per second. Green light has lower energy photons than blue but higher energy photons than red.

This thread was to make people aware of some myths believed to be facts in the cannabis world. Chief among them is the myth that plants do not use green light for photosynthesis and that all incident green photons are reflected. I do not see any reason to debate the merits of green light further, but, if you see a need feel free...

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You are clearly lacking in your undertanding of quantum mechanics and your use of power in place of flux is rather glaring but all in all I agree with much of what you had to say in this thread.

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spurr said:

@ highonmt:

One thing I forgot to mention: the main reason using an absorption spectra of Chl or accessory pigments is not sound in terms of light for photosynthesis (besides issues of in vitro vs in vivo and spectrophotometer vs integrating sphere/spectroradiometer and Pn/Chl fluorometer chambers) is a surprising amount of photons absorbed are not used for photosynthesis, much is lost as heat from the leaf. That is why using action spectra of photosynthesis (ASP) and relative quantum yield (QY), either alone or together, is much better than using the absorption spectra of Chl and some carotinoids as you posted above (as commonly found in vitro with a spectrophotometer). Out of all three, using a relative quantum yield curve is the best choice.

I have many papers on this topic, but below is a good intro and has good graphs showing differences in (normalized) spectra between "(a) leaf absorptance spectra, (b) leaf photosynthetic action relative to incident energy, and (c) leaf yield relative to quantum of energy absorbed (from McCree 1972a)". This paper was written by a plant physiologist I have spoken to many times, he works at Li-cor, the brand of quantum sensor I use to find irradiance in light studies and when setting grows rooms (i.e., umol/area/second).

That paper also discusses usage of an integrating sphere, i.e., absorbency, reflectance and transmittance. Fwiw, I plan on buying an integrating sphere/spectroradiometer to create action spectras of photosynthesis for cannabis, as well as creating quantum yield curves for cannabis after I buy a Pn/Chl fluorometer chamber. I still have much to learn about making ASP's and QY curves, but it's something I really want to accomplish for the betterment of the science regarding cannabis.

"Comparison of Quantum Sensors with Different Spectral Sensitivities"
http://www.licor.com/env/pdf/light/TechNote126.pdf​

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highonmt said:

Well done I wish you luck, and K+ for your scientific explorations of cannabis. OH And keep your patience stored up; a few days in front of a spectrophotometer can be maddening particularly when apparently identical experiments give vastly varying result. I watched a postdoc descend into a state of pathological paranoia while taking readings on a single compound he had to characterize to finish his thesus. He eventually changed his name to a Latin sounding phrase and dissapeared and I'm not shitting...Best of luck
HM

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spurr said:

I disagree, of course I only wrote about power (mW) in terms of photons per second to point out (in easy to understand terms) there are more red photons per X second than there are blue photons per X second at the same power (mW) due to lengths of waves and energy in photons.

The simple issue is it's better to provide the whole PAR range than simply blue and red. That said, I may have misunderstood your motives for posting, it seemed you were debating that green light isn't worthwhile in terms of Pn. It seemed to me like you were part of the LED crowd that has been hypnotized by the flawed usage of the Chl A/B absorption spectra. If that is not the case I apologize.

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