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Too much light?- what happens

Posted by zink 6a (My Page) on
Mon, Jan 24, 05 at 13:37

How do plants do to defend against excess light? New research.

http://www.lbl.gov/Science-Articles/Archive/PBD-plant-photo-protection.html

Zink

Here is a link that might be useful: Key Molecule in Plant Photo-Protection Identified


Follow-Up Postings:

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RE: Too much light?- what happens

thanks zink. very interesting stuff.


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RE: Too much light?- what happens

  • Posted by John_Z 7B Atlanta, Ga. (My Page) on
    Tue, Jan 25, 05 at 19:53

Zink, that was a great share. I read about the possibility that xanthophylls(carotinoid yellow pigments in leaves)might be important in protecting damage within the chloroplast as solar intensity increased a few years ago - now I know it is confirmed thanks to you. It was suspected that the zanthophyll pool of pigments existed as as violaxanthin in the plastids at dawn, and that compound was change to an intermediary compound called antheraxanthin and then zeaxanthin as sunlight intensity increased. It all got converted back for the next day if my memory serves me right. Amazing survival genes.

Light saturation point and light compensation point are basic keywords for those wanting to understand the essential process. But what was really cool to me was first learning about acclimation/acclimitization of plants to different light intensities - out from the yard into the growing area under artificial lights, for example. The disc-shaped chloroplasts move from a diagonal position in "higher light" and reorient themselves to a more horizontal position to capture more PAR in lesser intensities. The chloroplasts are no longer stacked up like plates, but become spread-out. Leaves therefore can become longer and thinner. If the red to far-red ratio is off, the plant becomes stretched ("internode elongation"). I wish I had the money to do the experiments under artificial light and photograph the effects of lamp spectral power distribution, PPFD, etc.

I thank you for sharing this, Zink. Also, you mentioned on a foot-candle measurement thread not reading articles about those who feed the lesser absorption areas with the right spectra to feed beta-carotene (the most abundant accessory pigment). Using quality spectroradiamtry equipment - I've never seen any carotinoid absorb outside the 400-500 (510) nm range, except for useless UV a&b. I still assert the "violet & blue" to "red" is 2:1.


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RE: Too much light?- what happens

This is my first year giving growing under light a "professional" try previously just used a couple of grow . Also what color bulb or K rating do you use. I have an old (1957) book on gardening under lights that talks about T-10 bulbs.. Also using a ratio of 3 watts florescent to 1 watt incadescent. Studies have found that florescent while giving green growth it's the incadescent invisable red rays that produces flowers. Granted this book is 40 years old and by now there must be grow lights that duplicate the sun

thanks

derfy@sbcglobal.net


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RE: Too much light?- what happens

"Also, you mentioned on a foot-candle measurement thread not reading articles about those who feed the lesser absorption areas with the right spectra to feed beta-carotene (the most abundant accessory pigment). Using quality spectroradiamtry equipment - I've never seen any carotinoid absorb outside the 400-500 (510) nm range, except for useless UV a&b. I still assert the "violet & blue" to "red" is 2:1."

Interesting.

Could you expand on this?

If I understood the article Zink supplied, the carotinoids are the 'shock absorbers' of the photosynthesis world.

Plants are evolved to prosper under the less than ideal light levels of Spring and Fall, so in Summer they get more than they need.

The cure is to ditch the excess energy to the carotinoids so the chlorophyl goes back to it's resting state.

If chlorophyl gets too revved up it would start oxidizing the plant's cellular structures and cause damage.

Exactly like revving a car engine too high, in fact. An engine run over it's safe range for too long will burn out.

But you mentioned :

"I've never seen any carotinoid absorb outside the 400-500 (510) nm range, except for useless UV a&b. I still assert the "violet & blue" to "red" is 2:1."

Now, as carotinoids aren't the main energy factory of a plant cell, why would it matter what light frequency they absorb?

They are picking up the excess energy chemically, not photonically. It's electron exchange.

Do carotinoids play a secondary role than shock absorber? If they act as secondary light converters, what specifically is that energy used for?

And what I really want to ask, is about the light balance you mention.

Plants use mainly Far Red. Violet and Blue are useful for phototropism, following the Sun, of course.

So, why would a 2:1 balance of Blue-Violet to Red be important?


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RE: Too much light?- what happens

I think its too simplistic to say red light is for growth and blue/UV for phototropism. The photosynthesis action spectrum specifically measures the metabolic rate, usually by CO2 usage, at each wavelength. This curve shows two peaks of more or less equal height in the far blue and far red, with a green trough dipping to 30%-60% of the peaks. Notice that light is used for photosynthesis across virtually the whole spectrum from 300nm to 700nm even though chlorophyll only absorbs strongly over perhaps 25% of that range.

The action spectrum for phototropism is weighted more towards the blue but that doesn't change the photosynthesis action spectrum. Blue and red both contribute strongly towards plant growth, and wavelengths in between a little less strongly. Blue also contributes strongly to phototropism with other wavelengths also causing phototropism but less strongly.

One function of carotenoids is to take care of energy not directly absorbed by chlorophyll, but it does much more. The carotenoids and other accessory pigments absorb light outsode the absorption range for chlorophyll and transfer it to the chlorophyll molecules. Then that energy is used just like light directly absorbed by the chlorophyll. In this way, energy in the green portion of the spectrum can be used for photosynthesis even though chlorophyll itself has very little ability to absorb light at these wavelengths.


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RE: Too much light?- what happens

"In this way, energy in the green portion of the spectrum can be used for photosynthesis even though chlorophyll itself has very little ability to absorb light at these wavelengths."

This is really interesting, thanks. So, the carotinoids act as energy buffers.

Do you know what the specific pigments in tree leaves are? The yellow and reds we see in the Fall, those are carotinoids too?

As I understand it, the reason chlorophyll can't use green light directly is that it IS green. Therefore green light mostly bounces off.

The analogy is that if I were aiming a green laser at you, and you were wearing a green shirt, 90% or more of that light would be reflected away.

I keep wondering if frequency splitting or doubling is going on here.

For example, if you triple red laser light by collimating it, you get blue light.

And if you use a chemical process to split blue light by flourescence, you can get the same amount of energy divided into lower wavelengths such as red.

So, it's pie in the sky theorizing, but I wonder if some of these compounds are flourescing and re-emitting the green light energy at frequencies the chlorophyll can use directly.

Or heck, maybe it's all electron transfer.


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RE: Too much light?- what happens

Do you know what the specific pigments in tree leaves are? The yellow and reds we see in the Fall, those are carotinoids too? Some of the pigments in autumn tree leaves are carotenoids which do not break down as quickly as the chlorophyll. Take out the green and you can see the yellow/orange carotenoid pigments clearly. There are oher pigments in autumn leaves, called anthocyanins, which are only created at that time of year and have stronger red tints.

As I understand it, the reason chlorophyll can't use green light directly is that it IS green. Therefore green light mostly bounces off. Yes, but it would be equally true to say that the reason chlorophyll is green is because it can't absorb green light :)

I keep wondering if frequency splitting or doubling is going on here. The pigments involved in photosysnthesis do fluoresce to some extent. Chlorophyll fluorescence is well-known, but the energy re-emitted is small compared to the original energy absorbed. Much of the energy transfer from the accessory pigments to the chlorophyll is through a process known as resonance energy transfer. This is not strictly the emission of light from one molecule and the absoprtion of it by another, but the process is similar. As a quantum effect, it is dangerous to try to describe it in strict classical terms. For example, the process does not occur over long distances and it is not possible to "intercept" the energy in the process of being transferred. The only observable effects are the input of energy to one molecule and the consequent output of it from a different molecule.


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RE: Too much light?- what happens

"For example, the process does not occur over long distances and it is not possible to "intercept" the energy in the process of being transferred. The only observable effects are the input of energy to one molecule and the consequent output of it from a different molecule."

Freaky. So, does anybody really know how it works? Spare electron swapping?


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RE: Too much light?- what happens

All these quantum, action at a distance, invisible things seem a bit freaky, but this is fairly well-understood.

Here is a fairly basic description, and here is a much more technical article.


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RE: Too much light?- what happens

This is ... interesting.

From what I gathered, it IS a form of flourescence that works by near contact.

"Fluorescence resonance energy transfer (FRET) is a nonradiative process by which the excitation energy can be passed from a fluorescent donor molecule (D) to an acceptor chromophore (A) over long distances, typically 10 100 "

(I don't really see why they say it's nonradiative. If it worked by direct contact, why not say so? If not, what is carrying the energy?)

The energy is captured by a 'donor' molecule and then re-transmitted to the 'acceptor' molecule. Which then uses it directly.

The distance is SO short that an outside observer won't see the photons jump.

It reminds me of a few things. There's a form of computer chip cooling that works in a similar way.

You charge a metal plate with high-voltage electricity. Not quite enough that the electrons will start 'jumping ship'.

Then, as the metal plate is attached to a computer chip, it gets hot. And that gives the electrons the nudge they need to boil off.

Since that uses energy, the metal plate cools off right away.

Other possibilities present themselves. The inventor Nikola Tesla was supposed to have a form of radiant lighting in his NY lab that was tubeless.

No plans of how it worked survive, but I wondered. He did a LOT with high-voltage AC. And technically, you could boost AC all the way into the visible light range.

That is, the wavelength and frequency of electricity could theoretically be in the same range as the wavelength of visible light.

So, I asked my electronics teacher about this idea. And basically, this super-high frequency AC, grounded to a metal plate, would radiate off into the air.

Maybe making the air itself flouresce. Or maybe the electricity would just pop photons off the surface of the metal. In whatever wavelength the AC was boosted to.

Then, a dim memory of Physics class. I was wondering if laser effects could happen in very large molecules.

The longest wavelength a laser can put out should be limited by the size of the atom. But if molecules could swap electrons .... !!

You could charge a 'donor' molecule just as any lasing gas would be charged, by flashlamp or electrically.

Then the 'donor' could spit that electron to another molecule a thousand angstroms away, resulting in Extra Low Frequency emissions.

No, I never got any teacher to answer these questions.


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