LED - 660nm + 730nm - Emerson effect = WtHeck :P

[Kassiopeija]

Fantastic information you have shared in this thread
This begs the question - Why are lighting systems of differing technologies being compared only with the metric of 400-700nm?

Thank you very much. It's either convenience or dis-knowledge of PBAR "Photo-Biologically-Active-Radiation" - a newer metric to better describe light-usage by plants 360-780nm.
The meaning of PAR ("photosynthetically-active radiation") is sadly misleading, plants can do photosynthesis beyond that region. Yeah, extending into both ends beyond 400-700nm.

But historically, photosynthesis wouldnt occur in the farred region when scientists dissolved chlorophylls in organic solvents to study it under glass.
But it's not the same compared to intact leaves.

And towards the UVA, there UV-induced photoinhibition also rendered rates down.
But now we know that plants can adapt via sunscreen pigments and chloroplast-travel, to harsh light-conditions. Though the UV inhibition is still real, they can cope with it if it's comparatively weak (like outside the sun's spectrum)

 

[Kassiopeija]

"Photo-Biologically-Active-Radiation

ironically PBAR is also somewhat misleading as it was coined to reflect that, "even though light outside of PAR wouldn't do photosynthesis", it's still effective via photosensors. Incorrect, and also incomplete range, see UV-B. Heck, yesterday read a study using UV-C pulses (very late in flower) to increase a healthy substance 'rutin' in vegetables.
Plus, even the UVR8 monomer will deliver absorbed photons to chemically conserve them. Although this, by no means, can compensate for the photo-inhibitory nature of UV light (esp. UV-C, UV-B, far UV-A... with N-UVA possibly either this or that, depending on its radiant flux)

So all lightwaves in between 280-780nm can, and should be, considered both photo-biologically active and photosynthetically usable.
Though the 750-780nm region is extremely ineffective. And, for Cannabis, the 330nm not really explored when it comes to effects using a monochromatic source.
I'd say 360-750nm + UVB is what need to be looked at always

 

[Kassiopeija]

Confirmation of a scientist (over at science.org) that investigated FR-light on photosynthesis that the Emerson-Effect is caused when whitelight (driving PS2) is enriched by far-red light (driving PS1)


the interesting quote is:
"... light in the 400 to 680 nm range (which stimulates PSII more than PSI)"

>> so, if farred is missing in all those cheap white-light bogus spectras, there will NOT be an equal 1<>1 photosynthetic stoichiometric response than under natural sun-light.

The question is:
How much is needed or: how much could be possibly added w/o causing too much trouble from the phytochrome reaction?

 

[PJ Diaz]

Emerson effect is basically shade avoidance. The plants get confused, as they sense the far red and think that they are in the shade, so they naturally elongate in search for more light. Somehow however they don't realize that they are not in the shade, and they are being blasted with light, so the elongation effect from shade avoidance becomes more of a productive expansion. I think for us cannabis growers, the real question is WHEN is this so called "Emerson Effect" useful to us? And perhaps more importantly, how are various cultivars affected differently, and how is the effect also different during the various life stages of cannabis from seedling to mature flowering?

 

[Kassiopeija]

Emerson effect is basically shade avoidance

Absolutely not, it refers to the photosynthetic drive & interplay of the 2 photosystems as is known as the Z-schematics, while the shade-avoidance-effect is triggered by the phytochrome-phototeceptor that causes an internal hormone change.
There are many studies measuring the farred photosynthetic effects without tapping into the phytochrome response - oftentimes, photosynthetic rates will just be measured on 1cm^2 on a single leaf, for a very brief time.

And the SAS can be suppressed by 660nm light or UV/blue... although granted, it has several levels or layers and I would think that some of the studies that arrive at the result that farred-enriched light promotes higher dry harvest mass or swifter flowering is partially due to that.

But a real SAS would see your plants stretch like mad like beans, no sideshoots and even the buds would look bizarre elongated. I remember Bugbee has a video on that on cannabis, too.

At one time I thought the SAS could be used for its strong boost, but that's been the wrong turn. The phytochrome-farred needs to be suppressed, otherwise no sunplant will grow normally

 

[Kassiopeija]

The interesting thing is that the region between 680nm & 700nm will still suppress Pfr but already stimulate PS1>PS2, so it should be feasable with on-point monochromatic diodes to harness this.

 

[PJ Diaz]

Absolutely not, it refers to the photosynthetic drive & interplay of the 2 photosystems as is known as the Z-schematics, while the shade-avoidance-effect is triggered by the phytochrome-phototeceptor that causes an internal hormone change.
There are many studies measuring the farred photosynthetic effects without tapping into the phytochrome response - oftentimes, photosynthetic rates will just be measured on 1cm^2 on a single leaf, for a very brief time.

And the SAS can be suppressed by 660nm light or UV/blue... although granted, it has several levels or layers and I would think that some of the studies that arrive at the result that farred-enriched light promotes higher dry harvest mass or swifter flowering is partially due to that.

But a real SAS would see your plants stretch like mad like beans, no sideshoots and even the buds would look bizarre elongated. I remember Bugbee has a video on that on cannabis, too.

At one time I thought the SAS could be used for its strong boost, but that's been the wrong turn. The phytochrome-farred needs to be suppressed, otherwise no sunplant will grow normally

Maybe I should have been more clear. It's a shade avoidance trigger. I have a 730nm only LED lamp, and have experimented with it on Cannabis in a variety of varying applications. Have you experimented with far red and cannabis yourself, or are you just relying on Bugbee? Frankly, Bugbee has a lot of good info, but I think he is often misguided as well. I don't always agree with his assessments, which is why I don't follow him.

 

[Kassiopeija]

I hope I didn't post that already but the above shows where Pf<>Pfr cancels out.

The SAS (shade avoidance sysndrome) looks also quite distinct from cannabis plants grown under FR-enriched full spectrum or "Emerson"-660/730nm added racks:

^^ the pure IR incandescant blended bulb that filters out most visible PAR light caused this massive stretching, long internodes, no sidebranching, and steep petiole angle (hyponasty).
these are the signs of a severe SAS even in the presence of 7200k white fluoros mainlight, enriched by 400nm + UVB.

Still, the apical gain was tremendous per day and maybe some SCROG could use. @Sedan used to call that "the fishing rod"-technique - although he used insufficient lightstrength for that, while I blasted them with high PAR & heat. Even night-light

 

[Kassiopeija]

Maybe I should have been more clear. It's a shade avoidance trigger. I have a 730nm only LED lamp, and have experimented with it on Cannabis in a variety of varying applications. Have you experimented with far red and cannabis yourself, or are you just relying on Bugbee? Frankly, Bugbee has a lot of good info, but I think he is often misguided as well. I don't always agree with his assessments, which is why I don't follow him.

Yes I have, and I understand both fine. I don't have to rely on Bugbee exclusively because I have the major academic schoolworks of photobiology at home. The one diagram showing the differences of PSI & PS2 is out of such a book... but most of this stuff is not very readily available for free on the internet, and Bugbee is a good source as he can explain complicated matters in popular words.

Read the studies above you will find all the proof of what I said here. I've even highlighted them for you.

The problem with the current state of affairs is that Emerson's significance got lost. From 1950 there were mostly HIDs available for lighting so there was no luxury to pick on isolated lightwaves anyway...

 

[PJ Diaz]

Yes, I have plenty of horticulture text books myself. I think sometimes folks get a bit lost in the academics of it all and end up not being able to see the forest for the trees. I work at a College and know plenty of academically intelligent folks who are professional idiots.

 

[Kassiopeija]

Yes, I have plenty of horticulture text books myself. I think sometimes folks get a bit lost in the academics of it all and end up not being able to see the forest for the trees. I work at a College and know plenty of academically intelligent folks who are professional idiots.

I'm with you, that's why I try to put things on test, albeit crudely.
There's also the problem that change in science happens slowly, it's normal that people just stick to what they learned at one point in time.

But I cannot quantify how much this effect later is, esp. because of a few other phenomenae - cyclical electron flow & LHC2 antenna travel. The photosystems have various other methods to balance them out.

But in a review on the green & farred enriched shadelight, there was a description of how the plants photosynthetic apparatus changes itself to optimize the harnessing of these photons. Even though they are not high in radiant flux, but they are more omnipresent in deeper layers. Esp. 700-780nm. Outside in nature a large portion of the total photosynthates actually stems from just this shadelight. Like 20-40% (on sunplants). Thing is, most of the direct photons are turned to heat, esp. in the first leaf.

 

[Kassiopeija]

Photosynthetic activity of far-red light in green plants

Abstract
We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, QA. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8 ± 0.5 kJ mol−1 and 12.5 ± 0.7 kJ mol−1 have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol−1 between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants

Fig. 1. Oxygen evolution rate traces measured on a sunflower leaf under laser illumination. The laser wavelengths in nm and absorbed light intensities in μmol m−2 s−1 units are indicated, as is the decrease of the wavelength during exposures. The leaf temperature was 22 °C; the background concentrations of CO2 and O2 were, correspondingly, 360 and 50 μmol mol−1. A double-headed arrow specifies the reading taken at 740 nm as an example. The light-saturated O2 evolution rate of the leaf was 20 μmol m−2 s−1.

>> this study covers alot of aspects in detail about the Oxygen-evolution (read: photosynthesis) though it will not investigate into the nature of the special 'darkred' (FR) chlorophylls - how these come into being.

Today we now know these are several chlorophylls working together to enable these special excitonic states.

 

[OneHitDone]

Photosynthetic activity of far-red light in green plants

Abstract
We have found that long-wavelength quanta up to 780 nm support oxygen evolution from the leaves of sunflower and bean. The far-red light excitations are supporting the photochemical activity of photosystem II, as is indicated by the increased chlorophyll fluorescence in response to the reduction of the photosystem II primary electron acceptor, QA. The results also demonstrate that the far-red photosystem II excitations are susceptible to non-photochemical quenching, although less than the red excitations. Uphill activation energies of 9.8 ± 0.5 kJ mol−1 and 12.5 ± 0.7 kJ mol−1 have been revealed in sunflower leaves for the 716 and 740 nm illumination, respectively, from the temperature dependencies of quantum yields, comparable to the corresponding energy gaps of 8.8 and 14.3 kJ mol−1 between the 716 and 680 nm, and the 740 and 680 nm light quanta. Similarly, the non-photochemical quenching of far-red excitations is facilitated by temperature confirming thermal activation of the far-red quanta to the photosystem II core. The observations are discussed in terms of as yet undisclosed far-red forms of chlorophyll in the photosystem II antenna, reversed (uphill) spill-over of excitation from photosystem I antenna to the photosystem II antenna, as well as absorption from thermally populated vibrational sub-levels of photosystem II chlorophylls in the ground electronic state. From these three interpretations, our analysis favours the first one, i.e., the presence in intact plant leaves of a small number of far-red chlorophylls of photosystem II. Based on analogy with the well-known far-red spectral forms in photosystem I, it is likely that some kind of strongly coupled chlorophyll dimers/aggregates are involved. The similarity of the result for sunflower and bean proves that both the extreme long-wavelength oxygen evolution and the local quantum yield maximum are general properties of the plants

>> this study covers alot of aspects in detail about the Oxygen-evolution (read: photosynthesis) though it will not investigate into the nature of the special 'darkred' (FR) chlorophylls - how these come into being.

Today we now know these are several chlorophylls working together to enable these special excitonic states.

Loving all of the deep science info your presenting here.
As of current - what is your preferred lighting rig?
Also, in your opinion how does the far red spectrums play into nutritional quality on edible crops?

 

[Kassiopeija]

Also, in your opinion how does the far red spectrums play into nutritional quality on edible crops?

Not much, it's mostly known for "mass-building" but it can keep a plant healthy (reduction of heat) or deliver higher access to photosynthates. Higher quality is usually associated with UVA. But I belief UVA & FR belongs together, in a way they antagonize each other while also mutually minimize potential harmfull effects.

 

[Kassiopeija]

No Interactive Effects Among Blue, Green, and Red Light
The Emerson enhancement effect describes a synergistic effect between lights of different wavebands (red and far-red) on photosynthesis (Emerson, 1957). McCree (1971) attempted to account for interactions between light with different spectra when developing photosynthetic action spectra and applied low intensity monochromatic lights from 350 to 725 nm with white background light to plants. His results showed no interactive effect between those monochromatic lights and white light (McCree, 1971). We tested different ratios of blue, green, and red light and different PPFDs, and similarly did not find any synergistic or antagonistic effect of different wavebands on any physiological parameters measured or calculated.

Frontiers | Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms

Red and blue light are traditionally believed to have a higher quantum yield of CO2 assimilation (QY, moles of CO2 assimilated per mole of photons) than gree...

www.frontiersin.org

>> A white light, or mixed blurple monochromatic light in the range of white light is not going to create an enhancement ("Emerson Effect")

 

[Kassiopeija]

https://febs.onlinelibrary.wiley.com/doi/full/10.1016/j.febslet.2005.04.088

In oxygen-evolving photosynthesis, the conversion of solar energy into electrochemical potential requires two photosystems, photosystem II (PSII) and photosystem I (PSI), operating in series. Light energy absorbed by the chlorophylls (Chls) of antenna systems of the two photosystems is rapidly transferred to the respective reaction centres where sequences of electron transfer reactions are triggered. For maximum efficiency an even distribution of absorbed light quanta between the photosystems is needed. This requirement is, however, severely degraded at the far-red end of the absorption spectrum of plants due to disparate spectral tuning of the centre pigments to about 680 and 700 nm for PSII and PSI, respectively [1]. Most of the antenna Chls are adjusted to 680 nm or shorter wavelengths, so that mainly excitation transfer occurs energetically downhill. Yet, in the PSI antenna a small number of far-red Chls is present, which absorb at even longer wavelengths than the PSI centre [2]. Occurrence of similar far-red Chls in the PSII antenna is not recognized. The unbalanced distribution of far-red light excitations between the two photosystems results in a well-known red drop of the photosynthesis quantum yield [3, 4]. Though this drop has frequently been observed in the earlier work [5, 6], its extension to the far-red spectral range has not been fully characterized.

>> this 'red drop' is what Emerson has been investigated as a pioneer of this phenomenum.

Here you can see the differential quantum yields of PS1 vs PS2:

it is no surprise that at around 680nm it changes abruptly as PS2 has its energetic "downhill" trap sitting there (p680)

and vs sunlight, leaf-absorption & background light:

>> the area-under-the-curve of PS2 of light <680nm creates an exciton-overflow in typical white light. This can be offset by light >680nm-750 (790max for some species).

Remarkably the 2 regions show a somewhat similar areal - which would lead to a 1<>1 photochemistry if the lightcolors represent the ratio of an equalized sun-spectrum.

Action spectra of photosystems II and I and quantum yield of photosynthesis in leaves in State 1

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochr…

www.sciencedirect.com

Far-Red light absorption by 'red-chlorophylls" in sunflower & bean:

^^ this is now investigated for PS2 to have a few of these far-red chlorophylls as well (in PS2-LHC), though studies suggest that these would only account for ~~10% of all captured far-red photons.

The (first) Emerson-Effect:

("first" relates to the increased photosythesis rate, available literature usually describes the 2nd E.E. as the red-shifted enhancement of the 'red-drop')

 

[Kassiopeija]

Now follows are excerpts from a short review which basically says anything on the subject:

Why Far-Red Photons Should Be Included in the Definition of Photosynthetic Photons and the Measurement of Horticultural Fixture Efficacy
by Shuyang Zhen, Marc van Lersel and Bruce Bugbee

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8258408/

Introduction
Photons above 700 nm have minimal photosynthetic activity when applied alone (Emerson and Lewis, 1943; McCree, 1971) and have thus been excluded from the definition of photosynthetically active radiation (PAR; 400 to 700 nm). However, those longer-wavelength photons have synergistic activity with photons in the PAR range (Emerson et al., 1957). Recent studies using lasers and LEDs with narrow-band spectra have provided new insights into the photosynthetic value of far-red photons (here defined as 700 to 750 nm). Far-red photons preferentially excite photosystem I (Zhen and van Iersel, 2017), at wavelengths at least up to 732 nm (Zhen et al., 2019). In crop-plant communities, far-red photons elicit photosynthetic activity equal to PAR photons when delivered at up to 30% of the total photon flux (Zhen and Bugbee, 2020a). The quantum yield of plant canopies (per 400 to 750 nm photons) is similar under blue + red or white LEDs with and without a 15% far-red photon substitution (Zhen and Bugbee, 2020b). The definition of photosynthetic photons, and efficacy measurements of horticultural fixtures, need to include far-red photons because this extended range (referred to as ePAR) better predicts photosynthesis.

Historical Background
Photosynthesis has long been known to be wavelength-dependent (Hoover, 1937; Emerson and Lewis, 1943). At low photon flux densities, McCree (1971) and Inada (1976) found that red photons (600–700 nm) drive photosynthesis more efficiently than green (500–600 nm), followed by blue (400–500 nm) photons. Because green photons penetrate deeper into leaves, more recent studies indicate that at higher photon flux densities red and green photons are used more efficiently than blue photons (Terashima et al., 2009; Liu and van Iersel, 2021). Longer-wavelength far-red photons (above 700 nm), on the other hand, are largely inactive for photosynthesis when applied alone (Emerson and Lewis, 1943; McCree, 1971) and have thus been excluded from the definition of photosynthetically active radiation (PAR; 400–700 nm).

The rapid decline in photosynthetic efficiency at longer wavelengths (above ~685 nm) was first observed by Emerson and Lewis (1943) in green algae (the “red drop”). Over a decade later, the same research group found that the photosynthetic rate under simultaneous illumination with photons above 680 nm and shorter-wavelength light was greater than the sum of the rates from applying each light separately (Emerson et al., 1957). This is now known as the Emerson Enhancement Effect. This enhancement effect among shorter- and longer-wavelength photons was later found to be due to the distinct excitation spectra of the two photosystems—PSI and PSII (Hill and Bendall, 1960; Duysens and Amesz, 1962). While the discovery of the Emerson Enhancement Effect contributed to the identification of PSI and PSII, the significance of the wavelength synergy in photosynthetic efficiency received little attention over the next 50 years and the spectral effects on photosynthesis continued to be studied under monochromatic lights. The main reason for this oversight is the belief that only the photosynthetic efficiency of longer-wavelength photons (~680 nm up to 720 nm) was improved by the supplementation with shorter-wavelength light, rather than a two-way synergistic interaction in which shorter- and longer-wavelength photons improve each other's photosynthetic efficiency; thus, the practical impact of the enhancement effect on photosynthesis was thought to be largely insignificant (Emerson et al., 1957; Myers and Graham, 1963; McCree, 1972a). As a result, the now widely accepted definition of PAR was developed without taking account of synergistic effects on PSI and PSII photochemistry between far-red and shorter-wavelength photons.

This 400–700 nm range was recommended by McCree (1972b) from among the most popular definitions of PAR in use at the time. He concluded that the photon flux density between 400 and 700 nm was “an acceptable definition of photosynthetic flux” for nine commonly used broad-spectrum lights. Interestingly, he found that photosynthetic rates, normalized based on PAR, were highest under a high-pressure sodium light (with most of the light in the red part of the spectrum) and a quartz-iodine light (rich in red and far-red photons). None of the definitions of PAR analyzed by McCree accounted for photons with wavelengths >710 nm. His study did not test whether including far-red photons in the definition of PAR would improve the correlation with the photosynthetic rate.

 

[Kassiopeija]

Recent Studies Indicate that the Classic Definition of Par Needs to Be Revised
Recent advances in light-emitting diode (LED) technology have enabled researchers to re-visit the Emerson Enhancement Effect and study not only the short-term photosynthetic responses of single leaves under low light but also the long-term responses of plant canopies under higher photon flux densities. Zhen and van Iersel (2017) found that adding supplemental far-red photons from LEDs (peak at 735 nm) to red+blue or white LED light synergistically increased the quantum yield of PSII and leaf photosynthetic rate over a wide range of light intensities (also see Murakami et al., 2018). The enhancement was slightly larger under the red/blue background light than under a warm white LED, probably because the warm white LED light already had 4% far-red photons

Zhen et al. (2019) studied the effects of photons from 678 to 752 nm using laser diodes that had a narrow spectral output [full width at half maximum (FWHM) of 2–3 nm]. As the wavelength of the photons increased from 678 to 703 nm, they increasingly excited PSI more efficiently than PSII. Photons up to 732 nm significantly enhanced photosynthetic efficiency by exciting PSI, but photons above 752 nm were not effective. There was a gap between 732 and 752 nm because laser diodes were not available in this region.

In a subsequent study, Zhen and Bugbee (2020a) measured the net photosynthetic rate of plant canopies (including leaves, stems, roots; with communities of plants inside 100 L gas exchange chambers) in 14 diverse species and found that photons from far-red LEDs (700–750 nm; peak at 735 nm) were as effective as traditional 400–700 nm photons when applied at up to ~30% of the total photon flux. As expected, far-red photons alone were not effective. Additional far-red photons applied at more than 30% of the total photon flux did not result in further increases in photosynthetic rate. The photosynthetic response to increasing far-red photon flux likely saturates, because under a light composed of mostly shorter-wavelength photons that over-excites PSII, only a certain number of far-red photons are needed to restore the excitation balance between PSI and PSII. An enhancement effect only occurs up to the point at which this balance is reached.

Zhen and Bugbee (2020b) followed this study with a long-term study with lettuce grown under either blue+red or white LEDs, each with and without 15% far-red photons from far-red LEDs. The total photon flux from 400 to 750 nm was equal among spectral treatments. Photon capture and canopy gas exchange were continuously measured, which allowed the analysis of canopy quantum yield (CQY; moles of CO2 fixed per mole of absorbed photons). CQY was equal among treatments from planting to harvest, confirming the important role of far-red photons for photosynthesis.

We have studied the effects of far-red photons at the photosystem, leaf, and canopy level, with consistent results at all three scales. Collectively, these findings provide compelling evidence for the photosynthetic value of far-red photons when combined with shorter-wavelength photons, as long as the far-red flux does not exceed about 30% of the total photon flux. This is equal to or higher than the fraction of far-red photons in sunlight under which plants have evolved.

Unfortunately, the Design Lights Consortium (2021), following ASABE Standard S640 (2017), recently decided to not expand the definition of PAR to include far-red photons, despite clear evidence of the photosynthetic efficacy of those photons. Design Lights Consortium kindly cites our research in their decision, but we do not believe that our research was interpreted accurately. One concern raised by Design Lights Consortium (2021) is that the enhancement effect may depend on the spectrum of the background light. As we explained earlier, the larger enhancement effect of far-red photons with red+blue background light compared to that under a white background light was most likely because the white light already had ~4% far-red photons. In addition to this, Zhen and Bugbee (2020b) found that the value of far-red photons was equal with either blue+red or white LEDs (contained ~1% far-red photons). As far as we know there is no experimental evidence suggesting that the spectral composition in the 400–700 nm range affects the magnitude of the enhancement effect.

Practical Limitations
Far-red photons typically cause significant stem, leaf, and/or petiole elongation, which will likely limit the maximum fraction of far-red photons to less than about 20% of the total photon flux for most crops. Because of these powerful effects, we recommend that LED manufacturers clearly indicate the fraction of far-red photons in fixture specifications [(700–750 nm)/(400–750 nm)].

>> this basically says it all. Up to 30% of total radiation could be far-red light, while for physiological reasons that number should be rendered down to 20%. That is still MUCH!

 

[Kassiopeija]

Simplified version of Photosystem 1 - LHC, chlorophylls, electron-transport chain & carotenoids; the red-chlorophylls are speculated to work together as dimers or trimers to enable the 'uphill' capture of far-red photons:

7.2. The `red' chlorophylls
In photosystem I there are chlorophylls which absorb at longer wavelength than P700 (so called `long wavelength' or `red' chlorophylls). Their number varies among different organisms [153,157,158].
PS I from S. elongatus contains nine to 11 Chla per monomer which absorb light at longer wavelengths (`red shifted') than the bulk of antenna Chla [157].

Two functional roles have been discussed for the `red' chlorophylls:
facilitation of efficient light energy capture under extreme environmental conditions, and focussing of the excitation energy to the reaction centre [113].
The red shift can be caused by several types of molecular interactions or properties of the chlorophylls: excitonic interaction of cofactors [159], deviations from planarity of the chlorine ring [160], charge effects [161], electronic interactions with polypeptide side chains, and changes in the dielectric constant of the local environment of the absorbing cofactors [162]. The structure at 2.5 As resolution can not provide the final answer to the question of where the 'red' chlorophylls are located.
However, tightly interacting chlorophylls, forming dimers and trimers of chlorophyll molecules, can be identifed. These chlorophylls, which are most probably excitonically coupled, are possible candidates for some of the long wavelength chlorophylls. In the following, the discussion will focus on a unique Chl trimer and three Chl dimers located at prominent positions in the complex. On the lumenal side of the membrane close to PsaX, a Chla trimer formed by aC-B31, aC-B32 and aC-B33 is located. The arrangement of the three Chla can be best described as a staircase, where the pigment molecules can be seen to be related to each other by an approximate translation, with their ring planes oriented roughly parallel (Fig. 9).
The interplanar separations between the chlorophylls are in the range from 3.5 to 3.7 As, with lateral centre to centre shifts of 8.3 As. Strong excitonic coupling is implied by this geometry of approximate translational symmetry, because the transition dipole moments of the chlorine rings are roughly parallel. PsaB coordinates only one of these three chlorophylls, aC-B31, directly via HisB470, whereas the Mg2‡ ions of the other two Chla molecules, aC-B32 and aC-B33, are coordinated to water molecules. It is remarkable, that the topography of this chlorophyll trimer is similar to small molecule crystal structures of chlorophyll derivatives [163], showing an absorption maximum at 740 nm within the crystals. For these chlorophyll derivatives, the extreme red shift compared to solutions was explained by strong excitonic interactions supported by interactions within the stacks. Only one Chla trimer is observed in the structure of PS I from S. elongatus. The cause of the lack of a chlorophyll trimer in PsaA at a position related by the pseudo-C2 axis could be an interference of the hypothetical PsaA counterpart with the monomermonomer contact within the trimer. Aî At the lumenal side of the membrane at the interface between PS I monomers, one pair consisting of chlorophylls aC-A32 and aC-B7 is located. These two chlorophylls are coloured red in Fig. 9. They are separated by a centre to centre distance of 8.9 and an interplanar distance of 3.5 Aî. Hydrophobic contacts are observed between this pair of chlorophylls and the C-terminal region of PsaL from the adjacent monomer. In 1998, Palsson et al. detected a loss of two chlorophylls absorbing at 719 nm, when the trimer of S. elongatus was split into monomers [157]. Possibly, these two chlorophylls could be identical with the chlorophyll pair aC-A32/aC-B7. These chlorophylls could also play a role in excitation energy transfer between the monomers. Two further Chla dimers are located at prominent positions near the stromal side of the membrane, which are related to each other by the pseudo-C2 symmetry. One dimer is constituted by aC-A38 and aC-A39, coordinated to PsaA, the other consists of ac-B37 and aC-B38 coordinated to PsaB. The dimer aC-A38/aC-A39 is located close to PsaF, aC-B37/aCB38 close to PsaL. In contrast to the majority of the bulk chlorophylls, their chlorine head groups are oriented parallel to the membrane. The centre to centre distances within the pairs aC-A38/aC-A39 and aCB37/aC-B38 are 8.2 Aî and 7.6 Aî, respectively. Their ring systems are stacked at 3.5 Aî separation so that their electron systems overlap partially. In close proximity to each of the two Chla pairs, one of the `connecting' Chla is located; the centre to centre distances between aC-A38/aC-A39 and aC-A40 and aCB37/aC-B38 and aC-B39 are about 16 Aî. Based on geometrical considerations, we can speculate that the excitation energy transfer from these Chla dimers to the ETC can be achieved at reasonable rates via the two `connecting' chlorophylls, thereby suggesting that these two dimers as well as the `connecting' chlorophylls may play a special role in excitation energy transfer.

Structure of photosystem I

In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrins…

reader.elsevier.com

 

[flyawayclyde]

Really good point. Especially considering that indoor growing started with stolen street lights and miracle gro!

I am actually almost guilty of that.....