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Comment by russfink | Hacker News Reader

Why wouldn't fluctuations of sunlight that cause instability in the green spectrum also cause instabilities in the red and blue spectra?

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They do. The paper's point is that plants want to choose two wavelengths with different amounts of average power. This gives the plant a window to tune within to get the desired amount of output power. If the total illumination drops, the plant compensates by tuning more towards the high power wavelength, and vice versa.

But the paper makes the point that the tuning mechanism isn't perfect due to internal noise. If the power level difference between the two input wavelengths is too great, then the random fluctuation of the tuning mechanism will itself create a lot of noise in the output. So basically there's an optimal value for the difference of average power between the two wavelengths.

Now, why the plant doesn't absorb the peak green wavelength and the closest wavelength whose power is exactly the optimal difference less was unclear to me. I think the idea was that the plant also wants to minimize the wavelength difference, and a given power delta can be achieved with less difference in wavelength in the steeper sloping sections of spectrum power graph.

I think the point is that since the power available at these wavelengths vary sharply with frequency in these regions of the solar spectrum, the plant can easily compensate for brightness fluctuations in these portions of the spectrum with minor tweaks to the target wavelength of the relevant photosystems

I'd this is the case, it's a shame the article didn't mention it.

It makes sense the changing environmental conditions for a molecule could affect the wavelengths of light it absorbs (for example, pH, temperature, etc), but I'm not sure this has been demonstrated for plant pigments?

I'm confused though -- how can the plant possibly vary this?

In principle plants have several different variants of the light absorbing molecules with different absorption spectra but I don't know how this would actually be regulated. But it is certainly imaginable that plants vary the relative abundance of the different light absorbing molecules or their efficiencies and this could be more or less automatic because of varying conditions like pH or whatnot that affects efficiency or lifetime of the molecules with a simple feedback mechanism or it could be actively regulated by a more complex feedback loop.

> It might be highly efficient to specialize in collecting just the peak energy in green light, but that would be detrimental for plants because, when the sunlight flickered, the noise from the input signal would fluctuate too wildly for the complex to regulate the energy flow.

> Instead, for a safe, steady energy output, the pigments of the photosystem had to be very finely tuned in a certain way. The pigments needed to absorb light at similar wavelengths to reduce the internal noise. But they also needed to absorb light at different rates to buffer against the external noise caused by swings in light intensity. The best light for the pigments to absorb, then, was in the steepest parts of the intensity curve for the solar spectrum — the red and blue parts of the spectrum.

I admit there's a bit of a jump there (gotta read the actual article, not the journalist retelling I suppose), but I assume the gist of the math is something like this:

Lets say direct green light delivers a maximum 100 "units of photo-energy" -- gonna play loose with the physics to demonstrate the math.

When a cloud passes over it, lets say the intensity drops to only 75%. Lets also say for now we always convert the energy at 100% efficiency.

So with green light, your 100 units of energy drops by 25 units with each passing cloud.

Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units.

So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units. Yeah, you might get less absolute energy (60 units rather than 75), but if the cost of energy swings in your process was very high, the tradeoff might be worth it.

You could also play with the efficiency-of-conversion (ie have higher conversion efficiency at the lower-swing points) for some fun second-order effects.

This is just a toy example of what the underlying dynamics could be. Gotta read the actual paper to understand the actual model they developed. https://arxiv.org/pdf/1912.12281.pdf

> Now lets say blue light delivers only 80 units of energy. When that same cloud passes over, 80 * 75% = 60 units of energy, or a drop of 20 units. So, if your process is sensitive to changes in absolute energy, you would rather have a swing of 20 units for every passing cloud than a swing of 25 units.

That's not exactly it. The idea is that the blue light absorption can be tuned (presumably by shifting the absorbed wavelengths by a few nanometer) to compensate for external changes. So if the overall light intensity drops 1%, the system responds by shifting the absorbed spectrum to get 1% more power.

This only works in parts of the solar spectrum where power varies sufficiently versus wavelength, hence the preference for blue and red.

Ahh I get it. Maybe the leaves even actually change colour when shaded, but our eyes just aren't sensitive enough?

I'm confused though -- how can the plant possibly shift this?

Biological light harvesting complexes[1] act as an antenna network. I guess the shape and other properties of such a complex change depending on conditions.

The paper doesn't really try to explain the biological tuning mechanism. Instead, they created a model of an antenna network capable of tuning. As in, this model had a bunch of parameters describing how it captured light and tuned. Then if they optimized those parameters for stability given the light spectra that different plants are exposed to, they found that their model would reproduce the actual absorption spectra of those plants. This strongly suggest that the plants have done the same optimization by evolution.

[1] https://en.wikipedia.org/wiki/Light-harvesting_complexes_of_...

And on what timescales does this timing occur? Are we talking seconds? Or days, i.e. accomplished by growing different structures?

FWIW, hardening seedlings to sunlight from a greenhouse is all about switching their spectrum, and it takes 3 to 7 days depending on what plant you're talking about. Gardener's rule of thumb. 4/6/8hr exposure etc.

I wonder if anyone has used RGB LEDs and tormented plants with short cycle color changes yet? There's much anecdata and debate over spectrum vs efficiency in indoor gardening, but it's faded as power density became more obviously dominant, i think.

Yeah, on the orders of days is what I'd expect. So if that's right, then what kind of meaningful tuning are talking about for plants? On the order of days you can simple grow more or less of the green chlorophyll; is that not tuning enough?

Near instantaneous. In the paper it is described as "noise-cancelling".

(waves hands) proteins (or something)

Interesting but why would it not apply the same shifting within the green light spectrum which seems to be quite stabile?

https://d2r55xnwy6nx47.cloudfront.net/uploads/2020/07/Plants...

That's kind of the point. The green section is flat - if you vary the frequency by a couple of nm, you do not see a meaningful change in power input.

But the slopes in the red and blue sections allow you to adjust the input power significantly by tweaking slightly the wavelength of the incoming light.

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