What does a light pigment (photoreceptor) absorption curve tell us?
I'm not a plant biologist and I have only a rudimentary understanding of plant receptors. But in simple terms, photoreceptors are pigments that absorb photons and pass the energy from those photons through an electron chain that is used for either photosynthesis (plant growth) or photomorphogenesis (plant structure and other compounds).
Photons of light are captured (absorbed) and the energy from those photons is harvested to make sugars (photosynthesis) or specialised cells (photomorphogenesis). There are by-products of each of these processes, but let's not worry about that for now.
So we know black surfaces absorb most light and white surfaces reflect most light. We know that a green surface absorbs red and blue light but reflects mostly green light. The absorption/reflection spectra are not absolute. A black surface still reflects some photons. A white surface still absorbs some photons. A green surface still aborbs some green photons. Etc.
In case you missed it earlier, this is the UVR8 pigment absorption curve. What it tells us is that when light shines on this receptor, it will absorb photons of certain wavelengths better than others. In the above graph you can see that the pigment readily absorbs photons in the 285nm range but does not absorb photons of other wavelengths as readily.
You can see above that – all things being equal – the UVR8 pigment will absorb 4x as many 285nm photons as 300nm photons. That means if you have equal amounts of 285nm and 300nm light and shine them both on the pigment, then 4x as many 285nm photons will be harvested and converted into electron energy to power whatever processes UVR8 is responsible for.
The other photons are either reflected or absorbed as heat energy (as opposed to excting the electron chain).
This is all relative of course because only a very small fraction of light that hits a plant is actually absorbed by its pigments. In the case of photosynthesis driven by the Chlorophyll A and B pigments, only about 4% of all light is converted to photosynthetic energy. And of that light, the majority is made up of the photons that coincide with the Chl A and Chl B absorption peaks – around 420-480nm on the blue side, and 620-680nm on the red side (there is debate about exactly where peak absorbtion in Chl A and B lies, with some studies claiming it is as low as 372nm/642nm for Chl A and 392nm/626 for Chl B).
However, peak absorption does not tell the full story. In fact, any photons that are within the curve can be absorbed and converted to electron energy to power whatever process the receptor is responsible for. It simply means that the lower the abosption rate, the more photons are needed to produce the same amount of convertible energy.
This explains why horticultural LED manufacturers go to such lengths to produce light that coincides with peak Chlorophyll absorption. Because it is the most efficient way to drive photosynthesis (and other processes if we look at other absorption curves).
HOWEVER, the efficiency of the light source must also be taken into account!
In the above UVR8 graph, if it takes more than 4x the energy to produce a 285nm photon compared to a 300nm photon, then it will take more energy to produce the same response in a plant using 285nm light than it will using 300nm light – even though the receptor is more sensitive to 285nm light.
Are you all with me so far? Good
All this is leading up to the fact that some light sources are more efficient than others (LEDs vs fluorescent bulbs, for example) and that it may be more efficient to produce a photon that
doesn't coincide with an absorption peak than one that does.
In mathematical terms it is simply the efficiency of the light source multiplied by the percentage of absorption multiplied by time that gives us the total response (assuming time to response in linear – plants simply stop photosynthesising when their pigments are saturated).
Let's go back to that Solacure spectral graph and have a look at the light produced around 310nm. Let's assume that 15% of the light is emitted from 305-315nm. If we check the UVR8 response curve, we can see that these wavelengths elicit a UVR8 response that is about 10% compared to 280-290nm absorption. 10% of 15% is 1.5%. That means 1.5% of the total light emitted by the bulb drives UVR8 response.
In the case of 280-290nm light, however, only 1% of the emitted light is in this range, and 1% of 100% is – you guessed it – 1%!
So, in this (hypothetical) case, 305-315nm light is 1.5% (50 per cent) more efficient at driving UVR8 than 280-290nm light – even though the response curve favours 280-290nm light!
I hope I haven't confused everyone and please, don't take the above figures as gospel – they are intended only to highlight the way photoreceptors work and to simplify the process so that we can all understand the correlation between photon absorption and lighting efficiency.
Not saying I understand it all but you ask and it will be explained by those involved in the design/build ( been given great info on getting the best from the lights)...
You said it. You have also been pretty adamant about UVR8 not being triggered by other wavelengths – first claiming only 285nm would do the job, and then UVB in general. But let's move on from there.
