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Below are some chlorophyll absorption spectra from other answers here. There is strong absorption at both the blue/violet end, and the red end of the spectrum, and presumably both of these contribute to steps in water splitting used in photosynthesis.

Do the two broad but widely-separated peaks correspond to different steps, so that both are required? Or is it sufficient to have light present in one band alone?

If both are required, how are they used differently?

If not, why do LED grow lamps have both? Would twice-as-bright light with either wavelength be just as effective and useful to plants?


That horribly un-natural purple light looks cool in high tech settings, but it's kind-of ugly in ones living room.

note: Originally I had though that Chlorophyl A and B corresponded to the two peaks, but clearly this is not at all the case. From here:

Both Chl-a and Chl-b primarily absorb red and blue light, the most effective colors in photosynthesis. They reflect or transmit green light, which is why leaves appear green. The ratio of Chl-a to Chl-b in the chloroplast is 3:1.

enter image description here

above: From this answer, also seen here.

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above: x2 From this answer.

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  • $\begingroup$ It's possible that white or yellow lighting might be less efficient for growing plants. (Then again, maybe not.) Still, perhaps you'd want to splurge on running possibly-less-efficient lighting, at least during the hours when you're sitting in your living room? $\endgroup$ – tealhill supports Monica Dec 26 '18 at 18:01
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tl;dr: Sort of?

Logically, either red or blue light should be sufficient. Chlorophyll a and chlorophyll b preferentially activate different photosystems, and both photosystems are required in green plants.

Practically, we're in luck and someone has actually done the experiment.

As the original study reports, plants need a little blue light to grow into the right shape. Red-only plants produce oxygen and grow, but they take weird shapes. It seems like plants can grow on red light, but they use blue light to work out where light is. See here for a reasonably lengthy treatment on most of the obvious combinations.

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  • $\begingroup$ These are very interesting, thanks! It certainly seems that red-only is suggested to be all that's needed for substantial photosynthesis (though it's not clear yet if there is a slight improvement in photosynthesis or not with blue added) Has it been stated anywhere that blue-only is not sufficient? Since the absorption spectra show big peaks at both ends, I'm asking if either one is fine. Will plants not grow with blue-only? $\endgroup$ – uhoh Apr 19 '17 at 0:30
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    $\begingroup$ Uh, long story short: I don't know and I also don't know how to find out. (Other than the obvious way) $\endgroup$ – Resonating Apr 21 '17 at 21:15
  • $\begingroup$ OK thanks. I think the idea that multiple bands of wavelengths are important for more than just photosynthesis rate is really interesting. I have read that the newest LED modules used on the International Space Station will now include Green light as well, though I'm not sure if that's for the plants, or for the astronauts to improve their ability to do visual inspection. $\endgroup$ – uhoh Apr 21 '17 at 23:59
  • $\begingroup$ I've slightly modified the wording of the question, changing "one color" to "either color" in two places. I didn't realize until now that it's did not read clearly. $\endgroup$ – uhoh Apr 22 '17 at 1:09
  • $\begingroup$ I'm not sure why I initially held off on accepting this answer. From a practical point of view, if plants grow funny without sufficient and presumably balanced light in both bands, then both bands are necessary for good grow lights! $\endgroup$ – uhoh Aug 16 '17 at 10:13

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