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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.

enter image description here

enter image description here

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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The plant uses light to produce energy but also as a signal of how and when to grow (phototropism, photoperiodism). In the context of your question I'll first cover light-harvesting in photosynthesis and then phototropism.

Tl;DR

Blue and red light are important for plant growth. Red light is the main one in photosynthesis and if a plant is exposed to another low intensity, full spectrum light source there is no (theoretical) need for a blue LED.

Photosynthesis

enter image description here

(from WTAMU)

The light from the sun arrives in many wavelengths, no one material can use all this energy. In photosynthesis light is intercepted by an antenna, a Light Harvesting Complex, made of different pigments, each intercepting photons in a unique energy level (wavelength). These antennas are situated around a reaction center and redirect the photons to it. Only the reaction center releases an electron to the electron transport chain. The pigment in the reaction center is modified chlorophyll a. In PSII (Photo System II, beginning of the chain) it's called P680 and in PSI it's called P700, the number denotes the excitation wavelength. The goal of the antenna is to gradually redirect photons to the reaction center and to its energy level (reducing the energy). enter image description here

(Pandit, Anjali & Holzwarth, Alfred & Groot, Huub. (2006). Harnessing Solar Energy for the Production of Clean Fuels.)

When a photon excites a chlorophyll molecule this energy needs to dissipate in some manner. Energy dissipation from a blue photon excitation is too fast for it to be captured by the electron chain reaction. All the of the energy difference between blue and red photons is wasted (regarding photosynthesis). enter image description here

(Dr. Shilo Rosenwasser presentation)

From the chlorophyll and other pigment explanation we understand that using any light except red is wasteful regarding energy conversion efficiency, the plant photosynthetic system will "ignore" any energy extras above the red photon.

Phototropism

The stem of the plant grows towards light and the roots grow away from light, this phenomenon is easily observed by looking at plants close to a window, they tend to "look out" of the window. This means that plants have a sensor for light. This observation was first correctly analyzed by Darwin (a historical review).

Without blue light plants will elongate in an proportional way called etiolation. Etiolation makes a deformed, fragile plant because of its mechanism. Without blue light a plant will etiolate, even with enough energy for photosynthesis.

It is important to note that the light intensity required in light-sensing responses is much lower than the intensity required for photosynthesis.
It is also important to remember that blue & red light take part in other critical steps in plant growth like germination and flowering.

In conclusion

Red light is the major contributor to photosynthesis. Blue light indicates to the plant that there is light and it should start growing normally and do photosynthesis.

In the LiCOR photosynthesis measurement instruments the default settings are 96% red light and 4% blue light, showing that empirically red light is relevant to photosynthesis and blue is for signaling.
If your plants are exposed to some natural light or white light (from a window or lamp in your living room) I don't think that you need to supply blue light as they will get the required amount from those sources. Growing in a controlled, closed environment does require the blue light.

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    $\begingroup$ Fascinating! Thank you for your thorough and well-organized answer. $\endgroup$ – uhoh Feb 16 at 7:36
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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
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    $\begingroup$ Hi, I've un-accepted your answer because both of your links to hortsci.ashspublications.org have broken/rotted and you only included minimal information from them of them within your answer itself, and actually not even the titles of the works nor the authors' names. This is now an unsupported answer; is it possible to find new links to support it? Thanks! $\endgroup$ – uhoh Feb 12 at 11:07

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