Science Questions

Moonlight Photosynthesis

If the sun is shining on the moon, and it’s a full moon, can plants photosynthesise with the reflected light? If so, can they survive on that light alone? Pete Charakas, Minneapolis

We put this question to Howard Griffiths, at the department of Plant Sciences, Cambridge University.

When I was first thrown this question about the role of moonlight in photosynthesis my initial response was not a chance! Because the light intensity that we get reflected off the moon is an order of 100-1000 times too little to support photosynthesis in most terrestrial pot plants and plants we have in our garden. However, I did a little bit of digging around and I looked at some latest analysis of photosynthesis rates in algae. Amazingly enough it does seem that some groups of very small phytoplankton might be able to photosynthesise using the light from the moon provided that it was in the tropics and provided that at wasn’t being attenuated by a water column which tends to absorb light exponentially. So the answer is still probably no because, obviously phytoplankton grow in a water column so they’re not really likely to be able to pick up the light intensity.

However, it also opens up a number of intriguing questions because plants do certainly try to avoid the light from the moon. I’m sure many of you are familiar with the folding of leaves that we see in the clover growing in your lawns and lots of plants in the garden fold up their leaves at night. Darwin was interested in this and thought that it was to do with the leaves trying to maintain their heat balance at night. What we think is happening now is that the leaves are trying to avoid moonlight so as to prevent their circadian rhythms being disrupted by those very light intensities because they certainly do respond to moonlight. In fact, it’s now known that lots of animals – animals as diverse as snakes and crocodiles and a whole array of plants and different systems including humans – are highly sensitive to moonlight and the way that it can interrupt our circadian control and our sensing of day length.

November 2007