[LudoA]

This is really interesting. I'm tempted to attach red LEDs to a battery and put it in the fridge. But it would be quite tricky to measure whether it works. (Almost impossible to do accurately, without different fridges.)

[londons_explore]

Put two cardboard boxes in a fridge. Put a leaf of lettuce in each, and a red LED in the top of each box.

Turn one LED on, while the other stays off.

Observe the leaves of lettuce after a week.