Though using the plants as an electrical source is cool ... consider using the potential difference between the soil and the air - actually, an antenna waaaay up in the air. There's much on the Web about this. Please use caution as there's quite a bit of juice that can be extracted from that set-up. Then there's the storage issue. Quite a pain. As we move away from the AC-based system, and even the typical DC system, there's much to learn when considering pulsed DC and cold electricity. (Yes Virginia, cold electricity does indeed work and it's real) You must consider Nathan Stubblefield when discussing the early inventors of ground-based electricity. (
http://www.angelfire.com/ak5/energy21/earthbatteries.htm) He was the Man, as it were. Be careful out there as there's many who will sell you a load of horse hoo-ey...and it will only run a few LEDs. History and doing the experimenting yourself is still the best - it does help to have someone to bounce a few ideas off of...
Cheers from here,
Col