If they're trying to experiment for real, it seems like there are some pretty obvious experimental controls that should be used which I don't see discussed.

The control plants should have a non-electrical-conducting wire planted in a similar manner. And in fact, another control should use a copper wire that is cut in various places to interrupt the circuit (and reconnected to be rigid with a non-conducting material)

Without those controls, the effect could be simply a chemical interaction with metal as others here are suggesting. Or even just an effect from how the plant is potted with a wire placed in the pot. Maybe the wire just adds air or makes it easier for the roots to grow or something who knows. Interesting.