7. Vegetable electricity

Some internet posts suggest that it is possible to generate electricity from photosynthesis by using a metallic network that winds through the roots of a plant. In response, a skeptic may claim that the setup is just a voltaic pile, and that therefore, photosynthesis is irrelevant. Explain the phenomenon and specifically determine whether photosynthesis is indeed involved. Determine the main factors affecting the phenomenon, and optimize them so as to generate the maximal power per unit of soil area.