However, for AE, since we don't need to convert to MIDI or other digital data out, we would not need most of that circuit.
From a quick glance, it's a galvanometer to measure resistance across the surface of the plant's leaf or stem. So, basically, it is using the plant's skin as a potentiometer with really tiny resistance.
So, what we need to do is have a way to amplify these tiny changes so they make a bigger change of voltage to feed into our synths.
I think that humans' skin can give bigger resistance changes because we sweat more and less at different times so the salty liquid conducts more when we perspire.
Anyway, I'll dig a bit deeper. It should be pretty easy to get something that gives a control voltage from your own changing skin resistance (GSR Galvanic Skin Response), something like the old polygraph or lie detector. To do it with plants is probably a matter of more careful control of noise etc. and offsets. also, fining some nice gold-plated wire to attach to the leaves would help.
There's a guy in Japan who has been doing this for decades. I have met him a few times and he plays with a friend of mine a lot.
This is the installation I saw in Tokyo (jeebus 1996! who's old???) of Yuji Dogane and my friend Maomoru Fujieda. Yuji had all these plants wired up and Mamoru had it linked up to synths via Max and a Mac. Ya tell kids that people used to lug CRT monitors to gigs and they don't believe you!
That gallery was a dream. It was right on a sharp angled corner so it was shaped like a wedge of cake and the floor was made of this gorgeous tightly packed red dirt. They would let artists dig holes in it etc.