Enzymes (or extra microbial activity) around roots is a nice idea.
Try getting your students to get hold of some unexposed but processed
color slide film. Best if mounted. This can be put into moist soil and
after about 4 to 7 days the soil bugs will have started to dissolve the
geletin holding the pigments and with careful washing will produce some
terrific "abstract" color pictures when projected.
NOW - having got a feel for how long to leave the slides (diapositives)
in the pot of soil, plant some rapid germinating and growing seeds and
let the roots grow down over the slide surfaces. There SHOULD be extra
activity and extra colour shifting in the root zones.
Anyway, it's cheap and fun and nothing beats getting the soil bugs to
"take pictures of themselves" or at least their enzyme activity !
Peter Harris, Reading, UK.