Hello hammerpamf,
I grew 6 foot tall pea plants in the 4 inch ADS irrigation tubing. I forgot to take into account getting the root mass out when I first built it. So I had to cut the tube in half to get it out. I attached a picture of the root mass that came out of one half of the tube. I don't know how you plan to construct the system, but good luck getting to the root system. You would need to be able to pull the whole plant out of the tube to prune the roots. If not you will be leaving dead/decaying roots to feed pathogens, bacteria, and fungi.
It will be just a mater of weeks before the roots begin to entangle with the other plants. That will make it impossible to pull the whole root system out to prune. After a month or so, you wont be able to pull the baskets up from the tube at all, and wont even be able to get to the roots. I had to take a stake knife and sliced the roots from the baskets as I slowly pulled them out. Once the baskets were out, I just grabbed the whole root mass and slid it out from the open end. Also fintuckyfarms has grown tomato's in 4 inch PVC tubing, and had a problem with their root mass clogging the water flow through it.
If you want to do a root pruning experiment, I would recommend using a water culture or
aeroponic system, designed for easy access to the roots, and being able to easily take out the cut/pruned roots.
I don't remember anyone doing any root pruning experiments in the past few years I've been on this forum. I cant say I really remember reading any research on it either. But what I do know is there's a fine line between pruning and causing the plant stress. I prefer to let the plants roots grow as big as they want. Just taking into consideration how big the root mass will get before I design the hydro system I put the plants in. What do you mean by "FAD"?
P.S.
I'm considering doing an experiment growing corn in a NFT or Flood & Drain type system. It wont be in the near future. But a friends wife told me about a variety of sweet corn that only grows 3-4 feet tall, grows 3-4 ears per plant, and matures fairly quickly. I would need to grow some in order to see how big the root mass will get. But I can see having rows of tubes with corn growing in them. Staggering the age of the plants so I have a continual supply of maturing corn plants, and thus fresh corn. The problem is I have many more ideas than money or space to do them. I have another plan to grow year round raspberries commercially on the list as well (even here in the desert).