I am an academician in Faculty of Agriculture. Roots development of plants is an important parameter in our experiments. So I need to display the root length without remove the plants from the pots. I need to find the differences between cultivars or treatments about root length, efficient root depth and root area. Is there any sensor that I can scan the roots without the damage the plants by using of arduino ? Coloring is not important for me, I need just selection of roots.
I'd be tempted to consider going low-tech. If you grew the plants in very narrow rectangular pots (glass or plexiglass) you would be able to see a fair fraction of the roots as they grew down along the inside of the walls. Would this give you a good enough estimate?
If not, ultrasound might be easier to deal with than x-ray systems. Also, (again using narrow rectangular pots) high intensity illumination on one side might show you shadows of the roots on the other, depending on your soil composition. But I'm just spit-balling here.
Good luck!
[Paul: If you found links to a sensor that addresses the OP's query (sensor to use with arduino to determine root length WITHOUT removing plants from their pots) please post them. I'm sure the OP would be very interested, and I would also.]