Here are two more root qualities that should be considered.
1) Root systems may have a primary taproot, multiple major roots, or rather shallow fibrous roots.
For example, regarding Rosa setigera, Shepherd (1954) wrote, "Fibrous root systems and wide adaptability to soil and climate have encouraged use of the various forms as understocks. Results have been somewhat disappointing, as the plants are difficult to bud, seeds germinate slowly, and cuttings do not root readily."
http://bulbnrose.x10.mx/Roses/breeding/ ... a1954.html
At the opposite extreme, there is 'Dr. Covell's Carrot-rooted Understock'.
https://www.helpmefind.com/gardening/l. ... 6265&tab=1
2) Root tips vary among different plants. Fitch (1913) discussed the great differences between the roots of potatoes and alfalfas.
If we compare the potato root habit and root cap with the root habit and root cap of the alfalfa, we find a striking contrast. The alfalfa root goes straight down through raw clay soils unpenetrated before by any root, and the alfalfa root cap is snug and loses very few cells when subjected to rubbing, but seems rather to cover the root like a glove finger.
I have no information on root tips and root caps in roses, but there are bound to be differences. Plants that wander gently through sand or gravel are probably not as well protected as their cousins that must battle heavy clay.
A meeting place for rose breeders.