A simple averaging model
Treat the root as sensing gravity through a slow filter with a response time of about 60 seconds. A gravity vector turning at angular speed omega then looks, on average, like a vector of size 1 / square root of (1 + (omega x 60)^2) g. At 0 rpm the root feels the full 1 g; at 2 rpm only about 0.08 g.
Spinning also creates an outward force of omega^2 x r. This is a teaching model; real gravity sensing involves many processes with different time scales, and studies use speeds anywhere from under 1 rpm up to about 60 rpm.
Try it: start at 0 rpm and step up by 0.5 rpm. The averaged gravity drops below 0.3 g by about 1 rpm and the roots stop bending down. Then push to 20 rpm with seedlings 15 cm out and watch the spin force climb past 60 milli g.
Simulated, not real, weightlessness
A clinostat removes the steady direction of gravity, not gravity itself. Cells still feel weight at every instant, so results are checked against true spaceflight experiments whenever possible.