How would one track a mineral nutrient in a plant in order to prove that nutrient has been re mobilized?
It is not possible to track individual molecules within a plant. However, one can track the level of radioactivity from place to place. So, in the case you are talking about, one would find that some level of radioactivity persisted for a time at one location, then it is observed to decline while it increases in another. There also might be a delectably increased level of radioactivity along the pathway between those two areas - regardless, one would logically connect decreased activity in one area with increased activity in another.