Welcome and thanks for your question.
First of all, it is best practice to ask one question at a time.
Secondly, many questions touch upon different disciplines; the physics of biology is a discipline that spans both fields, and et voila biophysics was born. Where to ask multidisciplinary questions on the SE network? Well, you have to look at the relative contribution of the different disciplines. Suppose you are wondering how NMR manages to characterize different types of chemical bonds in complex proteins, it may be best asked over at Chemistry.SE; when your question is why bond A yields a different NMR response than bond B, it's probably best off at Physics.SE; if you ask whether protein complex A and B can be analytically separated by NMR, Biology may be the place to ask. Does that make sense?
In this case, the question why/how fruit A sequesters isotope B, it's probably Biology. However, there may be a reason involved where one isotope is bound to a certain soil component in the bottom and the question may be more of a geological question and better of at EarthSciences!
When you ask how radioactive compounds can be enriched, it's Physics. It doesn't matter if it's raw uranium or bananas. Likely some kind of centrifugal force will be applied to make use of the different masses of the isotopes. This shows well why you should ask a single question at a time.
To determine what the relative weights are of the disciplines you need to do prior research and include that research in the question. If your question has fleshed out, assess yourself where the question is veering at.
And, if all falls apart, the community can guide you through the process further and mods are able to migrate posts across the network. But key to questions like this is prior research effort to generate focused, fleshed out questions.