Grouped here because they're all grown mainly for what's happening underground, not because they're related -- potatoes, sweet potatoes, carrots, beetroot and the rest come from entirely different plant families, so what applies to one rarely applies to the others. Seed-saving in particular varies enormously across this group: some regrow from a piece of root or tuber rather than true seed at all, and cross-pollination risk (where it exists) is specific to each crop's own family, not shared across the category the way it is with brassicas.
Potato
Guide available
Sweet Potato
Guide available
Carrot
Guide available
Beetroot
Guide available
Parsnip
Guide available
Swede / Rutabaga
Guide available
Turnip
Guide available
Celeriac
Guide available
Salsify
Guide available
Radish
Guide available
Jerusalem Artichoke
Guide available
These crops aren't botanically related, so treat each one's seed-saving and cross-pollination guidance separately. Potatoes, sweet potatoes and Jerusalem artichoke are grown from tubers rather than true seed at all; carrot, parsnip and celeriac share a family but different genera, and can cross with wild relatives like Queen Anne's lace; swede, turnip and radish each have their own isolation needs despite superficially resembling brassicas. Full detail is on each crop's own page above.