If it occurs in natural populations, breeds true from generation to generation, and doesn't (can't) interbreed with other subspecies of the same original population. That is basically what a specie is.
That's quite different. If A is crossbred to produce B, and B is crossbred to produce C, even if A and C cannot interbreed you won't accept C as a new species, because A and C can both interbreed with B.
Obviously, if I point out any man-made line of crossbreeding steps, each plant at each step can interbreed with those near it, so no example would ever qualify. And any naturally occuring line is invalid because you will accept no proof that any plant, such as coffea arabica, was actually derived from the source that I claim.
You realise that by this definition, you would consider A and C to be the same species, even if they can't interbreed! I suppose that makes as much sence as saying bison and cows are the same species...
We'll have to just let it go at this point. I have no idea what I could possibly offer you.