Species Concept

According to the biological species concept (Ernst Mayr), a species is a group of individuals that can interbreed freely among themselves in nature and produce fertile offspring, and that are reproductively isolated from other such groups.
So the conclusive test for two plants to belong to the same species is that they can reproduce freely with each other and form seeds that give fertile progeny.
Similarity of appearance or of secondary metabolites is not conclusive, because different species may look alike.
Different species can have the same number of chromosomes, so chromosome number is not conclusive either.
A high percentage of similar genes is also found between related but distinct species.
So the answer is that they can reproduce freely with each other and form seeds.

According to the biological species concept, a species is a group of natural populations whose members can interbreed and produce fertile offspring, and which is reproductively isolated from other such groups.
Members of a species can interbreed; true.
Variations always occur among the members of a species; true.
Each species is reproductively isolated from every other species; true.
Because the populations of a species can interbreed, genes move between them through migration and mating; this gene flow keeps the species together as one unit.
So the statement that gene flow does not occur between the populations of a species is not true.