The sequence of reactions of pollen tube and style tissue has to be co-ordinated. This co-ordination is impossible with pollen of foreign species. The consequence is a rejection called incongruent reaction. Usually causes the lack of one or several steps in a chain of events that starts with the encounter of pollen and style cells the incongruent reaction. Gymnosperms display no self-incompatibility, presumably because they have no style.
Gymnosperms do show incongruent reactions.
The protoplasts of incompatible plants are fused. It is tried to stimulate the fusion product to regenerate, an attempt that fails with most monocots. Such an experiment requires protoplasts of haploid plants that are difficult to obtain.