The recognition and rejection reactions in a SSI system occur on the stigma surface, placing barriers for pollen germination or penetration of pollen tube into the stigma. Thus the pollen grains either fail to germinate or the small tube they put out is inhibited by callose deposition at the tip. The papillae at the stigma also develop a lenticular callose plug within 10 min after pollination. Infact, in incompatible cases, inhibition of varying degrees operates at every level starting from pollen adhesion, hydration, germination and tube entry into the stigma. In some plants stigma is covered with a cuticular layer such that the incompatible pollen tube fails to penetrate.
Various studies have proved the necessity of enzyme cutinase in eroding the cuticle. This enzyme is activated only by the compatible cases. Incompatible stigmas may lack copious exudate, but dry stigmas have a hydrated layer called pellicle over the cuticle. This pellicle consists of a lipid layer where a mosaic of proteins floats. The pellicle probably originates by the protrusion on the surface of the papillae through discontinuities in the cuticle. As soon as pollen is received by the stigmatic surface the papillate cells exude moisture. The incompatibility is the result of interaction between protein fractions of exine and stigma.