Incompatibility in Amsinckia Grandiflora (Boraginaceae): Distribution of Callose Plugs and Pollen Tubes Following Inter‐ and Intramorph Crosses

The distribution of callose plugs and pollen tubes was investigated following inter‐ and intramorph crosses of Amsinckia grandiflora (Boraginaceae), a distylous species possessing cryptic self‐incompatibility. Callose plug distribution provided a good indication of the distribution of pollen tubes. Compared to intramorph crosses, many more callose plugs and pollen tubes were found in basal stylar regions following intermorph crosses, indicating that differential pollen tube growth is a likely cause of cryptic self‐incompatibility. The incompatibility response differed for the floral morphs: in the pin (long‐styled) morph pollen tubes were most likely to cease growth in the midstylar region, while inhibition was more likely to occur in the upper stylar region of the thrum (short‐styled) morph. There was no evidence of stigmatic inhibition of pollen tubes for either morph, although the incompatibility response in the Boraginaceae is normally located in the stigmatic region.