Gasteria has ovular incompatibility, and recognition of cross- and self-pollen takes place. Cross-pollination includes recognition and pistil activation and leads to seed set. Self-pollen germinate, and their pollen tubes penetrate the ovules but after fertilization they abort. A group of glycoproteins in the pollen coat seems implicated as signal molecules for recognition and activation. Pistil activation is expressed as extra water uptake in the pistil, especially in the fluid pollen tube pathway, and results in higher in vitro pollen tube penetration in the ovular micropyle. In the fluid pollen tube pathway of unpollinated styles a high concentration of sucrose, glucose and fructose is present. The level of these carbohydrates remains the same during the pollen tube growth of cross-pollen. This level decreases after the passage of self-pollen through the stylar channel, and the level of carbohydrates is restored. This implies extra carbohydrate influx in the pollen tube pathway after cross-pollination. Recognition and activation signals act together after cross-pollination of Gasteria. After self-pollination the utilization of carbohydrates lowers the pistil’s carbohydrate capacity, perhaps also leading to a late-acting incompatibility.
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