| Self-incompatibility (SI) in an inbreeding avoidance mechanism. It is most commonly associated with the rejection of self-pollen by the female tissues of the flower. In Theobroma cacao, the chocolate tree, SI is characterized by a lack of pollen tube inhibition. Compatible and incompatible pollen tubes grow equally well in the flower, and by 8 hours after pollination, the pollen tubes are in the ovary. Because there is no inhibition of pollen tube growth in the upper parts of the flower, SI in Theobroma has been defined as a late-acting system. However, there has never been demonstration of a physiological mechanism operating after pollen tubes have reached the ovary. Hence, initiation of the SI reaction could begin earlier in time and not be manifested until pollen tubes reach the ovary. The SI response is abscission of the flower--a hormonally regulated event. I investigated levels of three hormones that participate in the abscission reaction (ABA, IAA and ethylene) after non-pollintaion, compatible pollination and incompatible pollination. Hormone fluctuations indicate that unpollinated flowers and incompatibly pollinated flowers are physiologically indistinguishable. Compatibly pollinated flowers show a significantly different response 24 hours after pollination--16 hours after the pollen tubes are in the ovary. Thus we provide evidence for a system of SI that operates in the ovary and is based on the control of floral abortion by phytohormones. It is a novel system of incompatibility in which rejection of self occurs at the whole flower level, not as the inhibition of pollen tube growth.; An associated anatomical feature is the reduction in callose fluorescence in the hypostase after compatible pollination. This decrease in fluorescence is significant 24 h after compatible pollination. The removal of callose from the hypostase may represent the opening of a nutrition pathway for the developing embryo.; The SI system, considered in concert with other reproductive and ecological characteristics, is an adaptive incompatibility system that optimized fruit set and outcrossing rate. |
