Heterostyly and Pollen-tube Interactions in Luculia gratissima (Rubiaceae)

Abstract

Observations on the floral biology of Luculia gratissima (Rubiaceae) showed that this species is distylous with complementary positioning of anthers and stigmas in the two floral forms. Unusual features of distyly in this species include the larger size of the corolla, the stigma surface and the stigmatic papillae in the thrum flowers compared to the pin ones. Stigmatic surfaces have similar secretions but they appear more copious in thrum than pin. The floral dimorphism was accompanied by a very effective self-incompatibility system and no seed was set on selfing. Seed number per capsule on crossing was significantly greater in thrum flowers compared to pin. Incompatible pollen tubes were inhibited within 24 h at the base of the stigma/top of the style in both morphs. Amputation of this region of the gynoecium removed the self-incompatibility reaction in thrum but not pin flowers. Pollination with a mixture of compatible and incompatible pollen and sequential pollination with self followed by cross pollen showed that there were interactions between the two types of pollen tube. The presence of compatible tubes was found to cause the excessive swelling of the pollen-tube tip of the incompatible ones. The incompatible tubes did not appear to have any effect on the growth of compatible ones.