Structure of styles and pollen tubes of distylous Turnera joelii and T. scabra (Turneraceae): are there different mechanisms of incompatibility between the morphs?

We investigate the anatomy and fine structure of styles and pollen tubes of two distylous Turnera species, which possess a heteromorphic self-incompatibility system. We use fluorescence and transmission electron microscopy to provide the first description of the cellular aspects of pollen-pistil interactions and ultrastructural changes to pollen tubes during the self-incompatibility response of the morphs. No obvious ultrastructural differences occur between pistils and compatible pollen tubes. Conspicuous differences were, however, observed between incompatible pollen tubes of the morphs. Incompatible pollen tubes of the long-styled morph always appear to be intact, and pollen tube tips are often highly fluorescent when stained with aniline blue, a stain for callose. Swelling and loss of cristae of mitochondria, and circular rough endoplasmic reticulum, were observed for incompatible pollen tubes of the long-styled morph. For incompatible pollen tubes of the short-styled morph, the tube cell wall apex and plasma membrane often appear to be ruptured and no easily recognizable organelles, such as mitochondria, can be discerned. Our results clearly show ultrastructural differences between the morphs and support the hypothesis that different self-incompatibility mechanisms might operate between them.