The anther steps onto the stigma for self-fertilization in a slipper orchid

Abstract

Background: Due to the spatial separation between male and female pollen grains from the anther of most flowering plants, including orchids, pollens are transported by wind or animals and deposited onto the receptive surface of the stigma of a different plant. However, self-pollination is common in pollinating animal-scarce habitats. In such habitats, self-pollinations require the assistance of a pollinating agent (e.g., wind, gravity, or floral assembly) to transport the pollen grains from the anther onto its own stigma.

Methodology/principal findings: Based on observations on floral morphology and flowering phenology, tests of the breeding system, and a comparison of pollination mechanisms, a new self-pollination process was discovered in the hermaphroditic (i.e., possessing spatially separated male and female organs) flower of a slipper orchid, Paphiopedilum parishii. The anther changes from a solid to a liquid state and directly steps onto the stigma surface without the aid of any pollinating agent or floral assembly.

Conclusions: The mode of self-pollination discussed here is a new addition to the broad range of genetic and morphological mechanisms that have evolved in flowering plants to ensure their reproductive success. The present self-contained pollination mechanism is a possible adaptation to the insect-scarce habitat of the orchid.

Figure 1. Pollination mechanism of P. parishii.

(A) Structural characteristics of the column. The distance between the anther (red arrow) and stigma (blue arrow) is 2 mm to 3 mm (Figures 1B and 1E). (B) The flower moves backward and upward, making the both of anthers and the lateral lobes of the stigma lie in a horizontal plane, and solid-state pollinia begin to liquefy (red arrow). (C) The anther continues to liquefy close to the stigma and touches its margin (red arrow). (D) The anther liquid spreads onto the receptive surface of the stigma (red arrow). (E) Floral morphology and structure. The anther and stigma are shown in red and blue arrows, respectively. (F) Floral morphology and structure of P. dianthum pollinated by hoverflies. The anther and stigma are shown in red and blue arrows, respectively. (G) The column of P. dianthum, whose anthers do not liquefy (red arrow).