Abstract:

Aims Lobelia chinensis(Campanulaceae), a perennial herbaceous creeper that grows mainly on riverbanks or wet farmland in central and southern China, is characterized by stamen fusion. The anthers of its five stamens are fused tightly into a tube in which the stigma and style are packed, and the filaments are also fused at the upper parts, forming a column-like structure surrounding the pistil. The objective was to elucidate the adaptive significance of this growth pattern, the floral syndrome and breeding system using anatomical analyses, pollination surveys and bag experiments.

Methods Flowers were observed at different developmental stages to determine the growth patterns of each floral part, especially morphologies of sexual organs, dichogamy and numbers of pollen grains and ovules, and their possible changes in spatial position and morphology. Pollination surveys were conducted to find the primary floral visitors. Bag experiments were conducted to determine self-compatibility, autogamy and apomixy. These studies would reveal the breeding system.

Important findings Flower diameter was >1 cm. The lower parts of perianth were fused into an open tube. The stigma and style were enclosed in the anther tube and the united parts of filaments. The flower was protandrous, and the pollen grains were released into the anther tube and dropped on the stigma surface. Growth of the style sent the stigma out of the anther tube 1 or 2 days after opening and then it became receptive. The main floral visitors were ants, hoverflies and flies. The pollen-ovule ratio (P/O) was <100, suggesting facultative autogamy, but the outcrossing index suggested the breeding system is outcrossing with partial self-compatibility and a pollinator is needed. Bag experiments indicated no apomixy and no spontaneous autogamy, but self-compatibility. The species largely outcrossed but suffered pollen limitation in field. The low P/O in this stamen-fused species might be due to the higher siring probability of individual pollen grains resulting from the accumulation and collective pollination of pollen, a similar mechanism to pollen aggregation such as pollinia, tetrads and viscin threads. The correlation of stamen fusion with perianth tube, dichogamy and low pollen-ovule ratio in this and other anther-fused species suggested that these floral traits probably formed a floral syndrome adaptive for small pollinators.

Key words: synantherous stamen, pollen aggregation, pollination, pollen-ovule ratio, dichogamy