Bubbles help male flowers overcome obstacles during pollination in Hydrilla verticillata

Abstract

Premise Pollination in many aquatic plants takes place on the water surface, and the male flowers or stamens often produce gas bubbles underwater; however, the generation mechanism and function of these bubbles are unknown. Methods A common submerged plant, Hydrilla verticillata, was used as experimental material to observe the structure of male flowers, analyze the process of bubble generation, and simulate the movement process of the male flower with attached gas bubble in water. Results The aerenchyma inside the male plants of H. verticillata transported the gas produced by the plant's branches during photosynthesis to the male flower, and the formed gas bubbles became attached to the edge of the perianth. The gas accumulation rate in the attached bubbles increased with light intensity. Once the bubble diameter increased to approximately 3.3 mm, the male flowers with the bubble detached from the plant and floated to the water surface. The removal of the attached bubbles did not affect the male flower detached from the plant; however, the surfacing of male flowers without gas bubbles was easily prevented by the plant's branches in the water, and they could not reach the water surface to complete pollen dispersal. Conclusions The gas bubbles produced by male flowers of H. verticillata came from the gas produced by branches under light. These bubbles can help ascending male flowers bypass the obstacles in water and reach the surface to complete pollination.