Reversible anther opening enhances male fitness in a dichogamous aquatic plant Butomus umbellatus L., the flowering rush

We studied reversible anther opening in a dichogamous aquatic plant Butomus umbellatus L. (Butomaceae) to assess its consequence on male fitness. Light microscope observations indicated that stomium breakage was simultaneous in all the anthers within an opening flower; however, detachment of the epidermis and outward bending of the anther wall were asynchronous. SEM observations showed that epidermis cells alternated between orbicular and crinkly shapes in response to absorption and loss of water. This generated centripetal and centrifugal forces which were significant enough to cause inward and outward bending of the anther wall, thus causing opened anthers to close and closed anthers to re-open respectively, depending on relative humidity of the environment. Behaviour of in vitro and in situ anthers from different whorls within a flower was recorded under four regimes of relative humidity or under a water-spraying treatment. The three stages of anther opening namely stomium breakage, epidermis detachment and outward bending of the anther wall were affected differently by moisture levels. Outward bending of anther wall which was responsible for reversible anther opening was mainly dependent on environmental relative humidity levels. Notably, on sunny days, anther re-opening was inhibited at high relative humidity for in vitro anthers, but not for anthers in situ. Reabsorption, rather than evaporation was responsible for reversible anther opening for this aquatic plant. Water-spraying treatment indicated that flowers with re-closed anthers attracted fewer pollinators. Results using pollen stainability and germinability of pollen on stigmas for pollen grains from different treatments also showed the mechanism of reversible anther opening significantly prolonged pollen longevity by maintaining high viability in a simulated rainy. Discontinuous pollen presentation due to reversible anther opening avoided unfavorable pollination environment and pollen loss.

► We report a novel mechanism of reversible anther opening in Butomus umbellatus.
► Anthers respond to rainfall by temporarily closing up.
► Out- and inward anther wall bending is mainly due to the retractile epidermis cells.
► Water reabsorption is mainly responsible for anther opening of this aquatic plant.
► The mechanism prolongs pollen longevity and enhances efficiency of pollen export.