Exposure to water increased pollen longevity of pondweed (Potamogeton spp.) indicates different mechanisms ensuring pollination success of angiosperms in aquatic habitat

Pollen longevity in seven Potamogeton species representing different pollination systems (anemophily, epihydrophily and hydroautogamy) was assessed both under aerial condition and in contact with water to investigate how water impacts the sexual reproduction in these aquatic taxa. Stainability of pollen with MTT was considered as an indicator of pollen viability. The half-life of pollen longevity was calculated using exponential decay regression. Overall, pollen viability decreased relatively rapidly with time. Pollen grains of obligate anemophilic species had lower initial viability and shorter half-lives than those of facultative anemophilic species. Pollen in these latter species may take more time to reach the stigma. The pollen of Potamogeton may be categorized as partially hydrated pollen owing to its generally spherical shape and lack of furrows, rapid loss of viability, and fast pollen tube initiation. The half-life is positively correlated with pollen size. Smaller-sized grains are at greater risk of desiccation than larger grains. In contrast with the situation observed in most terrestrial angiosperms, contact with water increases pollen longevity in Potamogeton species. In our present study the half-lives of pollen longevity of Potamogeton species in which the pollen had come into contact with water (mean of 10.65 h) were markedly higher than those under aerial conditions (mean of 5.79 h, t = 2.622, P = 0.039). The results of our study contradict a widely held belief that water is detrimental to pollen viability in angiosperms and furthermore indicate that close proximity to water results in selection for wettability. The transition to a hydrated status together with its morphology, make Potamogeton pollen more adapted to the aquatic environment and thus serves to ensure reproductive process. Results of our present study may have direct implications for understanding the evolution of the sexual reproductive system in aquatic angiosperms.