Mechanism of underwater pollination in Najas marina (Najadaceae)

Abstract

Najas is the largest of the 18 hydrophilous angiosperm genera, however, study of pollination biology in the genus is largely ignored. In this study, the pollination mechanism of Najas marina L. was observed in two freshwater lakes as well as in aquaria. A unique mechanism of rapid pedicel elongation and curvature in the male flower of N. marina that brings the anther to lie opposite to the branches is reported for the first time. This process occurs before anther dehiscence and facilitates dispersal of pollen by water currents. A large number of pollen grains are dispersed in dense “clouds” for 5–10 min after anther dehiscence. Microscopic examination of pollen grains revealed the presence of numerous starch grains in the cytoplasm. These increase the gravity of the pollen grains and provide energy for pollen tube growth to a length of 2 mm in the water. The development of pollen tubes before contact with the stigma enhances the chances of the pollen to be captured by the branched stigmas. In addition, the pollen grains undergo a marked elongation of their polar axes just prior to release. The relatively long period of viability of both precocious and ungerminated pollen grains further enhances pollination. The high pollen loads observed on stigmas could account for the high seed yield in this submerged plant. The precocious pollen grains may be functionally equivalent to the filiform pollen occurring in submarine-pollinated seagrasses. This provides an example of convergent evolution in response to the pollination environment created by strong water currents.