Mechanism of underwater pollination in Najas marina (Najadaceae)

Najas is the largest of the 18 hydrophilous angiosperm genera, however, stu dy of pollination biology in the genus is largely ignored. In this study, t he 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 li e opposite to the branches is reported for the first time. This process occ urs before anther dehiscence and facilitates dispersal of pollen by water c urrents. A large number of pollen grains are dispersed in dense "clouds" fo r 5-10 min after anther dehiscence. Microscopic examination of pollen grain s revealed the presence of numerous starch grains in the cytoplasm. These i ncrease 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 b efore contact with the stigma enhances the chances of the pollen to be capt ured by the branched stigmas. In addition, the pollen grains undergo a mark ed elongation of their polar axes just prior to release. The relatively lon g period of viability of both precocious and ungerminated pollen grains fur ther enhances pollination. The high pollen loads observed on stigmas could account for the high seed yield in this submerged plant. The precocious pol len 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 wate r currents. (C) 2001 Elsevier Science B.V. All rights reserved.