Pollen Dimorphism in Heterostylous Species of Oxygonum (Polygonaceae, Polygoneae)

Abstract

The pollen dimorphism of eight African heterostylous species of Oxygonum Burch. ex Campd. is described for the first time using light and scanning electron microscopy. The pollen grains are mostly 3-colporate, subprolate to prolate with semitectate-reticulate exine. The pollen grains from short-styled (thrum) flowers are larger than those from long-styled (pin) flowers, with the ratios of thrum/pin pollen size varying from 1.01 to 1.35 in polar axis, and 1.07-1.62 in equatorial diameter. 0. sinuatum shows the most pronounced pollen dimorphism with respect to size and exine ornamentation. In other taxa, such as 0. dregeanum, 0. fruticosum and 0. stuhlmannii, the pollen dimorphism is weak. In the thrum flowers the exine lumina are slightly larger than in the pin flowers. There is, however, no significant difference in shape and in the number of apertures. Pollen polymorphism as a significant component of heterostyly and its functional significance and evolution are briefly discussed. More- over, heterostyly is reported here in four additional species of Oxygonum, i.e., 0. alatum, 0. frutico- sum, 0. pachybasis and 0. tenerum.