Tristyly involves three different forms of flowers that differ reciprocally in the heights of stigmas and anthers within flowers. Apart from the style and stamen lengths, heterostylous species also demonstrate pollen and stigma polymorphisms. We quantified stigma polymorphisms in tristylous Lythrum salicaria. We measured the stigma diameters, structure of papillae, density and distribution of papillae on the stigma from flower samples from 201 individuals belonging to the three morphs. The diameter of stigma and the distribution of papillae were quantified under a scanning electron microscope, and the structure of papillae was determined using a light microscope. The stigma diameter and the density of papillae in the long morph were significantly greater than both in the mid and short morphs, but the total number of papillae per stigma did not differ across the morphs. The length and diameter of papillae at the apex, neck and basal regions were significantly greater in the long morph followed by the mid and short morphs. A discriminant function analysis separated the long morph from the mid and short morphs based on the canonical scores of measurements of papillae structure. The polymorphisms of stigma coupled with those of pollen may play a functional role in self-incompatibility mechanisms.

Key words: Heterostyly, incompatibility, Lythrum salicaria, purple loosestrife, scanning electron microscopy, stigmatic papillae.