Quantitative evaluation of stigma polymorphism in a tristylous weed, Lythrum salicaria (Lythraceae)

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 polymorphism in tristylous Lythrum salicaria by measuring the stigma diameters, structure of papillae, and density and distribution of papillae on the stigma from flower samples of 201 individuals belonging to three morphs. The diameter of the stigma and the distribution of papillae were quantified using a scanning electron microscope, and the structure of papillae was determined using a light microscope. The stigma diameter in the long morph was significantly greater than in the mid and short morphs. While the density of stigmatic papillae was significantly greater in the mid and short morphs than in the long morph, the total number of papillae per stigma did not differ across morphs. The length and diameter of papillae at the apex, neck, and base 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 stigma polymorphism coupled with those of pollen may play a functional role in self-incompatibility mechanisms.