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

Tristylous breeding system consists of 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. Tristylous Lythrum salicaria L. bears trimorphic, striate, tricolporate pollen with three true colpi and three pseudocolpi. Using a scanning electron microscope, we quantified pollen polymorphism in L. salicaria by measuring the length and diameter of pollen, and the length of colpi and pseudocolpi. Pollen grains from the long stamens are significantly larger than those from the mid stamens, followed by those from the short stamens. Despite significant differences in pollen size among long, mid, and short stamens, pollen morphometric traits demonstrate overlap among morphs. Pollen diameter also differed significantly between source populations. A discriminant function analysis separated long-level pollen from mid- and short-level pollen based on the canonical scores of measurements of pollen structure. The polymorphism of pollen in conjunction with that of the stigma may play a functional role in self-incompatibility mechanisms.