Tristyly in Pontederia cordata (Pontederiaceae)

Abstract

Pontederia cordata L. (Pontederiaceae) is a perennial, entomophilous, emergent aquatic and possesses a tristylous breeding system. A survey of 74 North American populations indicates that populations usually contain the three floral morphs. A strong pollen trimorphism is associated with differences in stamen and style length among the morphs. Pollen size is proportional to stamen height, while pollen production is inversely proportional. There are two size classes of anthers in P. cordata. Long-level anthers of mid- and short-styled plants and mid-level anthers of the short-styled morph are significantly larger than short-level anthers of long- and mid-styled plants and the mid-level anthers of the long-styled morph. The larger mid-level anthers of the short-styled morph produce nearly twice as many pollen grains as the equivalent level in the long-styled morph. It is suggested that this difference may be associated with varying patterns of stamen insertion and development in the floral morphs.Comparisons of various reproductive parameters among the floral morphs in natural populations suggest that there are no significant differences in flowering patterns, inflorescence and flower production, seed production, and germination levels. In addition, bumble bees show no apparent preferences among the floral morphs during foraging activity. Tristyly appears to be a relatively stable breeding system in P. cordata in comparison with tristylous species of the related Eichhornia.