Populations of Pontederia rotundifolia in the Lower Amazon and Costa Rica are tristylous. A strong pollen trimorphism is associated with differences in stamen and style length in the three floral forms. The results of a controlled pollination programme with thirty individual plants of P. rotundifolia demonstrate that floral trimorphism is accompanied by a physiological self-incompatibility system typical of many heterostylous plants. Legitimate pollinations are considerably more productive of seed than illegitimate pollinations. Individual plants exhibit differences in the strength of self-incompatibility, but in general, short-styled plants possess the strongest self-incompatibility, long-styled plants are intermediate and mid-styled plants have the weakest self-incompatibility. Large differences in the strength of self-incompatibility are expressed in self-pollinations with pollen from the two anther levels of each style form.
Natural populations of P. rotundifolia often contain only a single style form or contain unequal proportions of style forms. Seed set, resulting from moderate self-compatibility, occurred in a Cosra Rican population that contained only short-styled plants. Two populations associated with rice cultivation in the Lower Amazon contained equal proportions of the three style forms. The seed set of the three forms within these populations was similar. The floral trimorphism and breeding system of P. rotundifolia are compared with those of the related P. cordata and Eichhornia crassipes.