Structural studies of trimorphic incompatibility in Pontederia sagittata Presl. (Pontederiaceae)

Heteromorphic characters and structural features of the pollen tube pathway were described in tristylous Pontederia sagittata (Pontederiaceae) to assess their influence on the pollination process and in mediating SI behaviour. Heteromorphic characters that distinguish the floral morphs include; style length, stigma depth, stigmatic papillae length, stylar parenchyma cell length, area of the stylar canal, stamen height, anther sac size, and pollen size. Floral features common to the floral morphs were: (1) a wet stigma, becoming so through secondary inundation by stylar exudates; (2) a hollow trilobed stylar canal separating into two sterile, and one fertile canal; (3) a single anatropous ovule with a highly elaborated integumentary obturator. The similarity in structural features of Pontederia sagittata to those seen in monocotyledonous taxa with gametophytic SI suggests that phylogenetic constraints may have influenced the evolution of trimorphic incompatibility in the Pontederiaceae.;A quantitative study was undertaken to describe the general properties of pollen tube growth in legitimate and illegitimate pollinations. The overall behaviour of legitimate pollinations was similar to that observed in illegitimate pollinations. There was no evidence of a unitary rejection response of the type observed in homomorphic sporophytic systems. Instead, pollen tubes ceased growth at different locations in the pistil depending on the size of pollen employed. Pollen tubes terminated growth in the style, with pollen of smaller size than the legitimate size class. This suggests that pollen storage reserves govern pollen tube lengths. In pollinations with pollen of larger size, than the legitimate size class, two patterns were observed. In the M morph high levels of seed set occurred, indicating weak expression of self-incompatibility. In contrast, in the S morph low levels of seed set were obtained with pollen tubes entering the ovary and growing past the obturator. This behaviour may result from a failure of pollen tubes to respond to an ovular signal. The variety of different pollen-pistil interactions observed in P. sagittata cast doubt on the evolutionary origin of heteromorphic from homomorphic incompatibility systems.