Tapetal and parenchymatic anther tissues participate in polyad adhesive production in Calliandra brevipes (Leguminosae)

Calliandra brevipes Benth. is an American shrubby species largely used for ornamental purposes. Like all other Calliandra species it has heteromorphic pollen grains shed in ellipse-shaped, calymmate polyads and a unique mode of pollen presentation by producing a sticky substance called “pollen adhesive”. The present study aimed to investigate in detail the origin of polyad adhesive in C. brevipes. Serial microtome sections of anthers in various developmental stages were used and histochemical tests applied to detect the adhesive production sites and investigate the subcellular characteristics of the anther cells under transmission electron microscopy. The pollen adhesive in C. brevipes is not only produced by parenchymatic cells of the anther transversal septum, as it is described for Calliandra angustifolia, but parenchymatic cells and tapetal cells next to the polyad apical pollen grain also participate in the pollen adhesive production. The cytoplasm of the degenerating tapetum cells contains oleoplasts and fibrogranular material inside the vacuoles which mixes with the adhesive produced by adjacent parenchymatic cells and which therefore contributes to its composition. Vacuoles containing fibrogranular material are very similar to those found in tryphine producing tapetal cells, and the subcellular structures of parenchymatic and tapetal cells are similar to each other. The fact that the pollen adhesive becomes solid in contact with the environment is attributed to dehydration and the presence of the protein fibrogranular material and lipid substances since resin could not be found in its composition. It seems that the sites of pollen adhesive production in Calliandra may vary among its members. Studies of polyad adhesive production in the genus should be standardized in order to verify the information already available in the literature.