Pollen biology

Pollen grains embody the male partners in sexual reproduction. They are generally shed in a desiccated condition and their moisture level is less than 20%. At the time of shedding, pollen grains are either two-celled – a large vegetative cell enclosing a generative cell; or three-celled – a vegetative cell and two sperm cells formed by the division of the generative cell. There is considerable variation in the size and shape of pollen grains (Erdtman 1966; Moore and Webb 1978; Iwanami et al. 1988; Faegri and Iversen 1989; Cresti et al. 1992). Although a majority of pollen grains are spherical, in some marine angiosperms, such as Amphibolis and Zostera, they are filiform (up to 5 mm) (Ducker et al. 1978). The wall of the pollen grain is made up of two layers: an outer, acetolysis-resistant exine composed of sporopollenin and an inner pectocellulosic intine. One of the conspicuous structural features of pollen grains is the ornamentation of the wall formed by the outer part of the exine (Cresti et al. 1992).

Pollen biology involves a comprehensive understanding of the structural and functional aspects of pollen grains. The main function of the pollen is to discharge male gametes in the embryo sac for fertilization and for subsequent seed and fruit development. This function depends on the successful completion of a number of sequential events. The following are considered the major events in pollen biology:

• Pollen development

• Free dispersed phase

• Pollination

• Pollen–pistil interaction

• Fertilization

Pollen grains develop inside the anther and are dispersed by dehiscence of the anther. After dispersal, pollen grains remain as independent functional units and are exposed to the prevailing environmental conditions for varying periods.