Question

(a) Name the organic material of the exine of the pollen grain. How is this material advantageous to the pollen grains?
(b) Still, it is observed that it does not form a continuous layer around the pollen grain. Give reason.
(c) How are pollen grains useful ?

Answer 1

a)It is an organic polymer that is resistant to oxidation and leaching. It is a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable and is usually well preserved in soils and sediments.
b) Yes, it is definitely observed that it does not form a continuous layer around the pollen grain. It is very obvious from the above diagram that exine which is the outer layer of the pollen grain is not continuous which also means sporopollenin is not present all around the pollen grain.
c) Pollen bank is a method where pollens are stored for germplasm collection. Different types of pollen are collected and stored here for future use. By using these pollens the plants that are facing extinction can be reproduced.

Complete answer:
a) The organic material of the pollen grain is sporopollenin. Sporopollenin is advantageous to pollen grains as pollen grains are dispersed by many different environmental factors, such as wind, water or animals. If the conditions are suitable the sporopollenin-impregnated walls of pollen grains and spores can persist in the fossil record for hundreds of millions of years, since sporopollenin is resistant to chemical degradation by organic and inorganic chemicals. Also sporopollenin resists adverse climatic conditions and are able to preserve themselves from getting destroyed or decayed or non functional.

b) Exine consists of germ pores at points where there are apertures in the layer. So the point at which exine is absent it is natural that there is no sporopollenin present there. It is beneficial for the pollen grain as it has to germinate through the germ pore to form the pollen tube to carry on the process of pollination. If there would be sporopollenin present as it is very resistant and non degradable, the grain would not be able to germinate and it will not be able to form the pollen tube thus inhibiting the process of pollination.

c) The ultimate goal of dispersed pollen is to reach the specific stigma of the carpel to germinate and develop male gametophyte.

During this period they must remain viable and survive to bring about fertilization. The viability of pollen grains differs from species to species. In the case of certain cereals such as rice and wheat the pollen grains remain viable only for 30 minutes, whereas the pollen grains of plants belonging to Rosaceae, Leguminosae and Solanaceae, remain viable for months.

Therefore pollen banks are used to basically preserve the viability of the pollen grains and also maintain a good amount of stock of that crops which are becoming extinct. These banks can be used to enhance food and crop production in the agriculture sector.

Note:
a) The chemical composition of sporopollenin has long been elusive due to its unusual chemical stability and resistance to degradation by enzymes and strong chemical reagents. Analyses have revealed a mixture of biopolymers, containing mainly long-chain fatty acids, phenylpropanoids, phenolics and traces of carotenoids. Tracer experiments have shown that phenylalanine is a major precursor, but other carbon sources also contribute. It is likely that sporopollenin derives from several precursors that are chemically cross-linked to form a rigid structure.

b) The size and shape of spores and pollen; exine microsculpturing and the number, position and characters of the apertures are of great taxonomic importance. These are studied under a separate branch of botany called palynology.

c) Presently, scientists have developed means for storing the viable pollen grains in liquid nitrogen (at -196 degrees) to be used as pollen banks in crop breeding programs.