In this article we will discuss about the composition of sporopollenin in spores and pollen.
Sporopollenin is the major component of exine of spores and pollen. ‘It is most extraordinarily resistant material known in the organic world’. It is extremely resistant to non-oxidative chemicals, enzymes and strong chemical reagents. It is chemically stable and remains unchanged in pollen grains that are 500 million years old.
It is well preserved in sediments. Sporopollenin remains unchanged when extant pollen grains are heated to 300°C or treated with concentrated acids and bases. Because of this resistance, the definite chemical structure of sporopollenin is inadequately known.
At present, it is assumed that sporopollenin is a polymer. Monomers of sporopollenin skeleton are linked by ether-linkage thus composing a complex polymer.
The sporopollenin polymer has a uniform composition, i.e., relatively small monomer moieties of similar primary structure are present. Kawase et al. (1995) report that the main structure of sporopollenin is a simple aliphatic polymer containing aromatic conjugated side chains.
The elemental contents of Carbon, Hydrogen and Nitrogen as determined from CHN analysis indicate that sporopollenins are N-free substances. Zetsche and Vicari (1931) estimated the presence of 90 Carbon, 142 Hydrogen and 27 Oxygen atoms in a sporopollenin molecule of Lycopodium. Sporopollenins differ specifically in composition and quantity in different pollen species.
It is to note that the very resistant Pinus pollen grains possess the greatest quantity of sporopollenin. Chemical analysis reveals that approximately half of the sporopollenin carbons are bonded as CH and the other half as CH2. The greater part of oxygen found in sporopollenin originates from hydroxyl groups and is derived from aliphatic and not from aromatics.
It is regarded that aromatics, long unbranched aliphatic components compose the complex structure of polymer. Hesse et al. (1995) report the occurrence of benzoic acid moieties in the sporopollenin of Typha angustifolia. Ahlers et al. (2000) found α, β unsaturated carbonyl group in the sporopollenin of Typha angustifolia.
Chemical analysis reveals that sporopollenin is a biopolymer containing mainly long chain fatty acids, phenylpropanoids, phenolics, carotenoids and xanthophyll. The presence of phenylalanine was detected by tracer experiments. Sporopollenin is close in structure to the potassium salt of glucuronic acid.
Infrared absorption spectra of the sporopollenin of Beta vulgaris reveal it. In the biosynthesis of sporopollenin the chain-elongating enzyme systems of lipid metabolism are involved. Biochemical studies indicate this by using thiocarbamate herbicide-the inhibitors of the chain- elongating enzyme systems of lipid metabolism.
The various properties of sporopollenin are:
i. It is resistant to acetolysis, high temperature, many chemicals, physical and biological treatments including fungal and bacterial attack.
ii. It is very durable compound and only strong oxidants such as hydrogen peroxide can destroy it.
iii. It is a very tough compound and may occur in pollen even 500 million years old. Due to sporopollenin pollen gets preserved during fossilization.
iv. It is arranged in complex forms in walls of pollen in such a way that it forms an elaborately sculptured characteristic pattern on sexine.
v. It checks natural decay of pollen.
vi. It has tendency to auto-oxidize. It picks up oxygen atom. This causes the pollen and spore wall to swell and eventually dissolute. The endexine is more resistant to oxidation than the ektexine (Rowley, 2001). It cannot survive in alkaline areas.
vii. Sporopollenin is less resistant to oxidation. It can be degraded by ozonolysis.