Which of the following is the hardest substance in the plant kingdom?

Sporopollenin

The correct answer is Option : (2) Sporopollenin

Pollen grains are generally spherical measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall.The inner wall of the pollen grain is called the intine. It is a thin and continuous layer made up of cellulose and pectin. The hard outer layer called the exine is made up of sporopollenin which is one of the most resistant organic material known. It can withstand high temperatures and strong acids and alkali. No enzyme that degrades sporopollenin is so far known. Pollen grain exine has prominent apertures called germ pores where sporopollenin is absent. Pollen grains are well- preserved as fossils because of the presence of sporopollenin. The exine exhibits a fascinating array of patterns and designs.