Question

Which stain is used for demonstrating lignin?
A. Aniline blue
B. Leishman’s stain
C. Methylene blue
D. Phloroglucinol

Answer 1

Hint: Lignin is a complex organic polymer recalcitrant in nature and is necessary for the formation of plant cell walls which require rigidity like the bark and wood cells. Different stains are used to identify different biochemical compounds in nature.

Complete answer:
Lignin is present in abundance in most of the vascular plants and algae and is the main reason behind the hardness in the plant parts. It is a cross-linked phenolic polymer, insoluble in water or alcohol but soluble in weak alkaline solution. It is rich in aromatic subunits.

To enhance the visualisation of cells and their components they are stained with different stains/dyes. Different stains are used for different types of cells which are particular for each other. For staining lignin phloroglucinol is being used. This stain is also known as Weisner’s stain which provides fuchsia or cherry pink colour to the cells having lignin. When the plant fragment is treated with this stain then it is simultaneously acidified with hydrochloric acid. Both ${\text{HCL}}$and phloroglucinol are corrosive and should be handled with care.

Hence, the correct answer is option (D).

Additional information:
• Lignin is present in covalent association with cellulose and hemicellulose.
• It provides support to the xylem and phloem and helps in the transfer of water to different parts of the plant stem.
• Lignin is the major roadblock in the process of biofuel production from lignocellulosic biomass, so it needs to be removed with the help of various treatments like enzymatic, chemical, and mechanical.

Note: Lignin is made up of three polymers – p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. All these polymers are known as monolignols. It can be easily broken down into toluene, benzene, phenol and xylene.