Phenolic Compound - Plant Physiology - Lecture Slides, Slides for Physiology. Biju Patnaik University of Technology, Rourkela


Description: These are the lecture slides of Plant Physiology. Key important points are: Phenolic Compound, Phenol Group, Hydroxyl Functional Group, Aromatic Ring, Biosynthesis of Phenolics, Shikimic Acid Pathway, Cinnamic Acid, Phenylalanine Ammonia Lyase, Types of Phenolics
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What’s a phenolic compound? A secondary product that contains a phenol group - a hydroxyl functional group on an aromatic ring.


Phenolics are a chemically diverse group: many different properties and functions.

Biosynthesis of phenolics Shikimic acid pathway is most common in plants. Converts simple carbohydrates into aromatic amino acids. Not present in animals.

Most plant phenolics are derived from cinnamic acid formed from phenylalanine by phenylalanine ammonia lyase

(PAL) enzyme.


PAL activity is inducible: fungal infection, low nutrient levels, high light

Major types of phenolics 1. Simple phenolics - e.g. coumarins 2. Lignin - 2nd most abundant compound in plants 3. Flavonoids - two aromatic rings, 2 pathways anthocyanins, flavones/flavonols 4. Condensed tannins polymerized flavonoids 5. Hydrolyzable tannins made of phenolic acids and sugars smaller molecules than condensed tannins

Fig. 13.10

1. simple phenolics

Involved in defense against insect herbivores and fungi, some may have allelopathic function.

Caffeic acid & ferulic acid implicated in allelopathy. Psoralen is one of several phototoxic furanocoumarins, (UV activated)

Simple phenolics

Fig. 13.11

Insects usually roll leaves for protection from predation and to create a habitat

Sometimes insects roll leaves to prevent furanocourmarins from being activated

Furoanocoumarins can certain light wavelengths. (common in Umbelliferae family – celery, parsnip)

Leaf rolling insects

Fig. 13.11

Simple phenolics

2. Lignin Second most abundant compound in plants.

Highly branched polymer of phenylpropanoid groups (benzene-C3)

- often found in vessel elements, tracheids, and stems; confers structural support. Primary structural


- Secondary role as a herbivore deterrent by reducing digestibility of plant matter

- also difficult for microbes

to degrade; its presence slows litter decomposition.

Fig. 13.10

3. Flavonoids - basic structure is two aromatic rings joined by a 3C bridge.

a) anthocyanins b) flavones c) flavonols d) isoflavonoids

a) Anthocyanidins and anthocyanins are pigments that give plant tissues red, blue, and purple color. Pollinator attraction Disperser attraction


b & c) flavones and flavonols UV absorbing protection against UV (280 - 320 nm) insect pollinator attraction

Flavonoids continued

How we see the golden eyes…

How honeybees see golden eyes… UV absorbing flavonols are present in the inner part of petals

Rhizobium is attracted to legumes through signaling by isoflavanoids released from roots.

d) Isoflavonoids common in legumes antimicrobial properties also involved in signalling e.g. attracting rhizobia

Tannins Condensed -formed by polymerization of flavonoid units -common in woody plants Hydrolyzable - contain phenolic acids: gallic acid, & simple sugars - smaller molecules than condensed tannins - more easily hydrolyzed and degraded Tannins reduce growth and survival of many different kinds of herbivores Also act as antioxidants - eat your isoflavonoids Johnny!

Many foods contains tannins (e.g. tea, red wine) and have some healthy side effects for humans (e.g. disallowing constriction of blood vessels)

Tannins also make protein less digestible. Animals can sense high levels of tannins in their

food and opt for another food resource (e.g. mule deer, beavers).

High levels of tannins in diet can actually kill some


Condensed tannins are polymerized flavonoids.

Hydrolyzable tannins are made of phenolics and sugars.

Fig. 13.15

The term “tannin” is derived from the tanning process

in which raw animal hides are preserved by rubbing tannins on them. The tannins help to complex the proteins and keep them from degrading.

This protein-binding property of tannins lends them

their toxicity to herbivores. • tannins can bind digestion enzymes in the gut of

herbivores. • tannins also form complex polymers when bound

to proteins which are difficult to digest, thus decreasing the nutritional value of the plant material.

Tannins can reduce nutritional value of tissues by binding to proteins, making them less digestible.

Care for a spot of milk in your tea?

Fig. 13.16

Creosote applications in herbal therapy · To dissolve urinary kidney stones. · Anti inflammatory for respiratory ailments (asthma) and arthritis · To eliminate gallstones · Against urinary infections · For the treatment of venereal disease · As an abortifacient · Against diabetes · Bronchitis and colds · Rheumatism · Against some types of cancer · As a mouthwash against tooth decay and halitosis

After life effects of phenolic compounds. Plant litter decomposition, and release of nutrients from decomposing litter, are strongly influenced by the chemical composition of the litter. Litter higher in tannins and lignin decomposes more


Lignin/Nitrogen ratio

Decomposition rate

Bill Shakespeare, secondary chemist

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