In this article we will discuss about 1. Simple Plant Phenolics 2. Lignin 3. Flavonoids and 4. Tannins.
Simple Plant Phenolics:
The phenolic compounds which contain basic carbon skeleton as C6, C6 – C1, C6 – C2, C6 – C3 and C6 – C4 are sometimes grouped as simple phenolics. Many of these compounds occur in plants as secondary metabolites e.g., caffeic acid, ferulic acid; coumarins such as umbelliferone; furanocoumarins such as psoralen; benzoic acid derivatives such as salicylic acid etc. Structures of some of these compounds are given in Fig. 24.16.
Some of these phenolic compounds such as furanocoumarins are phototoxic and have defensive role in plants (especially the members of the family Umbelliferae) against insect herbivores and fungi. Some phenolic compounds such as caffeic acid and ferulic acid have allelopathic activity.
These compounds are released by some plants into the soil which inhibit germination and growth of other neighbouring plants and thus act as agents of plant-plant competition. Salicylic acid and its methyl ester methyl salicylate are known to be involved in systemic acquired resistance (SAR) to plant pathogens.
Lignin:
Lignin is highly complex and branched polymer of simple phenolic compounds with C6 – C3 basic carbon skeleton.
The structure of lignin is obscure. However, three phenyl propanoid alcohols viz., coniferyl alcohol, sinapyl alcohol and p-coumaryl alcohol (Fig. 24.17) are believed to be building blocks of lignin. These three building units of lignin are not joined in a simple repeating manner but have a highly branched and complex arrangement to form lignin. The proportion, of these three compounds in lignin varies among species, plant organs and even in different regions of the same cell wall.
Lignin is a strengthening material which occurs chiefly in secondary walls of supporting and conducting tissue especially vessels and tracheids of xylem in all vascular plants. It may also occur in middle lamella and primary wall along with celluloses and other cell wall polysaccharides. After cellulose, lignin is second most abundant organic substance in higher plants. It comprises 15-25% of dry weight of many woody plant species.
Primary function of lignin is to provide mechanical support to plant. Besides this, lignin also protects the cell walls from physical, chemical and biological attack. Lignin is considered to have great evolutionary significance as an important adaptation of primitive plants to terrestrial environment.
Flavonoids:
Flavonoids are 15-C phenolic compounds widely distributed in plants and consist of C6 – C3 – C6 basic carbon skeleton. Positions on this carbon skeleton are numbered as shown in Fig. 24.18.
The two aromatic carbon rings at the left and right sides of the flavonoid molecule are designated as A and b rings respectively. The second ring B and 3-carbons (at positions 2, 3, 4) of the middle ring are derived from the shikimic acid pathway, whereas the ring A and oxygen of the middle ring are derived entirely from acetate units provided by acetyl-CoA through malonic acid pathway.
Flavonoids usually occur as glycosides and are soluble in water and mostly coloured being red, crimson, purple, blue and yellow. They accumulate in vacuole although they are syntheiszed outside the vacuole. Sometimes, flavonoids may also occur in chromoplasts and chloroplasts.
Hydroxyl groups (- OH) are usually present in flavonoids especially at positions 3′ and 4′ of ring B or positions 5 and 7 of ring A or 3rd position of central ring. In the latter cases, they serve as points of attachment of various sugars which make them soluble in water. Flavonoids perform variety of functions in plants including defense and pigmentation.
Based on degree of oxidation at various positions of the central ring, the flavonoids are classified into many subgroups. However, three of these subgroups are of particular interest in plant physiology viz., anthocyanins, flavonols & flavones and isoflavones (isoflavonoids).
(i) Anthocyanins:
The anthocyanins (from Greek anthos = flower; kyanos = dark blue) are coloured flavonoid pigments commonly found in blue, purple and red flowers. Sometimes, they may also occur in other parts of plants such as some fruits, stems, leaves and even roots. Although anthocyanins provide different colours to vast majority of flowers and fruits, but sometimes the colouration of flowers and fruits may be due to carotenoid pigments (tetraterpenes) such as in some yellow flowers and tomato fruits.
Anthocyanins are found dissolved in cell sap in vacuole as glycosides. Without sugar molecule, the rest part of anthocyanin (i.e., its aglycone) is called as anthocyanidin. Basic structures of anthocyanin and anthocyanidin are shown in Fig. 24.19.
Anthocyanidins contain hydroxyl groups at 3rd position of central ring and 5th and 7th positions of ring A. Sugars usually in the form of one or two glucose or galactose units are mostly attached at 3rd position of central ring or 5th position of ring A to form its glycoside i.e., anthocyanin. Sometimes sugar may be attached at 7th position of ring A.
Anthocyanidins also contain one or more hydroxyl groups (- OH) in ring B at 3′, 4′ and 5′ positions some of which may be methylated. These substituent groups on ring B give characteristic colour to the anthocyanidins.
Besides this, anthocyanin colour is also influenced by other factors such as:
(i) Occurrence of different anthocyanins in the same flower or plant organ,
(ii) Association among anthocyanins especially at high concentration,
(iii) pH of the vacuoles,
(iv) Co-existence of anthocyanins with other flavonoids such as flavonols and flavones (i.e., copigmentation) and
(v) Association of anthocyanins with chelated metal ions.
More than 22 different anthocyanidins are known. Of these, three are most common viz. pelargonidin, delphinidin and cyanidin. The colours and structural features of these and a few other anthocyanidins are given in Table 24.3.
Main role of anthocyanins in flowering plants is to attract insects and other animals for pollination and dispersal of fruits and seeds.
Anthocyanins and other flavonoids present in related species of a genus provide useful information to plant taxonomists in classifying and determining lines of plant evolution. These pigments are also of interest to many plant geneticists because sometimes it is possible to correlate many morphological differences in closely related species of a genus with the types of anthocyanins and other flavonoids which are found in them.
(ii) Flavones and Flavonols:
These are closely related in structure to anthocyanins except that they differ in central ring of their molecules as shown in Fig. 24.20.
Flavones or flavonols are mostly yellowish and ivory coloured pigments which are widespread in flower petals and may contribute to flower colour. Some of these pigments are colourless and appear to give ‘body’ to white, cream and ivory-coloured flowers. Flavones & flavonols are also widespread in leaves.
These pigments perform many biological functions in plants:
(i) Colourless flavones & flavonols absorb UV radiation and thus provide protection to cells against UV rays,
(ii) These may also be feeding deterrents to herbivores,
(iii) These pigments are not visible to human eye because they absorb UV light, however insects such as bees can see into UV range of spectrum and therefore, may respond to these pigments as attractant cues.
(iii) Isoflavonoids (Isoflavones):
These are found mostly in leguminous plants and differ from other flavonoids in structure in that the aromatic ring B is shifted and is attached to carbon at 3rd position of central ring instead to carbon at second position (Fig. 24.21.).
Isoflavonoids or isofloavones perform many biological functions:
(i) Some of them such as rotenoids have strong insecticidal properties (rotenone, an isoflavonoid from the roots of Derris elliptica, is widely used insecticide),
(ii) Isoflavonoids resemble in structure to some animal hormones estrogens such as estradiol, and cause infertility in mammals especially sheep,
(iii) Isoflavonoids are also known to act as phytoalexins. (Phytoalexins are antimicrobial substances which are produced in plants as a result of fungal or bacterial infection. Besides isoflavonoids, many sesquiterpenes in plants especially members of family solanaceae, are produced as phytoalexins).
Tannins:
The term “tannins” was introduced in 1796 to describe a group of compounds occurring in some plants which could tan animal skins or hides to produce leather. Tannins bind with collagen proteins of animal skins to make the latter more resistant to heat, water and microbial attack.
Tannins are plant phenolic polymers which are widely distributed in higher plants. Bark, leaves, wood and unripe fruits of some plants are good sources of tannins. The compounds having a mol., wt. between 500 – 3000 and containing sufficient phenolic hydroxylic groups (1- 2% M.W.) to form effective cross links with proteins are considered as good tannin agents. An extract from oak bark (Quercus spp.) is a common tanning agent.
Tannins are divided into two main groups, condensed tannins and hydrolysable tannins.
(i) Condensed tannins (flavolans):
These are polymers of 15-C phenolic compound with flavone or C6-C3-C6 type of basic carbon skeleton.
Condensed tannins are not easily hydrolysed:
On treatment with hydrolytic agents, they do not yield significant amounts of compounds of low mol. wt. Instead, they tend to be polymerised especially in acid, to form amorphous red coloured compounds called phlobaphenes.
(ii) Hydrolysable tannins:
These are heterogeneous polymers containing phenolic acids particularly gallic acid (Fig. 24.22) and simple sugars such as glucose. They have smaller mol. wts. in comparison to condensed tannins and can be hydrolysed comparatively more easily.
The simplest example of hydrolysable tannins is Chinese tannin from sumac (Khus sp.) which consists of eight Gallic acid resides connecting in various ways to one another and glucose. The hydrolysable tannins which consist of gallic acids as phenolic residues are also called as gallotannins.
Biological functions of tannins:
(i) Tannins act as feeding deterrents against herbivores because they cause astringency and also because they interfere with digestion and utilization of foods.
(ii) Tannins provide protection to plants against microbial attack.
(iii) Many gallotannins are known to inhibit plant growth and some of them act as al- lelopathic agents.