Tannins: What do they represent in plant life?

Abstract

Tannins were, and still are, the target of researches on their specific biological activities, especially in the role of antioxidants and anticarcinogens. Furthermore, there is ample proof of their anti-inflammatory, cicatrizant and anti-HIV functions. But, what is their real significance in plant life? When did they first appear in the evolutionary history of plants, and what ecological advantages do they bestow on these organisms? This chapter will present tannins from a 'plant point of view', in an attempt to point out their importance in plant survival, when acting against biotic herbivores, as well as pathogen and abiotic stress (air pollutants).

Full text

In: Tannins: Types, Foods Containing, and Nutrition ISBN: 978-1-61761-127-8
Editor: Georgios K. Petridis © 2010 Nova Science Publishers, Inc.
Chapter 10
TANNINS: WHAT DO THEY
REPRESENT IN PLANT LIFE?
Claudia Maria Furlan*, Lucimar Barbosa Motta
and Deborah Yara Alves Cursino dos Santos
Departamento de Botânica, Instituto de Biociências,
Universidade de São Paulo, São Paulo, SP, Brazil
ABSTRACT
Tannins were, and still are, the target of researches on their specific biological
activities, especially in the role of antioxidants and anticarcinogens. Furthermore, there is
ample proof of their anti-inflammatory, cicatrizant and anti-HIV functions. But, what is
their real significance in plant life? When did they first appear in the evolutionary history
of plants, and what ecological advantages do they bestow on these organisms? This
chapter will present tannins from a ‘plant point of view’, in an attempt to point out their
importance in plant survival, when acting against biotic herbivores, as well as pathogen
and abiotic stress (air pollutants).
INTRODUCTION
Tannins have been the target of researches with the aim of investigating their biological
activities, especially as antioxidants and anticarcinogens. Furthermore, there is ample proof of
their anti-inflammatory, cicatrizant and anti-HIV functions. But, what is their bearing on plant
life? When did they first appear in the evolutionary history of plants, and what ecological
advantages do they bestow thereon? It is important to know about tannins from the point of
view ‘plants’, in an attempt to understand their involvement in plant-defense against biotic
and/or abiotic stress, or rather, their importance in plant survival itself.
* Corresponding author: e-mail: furlancm@usp.br; lugalll@yahoo.com.br; dyacsan@ib.usp.br

Claudia Maria Furlan, Lucimar Barbosa Motta et al.
2
Tannins are chemically defined as secondary compounds synthesized through vegetal
secondary metabolism, or, for many authors, by special metabolism [Monteiro et al., 2005].
Secondary metabolites have been associated to plant-environmental interactions [Haslam,
1995]. Traditionally, tannins have been described as modulators in plant-herbivore
interactions and|or protection agents against infection, with the main function as herbivore
deterrents due to their acid taste and the property of precipitating proteins.
Khanbabaee & Ree [2001] provided a convenient classification of tannins based on their
specific structural characteristics and chemical properties, thereby avoiding the traditional
classification in hydrolyzable and non-hydrolyzable tannins. The authors classified tannins in
four groups as follows: 1. gallotannins, all those with galloyl units or derivatives bound to
diverse polyol-, catechin- or triterpenoid units; 2. ellagitanins, those in which at least two
galloyl units are C-C coupled to each other, without containing a glycosidically linked
catechin unit; 3. complex tannins, which present a catechin unit glycosidically bound to either
a gallotannin or ellagitannin unit; and 4. condensed tannins, all of which being oligomeric and
polymeric proanthocyanidins.
According to some authors, tannins can be used as chemotaxonomic markers, especially
for Angiosperm orders and families [Okuda et al., 2000; Okuda 2005]. Okuda et al. [2000]
correlated the orders, families and genera in the Cronquist system of plant classification with
the oxidative structural transformation of plant polyphenols. Accordingly, the biogenetic
transformation of hydrolyzable tannins is considered to start from gallotannins (galloyl group
in gallotannin I) to the hexahydroxydiphenoyl (HHDP) group in ellagitannin (II), continuing
with the dehydrohexahydroxydiphenoyl (DHHDP) group into dehydroellagitannin (III), and
finally transformed DHHDP groups into transformed dehydroellagitannins (IV). In their
correlation, tannins are absent in only one Cronquist subclass, the Asteridae.
It is important to point out that the Cronquist system of plant classification was accepted
and followed by many researchers until 1998, when macromolecular biology data (mainly
DNA sequences) gave rise to new insights regarding plant evolution. Nowadays, most of the
subclasses, orders and families proposed by Cronquist have been reevaluated, and angiosperm
systematics recircumscribed based on phylogenetic paradigms, thereby giving rise to a new
proposal - the Angiosperm Phylogeny Group [APG, 2009]. This new system presents a
synthesis of angiosperm phylogeny hypothesis, by emphasizing monophyletic groups based
mainly on the combination of DNA sequences (rbcL, 18S rDNA, atpB, matK). Even so, the
Asterids clade is still characterized by families with the presence of typical alkaloids and
iridoids (secoiridoids). Since this clade is mainly formed by herbaceous plants, tannins seem
not to be directly related to this form of life.
According to Agrawal [2006], the evolution of plant defense strategies is nowadays
viewed in a phylogenetic perspective: the biosynthetic machinery needed to produce plant
defense must be well-conserved and of single origin. Thus, in major classes the origin should
be monophyletic. This could be supported by the correlations between qualitative plant
defenses (alkaloids, iridoids, glucosinolates) and the absence of tannins (a quantitative
defense) [Hartman, 2008]. In other words, tannins are ubiquitous in woody plants, but almost
absent in herbaceous species [Haslam, 1988].
Although tannins are especially present and studied in flowering plants, it is important to
remember that non-hydrolysable tannins are also found in Monilophytes (mainly ferns), and
tannic substances in Phaeophyta, an algae group unrelated to Archaeplastida, a clade which
includes Glaucophyta, Rhodophyta, Chlorophyta and Land plants. Phaeophyta (brown algae)

Tannins: What Do They Represent in Plant Life 3
presents a characteristic group of polyphenols with tanning properties, the phlorotannins,
which constitute a structural class of polyketides found exclusively in these algae [Maschek
& Baker, 2008].
According to Popper & Fry [2004] and Popper [2008], and when studying the primary
cell wall composition of Lycophyta, Monilophyta (mainly ferns) and Spermatophyta
(Gymnosperm and Angiosperm), tannins may not be strictly a primary cell-wall component.
Nevertheless, the authors often came across tannins associated with primary cell-wall-rich
material, and some even deposited within the primary cell-wall itself. They also reported the
presence of non-hydrolyzable tannins (proanthocyanidins) in the alcohol-insoluble residue of
some of the analyzed monilophytes, although they were absent from lycophyte alcohol-
residues.
Vascular plants (or Tracheophytes) form a well-supported, monophyletic group, this
including the Lycophytes, Monilophytes and Spermatophytes [APG, 2009]. Ferns are the
earliest diverging plants in which proanthocyanidins began to predominate over flavonols [De
Bruyne et al., 1999]. Proanthocyanidins remained important in early diverging Angiosperms,
although their synthesis decreased in more advanced orders [De Bruyne et al., 1999], prior to
the Asterids forming the most divergent group of Angiosperms. It seems likely that the
production of proanthocyanidins evolved simultaneously in the monilophytes, contrary to
Bate-Smith [1977], who believed that these compounds appeared later on in plant
evolutionary history, rather than in the vascular condition.
Recent studies involve the search for connections between ecosystem processes and
genetic mechanisms. Schweitzer et al. [2008] assayed, not only the role of condensed tannins
in herbivore defense in a Populus, model-system, but also the presence of these compounds in
a genetic context. The authors pointed out that condensed tannins (complex flavonoid
polymers) occur across phylogenetically diverse plant groups, as well as in various
ecosystems (from Arctic to tropical). This wide distribution suggests a profound evolutionary
history. Variation in the production of condensed tannin has a clear genetic basis. The
possibility of a lack or even reduced synthesis of these compounds in knockout mutant
production is the key to clarifying the specific roles of tannins at individual and ecosystem
levels. A further observation was the importance of ‘underground’ condensed tannin in
regulating nutrient dynamics, a new view of tannins, outside traditional herbivore/pathogen
defense.
Almeida et al. [2005] studied the medicinal flora of the Caatinga, an arid Brazilian
biome, so as to verify whether the theory of ecological apparentness could explain choice and
ethnobotanical plant use. Five classes of chemical compounds (tannins, alkaloids, phenols,
quinines and triterpenes) were analyzed. Phenols and tannins were outstanding in all plant
species and habitats, possibly through their widespread distribution, especially among
lignified plants, common in the Caatinga environment. The authors pointed out that the
phenols have been related to several biological activities (antiviral, antioxidant, diuretic,
antirheumatic and others) [Grassmann et al., 2002]. On the other hand, tannins are used
against diarrhea, as antiseptics, vasoconstrictors, antimicrobial and antifungal, due to their
astringent activity [Gutiérrez et al., 2008; Falleh et al., 2008; Brandelli et al., 2009].

Claudia Maria Furlan, Lucimar Barbosa Motta et al.
4
Tannins vs. Biotic Stress: Herbivory and Allelopathy
Tannins are present in the leaves, bark, fruits and seeds of many plants. The main
function of these compounds is to provide protection against microbial pathogens, harmful
insects and other herbivores. The storage of proanthocyanidins in the endothelial layer of the
seed coat in many species may be seen as a classic example of a pre-formed protective barrier
[Lattanzio et al., 2004; Panjehkeh et al., 2009]. Aziz et al. [2004] suggested that the
proanthocyanidins present in glandular trichomes of alfalfa (Medicago sativa L.) act as a first
line of defense against insect predation. Coffee varieties that are susceptible to the fungal
pathogen Hemileia sp have lower levels of proanthocyanidins than those resistant. These
compounds, when isolated from pulp or leaves, inhibit fungal germination in vitro [Gonzalez
de Colmenares et al., 1998]. A strong negative correlation correlativeness was found between
the concentration of procyanidin, a condensed tannin, in leaf-bud petioles in seven genotypes
of groundnuts (Arachis hypogaea), and fecundity of the aphid Aphis craccivora on the same
genotypes [Grayer et al., 1992].
Developing seeds of Sesbania drummondii are attacked by nymphs and adults of the bug
Hyalymenus tarsatus (Heteroptera: Alydidae), which kill some and weaken others. Parasitism
by this piercing-sucking insect reduces the resources of future seedlings and affects seed
physiology, this including dormancy and the exudation of allelochemicals by imbibing seeds.
There is evidence that inducible proanthocyanidin accumulation, also present in leaves re-
acting against mammalian and insect herbivores, may represent a defense mechanism of some
seeds against piercing–sucking insects [Ceballos et al., 2002].
Condensed tannins are also considered to be an important inducible defense form against
mammalian herbivores. Ward & Young [2002] observed differences in condensed tannin
defense in Acacia drepanolobium submitted to distinct large mammalian herbivore
treatments. Contrary to expectations, tannin concentration in trees was higher in the upper
canopy, where there was little mammal induced damage.
Nevertheless, not all effects of proanthocyanidins are beneficial at the expense of
microorganisms and insects. For example, in the bark of Pinus densiflora, these tannins
function as oviposition stimulants for the cerambycid beetle Monochamus alternatus [Allison
et al., 1999].
Furthermore, hydrolysable tannins are believed to induce reduced insect performance.
Barbehein et al. [2009a] tested the hypothesis that higher foliar tannin levels also produce
higher concentrations of semiquinone radicals (from tannin oxidation) in the caterpillar mid-
gut, and radical enhancement could be associated with increased oxidative stress in mid-gut
tissues, with a consequential negative impact on larval performance. By testing various
concentrations of hydrolyzable tannins from hybrid poplars (Populus tremula x P. alba), the
authors observed that there was no measurable amount of semiquinone radicals in the mid-gut
of the larvae of Lymantria dispar caterpillars that ingested control leaves, whereas the levels
of these radicals greatly increased in those that ingested 15% tannin. Contrary to expectations,
larval growth rates were alike, whereby the surmise that tannins act as ‘‘quantitative
defenses’’, since high levels appear to be necessary for increasing levels of semiquinone
radicals.
According to Barbehein et al. [2009b], hydrolysable tannins are much more active as pro-
oxidants in caterpillar gut than condensed tannins. On studying the resistance of red oak
(Quercus rubra L.), a species that produces high levels of condensed tannins, and sugar

Tannins: What Do They Represent in Plant Life 5
maple (Acer saccharum Marsh.) which produces high levels of hydrolysable tannins, the
authors observed that when Lymantria dispar L. caterpillars ingested oak leaves coated with
hydrolysable tannins, levels of hydrolysable tannin oxidation increased in their mid-gut
contents. Once again, the emphasis on high levels of hydrolysable tannins being important for
producing oxidative stress, but that increased tree resistance to caterpillars depends on
additional factors, such as those producing nutritional stress.
Low-molecular-weight phenols (LMWP) and tannins have traditionally been studied in
their defensive role against plant herbivores and pathogens [Rooke & Bergström, 2001;
Barbehenn et al., 2009a, 2009b], as UV screens and anti-oxidants [Hässing et al., 1999;
Feldman, 2005; Falleh et al., 2008], and also as allelopathic agents [Rawat et al., 1998].
Esterified flavan-3-ols, such as (–)-epigallocatechin gallate, as well as oligomeric
proanthocyanidins, accumulate at very high levels in tea-plant leaves. The interest in tea
polyphenols is now mainly concentrated on their potential health-benefits [Khana & Mukhtar,
2007], although high levels of monomeric flavan-3-ols, such as (–)-epicatechin have also
been linked with plant resistance against fungal attack [Punyasiri et al., 2005]. Catechin was
recently described as a powerful allelochemical, responsible for the adaptive advantage of the
invasive species spotted knapweed (Centaurea stoebe) in North America [Chobot et al.,
2009]. (–). Catechin appears to induce oxygen production and a calcium-signaling cascade
that leads to root-death in susceptible species [Bais et al., 2003].
Prunus armeniaca is an important agro-forest tree planted on the boundaries of farm
land. Its negative influence on several crops is infered. Retardation in germination, growth
and yield were detected in wheat (Triticum aestivum) plants growing nearby [Rawat et al.,
1998]. The authors demonstrated that the effects on Triticum aestivum of light petroleum and
ethyl acetate extracts of Prunus armeniaca was higher than those of aqueous extracts. Among
the isolated compounds, proanthocyanidins displayed the maximum inhibition of wheat-
growth and germination in lab tests. Nevertheless, further recent studies, mostly under field
conditions, have shown that the allelopathic role of phenolic compounds might be
misunderstood. Authors have suggested that these compounds are more important in shaping
the soil nutrient environment for plants, than directly acting as growth inhibitors [Inderjit,
2006].
Phenolic compounds may affect the plant nutrient environment by distinct mechanisms,
due to their chemical reactivity. While low-molecular-weight-phenols (LMWP) reduce soil-
nutrient availability by stimulating soil respiration and the immobilization of N in the
microbial biomass, tannins bind to proteins and N-rich soil organic matter, mostly acting on
lowering gross soil-N mineralization and nitrification rates by binding to microbial enzymes
or enzyme substrates [Meier & Bowman, 2008]. Phenolic-rich plants may therefore
negatively modify neighboring plant growth by restricting N supply. Since phenolic
compounds may complexly affect soil nutrient availability for plants, some authors suggested
that the allelopathic influence of these compounds should be better addressed through field
experiments [Inderjit & Foy, 2001]. For these authors, laboratory bioassays can demonstrate
the possibility of an allelopathic relationship, and are only suitable for a partial understanding
of the process.
Nonetheless, Inderjit & Foy [2001] demonstrated that the inhibitive effect of mugwort
(Artemisia vulgaris L.) on red clover (Trifolium pratense L.) growth is directly attributable to
the phenolic compounds released by the former into the soil. It was observed that this

Claudia Maria Furlan, Lucimar Barbosa Motta et al.
6
inhibition was overcome by adding activated charcoal, which absorbed organic molecules
(e.g., phenols), not the case with nitrogen-based fertilizers.
By using two species that co-dominate alpine moist meadows as a model system (the
phenol-rich forb Geum rossii,and the fast-growing grass Deschampsia caespitosa), Meier et
al. [2009] demonstrated that phenol-rich root inputs could be an unappreciated factor
structuring plant communities, especially in N-limited systems dominated by phenol-rich
species themselves. In semi-arid environments, agro-forestry has the means of enhancing
agricultural production and buffer rural livelihood against drought. However, the
misapplication of phenol-rich plants in locations where soil nutrients are limited possibly
engenders economical problems [Nakafeero et al., 2007].
The understanding of how tannins function in plant-herbivore interactions depends to a
large extent, not only on our knowledge of the chemistry of these compounds, but also of
herbivore strategies when dealing with these substances.
Tannins vs. Abiotic Stress: Air Pollutants
There are changes in plant morphology, physiology, biochemistry and growth rate,
brought about by air pollutants [Iqbal et al., 1996; Viskari et al., 2000; Moraes et al., 2002].
Criteria for assessing this impact include analyzing visible injury [Oksanen & Holopainen,
2001] and the accumulation of toxic substances, as well as evaluating biochemical and
physiological pollutant-induced changes in parameters related to photosynthesis, respiration,
enzyme activities, and the syntheses of lipids, proteins and other metabolites, etc. [Viskari et
al., 2000; Calatayud & Barreno, 2001; Herbinger et al., 2002]. Air pollution can also induce
qualitative and quantitative changes in secondary metabolite composition [Kanoun et al.,
2001; Lopanen et al., 2001; Furlan et al, 1999, 2010].
In the late 1980’s, the study of secondary metabolites, when investigating plant pollution
stress, became more popular. Air pollution has been demonstrated as a potential cause of
qualitative and quantitative changes in many compounds from secondary metabolism. Katoh
et al. [1989a] were among the first to report a decrease in the levels of tannins in Cryptomeria
japonica growing near a steam-power station. They found a negative correlation between the
levels of foliar soluble sulphate and tannin content, thereby implying an association between
air pollution and inhibition of the shikimate pathway. The reduction in the amount of tannin
seemed to be related to an increase in feeding rate of Dasychira abietis argentata on C.
japonica. At the same time, the authors reported decreased amounts of soluble phenols in C.
japonica exposed to ozone, correlated to lower levels of glucose [Katoh et al., 1989b].
Later, Kainulainen et al. [1995] observed the same results, i.e., decreased levels of
glucose, frutose and soluble phenols, in needles of Pinus sylvestris and Picea abies exposed
to SO2, thereby infering the effect of air pollution on photosynthesis and carbohydrate
metabolism, with the consequential decrease in both carbon gain and the rate of synthesis of
secondary metabolites, especially those derived from the shikimic acid pathway.
In an open-top experiment using Phaseolus vulgaris L. cv. Nerina, Kanoun et al. [2001]
detected a significant decrease in the accumulation of a hydroxycinnamic acid derivative in
plants exposed to ozone fumigation at 65–85 ppb. These findings infer that ozone exposition
might alter plant–pathogen interactions. On the other hand, as cinnamic derivatives are

Tannins: What Do They Represent in Plant Life 7
important precursors of flavonoid synthesis, enhanced isoflavonoid composition was noted.
According to Kanoun et al. [2001], the production of ozone-induced phenolics seemed to be
closely related to leaf necrosis injury. Pollutants such as sulfur dioxide and ozone may
increase the amounts of leaf nitrogen [Viskari et al., 2000], hence stimulating herbivore
attack.
Furlan et al. [1999, 2004] observed a correlation between the increased percentage of leaf
loss due to herbivority, as observed in Tibouchina pulchra grown in the more polluted areas
of Cubatão (São Paulo, Brazil), and the increased leaf nitrogen content resulting from the
decrease in total soluble phenols and tannins. They suggested that, in addition to pollution
stress, plants growing in polluted areas underwent a further stress, viz., increased herbivorous
aggression.
Tannins have long been related to herbivore feeding, since the classic experiment of
Feeny in the late 1960´s. At the time, Feeny proposed that the deleterious tannin effects on
Operophtera brumata larvae feeding was the ability of these compounds to bind and
precipitate proteins. Tannins have been characterized as a form of quantitative defense against
insect herbivores [Moilanen & Salminen, 2008]. Later, in 1993, Appel postulated that
surfactants in insect-gut fluids inhibit tannin-protein interactions. The author observed that
most lepdopteran larvae have basic-gut (pH 9-12), which ionizes tannins and results in a loss
of hydrogen-binding capacity [Appel, 1993]. Hypothetically, tannin oxidative activity is
considered to be the real origin of their anti-herbivore function. Products of tannin oxidation
can deplete insect herbivore nutrients or produce cytotoxic effects [Appel, 1993, Hagerman et
al., 2003].
On the other hand, air pollutants tend to acidify intracellular pH or, the case of ozone,
cause leaf oxidative burst in sensitive species. According to Wohlgemuth et al. [2002], this
generates superoxide anion radicals and hydrogen peroxide. Reactive oxygen species (ROS)
not only provide protection against pathogens, but also induce an array of protective genes by
the oxidation of cell wall components and plasma membranes. Recently, ROS formation from
air pollution, UV-radiation or other abiotic stress factors has been shown to act as a signaling
pathway that overlaps hypersensitive response (HR), typical of plant infection by pathogens
[Rao et al. 2000; Langebartels et al. 2002].
Barbehenn et al. [2006] showed that the oxidative activity of tannins tended to
progressively diminish, group by group, from proanthocyanidins (condensed tannins),
through gallotannins and galloylglucoses to ellagitannins. However, many authors contest this
result, through ellagitannins having been the least studied. Moilanen & Salminen [2008], on
investigating 115 ecological tannin papers from 1985-2002, encountered 52 studies of total
phenols, 41 of proanthocyanidins, 13 of gallotannnins and only three related to ellagitannins.
The identification and characterization of leaf injury in well-adapted plant species are
crucial factors for realistically assessing risks by air pollution. Furthermore, bio-indicator
organisms react to atmospheric pollution and other biotic or abiotic stress. Field surveys have
recorded O3-like injury symptoms in numerous tree, shrub and forb species in Europe and
North America [Van der Heyden et al., 2001; Sans et al., 2002; Orendovici et al., 2003]. More
recently, Psidium guajava (the guava plant) was characterized as an ozone bio-indicator in
tropical regions [Furlan et al., 2007]. Visible foliar symptoms, in the form of dark-colored
(reddish) stippling among veins on the upper surface of older leaves, were highly correlated
with accumulated doses of ozone exposure (AOT40) [Furlan et al., 2007; Pina & Moraes,
2007]. Latter, Rezende & Furlan [2009] noted the higher incidence of anthocyanins and total

Claudia Maria Furlan, Lucimar Barbosa Motta et al.
8
tannins on Psidium guajava ozone fumigated plants, thereby indicating the cause of the
characteristic ozone leaf-injuries observed. This is of consequence, when contemplating the
less efficient traditional antioxidant system. Nonetheless, Dias et al. [2007] observed no
difference in the concentration of ascorbic acid and the activity of enzymes superoxide
dismutase (SOD) and peroxidase (POD) in leaves of P. guajava exposed to ozone in a
contaminated area of São Paulo, although according to Chalker-Scott [1999] there is a strong
relationship between the incidence of red leaves and stressful environments, when other
energy dissipation systems and free-radical scavenging have been exceeded, thereby
emphasizing its importance as part of the antioxidant pool.
When studying Psidium guajava exposed to high ozone concentration in São Paulo (the
city), using histochemical methods, Tresmondi [2010], observed that symptomatic and non-
symptomatic leaves accumulated phenol compounds, mainly anthocyanins and condensed
tannins, in upper layer mesophyll cells. Furthermore, the accumulation of H2O2 in mesophyll
cells as a result of oxidative stress was noted, thereby denoting the correlation between ozone
concentration and H2O2 content. Phenol accumulated on symptomatic leaves prior to the
appearance of leaf injury, thereby inferring their importance in chemical defense (mainly as
antioxidants) against ozone oxidative stress.
Tannins have also been related to environmental extremes, such as high altitude,
temperature and sunlight. Alonso-Amelot et al. [2007] observed an induced chemical
adaptive response in non-adapted plants growing at high altitudes. On comparing sun-exposed
and self-shaded fronds in Pteridium arachnoideum (fern), an accumulation of higher amounts
of phenols and tannins was noted, with the increase in altitude. Dry season water-stress also
caused an increase in phenols and tannins, thereby the conclusion that UV-B radiation and
water availability are important factors in non-adapted plant acclimation response to stress at
altitudinal gradients.
CONCLUSION
Tannins are undesirable compounds when plant-parts are consumed as food. On the other
hand, and from an agricultural view-point, they are useful as a protection from biotic-stress.
Tannins are also important in reinforcing plant tissues, their evolutionary presence pointing to
the emergence of Tracheophytes (Vascular Plants). This is especially patent in
proanthocyanidin-free mutant seeds of snapbeans, which proved to be more sensitive to
mechanical and water stress than seeds from the original cultivars.
It was also clear that phenols and tannins are synthesized in plants not only through
genetic determinants, physiological demands and evolution-controlled defense needs, but also
by the influence of environmental stress such as drought, UV-B radiation and atmospheric
pollution. Comprehending tannin functions in plant- interactions depends, to a large extent,
on our knowledge, not only of the chemistry of these compounds, but also of the strategies
that herbivores possess for dealing with these substances. This knowledge is also important
for finding tannin-rich species, or even producing mutants, which could increase the amount
of a particular biologically active metabolite.

Tannins: What Do They Represent in Plant Life 9
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