Lecture 3 – Plant Defences
: ALSO CALLED
Examples: WOODY STEMS ;
- SPINES, THORNS, PRICKLES, STINGING HAIRS.
RASPBERRY, BLACKBERRY, CACTUS, THISTLE, HAWTHORN
SMALL HOOKED OR CLUBBED HAIRS
= DENSE TANGLES
IMPEDE SMALL ANIMALS such as MITES and small INSECTS.
ONE TYPE - SEPARATELY STORED PHENOLS AND ENZYMES, WHEN
ANIMAL BRUSHES THEM THEY,BEAK OPEN, CONTENTS
MIX LIKE EPOXY
, ALSO RELEASES REPULSIVE
Plants have many external structural defences that discourage herbivory. Plant structural
defences on stems and leaves can deter, injure, or kill the grazer.
Some defensive compounds are produced internally but are released onto the plant's
For example, resins, ligning, silica, and wax cover the epidermis of terrestrial plants and
alter the texture of the plant tissue.
: The leaves of holly plants, for instance, are very smooth and slippery making
feeding difficult. Some plants produce gummosis or sap
(fluid transported in xylem cells
(tracheids or vessel elements) or phloem sieve tube elements of a plant) that traps insects
A plant's leaves and stem may be covered with sharp spines or trichomes
– hairs on the leaf
often with barbs, sometimes containing irritants or poisons.
Plant structural features like spines and thorns reduce feeding by large ungulate
herbivores (such as kudu, impala, and goats) by restricting the herbivores' feeding rate,
or by wearing down the molars as in pears.
The structure of a plant, its branching and leaf arrangement may also be evolved to
reduce herbivore impact
(constitutive, most quantitative metabolites are
digestibility reducers that make plant cell walls indigestible to animals)
a) WHEN EATEN, MAKES IT HARD FOR THE ANIMAL TO DIGEST THE PLANT
AND DEFENSIVE FUNCTIONS
b) Condensed tannins, polymers composed of 2 to 50 (or more) flavonoid molecules, inhibit
herbivore digestion by binding to consumed plant proteins and making them more difficult
to digest, and by interfering with protein absorption and digestive enzymes.
: Silica and lignins, which are completely indigestible to animals, grind down
insect mandibles (appendages necessary for feeding)
c) For example
, plants growing in nitrogen-poor soils will use carbon-based defences (mostly
digestibility reducers), while those growing in low-carbon environments (such as shady
conditions) are more likely to produce nitrogen-based toxins
3. STRUCTURAL ELEMENTS