Structural Defence Flashcards
Name some structural defense in plants
What are structural traits associated with?
“A trait can be viewed as a defence even though
defence is not its primary function”
Spines, hairs, tough leaves & minerals
Structural support, Reduce water loss, Assist with
Climbing, Thermal protection and UV protection
What do we mean by ‘Structural Defense’?
Any morphological or anatomical trait that confers a fitness advantage to the plant by directly deterring herbivores from feeding on it however does not include indirect defences such as mimicry, the growth of domatia ‘ant defence’ or resins & latex
Describe spinescence
Does size matter?
Spines – (holly) sharp-pointed, petioles, midribs, veins or stipules (coming from bits of leaf)
Thorns – sharp-pointed woody branches (grown up the stem, prevent herbivores accessing higher part of plant where leaves are
Relative size suggests spinescence is a better defence against large herbivores. If spines evolved to deter large herbivores then their removal should increase herbivory, When presented to captive kangaroos, Hakea plants with spines removed were eaten more than control plants.
Spines growth can also be induced e.g, more giraffes meant longer thorns. These defences can then therefore be upregulated (increased number of thorns per length)
Describe pubescence
Layers of trichomes on leaves, stems, flowers or fruits
Describe trichomes
What are the variety of defensive roles?
Trichomes – hair-like appendages extending from the epidermis
A multitude of forms – spiral, straight, stellate, hooked
They can also be glandular - i.e. secrete a chemical
defence
Reduce tissue ingestion (slugs), Reduce oviposition (moths) and Reduce movement/trap insects. Most effective on small, invertebrate herbivores
there is also induction (more and thicker, more dense leaves)
Describe Sclerophylly
hard leaved- Hardness due to high lignin and cellulose
Scleromorphic leaves reduce acceptability and digestibility
content (former is hard to digest)
Hard-leaved plants tend to be most common in Mediterranean-climate regions this could be because of high UV so needs to protect itself, also prevent water loss. More herbivores will spot the only green plant in a hot, dry summer
(dwarf palm is a good example of a tough indigestible plant)
Minerals (crystalline form in their tissues)
Silicon-based – plants absorb monosilicic acid (Si(OH)4) from the soil.
Transported to the shoot epidermis to form biogenic silica (SiO2 nH2O)
This forms opaline phytoliths (Grasses do it best)
Phytoliths are most common in the Poaceae
Increased leaf abrasiveness means more tooth/mouthpart wear; reduced digestibility (grass cuts) silica wears teeth down, it slows down digestive process
Good defence against mammal and invertebrate herbivores
Grasses and voles
Massey & Hartley (2009) - when grown in high Si soils, two grasses showed increased abrasiveness and reduced vole
attack
Same paper also evidences phytolith induction by voles
express more silica after attack
Calcium Minerals
Calcium oxalate CaC2O4 – found in most terrestrial plant families
Most heavily grazed Pacratium sickenbergeri populations had most CaC2O4 (Ward et al 1997)
Many inter-tidal algae secrete calcium carbonate
reduces attack by intertidal gastropods
What are the constraints for each defense mechanism?
Both chemical and structural defences require carbon and/or nitrogen
Herbivores aren’t the only thing a plant needs to worry about
Plants must grow and reproduce
Saying it with flowers
Many Hakeas have conspicuous, accessible inflorescences for bird pollination but pollinator attractiveness and accessibility leaves you vulnerable to flower eaters
Hakeas solve this by producing floral HCN AND Most Hakeas protect their inflorescences with spines
most Hakeas protect inflorescences with spines
Pollination syndromes and plant defence
Bird-pollinated
* Accessible inflorescences
* Low physical defence
* High chemical defence
Insect-pollinated
* Inaccessible inflorescences
* High physical defence
* Weak chemical defence
Describe defence linked to other selection pressures?
What about fruit defence?
Seems to be a trade-off between defence types
linked to pollination syndrome
Is pollination or herbivory a greater selection
pressure?
See Hanley et al (2009)
If hakea inflorescences are defended by spines what about fruits?
So defence seems to be water-tight!
But remember - For every defence, sooner or later a herbivore will overcome it – see Lamont et al (2016)
most plants don’t have enough resources to be used for both chemically and physically defensive
hakeas speculata, cockatoos, base of spines little fruits (parrots can still get around spines)
Summary
Defence or Not?
The defensive role of plant structures is less well studied than secondary metabolites
Structural traits have many varied functions
May have later been co-opted into defence
The Structural Arsenal
Spines and thorns reduce ingestion by large herbivores
Trichomes reduce invertebrate fitness in many ways
Hard leaves reduce ingestion and digestibility
Minerals reduce ingestion and digestibility
All are induced by tissue loss - supports defensive role
Interactions
There are constraints on allocation to defence
Growth and reproduction are important
Allocation choices depend on fluctuating circumstances
