Workshop 1 plants Flashcards
Defence Theory
1 – What are the 4 main hypothesis?
2 – Which of the theories on offer do you think best encapsulates what you know about plant-herbivore interactions?
Optimal Defence – McKey (1974); Feeny (1976)
Growth Rate – Coley et al (1985)
Carbon:Nutrient – Bryant et al (1983); Tuomi et al (1988)
Growth/Differentiation – Herms & Matson (1992)
Optimal Defence Theory.
1 – What are the three elements of the ODH?
2 – Which of a Mediterranean shrub or arable annual is more ‘apparent’? ]
3 – Think of an example where high-value
plant parts are well defended
Hakea inflorecences?
-Risk of attack (apparency)
-Value of the plant part (impact of loss on fitness)
-Cost of defence (reduced fitness when no herbivores)
Shrub – its big and there all the time
Hakea inflorecences?
ODH Issues
1 – ‘Apparency’ is difficult to quantify
2 – No consistent evidence for link between apparency and defence allocation in literature
3 – Only worth incurring defence costs when herbivores present, else there’s a fitness cost
Growth Rate Hypothesis
Defence is determined by plant growth rate - in turn
determined by available resources
GRH predicts that allocation to defence increases as
growth potential decreases
Thus plants in resource-poor areas, with inherently
slow-growth rates, tend to have long-lived leaves
that are well defended
GRH prediction
How does light limitation affect allocation to induced defence?
According to GRH which of a Mediterranean
shrub or arable annual is best defended?
Competitive shade-intolerant (fast-growing)
species exhibit higher induction than slow growing (shade-tolerant) species (Stamp 2003 pp 40)
shrub
Carbon:Nutrient Balance
Allocation determined by the Carbon:Nitrogen ratio
Plant growth in N-deficient soils more limited
than P/S – plants allocate excess C to carbon based defences (digestibility reducers) lignify structure.
Plants in low-C environments (e.g. shade)
more likely to produce nitrogen-based
defences (P/S more limited than growth)
plants become more cruddy in elevated CO2 as lignocellulose allocation and reduced N content also seen
Growth-Differentiation Balance
1 – What is the ‘dilemma’ at the heart of GDBH
2 – What is the cellular basis of this theory?
Grow (compete with neighbours) versus defend
(against being eaten)
There is a physiological trade-off between growth and
defence
Growth - cell division & enlargement
Differentiation - chemical & morphological changes
leading to cell maturation and specialisation (use)
Resources and the GDB
Where resources limit photosynthesis, C supply limits growth and defence
As resource increase, P/S requirements are met allowing CHO accumulation. But not enough to support growth, so C compounds used to synthesise C-based defence (phenolics, tannins)
When resource demands for growth met, C allocated to growth at the expense of secondary metabolism - fast growing plants have weak defences
The winner is GDB?
GDBH is probably the ‘most mature’ - comprehensive understanding of plant ecophysiology and trait
expression
However, the big 4 are not mutually exclusive and
its possible to integrate them See Stamp (2003) pp 46-49
Optimal Defence – Was useful, but difficult to test
Growth Rate and Carbon:Nutrient – effectively
subsumed into a better framework (GDBH)
but what about tolerance?
Many plants can simply re-grow (compensate)
following tissue loss - how does this fit into plant
defence theory?
Maybe GDBH has an answer?
“The physiological trade-off between growth and differentiation interacts with herbivory and plant
competition and manifests itself as a genetic trade-off between growth and defence in the evolution of plant life history strategies”
Herms & Matson (1992)
Defence theory - what have we learned?
ODH highly intuitive, but difficult to quantify aspects of the theory GRH & C:N BH simply compare resource
availability with allocation to defence –simplistic?
GDBH subsumes GRH & C:N BH and better encompasses elements of ODH
GDBH also trait-based - allows for tolerance etc…
Optimal defense hypothesis
The optimal defense hypothesis attempts to explain how the kinds of defenses a particular plant might use reflect the threats each individual plant faces.This model considers three main factors, namely:
-risk of attack
-value of the plant part
- cost of defense.
risk: how likely is it that a plant or certain plant parts will be attacked? This is also related to the plant apparency hypothesis, which states that a plant will invest heavily in broadly effective defenses when the plant is easily found by herbivores.Examples of apparent plants that produce generalized protections include long-living trees, shrubs, and perennial grasses.Unapparent plants, such as short-lived plants of early successional stages, on the other hand, preferentially invest in small amounts of qualitative toxins that are effective against all but the most specialized herbivores.
value of protection: would the plant be less able to survive and reproduce after removal of part of its structure by a herbivore? Not all plant parts are of equal evolutionary value, thus valuable parts contain more defenses. A plant’s stage of development at the time of feeding also affects the resulting change in fitness. Experimentally, the fitness value of a plant structure is determined by removing that part of the plant and observing the effect.[88] In general, reproductive parts are not as easily replaced as vegetative parts, terminal leaves have greater value than basal leaves, and the loss of plant parts mid-season has a greater negative effect on fitness than removal at the beginning or end of the season. Seeds in particular tend to be very well protected. For example, the seeds of many edible fruits and nuts contain cyanogenic glycosides such as amygdalin. This results from the need to balance the effort needed to make the fruit attractive to animal dispersers while ensuring that the seeds are not destroyed by the animal.
cost: how much will a particular defensive strategy cost a plant in energy and materials? This is particularly important, as energy spent on defense cannot be used for other functions, such as reproduction and growth. The optimal defense hypothesis predicts that plants will allocate more energy towards defense when the benefits of protection outweigh the costs, specifically in situations where there is high herbivore pressure.
Carbon:nutrient balance hypothesis
The carbon:nutrient balance hypothesis, also known as the environmental constraint hypothesis or Carbon Nutrient Balance Model (CNBM), states that the various types of plant defenses are responses to variations in the levels of nutrients in the environment. This hypothesis predicts the Carbon/Nitrogen ratio in plants determines which secondary metabolites will be synthesized. For example, plants growing in nitrogen-poor soils will use carbon-based defenses (mostly digestibility reducers), while those growing in low-carbon environments (such as shady conditions) are more likely to produce nitrogen-based toxins. The hypothesis further predicts that plants can change their defenses in response to changes in nutrients. For example, if plants are grown in low-nitrogen conditions, then these plants will implement a defensive strategy composed of constitutive carbon-based defenses. If nutrient levels subsequently increase, by for example the addition of fertilizers, these carbon-based defenses will decrease.
Growth rate hypothesis more
The growth rate hypothesis, also known as the resource availability hypothesis, states that defense strategies are determined by the inherent growth rate of the plant, which is in turn determined by the resources available to the plant. A major assumption is that available resources are the limiting factor in determining the maximum growth rate of a plant species. This model predicts that the level of defense investment will increase as the potential of growth decreases. Additionally, plants in resource-poor areas, with inherently slow-growth rates, tend to have long-lived leaves and twigs, and the loss of plant appendages may result in a loss of scarce and valuable nutrients.[97]
One test of this model involved a reciprocal transplants of seedlings of 20 species of trees between clay soils (nutrient rich) and white sand (nutrient poor) to determine whether trade-offs between growth rate and defenses restrict species to one habitat. When planted in white sand and protected from herbivores, seedlings originating from clay outgrew those originating from the nutrient-poor sand, but in the presence of herbivores the seedlings originating from white sand performed better, likely due to their higher levels of constitutive carbon-based defenses. These finding suggest that defensive strategies limit the habitats of some plants.
Growth-differentiation balance hypothesis
The growth-differentiation balance hypothesis states that plant defenses are a result of a tradeoff between “growth-related processes” and “differentiation-related processes” in different environments. Differentiation-related processes are defined as “processes that enhance the structure or function of existing cells (i.e. maturation and specialization).”A plant will produce chemical defenses only when energy is available from photosynthesis, and plants with the highest concentrations of secondary metabolites are the ones with an intermediate level of available resources.
The GDBH also accounts for tradeoffs between growth and defense over a resource availability gradient. In situations where resources (e.g. water and nutrients) limit photosynthesis, carbon supply is predicted to limit both growth and defense. As resource availability increases, the requirements needed to support photosynthesis are met, allowing for accumulation of carbohydrate in tissues. As resources are not sufficient to meet the large demands of growth, these carbon compounds can instead be partitioned into the synthesis of carbon based secondary metabolites (phenolics, tannins, etc.). In environments where the resource demands for growth are met, carbon is allocated to rapidly dividing meristems (high sink strength) at the expense of secondary metabolism. Thus rapidly growing plants are predicted to contain lower levels of secondary metabolites and vice versa. In addition, the tradeoff predicted by the GDBH may change over time, as evidenced by a recent study on Salix spp. Much support for this hypothesis is present in the literature, and some scientists consider the GDBH the most mature of the plant defense hypotheses.
