THEORIES OF PLANT DEFENSE
I. Introduction
A. The need for unifying theories
B. Lumping and Splitting
C. General categories of theories:
1. Post-hoc explanations
2. Adaptationist approaches to resource allocation
3. Importance of scale: Within Vs. between species
II. Theories explaining increased herbivore ability to exploit plants
A. Plant stress
1. Increased nutrient availability to insect: T.R.E. White
2. Decreased resistance: R.H. Waring
3. Indirect effects: P. Hughes, W. Mattson
4. Plant vigor: P. Price
5. Relationship of plant condition to insect feeding guild: S. Larsson
B. Plant genetic variability
1. Somatic Mutations: Edmunds and Alstad; T. Whitham
2. Hybrids: T. Whitham
III. Theories based on optimal plant allocation patterns
A. Introduction: Limited resources - competing allocation plans for
growth, reproduction, & maintenance (defense)
1. Selection for optimal allocation plan
B. Plant apparency
1. History: P. Feeney 1976, D. Rhoades & R. Cates, 1976
2. Likelihood of a plant or plant part being found
a. High likelihood: Quantitative defenses (tannins, terpenes)
b. Low likelihood: Qualitative defenses
3. Critiques:
C. Carbon: Nutrient Balance - Bryant et al. 1983 (intraspecific)
Resource Availability - Coley et al. 1985 (interspecific)
1. Tradeoff between growth & defense regulated by availability of
excess carbon
Optimal decision can be quantified:
dC = G * C * (1-kDa) - (H - mDb)
C = Biomass, G = Max. Growth Potential
D = Investment in defense; H = Herbivore pressure
2. Carbon based secondary metabolites positively correlated
with C/N ratio: phenolics, terpenes
Nitrogen based secondary metabolites inversely correlated
with C/N ratio: nonprotein amino acids, alkaloids
3. Premise: Moderate nutrient deficiency limits growth more than
photosynthesis. Hence, nutrient deficient plants accumulate
carbohydrates: increase C/N: secondary compounds
4. Predictions: Different responses to availability of light & nutrients
C
N
LIGHT
NUTRIENTS
5. Interpretations of observed patterns
A. Delayed Inducible Responses in Deciduous vs. Evergreen trees:
D. Growth : Differentiation: Tuomi et al. 1990, Herms & Mattson 1992.
1. Physiological explanation: Growth - Cellular enlargement and
reproduction vs. differentiation: Secondary metabolites and their
structures (glands, etc.)
Plant Relative Growth Rate = Net Assimilation Rate * Leaf Area Ratio
Assumptions
1. Plants export carbon from sources (leaves) to sinks (active meristem, etc.)
2. Photosynthesis and growth do not respond equally to gradients of water &
nutrients; Growth processes more sensitive than Photosynthesis.
3. Any factor that limits growth more than photosynthesis (moderate drought,
moderate nutrient limitation) will increase the carbon pool available for
allocation to secondary metabolism with little or no trade-off with growth.
Overall Predictions:
1. Secondary compounds are highest when plants are under intermediate stress
2. Fast growing plants generally less defended than slow growing plants
3. Fast growing plant parts generally less defended than slow growing parts
4. Plants in resource rich environment favor growth; resource poor favor defense
E. Criticisms of C:N, Resource Availability, Growth: Differentiation
1. Do not incorporate the Nutrient costs of making, storing, &
maintaining secondary compounds: Gershenzon
2. Focus on carbohydrates emphasizes substrate, not metabolism: Gershenzon.
3. Budget Vs. overall income Rapid growth and high concentrations often
correlated.
4. Does not incorporate inducible responses, particularly rapidly inducible
responses very well.. Lerdau; “Supply side”, not demand
F. Meta-analyses: What do they tell us?