Growth-defense trade-offs in two major defense traits of the common milkweed Asclepias syriaca

Tobias Züst and Anurag A. Agrawal

Costs of defense

• Key assumption of co-evolutionary theory

• Costs limit runaway selection

• Costs maintain genetic diversity

Asclepias syriaca

Defense traits of Milkweed

Latex

Cytoplasm

Extracellular space

Cardenolides

Defense traits of Milkweed

• Latex and cardenolides vary independently

Agrawal et al. (in press)Agrawal (2005)Agrawal et al. (2012)Bingham and Agrawal (2010) Mooney and Agrawal (2008)

0 0.5 1 1.5 2 2.5 30

1

2

3

4

5

Late

x ex

udati

on (m

g F

M)

Cardenolides (mg/g DM)

Defense traits of Milkweed

• Latex and cardenolides are inducible

Agrawal et al. (in press)

Defense traits of Milkweed

• Constitutive and induced levels trade off

Agrawal et al. (2012)

Questions

1. Do defense traits of milkweed convey allocation costs?

2. Is the presence of costs dependent on nutrient availability?

Growth as unit of costs

• A component of fitness in perennial plants

• Physiological measure of allocation

• Likely to show allocation costs

Plant growth rate

Plant growth rate

• RGR: Relative growth rate

• NAR: Net assimilative rate

• SLA: Leaf density

• LMR: Proportion of photosynthetic tissue

𝑅𝐺𝑅=𝑁𝐴𝑅×𝑆𝐿𝐴× 𝐿𝑀𝑅

Experimental design

• Plants grown for 45 days (24 genotypes x 2 nutrient levels)

• Frequent non-destructive size measurements

• Destructive harvests after 2, 4, and 6 weeks

• Quantification of defense traits at destructive harvests

Shoot cardenolides

Shoot latex exudation

Genotype-specific nutrient effects

Trait Low Fertiliser- Trait High Fertiliser

Genotype-specific nutrient effects

Plant-size effect on latex

• Latex exudation reflects storage capacity, not production

• Big leaves contain larger laticifers, more latex is stored

Conclusions

• Leaf cardenolide levels trade-off with growth at two nutrient levels, especially photosynthetic efficiency

• Nutrient-driven change in cardenolide synthesis also affects growth rate

• Latex correlates positively with growth and plant sizesimple exudation measure insufficient to quantify allocation costs

Acknowledgments• Sergio Rasmann• Elizabeth Davidson-Lowe• Chris Stieha• Yann Hautier• Collin Edwards• Georg Petschenka• Rayko Halitschke• Amy Hastings

Root cardenolides