Optimal Level of Chemical Defense Decreasing with Leaf Age

Area of Science:

• Plant Ecology
• Chemical Ecology
• Evolutionary Biology

Background:

• Pyrrolizidine alkaloids (PAs) are plant defense chemicals.
• PA concentration in biennial plants decreases with leaf age due to reallocation.
• Understanding optimal defense strategies is crucial for plant survival and growth.

Purpose of the Study:

• To determine the optimal age-specific pattern of defense chemical concentration in leaves for maximizing plant growth rate.
• To model plant growth under herbivory pressure and resource allocation constraints.

Main Methods:

• Utilized Pontryagin's maximum principle to calculate the optimal defense schedule.
• Developed a model incorporating leaf age, photosynthesis, herbivory, and defense chemical production/reallocation costs.
• Assessed the impact of various factors (e.g., herbivory intensity, production costs) on optimal defense levels.

Main Results:

• The optimal defense chemical level decreases with leaf age.
• Optimal defense concentration increases with leaf production costs and herbivory intensity.
• Optimal defense concentration decreases with defense chemical production costs, effectiveness, net productivity, and plant growth rate.

Conclusions:

• Plants should decrease chemical defense concentration in older leaves to maximize growth.
• Optimal defense strategies are influenced by the costs and benefits of defense and resource availability.
• In mixed herbivory scenarios, defense allocation is primarily driven by generalist herbivores.