A cornerstone of plant defense literature is the optimal defense theory, which predicts that young leaves should be better defended than old leaves because they are more valuable to the plant, as well as more vulnerable to attack if undefended. Alternatively, this pattern may be driven by physiological processes if: (1) defense follows allocation of carbohydrates into young tissue, which is the strongest carbohydrate sink, or (2) both young and old leaves contain the same total amount of defense, but these defenses become diluted as the leaf grows and expands. Here, we disentangle the relative importance of adaptation and physiology in driving defense allocation by comparing populations of Verbascum thapsus (an invasive, biennial weed) that differ in historic rates of herbivory. We used a common garden comprised of populations that historically experienced either low-risk or high-risk of insect attack (13 populations x 2 attack regimes x 3 time periods x 2 replicates = 156 plants). We measured defenses (trichome cover and length, leaf toughness, secondary chemistry) on young leaves, old leaves, and roots from each plant. To test whether differences in how plants defend their leaves is simply due to how plants grow, we standardized all defense measurements by biomass.
Results/Conclusions
Young leaves are consistently better defended than old leaves. For leaf toughness, this difference is most pronounced for plants that have historically experienced low levels of insect attack, especially early in the season when young leaves are likely most valuable. However, differences between how populations that have historically experience low- and high-risk of insect attack is less pronounced for other lines of defense, which suggests that plant physiology drives some, but not all of the observed variation in defense investment. These results imply that both plant physiology as well as coevolutionary history with insect herbivores can provide insight into how plants defend against being eaten. Overall, our data suggests we need to take a more nuanced approach to understanding the evolution of plant defense that investigates physiological constraints and risk of attack across multiple lines of defense.