Difference in defense strategy in flower heads and leaves of Asteraceae: multiple-species approach

Although a vast number of studies have investigated defenses against herbivores in leaves, relatively little is known about defenses in flowers. Using wild individuals of 34 species of Asteraceae, we investigated differences in five traits that are thought to affect the intensity of herbivory (C, N, P, water, and total phenolic contents). Combinations of these traits between flower heads and leaves were studied as well. We also evaluated phylogenetic patterns of flower head and leaf traits. Flower heads had higher P and lower total phenolics than leaves. Water and C contents were negatively correlated both in the flower heads and leaves. N, P, and water contents were positively correlated in the flower heads, whereas this pattern was not found in the leaves. Thus, the traits we measured were more tightly inter-correlated in flower heads than in leaves. Because the flower heads had a lower total phenolic content, the relative allocation of defensive compounds could not be explained solely by fitness values of the organs. Perhaps plants employ an escape strategy rather than a defense strategy to cope with floral herbivores and higher allocation in P may enhance their escape from herbivores by improving the growth rate of flower heads, though our result might be affected in part by the plasticity of plants growing at different sites. Moreover, we found weak phylogenetic signals in the defensive traits. Because we found significant differences in the flower head traits, these weak signals may imply that the traits we measured evolved frequently.