Defense traits vary across leaf ages in the thistles (Cardueae)

Plants produce a variety of structural and chemical defenses to deter
herbivores, which can covary across every scale of biological
organization. Along these lines, it has been suggested that young leaves
differ from old leaves in their defense strategies, but studies have
largely focused on woody plants and/or chemical traits. Thus, we lack an
understanding of how structural and chemical defenses ontogenetically
covary across a large portion of plant diversity. We investigate the range
of structural and chemical defenses of thistles (tribe Cardueae), which
comprise herbaceous species with a wide spectrum of defensive traits. We
collected structural and chemical trait data across species and leaf ages
from field collections, herbarium specimens, and the literature. Species’
young leaves tend to produce similar suites of traits, with greater
interspecific variation in older leaves. Young leaves are dominated by
chemical defenses, while older leaves produce a broader range of both
chemical and physical defense traits. Leaf spininess is associated with a
distinct chemical profile that differs from that of non-spiny species. Our
study demonstrates how defense strategies can vary across the leaves of
herbaceous species and their ages. Expanding such efforts will be critical
to further our macroevolutionary understanding of variation in plant
defenses.