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.

植物会演化出多种结构型与化学型防御策略以抵御植食动物，这类防御可在生物组织的各个尺度上发生协同变化。基于此，学界此前已提出幼叶与老叶的防御策略存在差异，但现有研究大多聚焦于木本植物或仅关注化学性状。因此，我们仍不清楚在大量植物类群中，结构型与化学型防御如何随个体发育发生协同变化。
本研究以蓟族（Cardueae）植物为研究对象，该类群均为草本植物，且防御性状谱跨度广泛。我们通过野外采集、馆藏标本及文献资料，获取了不同物种及不同叶龄的结构与化学性状数据。研究发现，物种的幼叶往往具有相似的性状组合，而老叶的种间差异则更为显著。幼叶以化学防御为主，而老叶则同时具备更丰富的化学与物理防御性状。具刺叶片拥有独特的化学特征谱，与非刺叶物种存在显著差异。
本研究阐明了草本植物叶片的防御策略如何随叶龄发生变化。后续拓展此类研究，将有助于我们进一步从宏观演化层面理解植物防御性状的变异规律。