Plant species with larger extrafloral nectaries produce better quality nectar only when needed and favour interactions with best ant partners

Few studies investigated the phenotypic plasticity of extrafloral nectary (EFN) functioning associated with indirect plant defense across species. Here, we experimentally investigate in three sympatric legume species the role of EFNs, hypothesizing that plant species with larger EFNs have higher induced nectar secretion after herbivory events, greater control over secretion, and are more likely to interact with more protective ant partners. We targeted 30 individuals of each legume species and estimated EFN size and activity in the field. We conducted field experiments to evaluate the phenotypic plasticity of nectar production after leaf damage and censused ant species feeding on EFNs. Plant species increased nectar after leaf damage but in different ways. Supporting our hypothesis, C. duckeana, with the largest EFNs, increased all nectar descriptors, taking its place as the most productive and intense post-herbivory induced response, attracting more dominant ants than the other plant species. The higher control over reward production in plant species with larger-sized EFN reflects an induction mechanism under damage that reduces costs and increases the potential benefits of indirect biotic defences. Together, these plant traits shape the patterns of ant attendance and defence against herbivores, possibly favouring the maintenance of plant protection mutualisms widespread in nature.

迄今鲜有研究探讨跨物种中与植物间接防御相关的花外蜜腺（extrafloral nectary, EFN）功能的表型可塑性。本研究以3种同域分布的豆科植物为实验对象，探究花外蜜腺的功能作用，并提出如下假说：花外蜜腺更大的植物，在遭受植食性昆虫取食后，其诱导型花蜜分泌量更高、对花蜜分泌的调控能力更强，且更易与具备更强保护作用的蚂蚁类群建立共生关系。我们为每个豆科物种选取30株个体，在野外环境中测定其花外蜜腺的大小与活性。我们开展野外实验，以评估叶片受损后花蜜产量的表型可塑性，并对取食花外蜜腺的蚂蚁类群进行调查统计。实验结果表明，各供试豆科物种在叶片受损后均提升了花蜜产量，但提升模式存在差异。研究结果支持我们的假说：花外蜜腺最大的C. duckeana，其所有花蜜相关指标均显著提升，表现出最强的植食后诱导响应，花蜜产量最高且诱导强度最大，同时相较于另外两种植物，该物种能吸引更多的优势蚂蚁类群。花外蜜腺更大的植物对花蜜这类防御奖励的产量调控能力更强，这反映了其在受损时的诱导机制：该机制可降低防御成本，并提升间接生物防御的潜在收益。综上，这些植物性状共同塑造了蚂蚁的访栖模式与植物对抗植食性昆虫的防御策略，或有助于维持自然界中广泛分布的植物-保护蚁互利共生关系。