Elevational gradients are useful ecological settings for revealing the biotic and abiotic drivers of plant trait variation and plant–insect interactions. However, most work focusing on plant defences has looked at individual traits and few studies have assessed multiple traits simultaneously, their correlated expression patterns, and abiotic factors associated with such patterns across elevations. To address this knowledge gap, we studied elevational variation in direct (phenolic compounds) and indirect (volatile organic compounds) constitutive defences and their inducibility after feeding by a specialist beetle Altica quercetorum in saplings of 18 wild populations of Quercus pyrenaica. We tested for: 1) clines in each defensive trait individually, 2) their patterns of correlated expression and 3) associations between any such clines and climatic factors. We found that constitutive direct defences (lignins and hydrolysable tannins) decreased with increasing elevation. We observed no elevational gradient for constitutive indirect defences (volatile organic compounds) or the inducibility of direct or indirect defensive traits when looking at groups of compounds. However, at individual tree‐level, increased induction of two monoterpenes (α‐fenchene and camphene) at higher elevation was shown. Furthermore, we show a significant pattern of co‐expression of constitutive and induced phenolics across populations, which weakened with increasing elevation. Finally, we found no evidence that climatic factors were associated with either individual or correlated trait expression patterns across elevations. Overall, these findings call for moving beyond elevational clines in individual plant defences, and argue that assessing elevational shifts in trait correlated expression patterns and their underlying mechanisms can increase our understanding of plant defence evolution and plant–herbivore interactions along environmental gradients.

中文翻译：

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基于个体性状及其相关表达模式的本构和诱导橡树防御中的上升梯度

海拔梯度是揭示植物性状变异和植物-昆虫相互作用的生物和非生物驱动因素的有用的生态环境。但是，大多数侧重于植物防御的工作都着眼于单个性状，很少有研究同时评估多个性状，它们的相关表达模式以及跨海拔的与此类模式相关的非生物因素。为了解决这一知识鸿沟，我们研究了由甲壳虫甲虫Altica quercetorum在18个野生栎种群的幼树中进食后直接（酚类化合物）和间接（挥发性有机化合物）本构防御的海拔变化及其诱导性。。我们进行了以下测试：1）每个防御性状中的风系，2）它们的相关表达方式，3）任何这样的风系与气候因素之间的关联。我们发现，组成性直接防御（木质素和可水解单宁）随着海拔的升高而降低。当查看化合物组时，我们没有观察到本构性间接防御（挥发性有机化合物）或直接或间接防御性状的可诱导性的海拔梯度。但是，在单个树级别上，显示了在较高海拔下两个单萜（α-芬琴和樟脑）的诱导增加。此外，我们显示出在人群中本构酚醛和诱导酚醛的共表达的显着模式，随着海拔的升高而减弱。最后，我们没有发现气候因素与海拔之间的个体或相关性状表达模式相关的证据。总体而言，这些发现要求超越单个植物防御中的海拔高度，并认为评估性状相关表达模式及其潜在机制的海拔变化可以增进我们对植物防御进化以及沿环境梯度的植物-草食动物相互作用的了解。