Abstract: Understanding how defense strategies differ within species across environmental gradients is crucial for predicting plant–herbivore interactions under shifting ecological conditions. Intraspecific variation in inducible defenses, particularly root–shoot systemic induction (RSI), may be influenced by ecological divergence, but its evolutionary potential remains largely unknown. We investigated ecotypic differentiation and genetic variation in RSI in Cardamine pratensis along an elevation gradient. Our approach combined a field survey of natural populations with a reciprocal common garden experiment using maternal families from both high- and low-elevation ecotypes. We measured constitutive and Jasmonic Acid (JA)-induced glucosinolate (GSL) production, as well as resistance to natural herbivory. Low-elevation ecotypes experienced greater belowground arthropod community abundance and produced higher baseline GSL levels, while high-elevation ecotypes invested less in constitutive defenses but exhibited stronger plasticity in specific GSL compounds. across both field and common garden experiments, JA application to roots consistently reduced leaf herbivory, indicating widespread RSI across ecotypes. However, RSI occurred independently of total foliar GSL levels, suggesting that systemic resistance is not explained by variation in measured GSL induction. Family-level variation in the effects of RSI on herbivory was observed, especially in high-elevation ecotypes, indicating genetic differences that could support the evolution of systemic inducibility. These findings collectively show that RSI contributes to herbivore resistance across different environments, although defense allocation patterns vary between ecotypes. Root-induced resistance appears to be a flexible part of plant defense, capable of changing across ecological gradients through both phenotypic plasticity and genetic variation differentiation.

Key words: ecotypic differentiation, Glucosinolates, elevation gradients, genetic variation, phenotypic plasticity, local adaptation, chemotypic divergence