Defense and Adaptive Strategies of Crithmum maritimum L. Against Insect Herbivory: Evidence of Phenotypic Plasticity

Insect herbivory exerts strong selective pressure on plants, yet no study has documented its effects on the halophytic Apiaceae <i>Crithmum maritimum</i> L. (sea fennel). Here, we present the first evidence of natural insect attack on this species, based on five Tunisian coastal populations distributed along a transparent bioclimatic gradient-from sub-humid to semi-arid-and exposed to different levels of herbivory. We implemented an integrative, multi-trait analytical design encompassing morphological, biochemical, mineral, and lipophilic datasets. Each dataset was explored through a suite of complementary multivariate analyses, including ANOVA coupled with Tukey's HSD, principal component analysis (PCA), partial least squares discriminant analysis (PLS-DA) with variable-importance-in-projection (VIP) scores, correlation matrices, hierarchical clustering, and distance-based redundancy analysis (dbRDA). This integrative strategy provided a robust framework for disentangling the complex trait associations underlying two distinct defense syndromes. Populations from low-herbivory, sub-humid sites (Tabarka, Bizerte, Tunisia) showed higher levels of phenolics, tannins, antioxidants, sterols, PUFA, and structural robustness, indicating a tolerance strategy. Conversely, high-herbivory, semi-arid sites (Haouaria, Monastir, Tunisia) were marked by elevated apiol and terpene levels, sodium and phosphorus accumulation, and reproductive adjustments, reflecting a resistance strategy. The site Cap Negro exhibited a transitional expression, revealing intermediate phenotypic plasticity. These findings show that herbivory intensity and bioclimatic conditions jointly influence the defense syndromes of <i>C. maritimum</i>, emphasizing its remarkable phenotypic plasticity and providing the first ecological evidence of insect herbivory in sea fennel.