Morphological, anatomical, and mechanical variation in cactus spines across species and stem developmental positions.

Cactus spines are highly variable structures that perform multiple ecological functions and have considerable taxonomic importance. However, their anatomical and mechanical properties remain poorly understood. A better understanding of spine structure may provide insights into the ecological function. In this study, we compared morphological, anatomical, and mechanical spine traits in three columnar cacti from Argentina (Cereus hankeanus, Stetsonia coryne, and Trichocereus terscheckii) and evaluated how these traits vary along a developmental gradient extending from the apical meristem. Spines were sampled from a single rib of 10 individuals per species. We measured spine length, fibre dimensions, cell wall thickness, and performed mechanical tests on oven-dried spines to calculate modulus of elasticity, modulus of rupture, and toughness. Significant differences in spine traits were detected among species. Stetsonia had the longest spines and exhibited the highest toughness, whereas Trichocereus showed lower mechanical resistance despite having thicker cell walls. Anatomical variation was also evident, with Stetsonia having smaller and more elliptical fibres than Trichocereus. In addition to interspecific differences, spine length in Stetsonia and Cereus increased with distance from the apex, indicating developmental changes along the stem. Among mechanical traits, only toughness varied along the stem, decreasing with increasing distance from the apex in all species, while anatomical traits remained unchanged. Together, these findings reveal substantial interspecific, intraspecific, and positional variation in cactus spine traits, highlighting the influence of developmental position along the stem and spine ageing, as well as species-specific differences in spine structure and mechanical performance.