Soil–Climatic Drivers of Anatomical and Metabolic Plasticity in Rheum tataricum L.f. Across Arid Landscapes of Kazakhstan

Climate change and progressive aridification represent a substantial threat to the sustainability of wild medicinal plant resources in Central Asia. Rheum tataricum L.f. (R. tataricum), a mesoxerophytic species with high pharmacological potential and a restricted distribution range, was selected as a model for investigating adaptive responses to combined climatic and edaphic stress. Relationships among climatic parameters, soil agrochemical characteristics, anatomical and morphological traits, and the metabolomic profile of roots and rhizomes were analysed across six ecopopulations distributed along latitudinal and altitudinal gradients in southern and western Kazakhstan. To quantify population-level vulnerability to climatic stress, a Climate Sensitivity Index (CSI) was calculated. All investigated ecopopulations exhibited high climate sensitivity (CSI = 0.30–0.40), indicating persistent climatic stress. Significant altitudinal dependence was detected for such anatomical traits, as primary cortex thickness, as well as for the accumulation of tannins, anthraquinones, and flavonoids. The metabolomic profile was strongly associated with seasonal precipitation, temperature, relative air humidity, soil agrochemical properties, and root elemental composition. These findings demonstrate pronounced anatomical and metabolomic plasticity in R. tataricum, which appears to function as a key adaptive mechanism in arid ecosystems. The results provide a scientific basis for sustainable bioprospecting, conservation of natural populations, and targeted cultivation aimed at obtaining specific metabolomic profiles.