Hyperaccumulators are plants capable of extracting and storing toxic concentrations of metals without stress symptoms, commonly thriving in ultramafic soils. In the Philippines, several Rubiaceae species are abundant in these habitats, suggesting potential hyperaccumulation ability. This study investigated heavy metal accumulation in selected Rubiaceae, particularly Timonius spp. and allies, from ultramafic areas in Ilocos, Dinagat, and Samar. Plant and soil samples were analyzed using Energy-Dispersive X-ray (EDX) and X-ray Fluorescence (XRF). Bioconcentration Factor (BCF) and Translocation Factor (TF) were calculated to evaluate hyperaccumulation capacity. Results revealed that many Philippine Rubiaceae species accumulate high concentrations of Ni, Cr, and Al, exceeding established thresholds. For example, Ni reached up to 6,926 ppm in stems, while Cr exceeded 6,700 ppm in leaves. Ultramafic species consistently showed higher BCF (>1) and TF (>1) than non-ultramafic species, highlighting their adaptation to metal-rich soils. This work represents the first systematic analysis of heavy metal accumulation in Philippine Rubiaceae, documenting both Ni and Al hyperaccumulation in the Guettardeae tribe. The findings suggest ecological roles in species diversification and potential applications in green technologies, particularly phytoremediation and phytomining.
