Heavy Metal Characterization of Edible Aroids (Colocasia Esculenta (Taro) and Xanthosoma Sagittifolium (Tannia) Growing in Parts of Rivers State

Edible aroids, particularly Colocasia esculenta (taro) and Xanthosoma sagittifolium (tannia), are crucial for food security and livelihood in developing countries, including Nigeria. However, environmental pollution, notably heavy metal contamination, poses a significant threat to their productivity and safety, especially in the oil-rich Niger Delta region. This study investigated the presence and accumulation of heavy metals in edible cocoyam genera and their growing soils across three local government areas in Rivers State, Nigeria: Obio/Akpor, Emohua, and Ikwerre.
The field experiment was carried out from June, 2021 to January, 2022 and repeated in June, 2022 and January, 2023 across three local government areas in Rivers State, Nigeria: Obio/Akpor, Emohua, and Ikwerre. Two genera of cocoyam, Colocasia esculenta (cultivars: NCe001 – Coco India, NCe002 – Ede ofe green, NCe003 – Ede ukpong) and Xanthosoma sagittifolium (cultivars: NXs001 – Ede ocha, NXs002 – Ede uhie, NXs003 – Okorokoro), were obtained from the National Root Crop Institute (NRCRI), Umudike. A Randomized Complete Block Design (RCBD) with four replications was used, with plant spacing of 50cm x 50cm. Pre-planting soil analysis revealed significant variations in certain physicochemical parameters across the locations. For instance, pH, electrical conductivity (EC), organic carbon (OC), phosphorus (P), calcium (Ca), acidity, aluminum (Al), exchangeable cation exchange capacity (ECEC), manganese (Mn), iron (Fe), copper (Cu), and zinc (Zn) showed significant differences (p<0.05) between locations, indicating varying soil conditions and potential for heavy metal presence. Heavy metal concentrations in soil, corms, cormels, petioles, and roots were determined using Atomic Absorption Spectrophotometry (AAS). The findings indicate that the levels of heavy metals varied across different plant parts irrespective of ascensions and locations, in corms as cobalt < nickel < zinc < lead < copper, in cormlets as cobalt < nickel < lead < zinc < copper, while in leaves, roots, and petioles, the metals followed the order: cobalt < lead < nickel < copper < zinc. Significantly, the lead content in all plant parts (corms, cormels, leaves, roots, and petioles) was found to be significantly different (p<0.05) across the locations, suggesting that lead levels might be influenced by the specific environmental or anthropogenic conditions of the different regions. This finding emphasizes the potential for lead contamination in these crops, which could pose a health risk to consumers. Similarly, copper content in the leaves, roots, and petioles was significantly different (p<0.05) between locations, although the copper levels in the corms and cormlets did not show significant differences (p>0.05). This suggests that copper may accumulate more in the aerial parts of the plant, such as leaves and petioles, potentially due to the absorption of copper through the soil or via atmospheric deposition. Interestingly, nickel and zinc concentrations in the leaves, roots, and petioles were also significantly different (p<0.05) across the locations, while their concentrations in the corms and cormels did not show significant variation. Finally, cobalt levels showed significant differences (p<0.05) in cormels, leaves, and roots, but no significant differences in corms and petioles (p>0.05). This finding suggests that cobalt may be more readily absorbed into the plant’s above-ground tissues than the tuberous parts.