Investigating the Phytotoxic Effects of Copper-contaminated Soil on Neermulliya (Hygrophila auriculata): Impact on Growth and Physiological Responses.

Abstract

Heavy metal contamination is a global environmental issue that affects environmental well-being and human health. Copper (Cu) is an essential micronutrient and a potential toxicant at high concentrations. This study aims to evaluate the phytotoxic effects of Cu on Hygrophila auriculata (Neermulliya) by assessing its impact on plant growth, chlorophyll content, and membrane integrity to determine the plant’s tolerance limits under varying Cu stress, due to its adaptability and phytoremediation characteristics, which was the reason for its selection. Cu was introduced to the soil at concentrations of 50, 100, 200, and 300 mg kg-1 using CuSO₄·5H₂O, with the untreated soil as a control. A total of 45 plants were cultivated under controlled environmental conditions for 12 weeks; the growth metrics, chlorophyll content, membrane integrity, and tolerance indices were examined. Results indicated a concentration-dependent response to Cu exposure. Compared to the control, 50 mg kg⁻¹ and 100 mg kg⁻¹ Cu treatments enhanced chlorophyll b synthesis without phytotoxicity (p = 0.011). Moderate concentrations (T3, 200 mg kg⁻¹) led to the highest total chlorophyll production (43.40 mg g⁻¹, p = 0.014). The highest tolerance index was observed at 50 mg kg⁻¹, particularly for root fresh weight (175.01%) and root length (131.55%). Conversely, plants exposed to 300 mg kg⁻¹ exhibited the lowest root and shoot growth, increased membrane damage (119.31 μS cm⁻¹, p = 0.001), and significantly reduced biomass production (p < 0.05), indicating severe phytotoxicity. Comparisons with previous studies support these findings, highlighting the plant's potential tolerance to moderate Cu contamination. Enhanced growth performance at moderate Cu levels suggests a threshold for toxicity, indicating Hygrophila auriculata’s ability to withstand moderate levels of Cu stress and it has the potential to remediate and survive in moderate Cu -contaminated environments.