Comparative Physiological Responses and Burden-Based Root–Stem Partitioning of Five Metals in Eichhornia Crassipes Under Citric-Acid-Assisted Hydroponic Exposure

Heavy-metal contamination of freshwater ecosystems has increased interest in aquatic plants for metal retention and phytoremediation. This study examined the physiological responses and root–stem partitioning of nickel (Ni), cadmium (Cd), lead (Pb), zinc (Zn), and copper (Cu) in water hyacinth (Eichhornia crassipes (Mart.) Solms) under hydroponic conditions. Plants received dosing solutions containing 100, 250, or 500 µM of each metal salt supplemented with 4.57 mM citric acid over four weeks. Untreated and fertilizer-only plants were included as reference treatments. Vegetative dimensions, tissue fresh and dry masses, dry-matter content, chlorophyll content index, toxicity symptoms, root and stem–petiole metal concentrations, and organ-specific metal burdens were assessed. Responses varied among metals and dosing levels. Lead and cadmium were associated with the clearest reductions in aerial growth and more evident toxicity symptoms at higher dosing levels. Chlorophyll content index declined by 49.0% for Ni and 43.6% for Cd between 100 and 500 µM, whereas Zn and Cu showed non-linear responses. At 500 µM, root Cu and Pb concentrations reached 859.74 and 430.76 mg kg⁻¹ dry weight, respectively. Mean root allocation of measured metal burden was 99.7% for Cu, 98.8% for Pb, 92.6% for Zn, 78.0% for Ni, and 69.4% for Cd. Cadmium showed the greatest mean stem–petiole allocation (30.6%), although its burden remained root-dominant. The findings highlight metal-specific physiological patterns and predominantly root-associated metal partitioning, particularly for Cu and Pb, and indicate the relevance of water hyacinth to root-focused metal retention in phytoremediation systems, supporting freshwater environmental management strategies.