Differential impacts of citric acid and malic acid on growth and rare earth element fractionation in Phytolacca americana L.

Rare earth elements are increasingly contaminating terrestrial ecosystems, accumulating in plants and thereby disrupting nutrient balance. The role of low molecular weight organic acids in mediating rare earth element fractionation and plant responses remains poorly understood. In this study, a hydroponic experiment was conducted using Phytolacca americana exposed to lanthanum (La) and yttrium (Y) in combination with varying concentrations of citric acid or malic acid, and plant growth, elemental concentrations and systemic responses were evaluated. Citric acid markedly promoted lanthanum accumulation in aboveground tissues, reversed the inherent plant preference for yttrium and increased the lanthanum to yttrium ratio, consistent with enhanced long distance transport. In contrast, malic acid did not significantly promote lanthanum translocation but instead enhanced root thickening and supported the coordinated uptake of essential nutrients, thereby maintaining ionic homeostasis and biomass. Multivariate analyses confirmed that citric acid induced a lanthanum centered fractionation pattern at the expense of nutrient balance, whereas malic acid sustained nutrient coordination. These findings provide mechanistic insights into the biogeochemical controls governing rare earth element mobility and have direct implications for the design of phytoremediation strategies based on low molecular weight organic acids for soils contaminated by rare earth elements.