Aluminum and iron interactions in commelina communis: additive chloroplast toxicity versus synergistic internal and...

Acidic soils (pH < 5.5) contain elevated levels of phytotoxic aluminum (Al³⁺) and iron (Fe²⁺/Fe³⁺); however, the mechanisms underlying plant adaptation to their coexistence remain poorly understood. Commelina communis, an invasive weed prevalent on acid soils of tea gardens, was exposed to Al (50 and 100 µM) and Fe (100 µM), individually and in combination (pH 4.0), for four weeks. Plant biomass remained unaffected under single or combined stress, accompanied by reciprocal reductions in foliar metal concentrations. However, chlorosis, reduced photosynthetic pigments, and altered photochemical parameters indicated exacerbated chloroplast damage under combined Al and Fe toxicity. Conversely, phenolic and anthocyanin contents in leaves and oxalate in roots peaked under dual stress, coinciding with significantly reduced stress markers. Histochemical analysis revealed that Al³⁺ and Fe²⁺ mutually inhibited binding to root tips, reducing membrane injury under co-treatment. Fourier-transform infrared (FTIR) spectroscopy of root tissues showed substantial decreases in cell wall-associated pectin and hemicellulose under Al stress, excess Fe and co-exposure, potentially limiting Al³⁺ and Fe²⁺/Fe³⁺ binding and mitigating rhizotoxic effects. Al treatment alone suppressed malate and citrate exudation, whereas Fe toxicity increased their release. In contrast, oxalate and phenolics exudation were enhanced by Al and Fe, with maximum excretion under combined exposure. These findings reveal two interaction patterns: additive toxicity affecting chloroplast function, and synergistic enhancement of internal and rhizosphere-based detoxification. Taken together, these findings explain the robust spread of C. communis in acid soils, provide a framework for future research on metal co-tolerance, and may inform acid soil management strategies and the development of stress-resilient cultivars.