Aluminum (Al) favors phosphorus (P) nutrition in Al-tolerant Amaranthus cruentus L. under P deficiency

Acidic soils are characterized by excess aluminum (Al) and low phosphorus (P) availability. However, the interactive effects of these elements within plants and the rhizosphere remain understudied. Red amaranth (Amaranthus cruentus L.), an Al-tolerant species, was precultured under sufficient (+ P) and deficient (‒P) phosphorus conditions, followed by treatment with Al for two weeks. A low ionic strength nutrient solution (pH 4.0) for avoiding Al-P precipitation, with an equal free Al3+ activity (60 µM) in the + P and −P mediums, was employed for plant treatment. Biomass and root elongation data indicated lower susceptibility of + P plants to Al3+ phytotoxicity compared with –P plants. Enhanced leaf accumulation of betacyanin and higher root exudation of phenolics, accompanied by less Al3+ binding to the root cell wall, were among the mechanisms for lower susceptibility of + P plants to Al3+ rhizotoxicity. However, our findings suggest the existence of some adaptive mechanisms acting under a combination of Al3+ toxicity with P deficiency. Phosphorus-deficient plants exhibited lower leaf Al accumulation, higher Al-induced H+ release, and acid phosphatase activity compared to + P plants. In addition, Al treatment enhanced the leaves’ total and free cytosolic inorganic P (Pi) levels and improved the plants ability to mobilize sparingly soluble phosphorus (Ca3(PO4)3). Our data suggest that although sufficient P nutrition is necessary for full expression of adaptive mechanisms to Al3+ toxicity, modifications in the rhizosphere of –P plants mediated by Al3+ may significantly improve Pi availability and help maintain Pi homeostasis in plants growing on acidic soils.