Aluminum Stress Responses in Quinoa Suggest Adaptive Potential in Acidic Environments

Abstract Aluminum (Al) toxicity is a major limiting factor for crop production in acidic soils, especially in tropical regions. Although quinoa ( Chenopodium quinoa Willd.) is widely recognized for its resilience to environmental stresses, its responses specifically to Al exposure remain insufficiently characterized, particularly for Brazilian cultivars such as BRS Piabiru. This study evaluated the physiological and biochemical adjustments of quinoa under increasing Al concentrations, addressing this knowledge gap by identifying dose-dependent stress thresholds under conditions of high Al availability. Quinoa plants were grown hydroponically at pH 4.0, a condition under which Al³⁺ availability is high, and exposed to six Al concentrations (0, 0.2, 1, 5, 25, and 125 mg L⁻¹). The lowest concentration (0.2 mg L⁻¹) was selected because it corresponds to the maximum Al level permitted in drinking water in Brazil and is consistent with the World Health Organization guideline. The subsequent concentrations followed a five-fold geometric progression (0.2 → 1 → 5 → 25 → 125 mg L⁻¹), allowing a controlled and proportional increase in Al supply. The highest concentration (125 mg L⁻¹) was chosen based on previous preliminary trials, in which it induced visible leaf chlorosis during the vegetative stage, thus representing a clearly inhibitory level. Fifty-four days after planting, growth parameters, biomass, oxidative markers (MDA and H₂O₂), antioxidant enzyme activities, and non-enzymatic indicators (proline, soluble sugars, carotenoids) were quantified. High Al concentrations (125 mg L⁻¹) severely inhibited growth, reducing root length by 92%, root volume by 95%, and total dry mass by 54%, while increasing MDA and H₂O₂ by 82% and 105%, respectively. Antioxidant enzymes, particularly GSH-Px, increased by 220%. In contrast, low Al exposure (0.2 mg L⁻¹) produced a mild stimulatory response, with root volume increasing by 22%, total dry mass by 5%, and proline by 296%, consistent with hormetic adjustment. Quinoa cv. BRS Piabiru exhibits a concentration-dependent response to Al, with low doses inducing mild metabolic stimulation and high doses causing pronounced oxidative damage and growth suppression. These hydroponic results clarify physiological tolerance limits and biochemical responses under conditions of high Al³⁺ availability; however, extrapolation to field soils should be made cautiously due to the complexity of soil chemical interactions not reproduced in hydroponics. Graphical Abstract