Root architectural traits as phenotypic markers of aluminum tolerance in wheat

Aluminum (Al³⁺) toxicity is a major constraint to wheat (Triticum aestivum L.) productivity in acidic soils, where it restricts root growth and impairs nutrient and water uptake. Characterizing root system architecture (RSA) offers an effective approach for early screening of genotypes capable of maintaining growth under Al³⁺ stress. This work aimed (i) to evaluate the effects of increasing Al^{3 +} concentrations on shoot development and RSA in wheat cultivars and (ii) to identify root traits that serve as reliable indicators of Al^{3 +} tolerance. Four cultivars, Anahuac, IAC-5, Onix, and Quartzo, were grown in a greenhouse for 15 days under increasing concentrations of Al³⁺ (0, 37.5, 75, and 150 µM). The cv. IAC-5 presented the highest tolerance to Al³⁺, verified by the maintenance of shoot dry weight and foliar area, nearly twice the average of the other cultivars and reductions of only 1.4% and 13.9%, respectively, at 150 µM Al³⁺ compared to 0 µM Al³⁺. Anahuac and Quartzo showed pronounced reductions in shoot and root growth (approximately 50%), with Anahuac being the most sensitive. Increasing Al³⁺ concentrations reduced total root length, number of forks, and the proportion of thin roots while increasing average root diameter; root thickening was particularly evident in Al-sensitive cultivars and in first- and second-order lateral roots. Al³⁺ tolerance was associated with sustained root emission and elongation, especially lateral roots during early vegetative state. Overall, RSA characterization, particularly root length, diameter, and branching traits, provides an efficient and rapid phenotyping strategy for selecting Al³⁺-tolerant wheat genotypes.