Root growth analysis: An underutilised approach to understanding aluminium rhizotoxicity

Despite numerous published studies, the mechanistic bases for Al rhizotoxicity and for intraspecific differences in Al tolerance remain elusive. Classical methods of growth analysis offer a means to describe quantitatively the onset of root growth inhibition in response to an Al challenge, and any subsequent recovery in growth rates (i.e. acclimation). Such information could help elucidate tolerance mechanisms (i.e. constitutive vs. inducible). Despite this potential utility, however, published data of high accuracy and precision are few, especially with wheat (Triticum aestivum L.). In this paper, a conceptual framework for analysing root growth responses to Al is presented. Specially designed tanks and a computerised video analysis system were used to monitor root elongation in a simple medium (1 mM CaCl₂, pH 4.3) containing varied Al concentrations. Root lengths of five wheat cultivars were measured every 2 to 5 h for 48 h after Al challenge. Growth rates typically exhibited a brief (1–2 h) lag phase prior to a sharp decline, followed by a recovery phase of 12 h duration. At low levels of Al (i.e. sufficient to cause a 10% reduction in net elongation) post-recovery growth rates generally equalled those of control roots, but were significantly reduced at Al concentrations yielding a 40 to 50% reduction in net elongation. All cultivars exhibited similar inhibition-acclimation response patterns and these responses did not explain differences in sensitivity to Al. Nonetheless, solution Al concentrations required to elicit the same growth responses varied by 20-fold across the five cultivars, consistent with the notion that root exclusion of Al is a significant component of differential Al tolerance in wheat.