Magnetic resonance imaging of absorbed aluminum in alfalfa roots

An estimated 30% of the world's arable soils are acidic and aluminum (Al) toxicity is often the primary growth-limiting factor. Excess Al is especially undesirable in sub-soils because it reduces rooting depth and branching and predisposes plants to drought injury. Liming the plow layer does not generally neutralize subsoil phytotoxicity and Al-tolerant cultivars offer an alternative or supplemental solution to the problem. Genetic diversity for acid soil tolerance in alfalfa (Medicago sativa L.) is limited and a better understanding of the basic tolerance mechanisms would facilitate the design of more efficacious breeding procedures. Evidence is accumulating that organic acids and proteins elicited by Al stress may complex and detoxify Al either within, or external to, the root. Because Al is a paramagnetic element that can reduce T2 relaxation times (inter-proton interactions) markedly, the mechanism of Al tolerance in alfalfa was investigated through T2-based Magnetic Resonance Imaging (MRI) of young lateral root sections of an Al-sensitive and an Al-tolerant alfalfa clone grown in nutrient solution (0 or 111 micromole Al; pH 4.5). Root sections that developed under phytotoxic levels of Al accumulated considerable Al in the epidermis and internal root tissue. Aluminum may have been complexed by low molecular weight proteins and organic acids in the tolerant clone whereas the sensitive clone appeared to have abundant free Al; however, variation among replications indicates that free Al may still have been present in tolerant roots and that other tolerance mechanisms may also be important. Root buds accumulated little Al compared to the remainder of the root, indicating that the pronounced effects of Al on lateral root development are indirect. Magnetic Resonance Imaging images evaluated in this study provided clues to the basic mechanisms of Al tolerance in alfalfa and, with further refinement could be used as one criterion for selecting Al-tolerant plants.