Magnetic resonance imaging of absorbed aluminum in alfalfa roots

An estimated 30% of the world's arable soils are acidic and aluminum (Al) t oxicity is often the primary growth-limiting factor. Excess Al is especiall y undesirable in sub-soils because it reduces rooting depth and branching a nd predisposes plants to drought injury. Liming the plow layer does not gen erally 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 desig n of more efficacious breeding procedures. Evidence is accumulating that or ganic acids and proteins elicited by Al stress may complex and detoxify Al either within, or external to, the root. Because Al is a paramagnetic eleme nt that can reduce T2 relaxation times tinter-proton interactions markedly, the mechanism of Al tolerance in alfalfa was investigated through T2-based Magnetic Resonance imaging (MRT) of young lateral root sections of an Al-s ensitive and an Al-tolerant alfalfa clone grown in nutrient solution (0 or 111 mu mol Al; pH 4.5). Root sections that developed under phytotoxic level s of Al accumulated considerable Al in the epidermis and internal root tiss ue. Aluminum may have been complexed by low molecular weight proteins and o rganic 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 toler ance mechanisms may also be important. Root buds accumulated little Al comp ared to the remainder of the root, indicating that the pronounced effects o f Al on lateral root development are indirect. Magnetic Resonance Imaging i mages evaluated in this study provided clues to the basic mechanisms of Al tolerance in alfalfa and, with further refinement, could be used as one cri terion for selecting Al-tolerant plants.