Optimizing techniques for selecting acid soil tolerant wheats: Case studies with aluminum and manganese tolerance

Numerous techniques exist for the selection of aluminum (AI) tolerant wheat genotypes, including hematoxylin staining (with and without root regrowth) and seedling growth measurements in solution culture or in acid soils (field or greenhouse). Rooting response measurements are superior to assays on above-ground tissues. Our experience with Kenyan and Canadian germplasm and soils indicates that these methods are all effective for improving AI tolerance. Breeding programs should use the optimum method for their own circumstances, reflecting budgetary, logistical and program resources. Empirical selection methods (field or laboratory) should only be used after characterizing the growth response of representative tolerant and sensitive germplasm to increasing AI stress. Without such information, development of a single protocol which maximizes differences in tolerance is not possible, especially when more than one gene may be involved. Recent studies document complex multi genic inheritance, which could account for the failure of physiologists to identify precise mechanisms of tolerance. Eleven different hypotheses, representing exclusion and internal tolerance mechanisms have been proposed, but no single hypothesis is yet favored. We have identified simple selection techniques and new sources of manganese tolerance. Tolerance sources all originate from common landraces in Brazilian regions of high soil acidity, and show complex inheritance.