We have been using nutrient solutions to study the genetics and mechanism of aluminum tolerance in maize. The screening technique has already been described (Agr. J. 69:755, 1977). A large number of inbreds have been classified by measuring relative primary root length (RRL) after two weeks growth.

The inbreds fall into four approximately equal classes when screened at 2.0, 1.0 and 0.5 mM Ca. Class I inbreds are the most tolerant, growing even at 0.5 mM Ca. Class IV inbreds are severely inhibited in the presence of Al, even when the Ca concentration is raised to 2.0 mM.

Tolerance was dominant in all crosses studied. Table I summarizes the F2 and BC1 data from crosses between inbred class levels. In every case, the progeny formed not a continuum but rather two discreet classes. A one-gene model was tested with the chi square statistic and found to be consistent with the data. We postulate that we are dealing with a multiallelic series at a single locus determining the root's ability to continue growth at a given Al stress level.

Table I.

To study the mechanism of aluminum tolerance it is preferable to compare tolerant and sensitive plants which are otherwise genetically identical. Previous work in this area has compared cultivars of different genetic backgrounds. We have selected for aluminum tolerance within a temperate composite, "Supermix," as an alternative to developing isogenic lines. Selections were transplanted to the field and randomly intercrossed. Equal numbers of seeds from each ear were bulked together to minimize genetic drift.

Another population was similarly selected from "Supermix" on the basis of vigorous root growth under non-stress conditions. This was done to distinguish selection for aluminum tolerance from "natural" selection for adaptation to nutrient media.

Graph 1 shows that selection for aluminum tolerance, as measured by primary root length, has been effective. The aluminum tolerant selection shows no improvement over the parent population under non-stress conditions but is superior at every stress level. Only at 2.0 mM Ca is the difference significant (Duncan's Multiple Range). However, if the values at all the stress levels are pooled and analyzed together, the difference is highly significant.

Graph I.

The control selection is not significantly improved in primary root length under non-stress conditions. Its average performance at the three stress levels is poorer (a = .01) than the original parent.

We feel that these populations will be useful tools in studying the mechanism of stress tolerance, especially after another cycle of selection. Differences between tolerant and sensitive inbreds can be re-examined with materials of closely similar background genotype. The interactions between aluminum, calcium and phosphorus response are of particular interest since aluminum tolerance has been variously described as calcium or phosphorus efficiency. The role of pH control as an avoidance mechanism will also be investigated.

E. William Stockmeyer, Herbert L. Everett and Dean Rhue

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