The objective of this study was to quantify the genetic variability and determine the inheritance of Al tolerance in lucerne (or alfalfa) using a 4-parent diallel mating design. Regrowth root length (RRL), along with root length (RL) and total root length (TRL), was measured to indicate relative Al stress response using a hydroponic system in a 3 lM Al solution at pH 4.5. A diallel analysis indicated the significance of general combining ability (gca) variance for RRL but not the specific combining ability (sca) variance; the same result was obtained for TRL but not for RL. For both RRL and TRL, genetic variance appeared to be more important than the environmental variance. For RRL, a strong but non-significant correlation was indicated between parental performance and their gca effects; while mid-parent heterosis and/or over-dominance were detected, as associated with the expressed sca effects in several combinations. These results suggest the complex genetic nature and expression of Al tolerance in the 4-parent diallel crossing system tested. Al-tolerant parent, GAAT'S', was the most promising parent, conferring the highest gca effects for RRL as well as for other two characters. The existence of significant gca variance in RRL may also suggest the feasibility of improving Al tolerance through enhanced root regrowth using phenotypic recurrent mass selection to pyramid desirable Al-tolerant genes, focussing on parental lines and/or elite individual plants expressing long regrowth roots.

Manganese (Mn) tolerance response in two aluminum (Al)-tolerant triticale (× Triticosecale Wittmack) varieties was characterized by measurements of growth and dry matter production of seedlings in nutrient solution culture containing 100 mg L -1 Mn. Root weight index (RWI) and total weight index (TWI) based on relative plant growth were two indicators of differentiating genotypic Mn tolerance; these two indices were used to make a comparative assessment of the degree of Mn tolerance in a group of eight Australian and South African genotypes which differ in apparent Al tolerance. The G4-95A was more Mn-tolerant than its Al-tolerant counterpart Tahara. A wide range of Mn tolerance was found in the eight genotypes, but few were tolerant of both Al and Mn stresses; measurements of RWI at 100 mg L -1 Mn stress differentiated them into three response types (i.e., Mn-tolerant, moderately Mn-tolerant/Mn-sensitive, and Mn-sensitive) at the two critical values of 0.30 and 0.60. Covariation analysis indicated no association between Mn tolerance and Al tolerance; selective breeding for acidic stress tolerance should focus on both stress tolerances.

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