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a International Maize and Wheat Improvement Center (CIMMYT), P.O. Box MP163, Mount Pleasant, Harare, Zimbabwe
b Pioneer SARL, 24 Rue du Moulin, F-68740 Nambsheim, France
* Corresponding author (K.Pixley{at}cgiar.org)
Quality protein maize (Zea mays L.) (QPM) can help alleviate human malnutrition and reduce costs of animal feed because it contains the opaque-2 mutation, which results in increased lysine and tryptophan concentrations and a higher biological value as a food than normal maize. To be commercially successful, however, QPM cultivars must be agronomically competitive with normal-endosperm alternatives while consistently achieving expected protein quality and endosperm modification (i.e., translucent or near-normal phenotype) standards. To assess stability of grain yield, protein content and quality, and endosperm modification of QPM cultivars, we evaluated 18 single-cross, 18 three-way, and 18 double-cross hybrids, and eight open-pollinated cultivars (OPCs) grown at 13 tropical locations on four continents. Hybrids averaged 13% higher grain yield than OPCs (5.97 and 5.17 Mg ha-1), whereas protein concentration in grain was 2% greater for the OPC relative to hybrid cultivars (94.6 and 92.4 g kg-1). Endosperm modification score and tryptophan concentration in protein were similar for all cultivar types. Genotype x environment interactions and sums of squares for deviations from linear regression 
for grain yield and protein concentration in grain were largest (indicating least stability) for single-cross hybrids, followed by three-way, double-cross, and open-pollinated cultivars (OPCs), successively. The reverse trend was observed for endosperm modification score, suggesting that more homogeneous cultivars had greater stability for this trait. Additive main effects and multiplicative interactions (AMMI) analysis indicated that genotype x environment interaction effects for grain yield and endosperm modification score were different for hybrids than OPCs; certain environments favored either hybrids or OPCs. In conclusion (i) protein quality and endosperm modification score were always within expected values for QPM and (ii) tryptophan concentration in protein was the most stable trait, followed by protein concentration in grain, then endosperm modification score and finally grain yield.
Abbreviations: AMMI, additive main effects and multiplicative interactions model • CIMMYT, International Maize and Wheat Improvement Center • GEI, genotype x environment interaction effect • IPCA, Interaction principal component axis (from the AMMI analysis) • OPC, open-pollinated cultivar • QPM, quality protein (opaque-2) maize
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