COMBINING ABILITY FOR YIELD AND PROTEIN-QUALITY AMONG MODIFIED-ENDOSPERM OPAQUE-2 TROPICAL MAIZE INBREDS

The opaque-2 mutation has been used in maize (Zea mays L.) breeding to develop cultivars with increased concentrations of lysine and tryptophan. Very little information is available, however, about combining ability for protein quality traits for modified/vitreous-endosperm opaque-2 maize (quality protein maize, or QPM), although such information is essential for planning an effective hybrid development program. This research assessed the relative importance of general and specific combining ability effects (GCA and SCA) for grain yield, protein concentration in grain, tryptophan concentration in grain, and tryptophan concentration in protein for various tropical, late maturity, white grained QPM lines. Four diallel trials, involving a total of 28 lines from five populations, were evaluated from 1988 to 1990, each at two to five locations during 1 yr. The best QPM hybrid in each trial exceeded the best normal-endosperm maize check by an average of 14% for grain yield, 48% for tryptophan concentration in grain, and 60% for tryptophan concentration in protein. Mean squares for GCA effects in across-location analyses were significant (P < 0.05) for grain yield, protein concentration in grain, and tryptophan con centration in protein for three of the four trials, and for tryptophan concentration in grain in two trials. Mean squares for SCA effects were not significant except for grain yield in one diallel. Location x GCA interaction was generally significant for all traits except tryptophan concentration in protein. Our results suggest that QPM hybrids with the highest values for protein quality traits will be crosses between lines with the highest GCA values. Concurrent improvement of yield and protein quality should be possible for this germplasm because phenotypic correlation coefficients were not significant for grain yield with concentration of protein in grain, concentration of tryptophan in grain, and concentration of tryptophan in protein.