HETEROSIS AND COMBINING ABILITY OF CIMMYTS QUALITY PROTEIN MAIZE GERMPLASM .1. LOWLAND TROPICAL

CIMMYT has developed a wide array of hard endosperm opaque-2 (o2) maize (Zea mays L.) germplasm, through the combined use of two genetic systems involving the o2 gene for improving protein quality and the genetic modifiers of the o2 locus for improving kernel phenotype and ameliorating other o2-associated defects. The objective of this study was to determine the heterotic patterns and combining ability of tropical quality protein maize (QPM) germplasm, and to identify superior germplasm suitable for hybrid development. Ten parents (four QPM pools, five QPM populations, and experimental variety PR 7737) were used in a diallel study in eight environments. Data on grain yield, time to silk, plant height, and endosperm hardness were recorded. General combining ability (GCA) effects were highly significant for all traits; specific combining ability (SCA) effects were significant for time to silk and plant height. Genotype x environment interactions and their partitions were significant for grain yield and endosperm hardness. Pool 24 QPM was the highest-yielding parent (6.48 Mg ha-1) and Pool 24 QPM x Population 63 was the highest-yielding cross (6.56 Mg ha-1) among hard endosperm parents. PR 7737, a soft endosperm opaque-2, was low yielding as a parent, but performed better in crosses with other hard endosperm parents and showed high heterosis. High-parent heterosis for grain yield was generally low in all crosses except those involving PR 7737 as a parent. Endosperm hardness ratings were intermediate relative to the parents, suggesting polygenic control. Crosses among white endosperm parents generally performed better than crosses among yellow endosperm parents. Crosses among dents and dent x flints yielded higher than flint crosses, but flint parents and their crosses had a superior endosperm modification compared with dents. Populations 62, 63, and PR 7737 showed significant positive GCA effects for grain yield; Pool 23 QPM, Pool 25 QPM, and Populations 62, 64, and 65 had significant negative GCA effects for endosperm hardness. Populations 62 and 63 among white endosperm materials and Population 65 among yellow endosperm could be used for initiating hybrid development work. The broad genetic base provides opportunities for developing intrapopulation interline hybrids.