Peer review

Maize (Zea mays L.) inbred lines with expired U.S. Plant Variety Protection Act certificates (ex-PVPA) can be an important source of favorable alleles to improve grain yield, root and stalk lodging, drydown, plant architecture, and reduced time to maturity of tropical hybrids. Understanding the heterotic patterns and combining ability of ex-PVPA lines with tropical lines is necessary to design effective breeding strategies using this germplasm. This study evaluated 10 Iowa Stiff Stalk Synthetic (BSSS) and 11 Non-Stiff Stalk Synthetic (NSSS) inbred lines in hybrid combinations with tropical mid-altitude testers in five mid-altitude environments (900?2000 masl) in Mexico and identified six hybrids with grain yield equal or superior to that of the check hybrids. Two NSSS inbred lines: PHR03 and PHN82 had significant (P < 0.01) and positive general combining ability (GCA) effects with inbred lines from CIMMYT?s Tuxpeño heterotic group. Similarly, the BSSS line PHG39 had a significant positive GCA effect (P < 0.05) with non-Tuxpeño lines. The lines PHN82 and PHK29 had positive GCA effects (P < 0.01) under drought stress and could be used as sources of drought tolerance in CIMMYT breeding programs. The mid-altitude lines CML537 and CML539 had positive GCA effects (1.45 and 0.69 t/ha, respectively; P < 0.01) with NSSS lines whereas hybrids of CML384 with BSSS lines yielded 0.64 t/ha (P < 0.01) more than the average of the testcrosses. These results showed that U.S. ex-PVPA lines can increase grain yield of CIMMYT?s tropical mid-altitude hybrids.

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