Determination of the Best Heat Stress Tolerance Indices in Maize (Zea mays L.) Inbred Lines and Hybrids under Khuzestan Province Conditions

Maize improvement for high temperature tolerance requires the reliable assessment of parental inbred lines and their combinations. Fifteen maize inbred lines were evaluated during 2007 and 2008 in Shushtar city (Khuzestan Province). The inbred lines were planted at two dates: 6 July, to coincide heat stress with pollination time; and 27 July, as normal planting to avoid high temperature during pollination and grain filling period. In addition, 28 hybrids from a combination of eight selected lines, were evaluated under the same conditions in 2008. Five stress tolerance indices, including mean productivity (MP), stress tolerance (TOL), stress susceptibility (SSI), stress tolerance index (STI) and geometric mean productivity (GMP) were used in this study. Data analysis revealed that the SSI, STI and GMP indices were the more accurate criteria for selection of heat tolerant and high yielding genotypes. The positive and significant correlation of GMP and grain yield under both conditions revealed that this index is more applicable and efficient for selection of parental inbred lines in producing hybrids to be tolerant to high temperatures and high yielding under both conditions. Based on two years’ data and using the STI, GMP and MP indices, K166B, K166A and K18×K166B proved to be the most heat tolerant lines and hybrid. Biplot analysis allowed us to distinguish groups of tolerant and sensitive inbred lines and hybrids. Based on the results of this study, the hybrid K18×K166B can be recommended for the Khuzestan region.