Rising temperature reduces the pollen viability and silk receptivity of corn resulting in poor seed set and reduced yield. Continuously rising temperature and less frequency and distribution of rainfall coupled with usual canal-closure particularly inPakistan have significantly been reducing the grain yield. This problem could be overcome by developing heat tolerant maize hybrids. For this purpose, five heat tolerant (lines), five heat susceptible (lines) and four heat susceptible (testers) corn inbred lines were hybridized artificially in a line x tester mating design. The 40 hybrids and 14 parents were evaluated for heat tolerance under high temperature field conditions by sowing on March 31 using triplicated randomized complete block design during spring 2004. Highly significant differences (P<0.01) were observed among 54 corn genotypes, 14 parents, 40 crosses, parent vs crosses and interaction term of lines x tester (L x T) for all the 14 maize plant traits. The inbred lines LI, L2, L3, L5 (lines), Tl, T3 (testers) and hybrids LlxT3, L2xT4, L3xT3 and L5xTl proved to be the excellent combiners with high GCA and SCA effects respectively, for most of the traits. Large differences in heat units (111 to 326) utilized by the parents and crosses under normal and high temperature conditions to mature physiologically suggested that inbred lines as well as crosses were photosensitive as they were not utilizing similar thermal units in both the environments. The dominance type of gene action was observed to be predominant for all the traits. The proportional contribution of lines was more for seven very crucial parameters. The estimates of heritability in broad sense were high for all the traits. Hybrid breeding is suggested as hybrid plants have higher capacity to tolerate heat stress in field conditions than their parents.
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