Study on gene action and combining abilities for thermotolerant ablilities of corn (Zea mays L.)

High temperature reduces the pollen viability and silk receptivity of corn resulting in poor seed set and reduced yield. Continuously increasing temperature and less frequency and distribution of rainfall coupled with usual canal–closure particularly in Pakistan have significantly been decreasing 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 × tester mating design. The 40 hybrids and 14 parents were evaluated for heat tolerance under moderate temperature field conditions (by sowing on March 31) using  triplicated randomized complete block design during spring 2004. Large differences in heat units (111 to 326) utilized by the parents and crosses under normal and moderate 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. Highly significant differences (P ≤ 0.01) were observed among 54 corn genotypes, 14 parents,  40 crosses, parent vs crosses and interaction term of lines × tester (L × T) for 14 maize plant traits. The inbred lines L1, L2, L3, L5 (lines), T1, T3 (testers) and hybrids L1×T3, L2×T4, L3×T3 and L5×T1 were proved to be the excellent combiners with high GCA and SCA effects respectively, for most of the traits. 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.