Genetic variation of heat tolerance and correlation with other agronomic traits in a maize (Zea mays L.) recombinant inbred line population

Heat stress is a critical environmental factor that affects agricultural production in the southwest of the United States, especially in West Texas. Heat stress can reduce plant height, kill the leaf tissues, cause abortion of pollen and kernel, and affect the cell activities. The objectives of this study were to determine the genetic variation of leaf firing, chlorophyll fluorescence, plant height, ear height, leaves above ear, number of tassel branches, days to flowering and yield per plant in a recombinant inbred line population, and to estimate the heritability of those traits. A total of 179 maize recombinant inbred lines from B76 x B106 were evaluated at Pecos, TX, in 2001, at Texas Tech University (TTU) experimental farm in 2002, and at TAMU Lubbock center in 2001 and 2002. Significant variation in leaf firing, chlorophyll fluorescence and other agronomic traits were found among the RILs. In the population, leaf firing ranged from 0% to 92.9% in Lubbock-TAMU 2001 test, 0% to 100% in Pecos 2001 test, 0% to 67.9% in Lubbock-TAMU 2002 test, and 0% to 100% in Lubbock-TTU 2002 test. The average of chlorophyll fluorescence of the RILs ranged from 0 to 0.79 (2001 Lubbock-TAMU), 0.14 to 0.78 (2002 Lubbock-TAMU), and 0.02 to 0.76 (2002 Lubbock-TTU). Leaf finng,chlorophyll fluorescence and other agronomic traits also showed transgressive segregation in the population in each environment. Only plant height, ear height, number of tassel branches (2001 and 2002 Lubbock-TAMU), and leaves above ear (2002 Lubbock-TAMU) showed the normality distribution.

Under heat stress, yield per plant was negatively correlated with percent leaf finng and days to flowering and positively correlated with chlorophyll fluorescence and number of tassel branches. Percent leaf firing was negatively correlated vMth chlorophyll fluorescence. Number of tassel branches was positively correlated with plant height and ear height. Leaf firing and chlorophyll fluorescence did not show strong correlation with plant height, ear height, number of tasel branches, and leaves above ear. The broad-sense heritability for leaf firing and chlorophyll fluorescence was 0.32 and 0.28 on the plot basis, respectively, and 0.85 and 0.77 on the mean basis. From their low heritability based on plot, high heritability based on means, and large genotype x environment interactions, we believe that the heat tolerance in maize is inherited quantitatively, and effective selection for heat tolerance requires evaluation genotypes in replicated trails in multiple environments.