Effect of high temperature on seedling growth and photosynthesis of tropical maize genotypes

Genotypic variability in relation to growth and photosynthetic CO2 assimilation rate (P-n) is well known for maize (Zea mays L.) under heat stress conditions. This study was, however, initiated to test whether genotypic growth variation is related to variations in individual leaf size, leaf extension rate (LER), and photosynthesis of the single leaf at high temperature. Six tropical maize genotypes selected from the International Maize and Wheat Improvement Centre (CIMMYT) with contrasting growth responses were grown for 9 days after emergence (DAE) in the first and for 15 DAE in the second experiment at 25/22 degrees C and 42/30 degrees C. High temperature caused a marked decrease in the growth parameters, and the genotypes showed high growth variations irrespective of temperature levels. Interestingly, genotypes did not follow a similar ranking in relation to biomass production between 9 DAE (heterotrophic growth phase) and 15 DAE (autotrophic growth phase) at 25/22 degrees C, but the pattern was similar at 42/30 degrees C. Total leaf area and daytime LER of leaves 2 (l(2)), 3 (l(3)), and 4 (l(4)) showed a tight correlation with biomass production at both temperatures, while the LER of the youngest leaf (l(4)) at night also showed the same correlation at 42/30 degrees C. A significant relationship between the areas i, and i, and biomass was observed only at high temperature and not at 25/22 degrees C. The P-n decreased markedly at high temperature and genotypic variability was pronounced. The genotypes maintained a similar ranking of P-n measured from l(2) at 8 DAE and from l(3) at 13 DAE under unfavourable conditions only and not at 25 degrees C. Of the six genotypes, F250 outperformed the others in relation to growth and P-n activity. A tight correlation between photosynthesis of different leaves and growth was detected at high temperature but not at the optimal temperature for growth. It is concluded that the areas i, or i,, daytime LER and P-n, all measured at high temperature stress conditions, can be regarded as good indicators of the thermo-tolerance of tropical maize genotypes at the seedling stage.