Selection for Drought Tolerance in Two Tropical Maize Populations

Drought is a major factor limiting maize (Zea mays L.) yield in much of the world. The need to breed maize cultivars with improved drought tolerance is apparent. This study compared two maize populations, ZM601 and ZM607 for drought tolerance during flowering, the most drought-vulnerable period for the maize plant. Cultivar ZM601 had been improved through recurrent selection for two cycles for drought tolerance at flowering plus one cycle under rainfed (“random”) drought stress, while ZM607 had been improved for high yield for three cycles under favorable growing conditions. A set of 143 random S1 lines from ZM601 was compared with 94 from ZM607 at two drought stressed and one well-watered environment for yield and secondary traits. The results did not show much difference in drought tolerance between ZM601 and ZM607. Differences between population means for grain yield, anthesis-silking interval (ASI), and number of ears per plant (EPP) were small. Frequency distributions for ASI and leaf rolling were different for the two populations. However, ZM601 had more lines with shorter ASI and less leaf rolling as compared to ZM607. Broad sense heritability of ASI was larger,while that of grain yield smaller, and the correlation between ASI and grain yield was larger at drought-stressed compared to unstressed sites. Absence of large differences in grain yield between ZM601 and ZM607 lines was attributed to little effect of the selection completed prior to this evaluation, to genetic similarities of the two populations prior to selection, and to large genotype-by-environment interaction between Mexico, where drought screening was conducted, and Zimbabwe, where this evaluation was conducted. Results confirmed the value of ASI (measured at drought-stressed sites) as an indirect selection criterion for improving grain yield under drought stress conditions.