Recent Advances in Breeding Maize for Drought, Heat and Combined Heat and Drought Stress Tolerance in Sub-Saharan Africa

Drought, heat, and combined drought and heat are important abiotic stresses constraining the production and productivity of maize ( Zea mays L.) in sub-Saharan Africa (SSA). In the face of climate change, these stresses are likely to occur simultaneously and put at risk food and economic security in SSA. This review describes maize breeding activities conducted by the International Institute of Tropical Agriculture (IITA) in partnership with national scientists under the Drought Tolerant Maize for Africa (DTMA) and Stress Tolerant Maize for Africa (STMA) projects, which together sought to develop and deploy multiple stress tolerant hybrids, and open-pollinated varieties. Emphasis was on (i) developing a reliable methodology for screening maize for tolerance to drought stress (DS), heat stress (HS), and combined drought and heat stress (CDHS) using key secondary traits and grain yield, (ii) use of appropriate breeding techniques for tailoring maize for tolerance to DS, HS and CDHS, (iii) exploring diverse sources of germplasm for genetic enhancement of maize, (iv) extensive multilocational evaluation to identify genotypes with stable performance under the stresses, and (v) application of genomic tools to accelerate genetic gains in maize breeding at IITA. At IITA, the performance of maize hybrids under stresses of DS, HS and CDHS have been improved using conventional breeding techniques/procedures. These techniques/ procedures have led to accelerated genetic gains in yield that were 26–49% higher than the best commercial hybrid checks under CDHS and DS. Additive gene action has been consistently found to be more important than the non-additive among early maize under DS and CDHS while both the additive and non-additive have been reported to be important for the extra-early maize. The most reliable secondary traits for selecting for improved grain yield under the stresses include anthesis-silking interval, ears per plant, and plant and ear aspects. Several early and extra-early landraces have been identified as potential sources of tolerance to DS, HS, and CDHS. Several quantitative trait loci (QTLs) associated with grain yield and key secondary traits have been identified via genome-wide association studies in landraces and inbred lines. Those desirable QTLs, upon validation, could be invaluable for genomics-enabled breeding.