Low maize (Zea maysL.) yields and the impacts of climate change on maize production highlight the need to improve yields in eastern and southern Africa. Climate projections suggest higher temperatures within drought‐prone areas. Research in model species suggests that tolerance to combined drought and heat stress is genetically distinct from tolerance to either stress alone, but this has not been confirmed in maize. In this study we evaluated 300 maize inbred lines testcrossed to CML539. Experiments were conducted under optimal conditions, reproductive stage drought stress, heat stress, and combined drought and heat stress. Lines with high levels of tolerance to drought and combined drought and heat stress were identified. Significant genotype × trial interaction and very large plot residuals were observed; consequently, the repeatability of individual managed stress trials was low. Tolerance to combined drought and heat stress in maize was genetically distinct from tolerance to individual stresses, and tolerance to either stress alone did not confer tolerance to combined drought and heat stress. This finding has major implications for maize drought breeding. Many current drought donors and key inbreds used in widely grown African hybrids were susceptible to drought stress at elevated temperatures. Several donors tolerant to drought and combined drought and heat stress, notably La Posta Sequia C7‐F64‐2‐6‐2‐2 and DTPYC9‐F46‐1‐2‐1‐2, need to be incorporated into maize breeding pipelines.

No abstract

Key message Intensive public sector breeding efforts and public-private partnerships have led to the increase in genetic gains, and deployment of elite climate-resilient maize cultivars for the stress-prone environments in the tropics. Abstract Maize (Zea mays L.) plays a critical role in ensuring food and nutritional security, and livelihoods of millions of resource-constrained smallholders. However, maize yields in the tropical rainfed environments are now increasingly vulnerable to various climate-induced stresses, especially drought, heat, waterlogging, salinity, cold, diseases, and insect pests, which often come in combinations to severely impact maize crops. The International Maize and Wheat Improvement Center (CIMMYT), in partnership with several public and private sector institutions, has been intensively engaged over the last four decades in breeding elite tropical maize germplasm with tolerance to key abiotic and biotic stresses, using an extensive managed stress screening network and on-farm testing system. This has led to the successful development and deployment of an array of elite stress-tolerant maize cultivars across sub-Saharan Africa, Asia, and Latin America. Further increasing genetic gains in the tropical maize breeding programs demands judicious integration of doubled haploidy, high-throughput and precise phenotyping, genomics-assisted breeding, breeding data management, and more effective decision support tools. Multi-institutional efforts, especially public–private alliances, are key to ensure that the improved maize varieties effectively reach the climate-vulnerable farming communities in the tropics, including accelerated replacement of old/obsolete varieties.

Worldwide, tropical maize (Zea mays L.) is commonly exposed to low N conditions. Identification of low N tolerance‐related traits would help to develop indirect selection for yield and marker assisted selection under stress. Tolerance to high plant population density has been proposed as an alternative breeding strategy to improve stress tolerance in maize. A better understanding of mechanisms underlying tolerance to high plant population density and low N is, however, needed. For this purpose, elite CIMMYT open‐pollinated varieties (OPVs), inbred lines, and hybrids were grown under optimal, high plant population density and low N conditions. Yield, yield components, and a set of morpho‐physiological traits (secondary traits) were assessed in the different treatments and germplasm types. Emphasis was placed on anthesis‐silking interval and traits related to senescence, dry matter partitioning, and ovule and grain number. Association was observed under low N conditions between grain yield and anthesis‐silking interval, delayed senescence as expressed by either chlorophyll concentration or the number of green leaves above the ear, and ear/tassel weight ratio. Under optimal, high‐plant population density and low N conditions, final grain number depended more on abortion rate than on the total number of ovules at anthesis. Under low N stress, grain yield was significantly negatively correlated with abortion rate. Under high plant population density, a positive association was noted between ovule number and abortion rate, suggesting a source limitation for C products. The effect of stress on yield components and the strength of association between secondary traits and yield varied greatly according to germplasm type.

No abstract