Breeding for Heat Stress Tolerance in Tropical and Subtropical Maize.

Climate projections suggest that elevated temperatures, especially in the drought-prone areas of sub-Saharan Africa and rainfed regions in South Asia, are highly likely to result in significant yield losses in tropical/subtropical maize. Compared to other abiotic stresses associated with climate change, especially drought stress, work on developing and deploying heat stress tolerant tropical/subtropical maize is still in its infancy. Recent studies by CIMMYT Global Maize Program, in collaboration with partners worldwide, indicated that current tropical/subtropical maize germplasm developed for drought tolerance may not perform well under drought stress at elevated temperatures.  Nevertheless, a few inbred lines with high levels of tolerance to drought as well as combined drought and heat stress, most notably La Posta Sequia C7-F64-2-6-2-2 and DTPYC9-F46-1-2-1-2, were identified and are presently being utilized in developing elite germplasm with tolerance to combined drought and heat stress in sub-Saharan Africa and Asia.

The recent findings that 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, has major implications in breeding heat stress resilient maize cultivars.  Several of the drought tolerant parents developed by CIMMYT and widely used in hybrid maize breeding in eastern and southern Africa were found to be highly susceptible to drought stress under elevated temperatures; a notable example is CML442 × CML444 that is used as the female parent in several commercial hybrids.  Therefore, intensive efforts are required to ensure that the most widely used drought tolerant inbred lines and hybrids also possess tolerance to combined drought and heat stresses, especially for deployment in drought-prone areas where temperatures are predicted to increase.  The product development strategies may also need to be reoriented to include screening under the combined effect of drought and heat rather than the individual stresses.

Studies undertaken by the CIMMYT-Asia team to identify heat stress tolerant tropical maize lines among the elite, drought tolerant maize germplasm developed in Mexico, Asia and Africa revealed high vulnerability of most of the tropical maize germplasm, including commercial cultivars in South Asia, to reproductive stage heat stress. Some promising lines with tolerance to high-temperature stress (coupled in a few cases also with drought tolerance) have been identified among the Asian-adapted maize germplasm for further evaluation and utilization.

Intensive multi-institutional efforts are also required to take forward the initial leads in terms of identification of trait donors to the development and deployment of heat stress resilient tropical/subtropical maize germplasm suitable for climate change vulnerable environments in sub-Saharan Africa and South Asia. CIMMYT is presently implementing two major research projects, supported by USAID, for developing and deploying heat resilient maize in sub-Saharan Africa and Asia.  The “Heat Tolerant Maize for Asia” (HTMA) Project brings together public and private institutions based in South Asia (Bangladesh, India, Nepal and Pakistan), besides Purdue University, USA, for accelerated development and deployment of heat resilient maize germplasm.

In summary, accelerated development and deployment of heat stress resilient maize varieties requires: (a) carefully undertaken field-based phenotyping in several relevant sites as well as under technically demanding managed-stress screens; (b) better understanding of the genetic architecture of heat stress tolerance as well as tolerance to combined drought and heat stresses; (c) utilization of modern breeding tools/strategies, including genome-wide association studies, genomic selection, and doubled haploid (DH) technology for rapid development of improved products; (d) identification of hot spots of climate vulnerability for germplasm deployment; and (e) multi-institutional efforts, especially public-private alliances, to ensure that the products (climate resilient varieties) effectively reach the climate change vulnerable farming communities.