Hot, dry climates call for resilient, high-performing wheat varieties

Public and private crop research organizations worldwide have worked behind the scenes for decades, bolstering the resilience of staple crops like maize and wheat to fight what is shaping up to be the battle of our time: feeding humanity in a biosphere increasingly hostile to crop farming.

Night heaters to increase night temperature in the field, as increasingly warmer nights are diminishing yield in many cropping systems. (Photo: Enrico Yepez/CIMMYT) 

In the case of wheat — which provides some 20% of carbohydrates and 20% of protein in human diets, not to mention 40% of total cereal exports — harvests spoiled by heat waves, droughts, and crop disease outbreaks can send food prices skyrocketing, driving world hunger, poverty, instability, human migration, political instability, and conflict.

Century-high temperature extremes and the early onset of summer in South Asia in 2022, for example, reduced wheat yields as much as 15% in parts of the Indo-Gangetic Plains, a breadbasket that yearly produces over 100 million tons of wheat from 30 million hectares of crop land.

Around half the world’s wheat crop suffers from heat stress, and each 1 °C increase in temperature reduces wheat yields by an average 6%, according to a 2021 review paper ″Harnessing translational research in wheat for climate resilience,″ published in the Journal of Experimental Botany, which also outlines nine goals to improve the climate resilience of wheat.

Simulating heat shocks in the field using portable plot-sized ‘heating tents’ (Photo: G Molero/CIMMYT)

Droughts and shrinking aquifers pose equally worrying threats for wheat, said Matthew Reynolds, a wheat physiologist at the International Maize and Wheat Improvement Center (CIMMYT) and lead author of the study. ″Water availability is the biggest factor influencing potential yield in a majority of wheat environments globally,″ Reynolds explained. ″Studies predict severe water scarcity events for up to 60% of the world’s wheat-growing areas by the end of this century.″

Science and sources to toughen wheat

Along with modernized, more diverse cropping systems and better farm policies, more resilient varieties are crucial for sustainable wheat production, according to Reynolds and a wheat breeder colleague at CIMMYT, Leo Crespo, who added that breeders have been working for decades to stiffen wheat’s heat and drought tolerance, long before climate change became a buzzword.

″Breeding and selection in diverse environments and at targeted test sites characterized by heat and natural or simulated drought has brought farmers wheat varieties that perform well under both optimal and stressed conditions and we’re implementing new technologies to speed progress and lower costs,″ said Crespo, mentioning that the Center’s wheat nurseries SAWYT and HTWYT target semi-arid and heat-stressed environments respectively and are sent yearly to hundreds of public and private breeders worldwide through the International Wheat Improvement Network (IWIN). ″Retrospective analysis of IWIN data has shown that heat tolerance has been increasing in recent years, according to a 2021 CIMMYT study.″

″Climate change is a serious driver of potential disease epidemics, since changeable weather can increase selection pressure for new virulent pathotypes to evolve,″ said Pawan Singh, a CIMMYT wheat pathologist. ″We must be ever vigilant, and the IWIN is an invaluable source of feedback on potential new disease threats and changes in the virulence patterns of wheat pathogens.″

In the quest to improve climate resilience in wheat, CIMMYT ″pre-breeding″ — accessing desired genetic traits from sources like wheat’s grassy relatives and introducing them into breeding lines that can be crossed with elite varieties — focuses on specific traits. These include strong and healthy roots, early vigor, a cool canopy under stress, and storage of water-soluble carbohydrates in stems that can be used as stress intensifies to complement supplies from photosynthesis, as well as an array of traits that protect photosynthesis including ‘stay-green’ leaves and spikes and pigments that protect the delicate photosynthetic machinery from oxidative damage caused by excess light.

Screening highly diverse lines – identified by DNA fingerprinting – from the World Wheat Collection under heat stress. (Photo: Matthew Reynolds/CIMMYT)

Though elite breeding lines may contain genetic variation for such traits, in pre-breeding researchers look further afield for new and better sources of resilience. The vast wheat seed collections of CIMMYT and other organizations, particularly seed samples of farmer-bred heirloom varieties known as ″landraces,″ are one potential source of useful diversity that cutting-edge genetic analyses promise to help unlock.

Rich diversity for wheat is still found in farmers’ fields in India, in the northern states of the Himalayan region, the hill regions, and the semi-arid region of Rajasthan, Gujarat, Karnataka. The landraces there show tolerance to drought, heat, and saline soils.

The so-called ″synthetic wheats″ represent another plentiful source of resilience genes. Synthetics are the progeny of crosses of tetraploid wheat (having four chromosomes, like the durum wheat used for pasta) with wild grass species. CIMMYT and other organizations have been creating these since the 1980s and using them as bridges to transfer wild genes to bread wheat, often for traits such as disease resistance and heat and drought tolerance.

The study, creation, and use of bridging lines, landraces, and seed collections with useful traits as part of pre-breeding is described in the 2021 paper ″Progress and prospects of developing climate resilient wheat in South Asia using modern pre-breeding methods,″ published in the science journal Current Genomics.

Lines with new sources of heat- and drought-tolerance from CIMMYT’s pre-breeding are also distributed to public and private breeders worldwide via the IWIN for testing as the Stress Adapted Trait Yield Nurseries (SATYNs), according to the paper. These special nurseries are grown by national and private breeders throughout South Asia, for example in Afghanistan, Bangladesh, India, Iran, Nepal, and Pakistan. Lines from the nursery have on occasion been released directly as varieties for use by farmers in Afghanistan, Egypt, and Pakistan.

A critical challenge in pre-breeding is to identify and keep desirable wild genes while culling the undesirable ones that are also transferred in crosses of elite breeding lines with landraces and synthetics. One approach is through physiological pre-breeding, where complementary crosses are made to improve the crop performance under drought and heat stress. The second approach is using genomic prediction, on the basis of seeds, or accessions, in the gene bank collection that have gone through genomic and phenotyping analysis for target traits such as heat and drought tolerance. These approaches can also be combined to boost the speed and effectiveness of selecting strong varieties.

Breeding revolutions

Wheat breeding is being revolutionized by advances in ″high-throughput phenotyping.″ This refers to rapid and cost-effective ways to measure wheat performance and specific traits in the field, particularly remote sensing — that is, crop images taken from vehicles, drones, or even satellites. Depending on the wavelength of light used, such images can show plant physiochemical and structural properties, such as pigment content, hydration status, photosynthetic area, and vegetative biomass. Similarly, canopy temperature images from infrared photography allow detection for crop water status and plant stomatal conductance.  ″Such traits tend to show better association with yield under stress than under favorable conditions″, said Francisco Pinto, a CIMMYT wheat physiologist who is developing methods to measure roots using remote sensing. ″A remotely sensed ‘root index’ could potentially revolutionize our ability to breed for root traits, which are critical under heat and drought stress but have not been directly accessible in breeding.″

Innovative statistical analysis has greatly increased the value of field trials and emphasized the power of direct selection for yield and yield stability under diverse environments.

Initial results from genomic selection programs, particularly where combined with improved phenotyping techniques, also show great promise. The potential benefits of combining a range of new technologies constitute a valuable international public good.

New initiatives

Launched in 2012, the Heat and Drought Wheat Improvement Consortium (HeDWIC) facilitates global coordination of wheat research to adapt to a future with more severe weather extremes, specifically heat and drought. It delivers new technologies — especially novel wheat lines  to wheat breeders worldwide via the International Wheat Improvement Network (IWIN), coordinated for more than half a century by CIMMYT.

HeDWIC is supported by the Foundation for Food and Agriculture Research (FFAR) and is part of the Alliance for Wheat Adaption to Heat and Drought (AHEAD), an international umbrella organization set up by the Wheat Initiative to bring the wheat research community together and to exchange new germplasm, technologies and ideas for enhancing tolerance to heat and drought.