Edited by: Jean-Marcel Ribaut, Generation Challenge Programme, Mexico
Reviewed by: Uener Kolukisaoglu, University of Tuebingen, Germany; Larry Butler, Generation Challenge Program, Mexico
*Correspondence: Philippe Monneveux, Research Management Officer, International Potato Center, Apartado 1558, Lima 12, Peru. e-mail:
This article was submitted to Frontiers in Plant Physiology, a specialty of Frontiers in Physiology.
This is an open-access article distributed under the terms of the
Wheat (
Wheat (
World wheat production increased at a rate of 3.3 percent per year between 1949 and 1978. Increases at the start of this period were due to both an expansion of production area and increased yields. However, starting in the 1960s, yield increases came mainly from the use of improved varieties and a greatly expanded use of irrigation, pesticides, and fertilizers. The rate of increase in world wheat production slowed to 1.5 percent per year between 1982 and 1991, one exception being China, which maintained a rate of increase in production of 2.6 percent per year and became the world's largest wheat producer. Also, wheat production increased at nearly 3 percent per year in India and Pakistan during the same period.
Today, world wheat production is 626 million tons [Food and Agriculture Organization of the United Nations (FAO),
Wheat is believed to have been domesticated in southwestern Asia (Gupta,
Today, bread or common wheat (
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Einkorn | C | Mountainous areas (France, Morocco, the former Yugoslavia, Turkey) |
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W | ||
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W | ||
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Poulard | C | Mediterranean countries |
Subsp. |
C | ||
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Emmer | C | Yemen, India, Morocco, Spain, Albania, Turkey, Italy |
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Durum | C | |
Subsp. |
W | ||
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Polish | C | Mediterranean countries |
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W | ||
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W | ||
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Timopheevi | C | Georgia |
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W | ||
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Bread | C | |
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Compact | C | Alpine countries and Southern Europe |
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C | Caucasus area | |
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Spelt | C | Northern and Central Europe |
Subsp. |
Club | C | India |
Being adapted to a wide range of moisture conditions from xerophytic to littoral, wheat is grown on more land area worldwide than any other crop. About three-quarters of the land area where wheat is grown receives between 375 and 875 mm of annual precipitation, but wheat can be grown in locations where precipitation ranges from 250 to 1750 mm. The optimum growing temperature is about 25°C, with minimum and maximum growth temperatures of 3–4°C and 30–32°C, respectively (Briggle and Curtis,
During the past 50 years, most of the yield progress in wheat has been due to the gradual replacement of traditional tall cultivars by dwarf and fertilizer-responsive varieties (Donmez et al.,
Since the HI in most modern cultivars seems to be close to its biological maximum, i.e., 60 percent, further genetic gain in yield potential is expected to come from biomass increases (Shearman et al.,
This requires, however, a good knowledge of the genetic and genomic resources available.
Wheat belongs to the
Genetic resources have been categorized by Frankel (
To address the need for general access to genetic maps, the International Triticeae Mapping Initiative (ITMI) was launched in 1989, to ensure that such maps would be available as a public good (Gustafson et al.,
There has been a significant increase in the productivity of wheat due to the application of Green Revolution technology. This has resulted in a doubling and tripling of wheat production in many environments, most notably in irrigated areas. In these locations, the high-yielding semi-dwarf statured wheat cultivars continuously replaced the older tall types at a rate of 2 m ha year−1 in the 1980s (Byerlee and Moya,
To examine the challenges facing wheat breeders more closely, Singh and Byerlee (
In recent years, breeders have been more successful in increasing the adaptation of wheat to dry environments. In developing countries, farmers have traditionally grown landrace cultivars that are well adapted to serious moisture stress conditions. However, these traditional cultivars generally give a poor yield in “good years” when rainfall is more plentiful. Modern cultivars now yield the same as the traditional cultivars in dry years as well as showing a better response to more favorable conditions of moisture and nutrient supply (Osmanzai et al.,
Further progress in developing drought tolerant germplasm depends on the efficiency of breeding and phenotyping methodologies. Accurate drought phenotyping implies precise definition of the target environment, choice and characterization of the testing environment, and water stress management and characterization.
Breeding work for drought-prone environments has been largely empirical to date, with grain yield being the primary trait for selection in wheat breeding programmes. However, most breeders select strongly for traits other than yield in the early segregating generations and do yield testing only at later stages, when a certain level of homozygosity has been achieved and large enough seed quantities are available. The decision to advance or reject a genotype is often complex and, in practical terms, breeders most often use a system of multiple cut-offs. In early generations, they select genotypes that, presumably, achieve the levels required for the primary traits evaluated in segregating populations (plant type, plant height, growth cycle, spike fertility, etc.).
When a breeding programme for drought adaptation is assisted by analytical selection, the conceptual model used considers yield under drought to be a function of: (1) yield potential; (2) flowering date (which indicates whether the crop will avoid drought stress); and (3) secondary traits that provide drought resistance. Physiological secondary traits can be used for the selection of parents to be included in the crossing block, as direct selection criteria for screening among a large number of genotypes (i.e., segregating populations) and/or when the amount of seed available is too small to carry out field trials with replications. Whereas intensive work is continuously being carried out by physiologists in the area of drought adaptation, few breeders routinely use physiological criteria in their mainstream breeding programmes. In the first place, the evaluation of some of the traits proposed by plant physiologists is time-consuming or expensive. This is not practical for application to the thousands of entries that comprise the segregating generations of breeding programmes. Then, the real value of a given trait may only be assessed by determining the genetic gain in segregating populations following selection, while many traits are not available in well adapted genotypes and their validation frequently requires the development of appropriate breeding material, which is again costly and time-consuming (Royo et al.,
Gene-based markers generated from gene sequence data, i.e., “perfect markers” can be used to screen large numbers of entries for a particular trait improving the efficiency and effectiveness of conventional breeding. Gene-based markers are particularly useful for introgressing genes whose expression is highly affected by the environment, such as genes for useful physiological traits that cannot easily be screened (e.g., root architecture traits), as well as for gene pyramiding. The most common situations in which marker-assisted selection (MAS) confers an advantage are: (1) when accurate measurement of the phenotype is expensive or difficult; (2) when multiple genes conferring a similar phenotype are being combined; and (3) when there is a need for rapid removal of donor chromosome segments in a backcrossing programme (Nelson et al.,
QTL estimation often spans several centimorgans, and hundreds of genes underlie a region of this size. The size of such a region can be reduced through a number of approaches, such as the use of high resolution crosses, or the development of near-isogenic lines (NILs) for small chromosomal segments across the putative QTL region (Nelson et al.,
For MAS to be useful, proper phenotyping is required and the evaluation of yield and relevant physiological traits should be done in conditions similar those of the target environment. An ecophysiological understanding of the traits in question and of how to measure them is crucial (Araus et al.,
Rainfall distribution patterns and evaporative demand over the crop cycle vary considerably among locations and years. The different sets of climatic conditions under which wheat is cultivated are characterized by breeders as “wheat mega-environments” (wheat MEs). ME delineation is based on water availability, soil type, temperature regime, production system, and associated biotic and abiotic stresses. Consumer preferences for grain color and industrial and end-use quality are also considered. CIMMYT has defined 12 MEs (Table
ME1 IR | IR | Temperate | Spring | 36.1 | 83 |
ME2 HR | HR (>500 mm) | Temperate | Spring | 8.5 | 25 |
ME3 AS | HR (>500 mm); AS | Temperate | Spring | 1.9 | 3 |
ME4 SA | LR (<500 mm) | Temperate/hot | Spring | 14.6 | 20 |
ME5 TE | IR, HR | Hot | Spring | 7.1 | 12 |
ME6 HL | SA | Temperate | Spring | 6.2 | 13 |
ME7 IR | IR | Cool | Facultative | − | − |
ME8 HR | HR | Cool | Facultative | 10.0 | 23 |
ME9 SA | SA | Cool | Facultative | − | − |
ME10 IR | IR | Cold | Winter | − | − |
ME11 HR | HR | Cold | Winter | 15.0 | 30 |
ME12 SA | SA | Cold | Winter | − | − |
The choice of the selection environment directly determines the potential genetic gains in the target environment. Ideally, the selection environment should mimic the target environment in all aspects: water distribution, profiles and potential evapotranspiration rates, and physical and chemical soil properties. Deviations may result in significant GEI between target and selection environments, and genetic gains achieved in the selection environment may not be expressed in the target environment. Geographic information system (GIS) tools can help considerably in describing the relationships between target and selection environments and establishing “homology maps.”
The crop facing water deficit simultaneously experiences a number of additional stress factors (e.g., micronutrient deficiency, soil compaction, salinity, nematodes, and fungal pathogens) that exacerbate drought stress. Such factors are hard to control and are generally not considered in field experiments. Hence, efforts should be made to remove all other constraints except drought. Soil surveys may allow the identification of selection sites or fields that avoid confounding factors. In some cases, these surveys may enable sites to be chosen where the selection pressure for these stress factors would permit the selection of genotypes targeted for regions where these stresses interact with drought. They could also identify the within-site distribution of e.g., nematodes (Nicol and Ortiz-Monasterio,
Target environments can also be mimicked if water is controlled by imposing a water regime by gravity or, better, by drip irrigation. Water stress management (timing, intensity, uniformity) and characterization (soil, plant measurements) are essential issues in drought phenotyping.
Moisture availability can itself be a complicating factor when comparing genotypes in field experiments. Although plots growing the different genotypes may receive the same quantity of water, the genotypes can vary in their water use and/or access to underground water, thereby confounding measurements associated with plant water relations. Study of water profiles (either experimentally or by using simulation models) can provide very useful information. Trait evaluation should preferably be carried out under field conditions, avoiding experimental situations (growth chambers, greenhouses, pots) that differ significantly from the agricultural growing environment. The ability to access water deep in the soil profile, which is an important drought-adaptive mechanism, is eliminated as a variable in pot conditions. Furthermore, the relative humidity of the air, which has an important influence on stomatal conductance (Ben Haj Salah and Tardieu,
When possible, drought tolerance evaluation should be done out-of-season, under irrigated conditions. This option allows better management of water stress but needs a dry season sufficiently long to cover the whole growth cycle. The photoperiod and temperature should not differ too much from the growing season, as is the case in the dry tropics, to avoid genotype-by-season interactions and allow results obtained from the out-of-season experiments to be extrapolated to the growing season conditions.
In the case of drought, some traits proposed by stress physiologists appear to be associated with crop survival. For example, comparison of old and new varieties has shown that, under drought, older varieties over-produce tillers many of which fail to set grain, while modern drought tolerant lines produce fewer tillers the majority of which survive (Loss and Siddique,
If the pattern of water deficit is predictable in a given region, selection for a flowering date that does not coincide with the period of water deficit is a very effective way of improving drought adaptation (Araus et al.,
Most of the traits currently mentioned in the literature associated with drought adaptation in wheat are shown in Table
Large seed size | Emergence, early ground cover, and initial biomass | Mian and Nafziger, |
+++ | ME4A |
Long coleoptiles | Emergence from deep sowing | Radford, |
+++ | ME4C |
Early ground cover (visual) | Decrease of evaporation and increase of radiation-use efficiency (RUE) | Hafid et al., |
+++ | ME4A |
Specific leaf dry weight | Thinner, wider leaves, early ground cover | Merah et al., |
++ | ME4A |
Growth habit (visual) | Lower soil evaporation and higher RUE | Richards et al., |
+++ | ME4A |
Tiller survival | Survival and recovery | Loss and Siddique, |
++ | Severe stress |
Long and thick stem internodes | Storage of carbon products | Loss and Siddique, |
+++ | ME4A |
Vegetation indices (normalized difference vegetation index; NDVI) | Green biomass | Royo et al., |
+ | |
Earliness | Drought escape | Blum, |
+++ | ME4A and ME4C |
Number of grain per spike around | Spike sterility | Hafsi et al., |
++ | Drought flowering |
Stomatal conductance | Extraction of water from soil | Farquhar and Sharkey, |
+ | |
Canopy temperature depression | Stomatal conductance, extraction of water from soil | Reynolds et al., |
++ | |
Carbon isotope discrimination | Stomatal conductance, extraction of water from soil | Monneveux et al., |
++ | |
Ash content | Stomatal conductance, extraction of water from soil | Misra et al., |
++ | |
Spike photosynthetic capacity | Grain filling | Evans et al., |
+ | ME4A, hot |
Leaf color (visual, SPAD) | Delayed senescence, maintenance of photosynthesis | Araus et al., |
+++ | |
Leaf waxiness | Lower transpiration rate and reduced photo-inhibition | Richards, |
+++ | Severe stress |
Leaf pubescence | Lower transpiration rate and reduced photo-inhibition | Richards, |
+++ | Severe stress |
Leaf thickness and posture | Lower transpiration rate and reduced photo-inhibition | Reynolds et al., |
+++ | Severe stress |
Leaf rolling | Lower transpiration rate and reduced photo-inhibition | Reynolds et al., |
+++ | Severe stress |
Glume pubescence | Lower transpiration rate and reduced photo-inhibition | Trethowan et al., |
+++ | |
Delayed senescence | Higher RUE | Hafsi et al., |
++ | |
Fructanes in stem | Storage of carbon products | Rawson and Evans, |
++ | ME4A |
Solute concentration in cells | Osmotic adjustment (OA) | Morgan and Condon, |
+ | |
Accumulation of ABA | Reduced stomatal conductance and cell division | Innes et al., |
+ | Severe stress |
Wheat faces different drought scenarios worldwide; consequently, the physiological traits that confer drought resistance in specific environments may be very distinct. The combination of yield data with data relating to secondary traits in multi-site field experiments ranging from well-watered to high stress levels may be useful at this stage by providing some light on GEI of traits related to drought tolerance. This is particularly the case when the heritability of the secondary traits is higher than that of yield, and the genetic correlation of these traits with yield in the target environment is high. Secondary traits can be classified according to their relationship to pre-anthesis growth, access to water, water-use efficiency (WUE), and photoprotection.
So far, studies have only been accomplished in recombinant inbred lines (RILs). CTD showed a significant association with yield under drought when measured pre-anthesis, suggesting an advantage from higher pre-anthesis growth rates. CTD also showed some association with final yield when measured during grain filling. Because a major role of transpiration is leaf cooling, canopy temperature, and its reduction relative to ambient air temperature are an indication of how much transpiration cools the leaves under a demanding environmental load. Higher transpiration means colder leaves and higher stomatal conductance, both aspects favoring net photosynthesis and crop duration. A relatively lower canopy temperature in drought-stressed crops indicates a relatively greater capacity for taking up soil moisture or for maintaining a better plant water status. Thus, higher transpiration is a positive trait when selecting for higher yield potential or better adaptation to moderate drought stress.
Measurement of carbon isotope discrimination or ash content of grain or other tissues can be used to estimate the WUE of the crop, since their signals are based on the integration of plant water status over a period of time (Condon et al.,
Spikes have higher WUE than leaves, and have been shown to contribute up to 40 percent of total carbon fixation under moisture stress (Evans et al.,
Genes that affect a greater relative partitioning of assimilates to the sink, resulting in a higher HI, would be expected to improve yield under drought, not being associated with the water cost of generating additional biomass. Plant height is usually negatively related with HI. However, there is a minimum height below which limitation on yield becomes evident (Slafer et al.,
To check for delayed senescence of leaves, particularly flag leaves, portable chlorophyll meters such as the Minolta SPAD
Decreased stomatal conductance in response to drought leads to warmer leaf temperatures and insufficient CO2 to dissipate incident radiation, both of which increase the accumulation of harmful oxygen radicals and photo-inhibitory damage. Photo-inhibition can be modified by some leaf adaptive traits such as waxiness, pubescence, rolling, thickness, or posture (Richards,
The effects of photo-inhibition can be alleviated by antioxidants such as superoxide dismutase (SOD) and ascorbate peroxidise, which have been shown to increase in quantity in response to drought stress (Mittler and Zilinskas,
While many traits have been studied for their use in breeding for drought resistance, there is a general consensus among breeders that only a few of them can be recommended for use in practical breeding programmes at this time (Table
Many drought-adaptive traits have been investigated in wheat. However, association of these traits with genetic gains for yield under drought has been poorly tested and documented. Most difficulties encountered in the identification of accurate drought tolerance traits are due to the fact that wheat is cultivated under very different climatic conditions and faces very different drought scenarios worldwide.
While some single traits have benefited from tremendous research efforts and have generated considerable debate in the literature (e.g., OA, ABA), relatively little emphasis has been placed on research that can be extrapolated and used directly to crop genetic improvement in target environments.
Most drought physiology research in wheat has been conducted in controlled environments and has been poorly integrated into breeding programmes. Multidisciplinary approaches involving physiologist, breeders, genebank managers, and biotechnologists are still scarce, holding back the exploitation of genetic diversity and the use of MAS for drought tolerance improvement.
Despite the tremendous potential offered by access to genetic resources from related species, and well-documented success in using them (e.g., 1B/1R translocation, synthetic wheats), wild
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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