GLOBAL IMPORTANCE OF WINTER WHEAT
Wheat is grown across a wide range of environments and is considered to have the broadest adaptation of all cereal crop species (Briggle and Curtis, 1987). This is, to a large extent, due to its tolerance to cold. Winter and facultative wheats are grown on 75 million ha of the 220 million ha devoted to wheat worldwide. The most winter-hardy wheat cultivars are required for areas in the northern Great Plains of North America, the Russian Federation and Ukraine. Other areas requiring wheat cultivars with high levels of winter hardiness are in eastern, central and northern Europe, eastern Turkey, northwest Iran and China.
Grafius (1981) stated that "little progress has been made in breeding for increased tolerance to low temperature stress since the introduction of the winter wheat variety Minhardi at the beginning of this century". However, this statement refers to the absolute minimum temperature at which wheat plants can survive. Most of the winter wheat-growing areas in the world do not require wheat varieties with such a high level of winter hardiness. Consequently, the main breeding objective in many winter wheat breeding programmes is to maintain, rather than increase, the winter hardiness level present in commercial cultivars. This was also the clear outcome of a survey of 76 winter wheat breeders addressing a total of 67 million ha. According to this survey, winterkill damages wheat crops in farmers fields in more than one out of ten years in the United States, Ukraine and Russian Federation; whereas in West Asia and North Africa, Central Asia and most of Europe, winterkill does not frequently cause damage (Braun et al., 1998). Thus, the wheat varieties developed by the programmes responding to the survey are mostly winter hardiness sufficient for the areas they target (Table 12.1), with only three out of the 76 programmes having reported winter hardiness as the most important breeding trait in their programme. Similarly, Fowler et al. (1993) concluded that plant breeders have successfully maintained the cold hardiness levels of cultivars for their target areas.
Unlike spring wheat, most winter wheat is grown under rainfed conditions. The approximate area in different moisture regimes is given in Table 12.2. Rajaram et al. (1995) have defined 12 wheat mega-environments (ME) to characterize global wheat-growing areas, of which MEs 7 to 9 describe three facultative wheat areas and MEs 10 to 12 describe three winter wheat areas (Table 12.3).
The term winter and spring wheats is often used in a confusing way. Originally, the term referred to the sowing time. A winter wheat was sown before winter and a spring wheat was sown in spring. Confusion arose in countries with mild winters, such as Australia, parts of China, India, West Asia and North Africa, Southern Africa and South America, where spring wheats grow best when sown in autumn, and farmers often refer to these spring wheats as winter wheats. In this chapter, the term winter and spring wheat is used as defined by Crofts (1989), that is based on the presence or absence, respectively, of genes controlling the vernalization requirement (Vrn). There exists no clear definition for what are called facultative or alternative wheats. According to Stelmakh (1998), they are usually characterized by strong photosensitivity and partial sensitivity to vernalization.Facultative wheats have, compared to true winter wheats, in general less cold tolerance, a shorter but distinct period required for vernalization, start growth in spring earlier and flower earlier. They are mainly grown in areas with milder winters, areas with late fall rains or when late sowing is required due to tight crop rotations. The importance of rapid spring growth is one reason why many widely grown wheat cultivars in winter wheat areas in developing countries are facultative, though with adequate winter hardiness for the target areas and often derived from spring x winter wheat crosses (Braun, 1997).
TABLE 12.1
Survey responses received from winter wheat breeders by region and frequency
within a ten-year period in which winterkill caused losses in farmers
winter wheat fields
Region |
Responses |
Countries |
Frequency out of 10 years |
||
2 or less |
3-5 |
6-10 |
|||
Central Asia |
9 |
6 |
5 |
3 |
1 |
West Asia and North Africa |
11 |
6 |
8 |
2 |
1 |
Russian Fed. and Ukraine |
4 |
2 |
1 |
2 |
1 |
East Asia |
5 |
2 |
2 |
1 |
2 |
Eastern and Central Europe |
12 |
9 |
9 |
3 |
0 |
Western Europe |
17 |
8 |
14 |
2 |
1 |
North America |
14 |
2 |
5 |
8 |
1 |
South America |
3 |
3 |
3 |
0 |
0 |
South Africa |
1 |
1 |
1 |
0 |
0 |
Total |
76 |
39 |
48 |
21 |
7 |
Source: Braun, 1997.
TABLE 12.2
Approximate area of winter and facultative wheats grown in different moisture
regimes
Moisture regime |
Area |
<400 mm annual rainfall |
15 |
400-650 mm annual rainfall |
29 |
>650 mm annual rainfall |
16 |
Irrigated |
15 |
Total |
75 |
Source: Braun et al., 1998.
STRESS FACTORS INVOLVED IN WINTERKILL
Reasons for winterkill in wheat, as well as the extent of the damage, vary greatly from region to region and from year to year. The main factors responsible for winterkill, occurring alone or in combination, are described below.
Damage can be due to low temperature, per se:
extreme air or soil temperatures below the critical temperature of the respective wheat cultivar;
inadequate hardening to cold temperatures due to late emergence in autumn or sudden decrease of temperature;
long periods of cold-induced desiccation (Gusta et al., 1997a);
prolonged periods of low sub-zero temperatures. In particular, temperatures below -15°C during mid-winter result in rapid loss of winter hardiness (Gusta et al., 1997b);
alternate freezing and thawing, which results in increased injury from ice crystal growth with each freeze (Olien, 1967).
Damage can be due to snow or ice causing ice encasement, which can be a major cause of plant death in areas of high rainfall and fluctuating temperatures during winter (Andrews et al., 1974). Ice has high thermal conductivity, so it can aggravate the effect of low temperatures, but having low gas permeability, it can, in extreme cases, cause smothering or suffocation of plants deprived of oxygen (Poltarev et al., 1992).
Indirect damage can be caused by low temperatures or snow:
Frost heaving can occur, a displacement of plants, accompanied by root damage, due to the formation of ice in the soil, which pushes the plants upward, breaking and exposing the roots.
Snow mould, a group of fungi that cause damage to overwintering grasses in areas with long-lasting snow cover, can occur. The most damaging fungus involved in winter survival is the pink snow mould Microdochium nivale (Ces. ex Berl. & Vogl.) Samuels & Hallett, previously known as Fusarium nivale (Fr.) Ces. (Hömmö, 1994). Microdochium nivale is tolerant to low temperature, and severe plant damage occurs in the temperature range between 0°C and 5°C, but the pathogen cannot survive freezing. Other, less important fungi causing snow mould are Typhula spp., the pathogen for speckled snow mould or Typhula blight, and Sclerotinia borealis, causing Sclerotinia snow mould.
The winter survival rate of a wheat cultivar is not only dependent on its ability to survive low temperatures, but also on how it is affected by various environmental and agronomic factors (Fowler and Gusta, 1978). Recently, zinc (Zn) deficiency was identified as a major constraint for winter wheat production on the Central Anatolian Plateau (CAP) in Turkey (Cakmak et al., 1996). Data (Yildirim, 1996, unpublished) from three locations on the CAP indicate that autumn application of Zn fertilizer to Bezostaya increased the number of plants/m2 by more than 10 percent.
Each of the above-mentioned winter stress factors can cause winterkill, but the basic process behind most events leading to win-terkill is freezing or the formation of ice in plant tissues (Figure 12.1). Freezing damage is in general not a consequence of the low temperature per se, but rather the result of cellular dehydration due to extra cellular ice crystallization. Cellular membranes have been recognized as the primary sites of freezing injury (Hincha and Schmitt, 1994).
Freezing tolerance is defined as the ability of plants to survive the formation of ice in tissues without significant damage to membranes or other cell components. For the sake of clarification, it should be noted that intracellular ice formation is always lethal. The chemical potential in the intracellular solution must be equal to the chemical potential of the external solution or the ice. This equilibrium is attained through removal of intracellular water. To avoid cellular dehydration under freezing stress, the osmotic potential of intracellular solutions is increased, respectively, the osmotic potential of extracellular solutions is decreased. Blum (1988) provides a detailed discussion of the physiological processes during freezing stress.
Freezing tolerance is the result of physiological, chemical and physical reactions and modifications of plant cell structure, which take place at appropriate developmental stages and under suitable environmental conditions. This process is called hardening or acclimation. Acclimation proceeds in two stages, dependent on the sequential action of chilling (more than 0°C) and freezing (-3° to -5°C) temperatures. A decrease of water potential in tissues, due to decreased osmotic potential (because of sugar accumulation in vacuoles), is the most important feature in the first stage of plant acclimation. It is correlated with a significant increase of the hormone abscisic acid (ABA) level and results in a modification of the protein synthesis. There are large differences among cereals regarding the above 0°C temperature at which acclimation is initiated. Winter rye starts at much warmer temperatures, prolonging the acclimation period compared to winter wheat. Spring wheat and spring barley do not initiate acclimation at temperatures above 2°C (Gusta et al., 1997a). In the frost-dependent stage of acclimation, reversible modifications of membrane properties, which result in a further decrease of water potential in parenchymatic tissue, seem to play a main role (Kacperska, 1994). Stresses other than low temperature (water stress, wind, etc.) can also induce a certain level of freezing tolerance.
TABLE 12.3
Area and major biotic stresses for facultative and winter wheat mega-environments
located in less developed in less developed countries
MEa |
Climate |
Region/Country |
Area ('000 ha) |
Total (96) |
Total ME ('000 ha) |
Major biotic stressesb |
|
Facultative wheat areas |
|||||||
7A |
Optimum environment; fully irrigated |
China |
6 000 |
18 |
8 950 |
YR, LR, PM |
|
7B |
Medium cold; supplementary irrigation |
Turkey, Iran |
600 |
2 |
|
YR, Bunt |
|
|
|
Central Asian Republics |
2 350 |
7 |
|
LR, Bunt, YR, LS |
|
8A |
More than 600 mm rainfall; medium cold; photosensitive |
South Chile |
150 |
0 |
2 000 |
YR, LR, Sept., PM, Fus., R-rot |
|
8B |
More than 600 mm rainfall; medium cold; photoinsensitive |
Transitional zones of Turkey |
800 |
2 |
|
YR, Bunt |
|
|
|
Thrace, Turkey |
500 |
2 |
|
LR, R-rot, PM, Bunt |
|
|
|
Central Asian Republics |
550 |
2 |
|
YR, LR, R-rot, PM, Bunt |
|
9A |
Medium cold; less than 400 mm rainfall; heat stress at grainfill; mainly non-semidwarf varieties cultivated |
West Asia, North Africa |
4 500 |
14 |
6 810 |
YR, Bunt, LR |
|
|
|
Central Asian Republics |
540 |
2 |
|
YR, Bunt, LR |
|
9B |
Medium cold; less than 400 mm rainfall; mainly semidwarf varieties cultivated |
China |
970 |
3 |
|
LR |
|
9C |
Less cold tolerance required than in 9a and 9b; mainly semidwarf varieties cultivated |
South America, South Africa |
800 |
2 |
|
LR, Russian wheat aphid in South Africa |
|
Winter wheat area |
|
|
|
|
|
||
10 A |
Optimum environment; fully irrigated |
China |
4 600 |
14 |
6 200 |
YR, LR, PM, BYD |
|
10 B |
Supplementary irrigation |
Turkey, Iran |
1 100 |
3 |
|
YR, Bunt, R-rot, Nematodes |
|
|
|
West and NW Iran |
300 |
1 |
|
YR, Bunt |
|
|
|
Central Asian Republics |
200 |
1 |
|
LR, Bunt, Smut, PM |
|
11AC |
More than 600 mm rainfall; photosensitive |
None in less developed countries |
|
|
90 |
YR, LR, PM, Sept., Fus., BYD, R-rot |
|
11B |
More than 600 mm rainfall; photoinsensitive |
Dem. People's Rep. of Korea |
90 |
0 |
|
LR, Fus., Sept., R-rot, BYD, YR |
|
12A |
300-450mm rainfall; cold; heat stress at grainfill; mainly non-semidwarf varieties cultivated |
Turkey, Iran, Afghanistan |
4 700 |
14 |
8 900 |
YR, Bunt, R-rot, Zinc deficiency, Nematodes |
|
|
|
Central Asian Republics |
1 100 |
3 |
|
YR, Bunt, LR |
|
|
Mainly semidwarf varieties cultivated |
China |
700 |
2 |
|
YR, PM |
|
12B |
300-450 mm rainfall; severe cold; medium heat stress at grainfill; mainly non-semidwarf varieties cultivated |
Turkey, Iran |
850 |
3 |
|
YR, Bunt |
|
|
Mainly semidwarf varietie cultivated |
China |
1 550 |
5 |
|
YR, Bunt |
|
|
Total |
|
32 950 |
100 |
|
|
aME = mega-environment.
bYR = yellow rust (stripe rust); LR = leaf rust; PM = powdery mildew; Bunt = common bunt; LS = loose smut; Sept. = Septoria spp.; Fus. = Fusarium spp.; R-rot = Root rots; BYD = barley yellow dwarf.
cThe area devoted to winter wheat in Eastern Europe, the Ukraine and the Russian Federation is an estimated 24 million ha.
Most of this winter wheat area is classified as ME 11 A and 11 B.
FIGURE 12.1 Diagram of the processes involved in winterkill and hardiness
SOURCE: Sãulescu and Braun, 1998
Freezing tolerance is not a static condition, but changes with time, temperature, soil and plant moisture, nutrition and physiological age and status. It depends largely on the cold acclimation or hardening processes. Differences in freezing tolerance of unhardened plants of different cultivars are negligible, while considerable differences can be detected after full hardening. The hardening process can be stopped, reversed or restarted. Generally, under natural conditions, the dynamics of freezing tolerance can be characterized by three stages (Prašil et al., 1994):
the hardening period, when cold tolerance increases in the autumn;
the period of tolerance maintenance, when the critical or lethal temperature varies depending on the fluctuation of temperature in the winter;
the period of dehardening, when plants lose their cold tolerance (end of winter).
GENETIC CONTROL AND TRAITS ASSOCIATED WITH FREEZING TOLERANCE
Freezing tolerance is the result of complex physiological mechanisms, which involve many cell and plant traits. The genetic control is complex, and as many as 15 out of 21 chromosomes in hexaploid wheat were reported to influence plant tolerance to low temperatures (Stushnoff et al., 1984). Major frost resistance genes are located on group 5 chromosomes and chromosomes 4B, 4D and 7A (Worland et al., 1987). Chromosome group 5 and chromosome 7A are also known to control vernalization. In particular, the gene Fr1, closely linked but separable from the gene Vrn1 on chromosome 5A, and gene Fr2, less tightly linked to gene Vrn3 on chromosome 5D, were shown to have a large effect on low temperature tolerance (Snape et al., 1997).
Freezing tolerance was also found to be associated with prostrate growth type. A gene controlling prostrate growth was found to be closely linked with Fr1 and Vrn1 on chromosome 5A (Roberts, 1990). Prostrate growth type can also be found in cultivars with low vernalization requirements but high photo-periodic response. These wheat genotypes are usually only moderately winter-hardy, but in barley some of the most cold-tolerant cultivars are known as day-length sensitive, with low vernalization requirement. Many other traits have been reported to be associated with cold tolerance, but none of the correlations is high enough to allow replacing the direct freezing tests (Sãulescu and Braun, 2001).
BREEDING APPROACHES
Handling a complex trait such as winter hardiness in a breeding programme is a difficult task because of the large number of genes involved and numerous interactions with the environment. However, the main difficulty in breeding cold-tolerant wheat is the general association of high freezing tolerance with lower yields and later maturity.
Many traits associated with freezing tolerance, such as delayed spring growth or small cellular size, are associated with negative effects on yield, especially in rainfed environments where rapid growth in early spring and earliness are important adaptive characteristics to avoid late drought and high temperatures. Therefore, the breeding objective should not be to maximize winter hardiness, but to develop cultivars with the minimum level of winter hardiness necessary for a given target environment. As Fowler et al. (1981) pointed out, in general, successful winter wheat cultivars have only marginally greater winter hardiness than the minimum required for the area in which they are grown.
Obviously, a breeding strategy regarding winter hardiness depends on the ratio between the hardiness level available in the gene pool used in the breeding programme and the minimal level necessary for the target area. If most parents used in crosses have a winter hardiness level equal to or higher than the accepted minimum, maintaining this level is a relatively easy task that can be accomplished through mild selection pressure against the rare less hardy segregants. On the other hand, if a large number of parents are not sufficiently winter-hardy, as is the case in programmes that use spring x winter crosses, higher selection pressure is advisable from the early generations to increase chances of recovering an acceptable level of hardiness (Braun, 1997). Since Fr1 and Vrn1 are closely linked with prostrate growth type, selection of cold-tolerant genotypes based on their growth habit should allow overcoming some of the problems related with insufficient differentiation in mild winters. This is applicable in particular for wheat breeding programmes utilizing spring x winter crosses.
Breeding for winter hardiness is much more difficult in areas marginal for winter wheat growing, where the minimum required hardiness is at or above the maximum available cold hardiness potential. As Grafius (1981) pointed out, there "has been a notable absence of improvement in the maximum cold hardiness potential of cereals in this century", and this "inability of plant breeders to increase maximum cold tolerance levels suggests that all of the available cold tolerance genes had been previously concentrated in hardy land races within winter cereal species". Cereals differ greatly in their ability to survive low temperatures; the most cold-tolerant rye cultivars are killed at around -34°C, wheat cultivars at around -23°C and barley at around -18°C. Recovering the maximum level of hardiness in higher yielding genotypes is only possible by applying very high selection pressure in large segregating populations.
Durum wheat has generally much lower winter hardiness than bread wheat, so breeding for freezing tolerance is more difficult. However, for areas where winter durum is superior to spring durum, breeding for winter hardiness has to be a high priority. Best winter hardiness is found in cultivars derived from interspecific crosses with bread wheat, and probably such crosses, as well as transgressive segregation in intraspecific crosses, can allow further progress in this respect.
CONCLUDING REMARKS
Winter and facultative wheat cover around 30 percent of the global wheat area. Although little progress has been made in breeding for increased tolerance to low temperature compared to old landraces such as Minhardi, the lack of sufficient winter hardiness remains a problem mainly in areas with very severe winters, such as the Great Plains of North America, the Russian Federation and Ukraine. For the majority of winter wheat breeding programmes, the main breeding objective is to maintain, rather than increase, the winter hardiness level present in commercial cultivars. This target is often reached through routine field screening. With the identification of genes controlling frost resistance and the development of markers, it is likely that some of the problems related to field testing and/or controlled environment screening will be overcome. However, field testing will remain for some time to come the final measure for the winter hardiness of a wheat cultivar.
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