1.1. Introduction: Wheat - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/3267/11/11_chapter 1.pdf · Introduction and Review of literature 3 1.1. Introduction: Wheat 1.1.1

Introduction and Review of literature

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1.1. Introduction: Wheat

1.1.1. Economic importance and taxonomy of the wheat

Wheat (Triticum aestivum L.) is the most important cereal crop for the

majority of world’s populations. It is the most important staple food of about two

billion people (36% of the world population). Worldwide, wheat provides nearly 55%

of the carbohydrates and 20% of the food calories consumed globally (Breiman and

Graur, 1995). Wheat is cultivated over a wide range of climatic conditions and

therefore understanding of genetics is of great value for genetics and plant breeding

purposes.

Wheat belongs to family Poaceae (Gramineae) which includes major crop

plants such as wheat (Triticum spp. L.), barley (Hordeum vulgare L.), oat (Avena

sativa L.), rye (Secale cereale L.), maize (Zea mays L.) and rice (Oryza sativa L.).

Triticeae is one of the tribes containing more than 15 genera and 300 species

including wheat and barley.

Linnaeus in 1753 first classified wheat. In 1918, Sakamura reported the

chromosome number sets (genomes) for each commonly recognized type. He

separated wheat into three groups viz. diploids (2n=14), tetraploids (2n=28) and

hexaploids (2n=42) chromosomes.

1.1.2. Wheat cultivation in India

Wheat cultivation in India started 5000 years ago (Feldman, 2001). Today, India

ranks second in wheat production with a harvest of 80.4 million ton during the season

08-09 (Fig 1.3).Its cultivation area is 28 M Ha.

Breeding programs are traditionally empirical, that is selection is generally based on

yield only which has limitations under stress environment. To meet the increasing

demand of wheat production without increasing area, there is need to incorporate new

physiological tools. These tools will help for the improvement of breeding programme

under abiotic stress environment. If specific physiological trait associated with yield

could be identified under stress environment, selection efficiency could be increased.

These traits will contribute to more objective screening of yield for selection in early

generations, when grain yield may not be properly assessed.

On the basis of agro climatic diversity wheat cultivation in India has been divided in

to six mega zones (fig 1.1). Maharashtra and Karnataka comes under Peninsular Zone.


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Wheat is grown in all the states in India except Southern and North Eastern states.

Uttar Pradesh, Haryana, Punjab, Rajasthan are the major wheat producing states and

accounts for almost 80% of total production in India. Only 13% area is rainfed. Major

Rainfed wheat areas are in Madhya Pradesh, Gujarat, Maharashtra, West Bengal and

Karnataka. Central and Peninsular Zone accounts for total 1/3rd of wheat area in India.

All India basis only 1/3 irrigated wheat receives desired irrigations and remaining is

limited irrigation only. Breeding programmes are generally aimed for rainfed and

irrigated environments and there is need to develop varieties which are responsive to

limited irrigation conditions.

Thus to increase the productivity of this region different physiological techniques

need to be adopted , for improving water use efficiency and breeding wheat genotypes

tolerant to water stress and heat.

1.2 Water stress Water stress is of common and wide occurrence in nature. It occurs whenever water

absorption by the crop is lower than the evaporative demand of the atmosphere. There

are two major processes involved in that i) water absorption by the crop which is

controlled by root characteristics and soil properties. ii) Crop evapotranspiration (ET)

which depends on atmospheric properties like net radiation, vapour pressure deficit

(VPD) and crop characteristics. Wheat may experience water stress in any

environment. CIMMYT has defined 12 mega environments (ME) as irrigated region,

high rainfall areas, acid soils, semi arid zones tropical areas and winter wheat zones

(Rajaram et al., 1995). ME definition is based on water availability, soil type,

temperature regime, production system and associated biotic and abiotic stress.

Taking into account all these factors India comes under ME4C.In this mega

environment wheat crop suffers from continuous or subcontinent type of drought

which is associated with stored moisture after monsoon rain that is rainfed condition.


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Fig 1.1: Different Wheat growing zones in India


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Fig 1.2: Monthly temperature variation during crop season

Fig 1.3: Area, production and productivity of wheat in India.

Source: Project Directors report DWR Karnal


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1.3. Drought tolerance 1.3.1. Drought

World food production is limited primarily by environmental stresses. It is

very difficult to find ‘stress free’ areas where crops may approach their potential

yield. Abiotic environmental factors are considered to be the main source (71%) of

yield reductions (Boyer, 1982). Drought is one of the most common environmental

stresses that affects growth and development of plants through alterations in

metabolism and gene expression (Leopold, 1990). It is a permanent constraint to

agricultural production in many developing countries (Ceccarelli and Grando, 1996).

Wheat production suffers from variability in yield from year to year and from

location to location. One of the main environmental abiotic stress responsible for

yield instability is drought stress, which may occur early in the season or terminally at

grain filling and grain development stages. Improvement of productivity of wheat

cultivars under drought conditions is one of the important breeding objectives in

wheat.

1.3.2. Mechanism of drought tolerance

Drought tolerance in wild plant species is often defined as survival, but in crop

species it is defined in terms of productivity (Passioura, 1983). Rosielle and Hamblin

(1981) defined drought tolerance as the difference in yield between stress and non-

stress environments, while productivity is the average yield in stress and non-stress. A

different definition regards drought tolerance as minimization of reduction in yield

caused by stress compared to yield under non-stress environments (Fischer and

Maurer, 1978;; Blum, 1983a; Blum, 1988). Also, it is defined as the relative yield of a

genotype compared to other genotypes subjected to the same drought stress (Hall,

1993). Drought tolerance comprises drought escape, dehydration avoidance and

dehydration tolerance mechanisms (Blum, 1988).

1.3.2.1. Drought escape

Drought escape through early flowering and/or short growth duration is advantageous

in environments with terminal drought stress and where physical or chemical barriers

inhibited root growth (Turner, 1986; Blum, 1988; Blum et al., 1989). On the other

hand, late flowering can be beneficial in escaping early-season drought, if drought is

followed by rains (Ludlow and Muchow, 1990). Under non-stress conditions, late-


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flowering varieties tend to yield higher than early-flowering varieties (Turner, 1986;

Ludlow and Muchow, 1990). This is because the early-flowering varieties are likely

to leave the yield potential unutilized (Muchow and Sinclair, 1986).

1.3.2.2. Dehydration avoidance

Dehydration avoidance can be defined as the plant’s ability to retain a relatively

higher level of ‘hydration’ under conditions of soil or atmospheric water stress (Blum,

1988). Levitt (1980) identified two plant types with respect to dehydration avoidance

i.e. ‘water savers’ and ‘water spenders’. Important features of these plants are root

characteristics (increased water uptake), leaf and stomatal characteristics (reduced

water loss) and osmotic adjustment to lower the osmotic potential (Blum, 1988;

Acevedo and Fereres, 1993).

1.3.2.3. Dehydration tolerance

Dehydration tolerance describes the ability of plants to continue metabolizing at low

leaf water potential and to maintain growth despite of dehydration of the tissue.

According to Hsiao (1973) and Boyer (1976), translocation is one of the more

dehydration tolerant processes in plants. It would proceed at levels of water deficit

sufficient to inhibit photosynthesis. When water stress occurs and the current

photosynthetic source is inhibited, the role of stem reserves as a source for grain

filling increases. Stem reserves may therefore be considered as a powerful resource

for grain filling in stress-affected plants during the grain filling stage.

1.3.3. A conceptual model for drought tolerance

The conceptual model was described by Reynolds et al., (2000). This model includes

following traits:

Large seed size

o Helps in emergence, early ground cover and initial biomass.

Long coleoptiles

o Helps for emergence from deep sowing (Radford, 1987). This enables

to help seedlings to reach the receding moisture profile and to avoid

high soil surface temperatures which inhibit germination.


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Early ground cover

o Thinner, wider leaves (i.e. with a relatively low specific leaf weight)

and a more prostrate growth habit help to increase ground cover, thus

conserving soil moisture and potentially increasing radiation use

efficiency (Richards, 1996).

High pre-anthesis biomass

o Up to 40% of available water may be lost by evaporation directly from

the soil surface in Mediterranean types of environments (Loss and

Siddique, 1994), so early ground cover and biomass production may be

useful to permit a more efficient use of soil water.

Good capacity for stem reserves and remobilization

o Stored fructans can contribute substantially to grain filling especially

when canopy photosynthesis is inhibited by drought (Rawson and

Evans, 1971). Traits that may contribute include long and thick stem

internodes, with extra storage tissue perhaps in the form of solid stems.

High spike photosynthetic capacity

o Spikes have higher WUE than leaves and have been shown to

contribute up to 40% of total carbon fixation under moisture stress

(Evans et al., 1972). Awns contribute substantially to spike

photosynthesis and longer awns are a possible selection criterion.

High RLWC/CTD during grain filling to indicate ability to extract water

o A root system that can extract whatever water is available in the soil

profile is clearly drought adaptive (Hurd, 1968), but that is difficult to

measure. Traits affected by the water relations of the plant, such as

relative leaf water content (RLWC) , canopy temperature depression

(CTD) during the day and C13 discrimination or ash content of grain or

other tissues, can give indications of water extraction patterns.

Osmotic adjustment

o Osmotic adjustment will help to maintain leaf metabolism and root

growth at relatively low leaf water potentials by maintaining turgor

pressure in the cells (Morgan and Condon, 1986).


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o

Accumulation of abscisic acid (ABA)

o It appears that plants can be pre adapted to stress by reducing stomatal

conductance, rates of cell division, organ size and increasing

development rate. The benefit of ABA accumulation under drought has

been demonstrated (Innes et al., 1984).However, high ABA can also

result in sterility problems since high ABA levels may abort

developing florets.

Leaf anatomy: waxiness, pubescence, rolling and thickness

o These traits decrease radiation load to the leaf surface (Richards,

1996). Benefits include a lower evapotranspiration rate and reduced

risk of irreversible photo-inhibition.

High tiller survival

o Comparison of old and new varieties have shown that under drought

older varieties over-produce tillers many of which fail to set grain

while modern drought tolerant lines produce fewer tillers most of

which survive (Loss and Siddique, 1994).

Heat tolerance

o The contribution of heat tolerance to performance under moisture

stress needs to be quantified, but it is relatively easy to screen

(Reynolds et al., 1998).

Stay green

o The trait may indicate the presence of drought avoidance mechanisms,

but probably does not contribute to yield .If there is no water left in the

soil profile by the end of the cycle to support leaf gas exchange, it may

be detrimental if it indicates lack of ability to remobilize stem reserves

(Blum, 1998). However, research in sorghum has indicated that stay-

green is associated with higher leaf chlorophyll content at all stages of

development and both were associated with improved yield and

transpiration efficiency under drought (Borrell et al., 2000).


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1.4 Identification of the physiological traits for use as a selection criterion The success of a physiological approach to improve yield under drought will

depend on the effective identification of the limiting trait and then on its genetic

complexity. The identity proposed by Passioura (1977) provides the most valuable

framework. Passioura proposed that grain yield in water- limited environments

is the product of three factors, viz:

Grain yield = crop water use x water-use efficiency x harvest index.

Improving any one of these should improve grain yield provided that the

components are largely independent on each other. This identity should be

considered in relation to the target environment.

The traits to be considered as candidates for yield selection must be

genetically correlated with yield. The trait should have greater heritability

than yield and less subject to genotype x environment interaction. Different

approaches can be used to identify potential traits for selection .One of the

approach is to identify the main physiological process involved in yield

determination and the plant attributes influencing them.

Yield can be divided into several integrative components. Yield itself is the

most integrative trait, because it is influenced by all factors that determine

productivity. However, there are many limitations in empirical breeding

approach based only on yield. Therefore, any breeding strategy based on a

physiological approach should use screening tools to evaluate the integrative

physiological parameters that determine harvestable yield. Although harvest

index has been the most successful trait when modified to improve yield.

Genetic advances in grain yield under rainfed conditions have been achieved

by empirical breeding methods and selection is generally based on yield.

There is urgent need to develop varieties responsive to limited irrigation. To

meet future challenges of increasing productivity under these situations

breeding strategies should incorporate the new physiological traits that can

increase water use efficiency. If specific physiological trait related to yield

could be identified, selection efficiency could be increased and may contribute

to meaningful screening for yield in early generations where grain yield may not be

properly assessed.


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Several morpho-physiological traits are traditionally used as screening criteria for

drought tolerance such as Relative Water content (RWC), stomatal traits, water use

efficiency (WUE) etc. Crop WUE can be increased by optimizing crop transpiration.

Under field conditions variation of environmental factors makes it difficult to reveal

genetic variation. Hence several workers have proposed carbon isotope discrimination

(CID), ash content and canopy temperature depression (CTD) as criteria for

transpiration efficiency. These may be used as surrogate measures for grain yield. The

proposed work therefore aims to assess the efficacy of these traits in breeding wheat

under water stress conditions.

1.5 Carbon Isotope Discrimination Carbon isotope discrimination is the ratio of 13C/12C. In C3 cereals like wheat, ∆ is

positively correlated to CO2 levels in intercellular air spaces (fig 1.4) (Farquhar et

al., 1982; Farquhar and Richards, 1984; Ehdaie et al., 1991) and, negatively related to

water use efficiency, (Farquhar and Richards, 1984; Hubick and Farquhar, 1989).

Plants with high WUE would be less able to discriminate against 13C, and thus

would accumulate more of the heavy carbon isotope in their tissues than less efficient

water users.

Carbon isotope discrimination provides an indication of WUE throughout plant

growth when measured in plant dry matter (Farquhar et al., 1982, 1989). Carbon

isotope discrimination has been proposed as a possible screening tool for identifying

variations in WUE in wheat (Farquhar and Richards, 1984; Ehdaie et al., 1991;

Condon and Richards, 1993) and barley (Hubick and Farquhar, 1989). The

relationship between WUE and ∆ indicate that ∆ may be useful for modifying the

WUE and yield under water stress condition. (Condon et al., 1987; Condon and

Richards, 1992, 1993).


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Fig 1.4: Carbon isotope discrimination under irrigated and dry conditions

STOMATA OPEN (irrigated conditions): Discrimination in favour of 12 co2fixation at high internal co2 concentration

STOMATA closed (moisture stress): Discrimination less favorable to 12Co2fixation as internal low co2 concentration falls


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1.5.1 Sample selection criterion

Considerable genotypic variations for carbon isotope discrimination have been

found in bread wheat (Condon and Richards, 1992), barley (Romagosa and Araus,

1991; Acevedo, 1993), and durum wheat (Araus et al., 1993a), but environmental

factors may cause even larger changes in the value of ∆, which could compromise

the effective

use of ∆ in breeding programs. Experiments carried out by Condon and Richards

(1992) concluded that assessing genotypic variation in ∆ would be most effective

under well-watered conditions. In this regard, Richards and Condon (1993) pointed

out that under WW conditions, ∆ is highly heritable and exhibits substantial

genetic variation and few genotypes x environment interactions.

In rainfed environments, Condon and Richards (1992) proposed sampling for ∆ at

early crop stages. However, the information available on rainfed environments does

not support the above hypothesis. When dry material from seedlings is analyzed

correlation between ∆ and yield is weak (Bort et al., 1998), although it increases

when upper plant parts are used in ∆ analysis. The best genetic correlations between

∆ and yield, together with the high broad sense heritability of ∆, have also been

reported for the upper parts of durum wheat (Araus et al., 1998b).

The effects of progressive stress after anthesis on yield revealed that the

correlation between yield and ∆ increases with plant age. On the contrary, ∆ usually

decreases from the oldest to the youngest plant parts, under well-watered

conditions (Hubick ‘and Farquhar, 1989; Acevedo, 1993). This decrease may lead to

stomatal closure in response to declining soil water or increasing vapor-pressure

deficit during the later stage of crop growth (Condon and Richards, 1992; Condon et

al., 1992; Araus et al., 1993b). Thus, mature kernels could be the most adequate plant

part to sample.

1.5.2 Carbon isotope discrimination (High or low)

In water-limited environments, genotypes with low ∆ should have greater

biomass and hence potential for higher yield, assuming that all genotypes use the

same amount of water for transpiration (Richards, 1996). Under same conditions,

selection for high WUE (Passioura, 1977) or low ∆ (Craufurd et al., 1991) has

been proposed as an important alternative in breeding strategies. However, under


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well-irrigated or rainfed conditions ∆ values often correlate positively with grain

yield and/or total biomass in wheat (Condon et al., 1987; Kirda et al., 1992; Araus

et al., 1993c, 1997b; Morgan et al., 1993; Sayre et al., 1995) and barley

(Romagosa and Araus, 1991; Richards, 1996).

From an agronomic point of view, a positive relationship between ∆ and

yield may exist if plants are not using all available soil water. Assuming the same

phenology, a genotype with high ∆ will be able to sustain a high level of

transpiration. Therefore, ∆ can be considered as an indicator of WUE, but also

depends on the water transpired by the crop. The positive association between ∆

and yield suggests that variations in stomatal conductance are predominant than

intrinsic photosynthetic capacity in determining ∆ (Romagosa and Araus, 1991;

Condon et al., 1992). High ∆ is related to a high level of CO2 in the cellular air

spaces due to greater stomatal conductance (Farquhar and Richards, 1984). This

leads to higher photosynthetic rates and, higher yield even in the absence of stress.

In this situation, decrease in WUE decreases (and ∆ increases) reduces

transpiration more than photosynthesis, even when yield may be positively

affected by low stomatal limitation.

Alternatively, lower stomatal conductance, may limit yield potential because of

the intercellular levels of CO2, resulting decrease in photosynthesis. These

genotypes will consistently show low ∆ values (Morgan et al., 1993). Stomata

that close only in response to severe water stress may be more useful in terms of

yield than stomata that permanently show low conductance values (Jones, 1987).

Selection for low ∆ i.e., high WUE may favor low-yielding genotypes under

drought conditions. Therefore, low ∆ may not be a good selection criterion for

improving yield in dry environments. Grain yield under drought conditions

depends not only on WUE but largely on the genotype’s capacity to sustain

transpiration (Blum, 1993).

1.5.3 Optimal yield conditions

Carbon isotope discrimination has been proposed as a useful trait to select

for yield potential (Araus et al., 1993c; Sayre et al., 1995; Araus, 1996). A positive

relationship between ∆ and growth has also been reported for seedlings grown

under adequate water conditions (Febrero et al., 1992; Lopez Castaneda et al.,

1995). On the other hand, increased early growth and leaf area development may


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be closely linked to decreased WUE (Turner, 1993) and thus resulted in higher ∆.

Blum (1996) reported that carbon isotope discrimination and yield support a

consistent positive relationship between crop yield and photosynthetic capacity for

wheat and other crops. He also reported that higher crop productivity brings an

increase in stomatal conductance, with related increase in ∆ (or a decrease in crop

WUE).

Carbon isotope discrimination (∆) has been proposed by several workers as a

predictive selection criterion for grain yield under drought (Araus et al., 2001;

Merah et al., 2001b; Condon et al., 2002; Monneveux et al., 2005;Misra et al.,

2006; Xu et al., 2007, Monneveux et al., 2007, Cabera –Bosquet et al.,2009). In C3

species, ∆ positively correlates to Ci/Ca, the ratio of internal CO2 leaf

concentration to ambient CO2 concentration (Farquhar et al., 1989).It also

provides an integrated measurement of long term Ci/Ca ratio and transpiration

efficiency (Condon et al., 1990). Carbon isotope discrimination in grain was found

to positively correlate to yield in the conditions of South Australia (Condon et al.,

1987), North West Mexico (Sayre et al., 1995; Monneveux et al., 2005), Spain

(Araus et al., 1998) and South of France (Merah et al., 2001c). All these conditions

correspond to Mediterranean-type environments, characterized by limited

irrigation water stress and moderate heat during grain filling (Rajaram et al., 1995).

Under limited irrigation significant correlation was found between grain ∆ and

yield in the conditions of North West Mexico (Monneveux et al., 2004). He also

reported significant correlation between grain ∆ and yield at pre-anthesis and

residual moisture water stress (Monneveux et al., 2005). However, the magnitude

of the correlation was lower than under post-anthesis water stress. Under severe

post-anthesis water stress, significant correlations were found between grain yield

and ∆ in flag leaf (Merah et al., 1999) and awns (Merah et al., 2001c). Under

residual moisture stress, negative correlation was found between grain yield and ∆

in young seedlings by Condon and Hall (1997).


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1.5.4 Role of phenology in genotypic differences in carbon isotope discrimination

In the absence of stress, ∆ in wheat is independent of phenological

differences (Sayre et al., 1995). In Mediterranean environments phenology is the

most important factor that accounts for increasing wheat yield, as it affects

assimilate partitioning, the pattern of water use. (Slafer et al., 1993; Loss and

Siddique, 1994). In addition, some of the genotypic differences in ∆, as well as

their positive association with yield, can be due to phenology. Thus, early

flowering lines are more likely to have high ∆ than late-flowering lines due to the

lower transpirative demand, which maintains higher stomatal conductance

(Ehdaie et al., 1991; Acevedo, 1993). In, short early flowering in wheat and other

cereals is related to higher yield, which is in accordance with higher ∆ in the early

genotypes.

1.6 ∆ Surrogates The cost and technical skills involved in carbon isotope analysis is very high

therefore, different surrogates such as mineral accumulation in vegetative plant

parts, leaf structure, ash content and canopy temperature depression (CTD) have

been proposed. These selection criteria allow the evaluation of traits other than

WUE that determines yield. Regarding the mineral accumulation, if passive

transport driven by transpiration is the mechanism of mineral accumulation in

vegetative parts, then mineral content will also be an indicator of the total water

transpired. The second trait i.e. leaf structure corresponds to structural criteria that

indicate the amount of photosynthetic tissue per unit leaf area and is therefore

related to photosynthetic capacity. The third trait canopy temperature depression is

a good indicator of a genotype's physiological fitness, since a high value is

indicative of good expression of all of those traits under a given set of

environmental conditions. When water evaporates from the surface of a leaf it

becomes cooler, and the rate of evaporative cooling is directly affected by stomatal

conductance (besides VPD), which is itself affected by feedback mechanisms of

other processes such as photosynthetic metabolism and vascular transport.


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1.7 Ash content Ash content accumulated in vegetative tissues have been proposed as surrogates

of ∆ in cereals, forage crops, and soybean (Walker and Lance, 1991; Masle et al.,

1992; Mayland et al., 1993; Mian et al., 1996). Masle et al., (1992) reported for

all the herbaceous C3 species they assayed a positive linear relationship between

total mineral content of vegetative tissues and the inverse relationship with WUE

or ∆. Therefore, the amount of minerals accumulated by plants in the field could

be a potentially useful indicator of ∆ and WUE (Walker and Lance, 1991; Masle

et al., 1992; Mayland et al., 1993). In theory, total mineral and ash content seem

to be better surrogates than the content of any single mineral, such as silicon or

potassium (Masle et al., 1992; Mayland et al., 1993). Therefore, estimating plant

mineral content, especially ash content, which requires a muffle furnace, might

be a good alternative to ∆ for preliminary screening of large, genetically diverse

populations (Masle et al., 1992; Araus et al., 1998b).

1.7. 1 Determination of ash content

Ash content was analyzed by using the Method 08-01 of the American Association

of Cereal Chemists, (1995). Ash content is expressed (on a concentration basis) as

a percentage of sample dry weight as follows:

[(Burnt crucible mass – empty crucible mass)

% Ash = ………………………………………………… x100

[(Filled crucible mass-empty crucible mass)]

1.7.2 Choice of environment and type of sample

The positive correlation between ash content and ∆ may indicate that plants

which are able to maintain higher stomatal conductance and transpiration will

accumulate more ash in a transpirative organ. Mineral accumulation seems to be

better related to ∆ and yield under well-watered conditions (Masle et al., 1992;

Mayland et al., 1993), as well as under drought conditions (Araus et al., 1998b).

Ash accumulated in the flag leaf may be positively related to the ∆ of kernels

(Araus et al., 1998b). Leaves must be mature to allow minerals accumulate, but

not senescent because minerals can remobilize to other plant parts. Ash content

measured at maturity in straw did not correlate with either ∆ of mature grains or

yield (Voltas et al., 1998). This may be explained by the fact that ash content on a


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kernel mass basis may be an indirect indicator of total reproductive sink per culm

attained at maturity (Araus et al., 1998b). Under Mediterranean conditions kernel

ash has been proposed as complementary to kernel ∆ in assessing genotypic

differences in cereal yield (Febrero et al., 1994; Voltas et al., 1998). In theory the

pattern of ash accumulation in kernels is different from that in vegetative tissues

because, unlike mineral accumulation in vegetative tissues, grain filling does not

take place via the xylem (driven by transpiration) (Slafer et al., 1993). Such

differences in mineral accumulation could explain the interrelationship of ash

content and ∆ kernels as necessary traits predicting grain yield (Febrero et al.,

1994; Voltas et al., 1998).

In short, kernel ash combined with either kernel ∆ or leaf ash can be independent

trait when assessing differences in grain yield (Febrero et al., 1994; Araus et al.,

1998b; Voltas et al., 1998). Thus selecting for low ash content in kernels,

combined with high ∆ in kernels or, high ash content in the flag leaf, may be

useful in wheat breeding programme. This approach, if coupled with a new

analytical technique such as near infrared reflectance spectroscopy (NIRS) would

allow a fast, reliable estimation of ash content and ∆ in intact kernels (Araus,

1996).

Correlation was found between ∆, leaf ma and grain yield in different C3

species by Masle et al., (1992), Mayland (1993) and Merah et al., (1999, 2001a).

Negative correlation was reported between ma in grain and grain yield by

Febrero et al., (1994), Voltas et al., (1998) and Merah (1999, 2001a), Misra et al.,

(2006), Xu et al., 2007 and L.Cabera-Bosquet et al., (2009) in Zea Mays.

1.8 Canopy temperature depression (CTD) Canopy temperature depression data was recorded at different stages of crop

growth for unirrigated crop using a portable infrared thermometer (Model AG-42,

Telatemp Corporation, Fullerton, CA).Reynolds et al., 2001b showed a clear

association of CTD with yield in both warm and temperate environments. Work

has been carried out to evaluate its potential as an indirect selection criterion for

genetic gains in yield. While CTD is affected by many physiological factors

making it a powerful integrative trait, its use may be limited by its sensitivity to

environmental factors.


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Table 1.1: Factors affecting canopy temperature depression (CTD) in plants

Sr.No. Biological factor Environormental factor

1 Partitioning of assimilates Radiation

2 Evapotranspiration Wind

3 Soil water availability Clouds

4 Carbon metabolism Temperature

5 Vascular transport Moisture

1.8.1 Factors affecting expression of CTD

Leaf temperatures are depressed below air temperature when water evaporates from

their surface. It is one of the factors determining evapotranspiration which is directly

related to stomatal conductance. This process is regulated by carbon fixation and

vascular transport of water. Canopy temperature depression is a good indicator of a

genotype's physiological fitness. Its high value will be indicative of good expression for

all of those traits under a given set of environmental conditions. CTD is also affected by

the ability of a genotype to partition assimilates to yield.CTD is a function of number

of environmental factors like soil water status, air temperature, relative humidity and

incident radiation. The trait is best expressed at high vapour pressure deficit (Amani et

al., 1996), low relative humidity and warm air temperature.CTD will not be useful

trait for cool and humid conditions and it is quite sensitive to environmental changes.

Therefore, it is important to measure the trait when it is best expressed that is on

warm and relatively cloudless day.

The trait can be measured in a few seconds with an infrared thermometer,

which measures the surface temperature of a field plot. Since the reading integrates the

temperatures of plant leaves and spikes over a small area of the canopy, error associated

with plant to plant variability is reduced. CTD measured on irrigated yield trials showed

a good association with plot performance, but in addition a good predictor of yield.

1.8.2 Combining selection for CTD and leaf conductance

Canopy temperature depression and leaf conductance show an association with each

other and with yield (Amani et al., 1996). Combining selection for both the traits is a

good criterion. At CIMMYT, work is in progress to evaluate the application of CTD

for yield potential. The objective is to evaluate families of F3 for higher expression of


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CTD. From these, individual plants were assessed for leaf conductance using a

viscous flow porometer (Thermoline & CSIRO, Australia). This instrument can give a

relative measure of stomatal conductance in a few seconds (Rebetzke et al., 1996),

permitting the possibility of identifying physiologically superior genotypes within

bulks that have already been selected for CTD and other important selection criteria.

Thus CTD can be used as an efficient tool for screening in early generation which can

be further used for evaluating advanced lines. Experiments carried out by Reynolds et

al., (1997, 1998a) suggest that CTD can also be used for selections in smaller plots.

Fig 1.5 Potential use of CTD in a breeding programme

1.9 Water Use Efficiency (WUE) Carbon isotope discrimination (∆) has also been proposed by several workers for

estimating water use efficiency (WUE). WUE of a crop has been defined, as the ratio

of total biomass, or above-ground biomass, or harvested yield, against total available

water, or evapotranspiration, or plant transpiration (Jones, 2004). According to

Gregory et al. (1997), the ratio of total biomass produced against total available water

may be expressed as:

WUEB = M / (Es + T + R + D)


22

Where M, Es, T, R and D are the biomass produced, the evaporation from the soil

surface, the transpiration during the growing season, the runoff, and the drainage

below the root zone, respectively.

Under drought conditions, carbon isotope discrimination (∆) was shown to be

negatively correlated to transpiration efficiency (Farquhar et al., 1982).It is also

negatively related to M/T (the ratio of biomass produced to water consumed, at the

plant level) and WUE (biomass or grain) under water stress conditions. A significant

negative correlation has been obtained in wheat between ∆ and M/T (Condon et al.,

1990; Ehdaie et al., 1991). In rice, Impa et al., (2005) also found a negative

association between leaf ∆ and WUE. Kirda et al. (1992), by using water balance

model (Hatfield, 1990) to evaluate evapotranspiration and by measuring soil water

with a neutron probe, reported a highly significant negative association between ∆ and

WUE in durum wheat grown in Austria. However, for bread wheat no significant

correlation was observed between ∆ and WUE in Australia (Condon et al., 1993).

WUE and the other traits can be used as an indirect selection for yield particularly in

drought prone environments (Monneveux et al., 2005; Misra et al., 2006; Xu et

al., 2007).

1.10 Molecular mapping in plants Biotechnology brings new and powerful tools to plant breeders, to meet

the increasing demand of food production. One method receiving growing

attention is the mapping of Chromosomal regions affecting qualitative or

quantitative traits. Polygenic characters, which were very difficult to analyse using

traditional plant breeding methods, can now be tagged using DNA molecular

markers. Molecular markers allow geneticists and plant breeders to locate and

follow the numerous interacting genes that determine a complex trait. Genetic

linkage maps can provide a more direct method for selecting desirable genes via

their linkage to easily detectable molecular markers (Tanksley et al., 1989).

Combining marker-assisted selection methods with conventional breeding schemes

can increase the overall selection gain and, therefore, the efficiency of breeding

program. With the use of molecular techniques it is possible to hasten the transfer of

desirable genes between varieties and to introgress novel genes from wild species

into crop plants. The plant breeder would like to exercise indirect marker aided

selection (MAS) at the seedling stage in early generations, if possible.


23

Availability of tightly linked molecular markers for a trait will facilitate such an

indirect selection and help plant breeding by saving time and expense.

1.10.1 Molecular marker technologies for genetic mapping

The development of molecular marker technologies during the last ten

years has revolutionized the genetic analysis of crop plants. A significant progress

has been made towards the use of molecular approaches in plant breeding. From

the time of Gregor Mendel until the mid-eighties, morphological characters had been

the major types of markers readily available for genetic mapping. Molecular marker

technology has changed dramatically during the past two decades. The first molecular

markers were isozyme markers, which based on the different mobility of differently

charged protein with the same enzymatic function on the gel. Enzyme markers have

limited genome coverage and numbers. The term molecular marker is taken here to

refer to markers identifying variation at the level of DNA, though biochemical

markers such as isozyme have made a valuable contribution to the development of

genetic maps in the late seventies and eighties for example, of tomato (Tanksley and

Rick, 1980) and maize (Edwards et al., 1987).

The molecular markers may be used for improving the efficiency of

traditional plant breeding by facilitating indirect selection through molecular

markers linked to genes for the traits of interest. These markers are not

influenced by the environment and can be scored at all stages of plant growth.

This saves time, resources and energy that are needed for raising large

segregating populations for several generations. In addition to these applications,

DNA markers can also be used for germplasm characterization, genetic

diagnostics, study of genetic diversity, study of genome organization, etc.

(Rafalaski et al., 1996). Molecular markers have already been used not only for the

preparation of molecular maps but also for tagging genes, controlling traits of

interest, for use in marker assisted selection (MAS). In plants, using markers,

several genetic maps were initially constructed in tomato (Bernatzky and

Tanksley, 1986). Subsequently, maps were constructed in different crops such as

rice (McCouch et al., 1988, Kishimoto et al., 1989); maize (Burr et al., 1988;

Beavis and Grant, 1991; Burr and Burr, 1991), barley (Heun et al., 1991; Graner et

al., 1991; Hinze et al. 1991) or wheat (Chao et al., 1989; Liu and Tsunewaki, 1991;

Liu et al., 1992; Devos et al., 1992; Devos and Gale, 1993; Röder et al., 1998).


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1.11 Progress in wheat molecular genetics

1.11.1 Use of molecular markers for mapping and gene identification

Progress in gene identification and marker development has been slow in wheat due

to its hexaploid nature and the large size of its genome. However, in the recent past,

a significant number of genes involved in various functions have been mapped to

specific wheat chromosomal regions. Characterizing genes that control flowering in

wheat has benefited from chromosome manipulations involving aneuploidy as well

as molecular markers.

Using intervarietal chromosome substitution lines and single-chromosome

recombinant line populations, genes controlling vernalization response Vrn1 and

Vrn3 have been located on the long arms of chromosomes 5A and 5D, respectively

(Law et al., 1976), and VrnB1 on chromosome 5B (Zhuang, 1989). Similar

procedures have been utilized to identify genes controlling photoperiod response

(Ppd genes) (Worland and Law, 1986). Plant height, important for determining

adaptation and yield in wheat, is genetically complex; so far about 21 genes have

been identified to be associated with this trait (McIntosh et al., 1995). A

microsatellite marker has recently been developed that is linked to Rht8 (Korzun et

al., 1998).

Efforts have been made to genetically dissect complex physiological traits

associated with drought tolerance such as accumulation of abscisic acid in rice and

to investigate possible relationship between rice and wheat homologous loci

controlling abscisic acid accumulation (Quarrie et al., 1997). In wheat, using single-

chromosome recombinant lines, Quarrie et al. (1994) located a genetic factor

controlling drought-induced abscisic acid production on the long arm of

chromosome 5A. Molecular genetic tools have also been used to study complex

traits such as carbohydrate metabolism and the association between abscisic acid

concentration and stomatal conductance (Prioul et al., 1997). Comparative RFLP

mapping in cultivated and wild wheat (Triticum dicoccoides) has led to the

identification of molecular markers associated with resistance to the herbicide

chlorotoluron which is a selective phenylurea herbicide (Krugman et al., 1997). A

list of wheat genes that control various physiological and agronomic parameters that

have been identified with the use of molecular markers is presented in table 1.2.


25

The existence of numerous sets of wheat near-isogenic lines (NILs) differing in the

presence/absence of a resistance allele for various biotic stress factors (diseases and

pests) has facilitated the mapping of genes for which such lines exist. Large number

of genes conferring disease or pest resistance have been identified and associated

with molecular markers (reviewed in Hoisington et al., 1999). When the

chromosomal location of a particular gene is known from previous genetic studies

but no NILs are available, markers mapped to that chromosome (Anderson et al.,

1992) can still be used to score parental lines for polymorphisms, construct a single-

chromosome map, and determine which markers are close to the gene of interest.

This strategy was followed by Dubcovsky et al. (1996) to tag the Kna1 locus in

wheat, which is responsible for higher K+/Na+ accumulation in leaves, a trait

correlated with higher salt tolerance.

In wheat, bulked segregate analysis, initially used mostly with RAPDs, can now be

used with any type of marker including AFLPs (Goodwin et al., 1998; Hartl et al.,

1998), which have the advantage that a high number of DNA fragments can be

amplified with one primer combination. Also, with AFLPs the problem of highly

repetitive DNA is overcome by using methylation sensitive endonucleases such as

PstI and SseI.Many genes that have been tagged with molecular markers in wheat

have been introgressed from alien species (Hoisington et al., 1999). In the case of

translocations from wheat’s wild relatives known to carry genes for agronomically

important traits, markers can be successfully established due to the high level of

polymorphisms between the wheat and introgressed genome and the low level of

recombination between the translocated segment and the corresponding wheat

chromosomes.

1.12 Mapping populations Mapping is putting markers (and genes or QTL) in order, indicating the

relative distances among them and assigning them to their linkage groups on the

basis of their recombination values from all pair wise combinations. Knowledge

about the genetic concepts of segregation and recombination is essential to the

understanding of mapping. The construction of a linkage map is a process that

follows the segregation of molecular markers in a segregating population and put

them in linear order based on pair wise recombination frequencies. Thus, a mapping

population with high number of polymorphisms over the total genome is highly


26

desirable. Towards this end, various ways have been used to create mapping

populations. Populations used for mapping are usually derived from F1 hybrids

between two lines (either homozygous or heterozygous), which show allelic

differences for selected probes. Genetic maps of plants are constructed based on

several different kinds of populations (Paterson, 2002), with each population structure

having unique strengths and weaknesses. Four types of population are commonly

used for map construction and mapping experiment. These are F2 population, back

cross population (BC), double haploid (DH) population, and recombinant inbred

lines (RILs). Most genetic mapping populations in plants have been derived from

crosses between largely homozygous parents.

1.13 Mapping QTLs in wheat Utilizing a base map and linkage data from a range of other segregating wheat, rye,

and barley populations, a consensus map with more than 1000 data points has been

developed (Gale et al., 1995). This detailed linkage map has confirmed that the order

of genetic loci across the A, B, and D genomes has been conserved (Gale et al., 1995).

A RIL mapping population developed utilizing ‘Opata 85’ and a synthetic hexaploid

from CIMMYT has been used extensively in mapping and genome relationship studies

(Van Deynze et al., 1995; Nelson et al., 1995a, b, c). The genetic map of this

population, developed by the International Triticeae Mapping Initiative (ITMI), contains

over 1000 RFLP loci. Two other published maps are available in wheat (Liu and

Tsunewaki, 1991; Cadalen et al., 1997). Linkage maps in wheat have confirmed

evolutionary chromosomal translocation rearrangements involving chromosomes 2B,

4A, 5A, 6B, and 7B, which were based on cytological evidence, and have established

synteny among closely related grass species such as rice, maize, oats, and wheat (Ahn

et al., 1993; Devos et al. 1994; Van Deynze et al., 1995; Borner et al., 1998).

The low number of quantitative traits dissected into their QTLs in wheat is a

reflection of the focus on simply inherited traits and the difficulty of building

comprehensive linkage maps. ITMI map is one of the densest and the population from

which it was developed is segregating for a number of traits. It has been used to map

important traits and several major genes. Known genes include vernalization (Vrn1 and

Vrn3), red coleoptile (Rc1), kernel hardness (Ha), and powdery mildew (Pm1 and Pm2)

genes (Nelson et al., 1995a), as well as genes conferring and suppressing leaf rust

resistance (Nelson et al., 1997). Quantitative trait loci have been identified for kernel


27

hardness (Sourdille et al., 1996), Karnal bunt (Nelson et al., 1998), and tan spot (Faris

et al., 1997).

A large number of studies have reported QTL for yield and yield components in

wheat in different environments including water-limiting stress environments. QTL

for grain yield, anthesis, plant height, grain weight, grain number have been reported

(e.g. Huang et al., 2006;Marza et al., 2006; Snape et al., 2007; Rebetzke et al., 2008;).

Three significant QTLs have been identified for CID in Barley by This D (2005). ,

two of them being antagonist on chromosome 4, and one other located on

chromosome 5. Arvindkumar et al., (2008) has identified the presence of significant

QTLs for ∆ on chromosomes 1B, 3A and 5A.

Research on developing molecular markers for traits associated with drought

tolerance in wheat started recently at CIMMYT. A RIL population is being utilized to

identify genomic regions associated with a range of physiological parameters

controlling drought tolerance.


28

Table 1.2 Genes identified and mapped with molecular markers for physiological and agronomic traits in wheat.

Traits Genes Species Chromosomes References

Physiological and agronomic

Preharvest sprouting QTL Triticum aestivum Anderson et al., 1993

Galiba et al., 1995; Korzun et al., 1997 Vrn1 5AS

Kato et al., 1998 Vernalization

Vrn3

5DS Nelson et al., 1995a

Photoperiod response Ppd1 T. aestivum 2DS Worland et al., 1997

Ppd2 T. aestivum 2BS Worland et al., 1997

Dwarfing Rht8 2DS Korzun et al.,1998

Rht12 5AL Korzun et al., 1997

Cadmium uptake Penner et al., 1995

Aluminum tolerance Alt2 4D 4DL

Luo and Dvorak, 1996; Riede and

Anderson, 1996

Drought induced ABA 5AL Quarrie et al., 1994

Na+/K+ discrimination Kna1 T. aestivum 4D 4DL Allen et al., 1995; Dubcovsky et al.,

1996

Water-soluble carbohydrate Wheat 1A, 1 D, 2D, 4A, 6B,

7B, 7D

Yang et al., 2007

Carbon isotope ratio, osmotic potential,

chlorophyll content, flag leaf rolling index

Wheat 2B, 4A, 5A, 7B Peleg et al., 2009


29

1.14 Genesis and structure of the thesis: Very few efforts have been made to understand the correlation between yield and

physiological parameters under water stress conditions in India. Based on the agro

climatic diversity, wheat cultivation in India has been divided in to six mega zones.

Maharashtra comes under Peninsular Zone. Wheat is grown in all zones except Southern

and North Eastern states. Uttar Pradesh, Haryana, Punjab, Rajasthan are the major wheat

producing states and accounts for almost 80% of total production in India. Central zone

and Peninsular zone accounts for 1/3rd of total wheat area in India. On all India basis only

1/3 irrigated wheat receives desired irrigations and remaining 2-3 irrigations (limited

irrigation). Breeding programmes are generally aimed for rainfed and irrigated

environments and there is need to develop varieties which are responsive to limited

irrigation condition. Breeding strategies in this region should aim for limited irrigation

requirement with high yield. Thus, to improve the productivity of this region the

technologies need to be adopted, to improve water use efficiency and breed for wheat

genotypes having higher tolerance to water stress and heat.

Several morpho physiological traits are traditionally used as screening criteria for drought

tolerance such as relative water content (RWC), stomatal traits, water use efficiency

(WUE) etc. Under field conditions variation of environmental factors makes it difficult to

reveal genetic variation. Several workers have proposed carbon isotope discrimination

(CID), ash content and canopy temperature depression (CTD) as criteria for transpiration

efficiency, Misra et al., (2006, 2010), Zhang et al., (2009), Blum et al., (2009). These are

the surrogate measure for grain yield. Present work is undertaken to identify suitable

physiological traits which can be useful in wheat breeding under water stress conditions,

parameters related to grain yield and water use efficiency and further identification of

QTLs for selected RILs. Therefore the objectives selected for present work are-

1.14.1 Objectives:

The main objective of the proposed work is to identify suitable

physiological traits which can be useful in wheat breeding under water stress

environment.


30

To analyze the relationship between carbon isotope discrimination,

ash content, grain yield and water use efficiency components in durum wheat.

Identification of QTLs for carbon isotope discrimination, ash content

and canopy temperature depression.

In order to achieve these objectives review of literature pertaining to carbon isotope

discrimination, ash content and canopy temperature depression and QTL analysis for

these traits was carried out. The work regarding these parameters has been organized in

subsequent chapters-

Chapter 1 Introduction and review of literature

Chapter 2 Utilization of carbon isotope discrimination and ash content to select

wheat under water stress condition.

Chapter 3 Relationship between carbon isotope discrimination, ash content, canopy

temperature depression and grain yield in wheat under water stress

condition.

Chapter 4 Analysis of the relationship between carbon isotope discrimination, ash

content,

grain yield and water use efficiency components in durum wheat.

Chapter 5 Identification of QTLs for carbon isotope discrimination, ash content and

canopy temperature depression.

Chapter 6 Summary and Conclusion

Bibliography

Curriculum vitae

Documents

1.1. Introduction: Wheat - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/3267/11/11_chapter 1.pdf · Introduction and Review of literature 3 1.1. Introduction: Wheat 1.1.1