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Drought tolerance in sorghum was estimated in
several ways: evaluation of grain yield under drought, the stability of yield, rate
and duration of grain fill, seed weight, stay green and associated traits.
Cultivars that tolerate pre-flowering water deficit are advantageous for the
reason that yield components such as stand, tiller number, the number of heads,
and especially the number of seed per head are determined during this period.
Green leaf area at maturity is used as an indicator of post-anthesis drought resistance
in sorghum breeding programs. For a successful selection of sorghum for drought
resistance, the presence of considerable magnitude of variability in the
available germplasm is a prerequisite. There is a positive correlation between
drought tolerance and root length in sorghum. Moreover, water stress at
seedling stage significantly affects the root to shoot ratio that seedling
under water stress cause an increase in root length with a reduced diameter. In
addition, numerous seedling traits have been suggested as important relative to
drought tolerance including root weight, lateral root number and root-to-shoot
ratios. Therefore, the root to shoot ratio and its characteristics can be an
indication of selection criteria for sorghum in respect of drought resistance.
Molecular markers linked to QTL for drought tolerance could be used in
increasing efficiency of breeding efforts to select sorghum germplasm with
enhanced drought tolerance once these markers are identified through carefully
monitored characterization of appropriate germplasm under stress conditions.
Therefore, in this study the scientific results were evaluated and summarized.
Keywords: Drought; Sorghum;
Drought resistance; Marker assisted selection; QTL
INTRODUCTION
A drought is actually a meteorological event
which implies the absence of rainfall for a period of time, long enough to
cause moisture-depletion in soil and water deficit with a decrease of water potential
in plant tissues [1]. But from an agricultural point of view, its working
definition would be the inadequacy of water availability, including
precipitation and soil-moisture storage capacity, in quantity and distribution
during the life cycle of a crop plant, which restricts the expression of the
full genetic potential of the plant [2]. It affects plant growth, survival and
productivity in the world [3, 4]. Its effect is more pronounced in the
semi-arid tropics (SAT), where rainfall is generally low and erratic in
distribution.
In
Eastern Africa, more than 70% of sorghum is cultivated in the dry and hot
lowlands where soil fertility low, poor stand establishment, in addition to
serious water deficit are among the major production constraints [7]. According
to Bramel-Cox et al. [8], Ethiopia is the centre of origin and diversity for
sorghum. In Ethiopia, sorghum is grown as one of the major food cereals. It is
utilized in various forms such as for making local bread (Injera) and for
preparation of local alcoholic beverages (tela and areke). It is also consumed
as roasted and boiled grain. Sorghum stover is used as feed for animals and as
housing and fencing material. Sorghum is annually cultivated at 1.3 million ha
contributing 1.7 million MT to annual grain production in the country [9]. It
is grown in the major agro-ecological zones including the lowland, mid and high
altitude areas of the country [10]. But it is predominantly cultivated in dry
areas that cover nearly 66% of the total area of the country [6]. Because of
the recurrent drought, other cereals rarely give harvestable yield in these
areas.
Biotechnological
Approach for Drought Resistance
Three breeding approaches for drought
resistance have evolved. The first is to breed for high yield under optimum
(water-stress-free) condition. As the maximum genetic potential of yield is
expected to be realized under the optimum condition and a high positive
correlation exists between performance in optimum and stress conditions [16], a
genotype superior under optimum level will also yield relatively well under
drought condition. This is the basic philosophy of this approach.
However, the concept of expression of maximum genetic potential in
optimum condition is debated [17] as genotype-environment interaction may
restrict the high yielding genotype to perform well under drought. Thus, the second approach, i.e., to breed under
actual drought condition has been suggested [18].
The second approach suffers from the problem that the intensity of
drought is highly variable from year to year and as a consequence,
environmental selection pressure on breeding materials changes drastically from
generation to generation. This problem compounded with low heritability of
yield makes for the complicated and slow breeding programme[19].
An alternative approach to the above two would be to improve drought
resistance in high-yielding genotypes through the incorporation of
morphological and physiological mechanisms of drought resistance. However,
transferring drought resistance in high-yielding genotypes is complicated due
to lack of understanding of the physiological and genetic basis of adaptation
in drought condition. In contrast, improving the yield potential of an already
resistant material may be a more promising approach, provided there is genetic
variation within such a material. Simultaneous selection in the non-stress
environment for yield and in a drought condition for stability may be done to
achieve the desired goal of evolving drought resistant genotype with high
yield.
As such, the breeding methodology to be applied for drought resistance
is the same as that applied for other purposes. In general, pedigree and bulk
method could be used for self-pollinated crops and recurrent selection for
cross-pollinated crops. However, if the transfer of few traits relating to
drought resistance to a high-yielding genotype is the aim, then backcross is
the appropriate methodology. On the other hand, biparental mating (half-sib and
full-sib) maintains the broad genetic base as well as provides the scope to
evolve the desired genotype of drought resistance [20]. The success of any
breeding programme depends on the availability of the screening technique,
especially for drought resistance.
Marker-assisted selection for drought resistance
In most breeding programmes, the genetic improvement for drought
resistance is accomplished through selection for yield and because of low
heritability of yield under stress and the spatial as well as temporal
variation in the field environment, conventional breeding approaches are slow.
Whereas molecular markers such as restriction fragment length polymorphism
(RFLP), random amplified polymorphic DNA (RAPD) and isozyme will facilitate
development of drought-resistant genotypes more effectively as their
expressions are independent of environmental effects. After identification of
the molecular markers associated with yield or other morphological traits
related to drought resistance, those markers could be used as selection
criteria for drought resistance. The application of marker-assisted selection
in evolving drought resistant genotypes is in an experimental stage; more
specifically just identification of RFLP markers associated with osmotic
adjustment, stay green, root traits has been achieved[22].
Screening techniques for drought resistance
Any effort for genetic improvement in drought resistance utilizing the
existing genetic variability requires an efficient screening technique, which
should be rapid and capable of evaluating plant performance at the critical
developmental stages and screening a large population using only a small sample
of plant material [23]. Drought resistance is the interactive result of
different morphological, physiological and biochemical traits and thus, these
different components could be used as selection criteria for screening
appropriate plant ideotype. A combination of different traits of direct
relevance, rather than a single trait, should be used as selection criteria
[24].
Drought as major constraint to sorghum production
In most areas where crop production is dependent on rainfall, there is
always the risk of crop failure or yield loss due to moisture stress. In the
semi-arid tropics, the loss mainly arises from the availability of low moisture
to support growth and development of crops [28]. In these areas, moisture is
always inadequate for crop growth because of low precipitation and erratic
distribution and poor soil moisture storage capacity of soils. In severe cases,
the stress could lead to total crop loss [2]. Sorghum is mainly grown in areas
of inadequate rainfall and is the principal source of food for millions of
people living in these areas.
In Africa over 24 million hectares of land is allotted for sorghum
production annually with a mean yield of 0.8 tones/ha [29]. Although several
factors such as low soil fertility, poor pest and disease control and low
yielding potential of local varieties contributed to low yield, much of the
reduction in yield is thought to be due to severe drought stress [3]. Efforts
have been underway to mitigate the effect of recurrent drought through soil and
moisture conservation and tillage practices and development of varieties
adapted to the dry land condition. Previous reports indicated that significant
morphological and genetic variability attributes to drought tolerance were
detected among African sorghums [30].
Mechanisms of
Morphological and Physiological Responses of Sorghum to Drought Resistance
Sorghum is known for its ability to tolerate drought better than most
food cereals and yet is the most affected crop by drought [34]. Efforts have
been going on to improve drought tolerance of sorghum in order to enhance
dryland agriculture. Significant progress has been made in identifying key
traits for drought tolerance and in understanding the reaction of genotypes to
drought at different stages of growth. In an effort to better understand the
underlying mechanisms scientists have dissected drought tolerance into
pre-flowering and post-flowering stress tolerance which is perhaps regulated by
different genetic mechanisms [35]. Cultivars that tolerate pre-flowering water
deficit are advantageous for the reason that yield components such as stand,
tiller number, the number of heads, and especially the number of seed per head
are determined during this period [36]. These are important components of both
yield and water use efficiency [37]. Pre-flowering leaf photosynthetic rate
correlates with biomass and grain production under both well watered and
water-limited conditions, whereas, post-flowering drought tolerance expressed
when moisture stress occurs during grain filling stage. Post-flowering drought
stress in susceptible genotypes may become very severe in that it interferes
with fixation of CO2 and the subsequent translocation of
carbohydrate to the grain. The drought that occurs during grain filling stage
usually results in rapid premature plant senescence [38].
Maintaining Green Leaf Area in Sorghum
Improve Yield under Drought
A mechanism of resistance, known as
stay-green [42],as indicated by maintenance of green stems and upper leaves when
water is limited during grain filling. Green leaf area at maturity is used as
an indicator of post-anthesis drought resistance in sorghum breeding programs
in the USA [43]and Australia [31]. Green leaf area at maturity and its
components have been found to vary with both water regime and genotype [40].The
critical issue is whether retention of green leaf area underpost-anthesis
drought actually increases grain yield in stay-green compared with senescent
hybrids. Positive associations between green leaf area duration and grain yield
have been observed in a range of cereals, including wheat, Triticumaestivum
L.maize, Zea mays L. and sorghum [31,44].
Green leaf area at maturity is accepted as a
key indicator of post-flowering drought tolerance in sorghum [31]. Green leaf
area and grain yield have been shown to be positively associated. Genetic
variability exists among sorghum genotypes for no senescent (stay green)
property and the relationship of this trait to drought tolerance has been well
understood. Visual scoring of stay green trait should be done at or right after
physiological maturity. The scoring procedure is relatively easy and not time
consuming but it is subject to individual bias and the difference in ratings
among individuals [45]. Visual ratings for percentage green leaf area and a
number of green leaves were highly correlated with measured green leaf area
under drought stress [46]. Consequently breeding for stay green trait is
becoming a fundamental strategy for increasing crop production in water-limited
conditions [47]. Progress has been made in the genetic improvement of
post-flowering drought tolerance of sorghum through manipulation of the
stay-green trait [48]. Genotypes possessing the stay-green trait maintain more
photosynthetically active leaves than genotypes not possessing the trait [43].
The longevity and photosynthetic efficiency of the leaves of stay green plant
were shown to be associated with the nitrogen status of the leaves [49],
increased leaf area at maturity and higher transpiration efficiency [47].
Genetics and Genetic
Improvement of Drought Resistance in Crop Plants
Drought resistance is a complex trait, expression of which depends on
action and interaction of different morphological (earliness, reduced leaf
area, leaf rolling, wax content, efficient rooting system, awn, stability in
yield and reduced tillering), physiological (reduced transpiration, high
water-use efficiency, stomatal closure and osmotic adjustment) and biochemical
(accumulation of proline, polyamine, trehalose, etc., increased nitrate
reductase activity and increased storage of carbohydrate) characteristics.
The identification of genes responsible for morphological and
physiological traits and their location on a chromosome has not been possible,
but their inheritance pattern and nature of gene action have been reported.
Polygenic inheritance of root characters is reported by Ekanayake et al. [54]. According to
Armento-Soto et al. [55], the long root and high root numbers are controlled by
dominant alleles and thick root tip by recessive alleles. However, leaf rolling
and osmotic adjustment have shown monogenic inheritance. Tomar and Prasad
[56]reported a drought resistance gene, Drt1 in rice, which is linked
with genes for plant height, pigmentation, hull color and awn, and has a
pleiotropic effect on the root system. Similarly, in cow pea drought resistance
is reported to be controlled by a single dominant gene [57].
Genetic basis of
drought response in sorghum
In a diallele study conducted to estimate the
inheritance of the stay-green trait by dissecting into two components which
determine the occurrence of the trait, it was suggested that inheritance of the
onset of senescence was additive, whereas for the rate of senescence a slow
rate was completely dominant over the fast rate [60]. Another study on the
genetic basis of osmotic regulation revealed the existence of significant
variation among different sorghum genotypes [61]. A biparental progeny genetic
study revealed that two independent major genes (oa1 and OA2) were involved in the
regulation of osmotic adjustment in sorghum [62]. Another study conducted using
a different population set, a monogenic inheritance has been reported to
control the trait.
Quantitative Trait Loci (QTL)
Mapping and Analysis of Drought Tolerance
Phenotypic Selection for Drought Tolerance
We have made slow but significant progress
via empirical breeding of sorghum for drought tolerance by breaking the trait
of drought tolerance into specific phenological stages. The approach has been
to break down the complex trait of drought tolerance into simpler components by
studying drought stress expressions at specific stages of plant development. We
have been particularly interested in midseason (pre-flowering) and late-season
(post-flowering) drought expressions in sorghum germplasm. Our rationale is
that if individual components associated with a complex trait can be
identified, we can measure the contribution of each of the factors or
mechanisms independently without the confounding effect of other factors. Using
this approach, we can identify sorghum germplasm that is uniquely pre-flowering
or post-flowering drought tolerant and few that combine tolerance at both
stages. We have developed new improved drought-tolerant sorghum lines in
diverse and elite germplasm background. Some of these lines have been
officially released and distributed to both public and private sorghum research
concerns. Several more await release and distribution following further
characterization and cataloguing to facilitate specific mode of utility. The
breeding and selection effort was based on reliable phenotypic markers
associated with morphological and yield-related symptoms that occur at
pre-flowering and post-flowering stages of crop development. Some of these
marker traits are simply inherited and others appear quantitative rendering
them amenable to QTL marker analysis and introgression.
Development of Hybrid Sorghum for
Marginal Environments
Globally estimated area planted with hybrid
sorghum was 48% which contributed to a minimum of 40% yield advantage over
open-pollinated varieties. Hybrid cultivars of sorghum are often preferred
because they give higher yield and have more stable performance under a wide
range of environmental conditions [68]. They have the advantage of giving
higher grain yield than open-pollinated varieties both under optimum and stress
environments with the advantage being higher under stress environment [30]. In
Kenya hybrid sorghum reported to give up to 50% yield advantage over
open-pollinated varieties under extreme drought situations (https://www.africancrops).
The performance of hybrids tested for several years at MARK exhibited a
consistent yield advantage of over 100 % compared to standard checks [68]. A
recent report confirmed that hybrids exhibited 68-131% yield more than the
open-pollinated check variety. Commercial production of hybrid sorghum became
only possible after the discovery of cytoplasmic male sterility system in the
1950s [69].
Different male sterility systems which
include A1, A2, A3 and A4 have been identified in sorghum [70]. But the A1
sterility system is widely used in hybrid sorghum program [71]. The A2
cytoplasm can be potentially useful for hybrid seed production provided that
suitable A2 sterile females and corresponding restorer are identified. However,
the A3 system was kept out of use because of limited source of fertility
restoration genes and the A4 cytoplasm is not sufficiently characterized [71].
Due to expanded use of hybrids, sorghum yield in the United States has improved
over 300% between the 1950 s and 1990s. Following the advent of hybrids in USA
35-40%, the genetic gain was estimated on grain sorghum [72]. Hybrid cultivars,
besides their superior yielding potential over the pure line varieties, have a
strong role in motivating private seed growers to engage in commercial seed
production [73]. In Sudan, there was a significant turnaround in seed
production following the release of Haggen Dura-1 (HD-1), the first commercial
hybrid released in 1983. This cultivar has excellent grain quality and stable
performance in areas where lack of moisture limits production. Thus the acreage
under this cultivar increased from year to year with the current statistics
showing 1 million ha of land put to the cultivation of this hybrid [29].
Considering the advantages of hybrid sorghum, several national programs in the
semi-arid regions have shown increased interest in hybrids [74].
Selection for Grain Yield in Sorghum under Moisture Stressed
Environments
Sorghum is an important food crop in the
semi-arid tropics(SAT), where rainfall is generally insufficient and erratic
and soil fertility status is very poor [75].According to Kawano and Jennings
[75] to improve and stabilize crop production in these areas, the genetic
potential of crop germplasm needs to be adjusted to available environmental
resources. Strategies are available for improving performance in moisture
deficit soils of the SAT. One of the approaches assumes that selection of plant
genotypes under optimal moisture supply may maximize genetic gain in low input
production environments. Testing the usefulness of this approach will be
important both in the stress-prone SAT as well as in temperate environments
where stress is infrequent but farmers often look for ways to reduce production
costs [76,77].
A review by Bramel-Cox et al. [8] presented conflicting
results regarding the usefulness of selection under non-stress conditions to
identify genotypes for use in low input environments. The amount of genetic
progress from selection for broad adaptation in both favorable and adverse
production conditions diminishes as the intensity and frequency of stress
increases in the unfavorable production environments [8]. These conclusions
were drawn from sorghum studies which were conducted in either moisture stress
or limited soil fertility. However, crop breeders in the semi-arid tropics most
frequently confront a combination of moisture and nutrient stresses in their
target production environments. Therefore, evaluating breeding materials under
both limited moisture and nutrient supply may increase the chance of
identifying lines which are adapted to one or both stress conditions.
CONCLUSION
Drought limits the agricultural production
the crop plants for example sorghum, by preventing them from expressing their
full genetic potential. Three mechanisms, namely drought escape; drought
avoidance and drought tolerance are involved in drought resistance. Various morphological,
physiological and biochemical characters confer drought resistance.
Morphological and physiological characters show different types of inheritance
pattern (monogenic and polygenic) and gene action (additive and non-additive).
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