Major historical droughts and its impact in Bangladesh .
Wheat is one of the major cereal crops in Bangladesh. Over the last two decades, wheat consumption has passionately amplified in Bangladesh but its production has declined due to various stress environments. Recurrent drought event due to climate change that threatens the country’s food safety has become a serious concern. To safeguard the food security, adopting suitable breeding strategies can add momentum. Developing drought tolerant wheat varieties are the definitive means of protecting the crop against hostile effects of drought. Plant breeders are exploring various breeding strategies to breed for the varieties that can cope with water deficient conditions well. Besides, breeders are consistently looking for new prospects and strategies that can boost genetic gain in yield. To endorse drought tolerance in wheat, understanding the physiological and genetic adaptation mechanisms of wheat cultivars during drought stress would provide the estimated benchmarks to adjust for suitable breeding programs. The efforts of developing drought tolerant wheat genotypes could be supported by different breeding strategies including in vitro haploid and double haploid protocols, polyploidization, development of various types of hybrids and induced mutants by utilizing both classical and molecular breeding techniques. The proposed book chapter shall discuss the pattern of drought-stress in the wheat growing regions, effects of drought stress on wheat production and suitable breeding strategies for developing drought tolerant genotypes in Bangladesh.
- wheat breeding
- drought stress
- tolerance mechanisms
- breeding strategies
Bangladesh is a small country geographically situated in between Himalaya and Bay of Bengal. It is among the most vulnerable countries in world to future climate change due to the flat deltaic topography, very low elevation (below 10 meters above sea level) and high population density [1, 2, 3]. Eating a lot of rice is the primary food habit of Bangladeshi people. Next to rice, wheat is the second most important cereal crops in Bangladesh for attaining food and nutritional security . Although being one of the major rice producers and consumers in world , consumption and import of wheat in Bangladesh are growing significantly over the years [5, 6, 7, 8, 9]. The speedy economic growth, swift urbanization, and the associated alterations in lifestyle are accountable for the increased consumption of wheat which is not going to change . Instead the demand of wheat will be enhanced in near future . Despite increasing yield, gradual decrease of wheat growing area make the domestic wheat production curve more or less static . At present, the domestic production of the country can only encounter around 20% of total wheat consumption [11, 12] and import is the only way for meeting her demand–supply gap . Several periodic natural calamities such as salinity, drought, high temperature stress, flash floods and cyclones have been accelerated due to climate change in recent years [4, 13]. Among the abiotic stresses, drought is the most prominent and prevalent limiting factors of wheat production [14, 15, 16]. Rising temperature and changing in precipitation pattern lead to increasing incidence and intensity of drought events in country like Bangladesh [17, 18, 19, 20, 21]. Drought employs expressively adverse effects on production of winter crop wheat in northern and central part of Bangladesh [22, 23]. Around 3.5 million ha land are vulnerable to crop production due to drought and wheat is one of the major cereal crops under the radar of this threat . Considering these facts, drought should be highly preferred in future wheat improvement programs. For attaining self-sufficiency in wheat production, wheat breeders of Bangladesh have no alternatives but to develop well adapted drought tolerant varieties . In spite of the polygenic nature, there are ample opportunities to increase drought tolerance of wheat through making some alterations in genetic and molecular levels. Therefore recent wheat breeding programs for drought stress should focus on utilization of both conventional as well as advanced molecular techniques.
2. Pattern and distribution of drought stress in Bangladesh
In Bangladesh, drought is defined as the period when soil moisture content is less than the required amount for satisfactory growth of a crop during a normal crop growing season . According to assessment of Intergovernmental Panel on Climate Change (IPCC), by the year of 2050 about 8 million people of Bangladesh will be affected by droughts . Due to tropical humid type climate, Bangladesh faces widely varying seasonal rainfall pattern, moderately warm temperatures and high humidity . Irregular and varying rainfall pattern due to climate change and lack of surface water is the main reasons of recurrent devastating drought events in many areas of Bangladesh [28, 29]. Among the meteorological droughts, seasonal drought due to asymmetrical distribution of standard rainy and dry season and contingent drought due to irregular rainfall are more predominant in Bangladesh . Due to high variability in pattern and distribution of rainfall, the north-western part of Bangladesh become more susceptible to droughts [30, 31]. In addition, groundwater resources are continuously abused by the farming communities causing scarcity in surface water [32, 33]. Over the last 2–3 decades, the northwestern part of Bangladesh (Barind tract) has been more exposed to recurrent drought events than the other parts . Majority of the parts of greater Dinajpur, Rangpur, Pabna, Rajshahi, Bogura, Naogaon and Joypurhat districts are included in Barind Tract shown in Figure 1 characterized by relatively less rainfall (average annual rainfall 1329 mm), shortage of surface water and high temperatures [25, 35]. One of the most vulnerable districts to droughts in Bangladesh is Rangpur .
Because of the extreme climate fluctuations mainly in the patterns of rainfall, Bangladesh is predicted to face increased rainfall upto 5–6% by 2030 resulting prolong flood during monsoon season and severe drought outside the monsoon season [13, 34]. Inadequate pre-monsoon shower, a delay in inception of rainy season or a quick advent of the monsoon season may accelerate the drought risk in Bangladesh . Bangladesh experienced 20 different drought events over the last 50 years and among them the droughts of 1973–1974, 1975, 1978–1979, 1981, 1982, 1989, 1994–1995, 2000 and 2006 are most hazardous [34, 38]. Effects of some major historical drought events of Bangladesh are presented in Table 1.
|Happening year||Drought impacts|
|1973–1974||One of the most severe drought events in the century that caused famine in 1974 in northern part of Bangladesh|
|1975||Affected 47% of area and half of the total population of Bangladesh|
|1978–1979||Affected about 42% of the cultivated land and 44% of the total population. Caused severe damage to crop production especially rice (reduced about 2 million tons production)|
|1981||Adversely affected crop production|
|1982||Caused severe reduction in rice production (reduced about 53,000 tons production)|
|1989||Dried up most of the rivers in north-western regions of Bangladesh with dust storms in Nawabganj, Naogaon, Nilpahamari and Thakurgaon districts|
|1994–1995 and 1995–1996||Caused immense crop damage, especially to the main crops of northwest Bangladesh like rice, jute and bamboo clumps. The most persistent droughts in recent times|
In Bangladesh, the spatial pattern of pre-monsoon droughts are more recurrent in northwestern part . An analysis on monthly pattern drought from 1971 to 2010 has suggested that Dinajpur, Kushtia, Rajshahi, Rangpur and Bogura are the highest drought-prone parts of the country . Further drought trends investigation has revealed the declining trends in rainfall and increase in dryness at Ishurdi, Bogura, Sayedpur and Rangpur . Investigation on spatiotemporal drought patterns on a regional scale has exposed that higher intensities and frequencies of drought events in the northwestern part make the area more vulnerable to both drought severity and extremity . Recent assessment of droughts from 1960 to 2011 in context of changing climate using drought hazard index (DHI) and drought index (DI) has disclosed that the northern part of Bangladesh are more drought-prone and there is a probability for the area of experiencing more extreme drought events in near future . The studies on changing pattern of meteorological droughts indicates the rising trend of more extreme droughts in cropping season and also reveals the possibility of changing the drought occurrence pattern in both areas where it historically affected most (northwestern part) or the areas with fewer droughts (other parts) [44, 45]. Huge uncertainties are noticed in the possible future changes in droughts and also that would expand from north-western to central, western and south-western regions in Bangladesh [46, 47].
3. Cropping pattern in the drought-affected zones
Cropping pattern of an area is normally determined by its climatic parameters related to a particular time of a year. Bangladesh is situated in subtropical region giving it a suitable temperature range which makes it favorable for year round crop cultivation. However, Bangladesh has a complex and intensively diverse cropping pattern and that pattern is evolving and changing at a continuous basis . Depending on cultural method, the whole crop-growing period of Bangladesh is distributed into two major seasons i.e. Kharif season and Rabi season. Beside these two, there is a transitional season named pre-kharif (shown in Table 2) . Kharif crops like rice, jute, maize, millets etc. are grown in Kharif season and Rabi crops like wheat, mustard, chickpea, lentil etc. are grown during Rabi season .
|Cropping season||Occurring month||Characteristics|
|Kharif or Monsoon (also known as kharif-2 or aman)||June/July to September/October|
|Rabi or Winter||October/November to February/March|
|Pre-kharif or pre-monsoon or spring (also known as kharif-1)||March/April to May/June|
In Bangladesh, all the cropping season are more or less affected by drought. But pre-monsoon and post-monsoon period are mostly prone to drought events . Kharif drought negatively affects the critical reproductive stage of transplanted Aman rice where all of the Rabi crops are affected by pre-kharif/rabi droughts . Assessment of drought in northern area of Bangladesh for the period between 1971 and 2008 reveals that most extreme drought conditions have been experienced in Rabi season including pre-monsoon . Increasing trend in precipitation change in Bangladesh causes more rainfall in monsoon and less rainfall in winter resulting in droughts in winter season. Thus yield of various crops like HYV boro rice, aus rice, wheat, sugarcane, pulses and potatoes growing in Rabi and pre-kharif season are badly affected by droughts [35, 50]. In recent decades, the drought condition in northwestern Bangladesh severely affected the production of rice and all Rabi crops (wheat, tobacco, sugarcane etc.) . Rice-rice, rice-wheat and rice-maize are the dominating cropping patterns in Bangladesh in the drought regions [51, 52]. In late October to early November, certain areas of lands in Bangladesh become empty because of using short duration rice varieties which is appropriate for wheat cultivation . For decades, wheat is grown in wheat-fallow-T. aman rice cropping pattern in north-western part of Bangladesh with some exceptions like wheat-jute-T. aman rice cropping pattern .
4. Adverse effects of drought on wheat production
Drought is one of the most limiting stress factors for crop growth and development, dry matter production and potential yield [15, 54]. The major processes required for plant growth and development are hampered by the drought condition. Water deficit conditions lessen the rate of photosynthesis by inhibiting chlorophyll synthesis, impede cellular elongation and metabolism, decline the CO2 assimilation rates due to reduction in stomatal conductance and gaseous exchange, reduce dry matter biomass production and alter root morphology [54, 55]. As a result leaf size, stem elongation, root production and finally the rate of growth and yield are affected by drought .
Drought is not a static stress, it can occur at any crop growing period, its severity and frequency can vary and also it can recurrently happens in combination with other abiotic stresses, such as salinity and heat . Drought stress can fluctuate diurnally (high during peak photosynthetic period and low overnight) and different organs of plants respond differently to drought stress . Yield contributing traits vary according to growth stage of plant, so the level of seriousness of drought stress eventually relies on the particular growth stages that are impacted by drought. The nature of plants’ response also differ depending on whether the plant is experiencing stress for the first time or after several exposures and whether they are recovering from stress after a rainfall or irrigation event .
Water is needed for the entire growth period of wheat but some specific stages are more sensitive to water limitations. Various morphological, physiological and biochemical alterations are occurred in plants body under drought environment (see Table 3). In case of wheat, the extent of drought stress may vary according to different growth stages. Specific critical growth stages of wheat plants such as germination and seedling stages ; tillering and stem elongation stages [60, 61]; heading, anthesis and grain filling stages [16, 59] may be more vulnerable to drought stress. Long term droughts (starting from stem elongation through to maturity) cause more drastic yield reduction compared to those initiating at later stages through to maturity . Although the influence of drought stress on heading and grain filling stages are more severe in terms of yield, drought can also negatively affect the multiple growth stages of wheat comprising germination, tillering, booting, heading, anthesis, and maturity .
|Drought stress in wheat||Morphological alterations||Limited plant size, ceased plant height, reduced leaf extension, lessened leaf size and number of leaves, decreased leaf area, reduced leaf longevity, prompt maturity, augmented root-to-shoot ratio, condensed total shoot length, lowered yield|
|Physiological alterations||Stomata closure, reduction in photosynthesis, swift in oxidative stress, alterations in cell wall integrity, decrease in leaf water potential, lessen growth rates, reduced transpiration rates and relative water content, developed water use efficiency|
|Biochemical alterations||Reduction in rubisco efficiency, decrease in photochemical efficiency, production of reactive oxygen species (ROS), increase in oxidation damage, hampered antioxidant defense system, reduced chlorophyll content|
5. Crop traits and mechanisms adaptive to drought stress
In drought condition, sometimes very swift growth responses are generated even due to a little water pulse that can vigorously activate plant growth and safeguard survival . Constantly fluctuating nature of drought events makes it indispensible to understand the plants’ aptitude to adapt and recover from the stress . To overcome the harmful effects of drought stress, naturally plants are well furnished with various adaptive mechanisms. These adaptive mechanisms support plants for an optimal maintenance of growth for metabolic regulation and survival . The more the extent of these mechanisms, the more will be the plants’ capability to overcome stress condition. But the adaptive mechanism is not as simple as it sounds; it comprises diverse morphological, physiological and anatomical modifications in plant under stress condition. Morphological and metabolic adaptation processes of plants vary according to cultivars in response to water deficit condition. As, plants’ may have unique adaptation capabilities irrespective to cultivars . Different physiological processes in plant such as photosynthesis, heat dissipation and chlorophyll fluorescence are occurred in rivalry with each other in response to drought events i.e. any upsurge in the efficacy of one will bring diminution to others . The normal fluctuation values of these physiological processes can denote plant fitness with the magnitude of environmental stress .
Drought adaptation is complicated that experiences diverse anatomical and morpho-physiological and biochemical amendments in plants such as alterations in leaf traits or canopy cover, leaf water relations with modification of growth rates, reduction of stomatal opening and associated components [70, 71]. The plants’ response to water deficit condition has been extensively studied to recognize tolerance mechanisms . So, detailed knowledge about underlying behavior of plants under drought stress is required to develop drought tolerant plants. Although being complex, mainly three kinds of drought-resistance mechanisms are exhibited by plants to evade the resulting devastating effects of droughts: (i) drought escape (ii) drought avoidance and (iii) drought tolerance . Drought escape happens when plants grow quickly and reproduce before severe drought conditions. In this mechanism, plants evades drought season by modifying flowering time thus they try to complete their life cycle before drought condition. In drought avoidance mechanism, plants avoid water-deficit situation by enhancing their water-use efficiency (WUE) through closure of stomata, reduction of transpiration, limitation of vegetative growth, or by increment in root growth. In case of drought tolerance, drought stress is fought by plants at cellular level through osmotic adjustment by developing antioxidants and production of molecules that stabilize proteins .
Wheat plants exhibit a tight network of morpho-physiological and photo-protective mechanisms to alleviate the drought stress . To escape reproductive failure from severe drought stress, plants displayed phenological alterations of earlier anthesis and maturity . Previous literature revealed constitutive traits that confer dehydration avoidance mechanisms in plants include leaf waxy layer, leaf rolling and osmotic adjustment , high root length density  and high fine roots with small diameters , a deep root system and the number of seminal roots [77, 78, 79], high total root length and total root surface area [80, 81], root-to-shoot dry matter ratio  and root partitioning of assimilates to shallow or depth roots in response to drought . There are range of morphological, physiological and biochemical derivatives of drought tolerance in wheat  (shown in Figure 2). Water use, water use efficiency, biomass yield and flag leaf relative water contents are the important drought tolerance traits in wheat [84, 85]. Selection of wheat plant with high transpiration efficiency, high percentage of relative water contents and cell membrane thermo-stability and greater osmotic adjustment capacity leads to produce drought tolerant plants [64, 86]. When drought stress is imposed on seedling stage of wheat, cell membrane thermo-stability, fresh and dry weight of seedlings are considered major traits to govern drought responses under stress conditions . Therefore, greater morphological adaptation with limited down-regulated physiological activities followed by high recovery in wheat cultivars designate its capability to effectively endure drought events .
6. Prospects of breeding for drought tolerance in wheat
Plant breeders around the world have to deal with great challenges to work with drought stress. Polygenic nature of drought makes the breeding efforts more complicated than other abiotic stresses . Global climate change will result in frequent drought events as per predicted in country like Bangladesh . So, for improving wheat production in Bangladesh, research priority should be focused on breeding new high yielding drought tolerant wheat varieties. Majority of the studies under drought stress focus on the response of natural drought in field conditions where drought events are ambiguous and irregular using conventional techniques. Generally the conventional breeding techniques such as introduction, selection, hybridization and mutation are being used by the breeders of Bangladesh. Whereas throughout the globe, wheat breeders are now using different novel breeding methods including
In recent times, as part of empirical breeding based programs, breeders have been embracing replicated, multi-locational and multi-year variety testing for finding out the best adaptive varieties to stress environments. Expanding grain yield under drought stress can be performed to a limited extent through selection process [91, 92]. For being recurrent and season indefinite stress event, trait evaluation under drought condition may cause losing of potential genetic resources which perform better in normal wheat-growing environments . This may ultimately hamper the variety development process. Therefore, evaluation including diverse testing environments including both normal and stressed conditions will be more suitable and competent for the development of high yielding, stable varieties amended to water-deficit conditions .
7. Breeding strategies for drought tolerance in wheat
It is very challenging for the plant breeders of Bangladesh to develop drought-tolerant wheat varieties . For ensuring future food security of Bangladesh, the scientists of Wheat Research Center (WRC) of Bangladesh Agricultural Research Institution (BARI) are trying hard to develop wheat varieties that can be suited well in abiotic stress environments . But alongside using a range of conventional breeding strategies for developing stress tolerant variety, breeders always search to produce new genetic variant to increase of genetic gain through advanced molecular approaches.
For maintaining the consistency of wheat production in Bangladesh adaptive to future climate change, the wheat varieties of next generation should possess high yield potentially even under stressed conditions. Yield potentiality can be enhanced through strategic crosses depending upon pyramiding yield potential traits and related physiological traits to stress tolerance in well adapted genotypes . Breeding for drought tolerance in wheat initially requires satisfactory amount of variability among the source populations. Conventional hybridization is the most widely used breeding procedure in wheat, where genetic variability is created through combination and recombination of desirable genes in the background of diverse adapted genotypes followed by a selection of desirable plants in subsequent generations to develop improved varieties for the target environment . Generally grain yield is the primary basis for selection for drought tolerance but indirect selection based on related yield-contributing and physiological traits can be more effective for developing drought tolerant varieties [89, 93, 94, 95]. In this connection, several wheat lines collected from various national and international sources especially CIMMYT (International Maize and Wheat Improvement Center) are evaluated for their performance in diverse growing environments of Bangladesh . Screening of drought tolerant wheat genotypes has been commenced at Barind area of Rajshahi region of Bangladesh where incorporation of related traits to drought tolerance into adapted varieties is also undergoing . Although being the main breeding procedures with some advantages, conventional techniques are slow, labour-intensive and economically unfeasible .
In contrast to time-consuming conventional breeding methods for accomplishing homozygous lines to develop wheat varieties, double haploid breeding instantly enables development of homozygous lines from a crop plant. Hence, double haploid breeding can be also an effective method in wheat breeding since selection efficiency relies on uniform homozygous line production. But, unwanted genetic modifications due to gametoclonal variation negatively affect the selection of population [97, 98, 99]. Interspecific crosses can also produce double haploids of wheat. Recently, WRC of BARI (now, Bangladesh Wheat and Maize Research Institute) has embraced the double haploid breeding technique through cross-pollinating wheat and maize . For speeding up the variety release process, scientists are being trained for efficient targeted crosses to produce double haploid plants . Mutation breeding offers another way to produce drought tolerant wheat varieties in Bangladesh. Induced mutations by gamma-ray is very efficient in augmenting genetic variability which provide a great opportunity for the wheat breeders to select for drought tolerance in M2 (mutant generation 2) and next mutated generations [100, 101, 102]. Recently, in bread wheat, drought tolerant mutants are formed using gamma rays that lead to the release of 26 varieties worldwide . Incorporating with several improved traits, these varieties can survive the stress environments. Thus, high potentiality of developed wheat mutants for direct release and inclusion in hybridization breeding programs is the major benefit of mutation breeding .
Molecular mechanism of drought tolerance is very complicated to understand. Numerous drought-responsive genes are involved in making plant drought tolerant, furthermore expressions of these genes also differ with various plant growth stages [74, 105]. Various genes and their related enzymes and proteins including late embryogenesis abundant (lea), responsive to abscisic acid (Rab), rubisco, helicase, proline, dehydrins, vacuolar acid invertase, glutathione-S-transferase (GST) and carbohydrates provide the molecular basis for drought tolerance in wheat . It points towards challenges and uncertainties remain in breeding for drought tolerance. Hence, inclusion of innovative molecular and biotechnological methods like molecular marker methods, quantitative trait loci (QTL) mapping strategies, expression patterns of genes and genetic engineering should be practiced for the development of drought tolerant wheat genotypes. Currently, molecular markers are extensively used for detecting the location of drought-induced genes. Genome mapping and tagging of various traits aided by molecular markers are utilized in Marker-assisted breeding in wheat for developing drought tolerance . Marker techniques allow indirect selection independent of crop developmental stage specially when dealing with polygenic trait like drought tolerance. In the previous few decades, molecular markers like isozymes, SDS-protein and sequence based DNA markers are exploited in wheat breeding for assessing gene diversities, precise mapping of their respective QTLs on chromosomes and finally for selecting quantitative traits like drought tolerance [107, 108, 109, 110, 111]. Even though large genome size of wheat, polygenic nature of the trait, instability of some QTL ultimately make the mapping process very challenging to execute for drought tolerance [106, 112, 113].
Now-a-days, modern biotechnological approaches have been involved in developing transgenic plants that can withstand the severity caused by drought. Since, these biotechnological strategies enable more understanding about the drought responses of crops at the entire plant and molecular levels . It is evident from previous study that in field conditions, genetically modified wheat exhibits high tolerance to drought . Plant tissue culture, hydroponic culture,
As it is an urgent call for upgrading wheat production under increasing potentiality of drought events, wheat breeders of Bangladesh need to emphasize on integrating more breeding techniques to make drought tolerant varieties. Majority of the breeding approaches here are concentrating on conventional techniques. So, it is high time to combine the conventional breeding methods with the modern techniques to develop wheat genotypes for the next generation. New advanced screening, hybridization and selection techniques shall need to be incorporated with conventional techniques. To maximize the breeding efficiency for drought tolerance in wheat, advanced precision phenotyping accompanied by genetic and molecular approaches should be integrated in breeding programs.
Conflict of interest
“The authors declare no conflict of interest.”
This research was supported by the University Grants Commission of Bangladesh (Grant No. 2019/829/UGC).