Genetic Analysis of Drought Tolerance in Cowpea [vigna Unguiculata (L.) Walp]
Agriculture in Sub Saharan Africa (SSA) is under serious threat due to water shortage, population pressure and climate change. Cowpea, a protein-rich legume crop complements staple cereal and tuber crops in the diets of rural and urban people of the tropical and sub- tropical regions of the world. It therefore, plays a significant role in the sustainability of food and nutrition security in SSA. Cowpea, though reported to be inherently drought tolerant; but because it is mostly grown under rain-fed conditions towards the end of the rainy season in the drier parts of Nigeria, its productivity is still being adversely affected by the erratic pattern of rainfall which occurs frequently in these areas. Increasing the level of drought tolerance in existing cowpea varieties that will possess farmers’ preferred traits will increase farmers’ adoption of these varieties and ensure high and stable yield from farmers’ fields under the ever changing climatic conditions. The objectives of this study were therefore, to: (i) identify the impact of drought on cowpea production and farmers’ preferred traits in new cowpea varieties (ii) assess the diversity of cowpea germplasm for drought tolerance (iii) assess the combining ability of cowpea lines under drought and well-watered conditions and (iv) determine the gene action controlling drought tolerance in cowpea. The results of a Participatory Rural Appraisal (PRA) conducted in fifteen cowpea growing communities of Kano State, Nigeria established that drought, pests and diseases were major constraints to cowpea production. Drought reduced grain yield and fodder yield to about 62% and 56% of realizable yield under normal condition respectively. Fifty-eight percent of the farmers confirmed drought at the flowering / pod-filling stage was more devastating than drought occurring at the vegetative stage (32%) while 10% of the farmers confirmed that both growth stages were both growth stages are susceptible to
drought. Consumer-based traits such as large seed, short cooking time and dual-purpose varieties which increase farmers’ income were identified as important preferred traits as well as traits for biotic and abiotic tolerances in new cowpea varieties. Ninety-one cowpea varieties were screened for tolerance to drought using the wooden box screening technique with the aim to identify parents to be used for genetic analysis studies. Twenty lines were selected based on their responses to the screening and were mated in a North Carolina Design II to generate 100 single F1 crosses. The F1 progenies and their parents were evaluated under drought and well-watered conditions at two locations. Grain yield of the F1 progenies ranged between 2533 kg ha-1 for TVu6707 x TVu9797 and 18 kg ha-1 for TVu11986 x TVu2736 under drought stress, 3786 kg ha-1 for TVu6707 x TVu9797 and 45 kg ha-1 for TVu633 x TVu2736 under well-watered conditions. General Combining Ability (GCA) and Specific Combining Ability (SCA) mean squares were significant for grain yield and other traits across all research environments indicating that both additive and non-additive gene effects were important in the control of grain yield and other drought adaptive traits across all research environments. The contribution of GCA (71%) to the total sum of squares was higher than that of SCA (21%) for grain yield under drought stress indicating that additive gene action was more important in the inheritance of grain yield under drought stress. Similarly, the superior positive GCA (GCA-female and GCA-male) effects for 100-seed weight, number of seeds per pod, Normalized Difference Vegetation Index (NDVI) measured at three different growth stages, the number of pods and seeds per plant under drought stress indicated that additive gene action was more important in the inheritance of these yield related traits under drought stress. The lines TVu79, TVu6707, TVu9693 and TVu9707 were identified as general good combiners with outstanding
positive GCA effects for grain yield under drought stress. These can be used as parents to generate improved cowpea varieties for drought tolerance. Considering both mean yield and stability performance, TVu8670 x Sanzi, IT89D-288 x TVu8670, TVu6707 x TVu79 and TVu8670 x TVu79 can further be advanced for development of novel drought tolerant varieties.
1.0 GENERAL INTRODUCTION
Cowpea [Vigna unguiculata (L.) Walpers] is the most economically important indigenous African grain legume producing a source of economic livelihood and nutritional well-being for rural poor and urban consumers (Agbicodo et al., 2009; Langyintuo et al., 2003; Timko, 2006).
Cowpea plays a critical role in the lives of millions of people in Africa and other parts of the developing world, where it is a major source of dietary protein that nutritionally complements staple low-protein cereal and tuber crops, and is a valuable and dependable commodity that produces income for farmers and traders (Singh, 2002; Langyintuo, et al., 2003). Cowpea is a valuable component of farming systems in many areas because of its ability to restore soil fertility through nitrogen fixation, for succeeding cereal crops grown in rotation with it (Carsky et al., 2002; Tarawali et al., 2002; Sanginga et al., 2003). The N contribution to a cropping system by a cowpea cover crop was reported to be about 145.7 kg N/ha per season if the crop is turned under (Valenzuela and Smith, 2002). Early maturing cowpea varieties can provide food earlier than any other crop (in as few as 55 d after planting), thereby shortening the “hunger period” that often occurs prior to harvest of other crops in farming communities in the developing world. Cowpea haulms and chaff are used as livestock feeds and are also beneficial in maintaining soil fertility thus making it an important component of any cropping system (Sanginga et al., 2000, Muchero et al., 2008). Dry grain for human consumption is the most important product of the cowpea plant; these grains can either be boiled or converted into other food products such as moin- moin, akara, bean soup etc. in Nigeria. The green leaves/twigs, are also used in preparing
nutritious vegetable soup; the fresh pods and peas are used for salad in vegetarian diets (Timko et al., 2007).
The estimated world cowpea production area is over 14.5 million ha, with an annual production estimated at about 7.64 million tonnes. Out of this estimate, West and Central Africa (WECA) account for over 9 million ha and 3 million tonnes. West Africa is the key cowpea production zone, mainly from the dry savanna and semi-arid agro-ecological zones. In West Africa, Nigeria and Niger Republics are the major cowpea producers with Nigeria contributing over 60% of the total production (FAOSTAT, 2015). Compared with many other crops, cowpea is reported to thrive in places considered too dry for the production of other grain legumes but because it is mostly grown under rain-fed conditions on sandy soils having low water-holding capacity in the drier regions that receive between 300 – 600 mm annual rainfall, its productivity is adversely affected by erratic rainfall patterns which occur frequently in these areas (Belko et al., 2013).
Drought can cause direct reduction of about 50 – 67% in cowpea grain yield (Fatokun et al., 2012; Sanda and Maina, 2013). In addition to the direct effect on yield, many aspects of plant growth are affected by drought stress (Hsaio, 1973), including leaf expansion, which is reduced due to the sensitivity of cell growth to water stress. Water stress also reduces leaf production by promoting senescence and abscission (Karamanos, 1980), resulting in decreased total leaf area per plant. Reduction in leaf area reduces crop growth and thus biomass production. Seed production, which is positively correlated with leaf area (Rawson and Turner, 1982), may also be reduced by reduction in leaf area caused by drought stress.
There are various ways of reducing the effect of drought or addressing the problem of drought stress including irrigation and breeding. However, irrigation requires large capital outlay and availability of water throughout the growing season, especially at flowering and pod filling stages. This makes it less feasible especially for small scale farmers in Africa. Developing drought tolerant varieties is a more sustainable option of managing drought since there would be no additional cost to the farmer once drought tolerant seeds are available. Breeding for drought tolerance and grain yield however is complex because they are governed by minor genes whose effects are often confounded by interaction of morphological, physiological and biochemical characters of the crop with the environment thus making genetic improvement of these traits in crops a slow and difficult process (Fatokun et al., 2012; Mir et al., 2012).
In cowpea research, drought tolerant factors have been separated into shoot and root tolerance using simple, rapid and cheap screening methods (Singh and Matsui, 2002; Hall et al., 2003). For selection, two classical approaches are followed when breeding for drought tolerance: (i) utilization of grain yield as selection criteria, and (ii) identification of physiological traits that might contribute to yield production under drought (Singh et al., 2003; Hamidou et al., 2007; Badu-Apraku et al., 2011).
In order for a rapid progress to be made in the development of more drought-tolerant cowpea varieties, it would be necessary to identify easily recognized and measurable characteristics that are associated with the physiological traits upon which selection could be applied. Numerous efforts in this direction have led to discovery of some traits such as stomatal conductance, lower leaf area development and lower canopy conductance, stay
green or delayed-leaf-senescence (DLSC) that are associated with drought tolerance (Agbicodo et al., 2009; Belko et al., 2013; Muchero et al., 2013). Studies on shoot tolerance in cowpea using wooden boxes placed in the greenhouse revealed positive significant correlation between drought tolerance at the seedling stage and drought tolerance in the field (Ewansiha and Singh, 2006; Muchero et al., 2010). They concluded that lines possessing shoot tolerance should also perform well under drought in the field. However, this does not exclude field evaluation in order to identify lines with reproductive-stage drought tolerance and stay-green characteristics, both of which could enhance better performance of cowpea under drought.
Breeding for earliness is important in cowpea adaptation in water-limited areas since early maturing crops are bred to escape terminal drought stresses. However, they are susceptible to water deficits occurring during the flowering and reproductive stages (Cisse et al., 1995, 1997; Ehlers et al., 2000). They can suffer significant reductions in plant stature that lower potential yield when stress occurs at the seedling stage. Because it has become more frequent for farmers’ fields to experience irregular rainfall during cropping seasons, varieties that combine earliness and stay-green characteristics should be able to give farmers some grain yield even with irregular rainfalls. While early flowering is a drought escape mechanism, drought tolerance at the seedling stage and ‘delayed leaf senescence’ (DLSC) should enhance the plants’ ability to survive drought during early, mid-season and at pod filling stages. In Senegal, an early DLSC cowpea variety was reported to produce higher yield because of its ability to produce two flushes of pods (Hall et al., 2003). The prolonged life span that DLSC confers on plants would also add to the plants’ ability to tolerate terminal drought better. It can as well serve as an indirect selection for grain yield
and biomass under drought (Gwathmey et al., 1992a, c; Hall, 2004, Muchero et al., 2013). If cowpea varieties that combine the above attributes become available, they will enable farmers to obtain a better grain yield in those years when rainfall is irregular.
To be able to develop tolerant cowpea varieties that will be adopted by farmers, their involvement at the beginning of breeding effort is very crucial for adoption of developed varieties (Thagana et al., 2009; Kaloki, 2010). Their participation will provide the demand- pull necessary to ensure that the effort in breeding work is focused on key issues of value to the farmers and consumers (Witcombe et al., 1996, 2006). In this regard, a Participatory Rural Appraisal (PRA) was conducted to identify farmers preferred traits that should be bred into drought tolerant varieties in order to increase cowpea productivity.
Polygenically controlled traits such as yield, quality and drought tolerance are complex traits because they are under the control of many genes each contributing small effects to the phenotype and thus cannot be easily identified (Babu et al., 2004). The regions within genomes that contain genes associated with a particular quantitative trait are known as quantitative trait loci (QTL). Identification of these QTL based on phenotype observation is impossible because they exhibit significant genotype by environment interactions (G x E), which necessitate extensive multi-location testing and breeding efforts targeted to specific production environments (Heffner et al., 2009; Bharadwaj et al., 2011). Multi- location testing, however, usually results in genotype-by-environment interactions that often complicate the interpretation of results obtained and reduce efficiency in selecting the best genotypes (Annicchiarico and Perenzin, 1994). Better understanding of the level of G × E interaction and performance stability in crops serves as a decision making tool,
particularly at the final stage of the variety development process, to generate essential information on pattern of adaptation in breeding lines as well as new varieties for release, and to determine the recommendation domains where a given cultivar would be better adapted (Yan, 2011).
Rapid progress in the development of polymorphic molecular markers has led to the intensive use of QTL mapping in genetic studies for quantitative traits (Wang et al., 2007). The principle of QTL analysis is based on detecting an association between the phenotype and the genotype or the marker tightly linked to the trait of interest. QTL analysis effectively improves breeding for difficult characters (Collard et al., 2005, Muchero et al., 2013). Five stay-green QTL showing positive pleiotropic effects between delayed senescence, increased biomass, and grain yield have been reported by Muchero et al., (2013). Three of these five QTL: QTL, Dro-1, 3, and 7 were identified in RIL population and diverse germplasm suggesting they may be invaluable targets for marker-assisted breeding in cowpea. Early vegetative delayed leaf senescence was co-located with biomass and grain yield suggesting the possibility of using delayed senescence at the seedling stage as a rapid screening tool for post-flowering drought tolerance in cowpea breeding (Agbicodo et al., 2009; Muchero et al., 2013). Investigations on G x E interactions at important crop growth stages for yield components would help to develop strategies that integrate conventional plant breeding with modern molecular marker-based selection for tailoring cultivars for high yield and target environments.
Although grain yield between 2500 kg/ha to 4000 kg/ha is achievable for cowpea, several constraints have kept farmers’ yields constantly low at levels between 350 and 700 kg/ha
(Ajeigbe et al., 2010a). If the yield barrier is to be overcome, strategies to improve the genetic potential of cowpea plants by introducing novel genes is required. For this to be achieved, genotypes with potential for higher yield and other desirable traits are needed as parent lines to develop improved varieties (Aremu, 2005).
The identification of suitable parental genotypes, potentially generating superior lines with traits contributing to the overall yield of a crop, is an important step in the development of improved varieties because if parents are precisely selected, the desired recombinants will be found in the segregating generations (Moalafi et al., 2010; Ayo-Vaughan et al., 2013). Knowledge of the genetic control of complex quantitative traits and the magnitude of genetic variability that exists among available germplasm are therefore important for selection and genetic improvement of crop plants. Selection of parents based on combining ability has been used as an important breeding approach in crop improvement. The combining ability and gene effects of yield and its components have been studied by many researchers. Basbag et al. (2007) suggested that combining ability analysis is an important tool for selection of desirable parents together with the information regarding nature and magnitude of gene action controlling quantitative traits. Combining ability study provides useful information on how two inbred lines can be combined to produce a productive hybrid or breed novel varieties. Selection and development of parental lines with high combining ability is one of the most important breeding objectives whether the goal is to create a hybrid with strong vigour or develop a pure line cultivar with improved characteristics compared to their parents (Kadam et al., 2013). The genetic variability among the crosses is partitioned into effects due to additive (GCA) or non-additive (SCA) variances (Shiri et al., 2010). In self-pollinated crop species like cowpea, lines with high
and positive GCA estimates for a character will be good candidates to be used as parents. Preponderance of GCA was reported for hundred seed weight, pod per plant and grain yield of cowpea under moisture stress (Carvalho et al., 2012; Alidu et al., 2013), for inheritance of resistance to cowpea-aphid borne mosaic virus (Orawu, 2013). These results showed that these traits can be improved by exploiting additive gene effects through selection schemes such as recurrent selection capitalizing on favourable additive variation. Rupela and Johnansen (1995) used pure line selection to improve nodulation in pigeon pea as a result of large GCA effects. Cho and Scott (2000) reported predominance of additive effects for seed vigour and yield in soybean. Nkalubo et al. (2009), who studied the genetic control of anthracnose resistance in common bean, obtained both additive and non-additive effects controlling this trait, but with a slight predominance of additive genetic effects over dominance effects. Information regarding combining ability and nature of gene action governing the inheritance of desirable traits are therefore efficient ways to achieving maximum genetic gain when developing high yielding cowpea cultivars with higher and more stable yields across drought prone regions where it is grown. This study is among the few studies to provide information on the combining abilities of cowpea lines of diverse germplasm under contrasting soil moisture regimes in order to be able to efficiently develop novel varieties that meet farmers’ demand and high yielding under varying moisture stress.
The objectives of this study were to:
- Elicit farmers’ understanding of drought and its impact on cowpea production and productivity.
- Assess the genetic diversity for drought tolerance in cowpea
- Assess the performance of the F1 crosses under drought and well-watered
- Determine the gene action controliing yield and drought tolerance in
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