Cowpea [Vigna unguicalata (L.) Walp.] is a diploid legume species (2n = 2x = 22) widely grown in Africa, Asia, the Middle East, southern Europe, southern and western United States, and Central and South America. Worldwide cowpea production is estimated to be 5.4 million tons of cowpea grain annually and Africa is the leading producer (Olufajo 2012). Cowpea is cultivated on more than 14 million hectares (Singh et al., 2003). It provides good-quality nutrition for human consumption (Frota et al., 2008). In addition, cowpea can contribute toward protecting soils from being eroded because it is an excellent cover crop. In the western part of the United States, a growing interest in using cowpea as a cover crop has been noticed since cowpea can tolerate drought conditions (Agbicodo et al., 2009). However, increasing concerns due to salinity in this part of the country can limit the use of cowpea as a cover crop (Wilson et al., 2006). In semiarid regions in which cowpea cultivation is predominant, the low rainfall frequency could lead to salt compounds not being properly leached out, hence accumulating within soils and exacerbating salinity-related issues (Zhang et al., 2012).
Salinity is one of the major limiting factors that have been constraining agricultural production globally (Allakhverdiev et al., 2000). In croplands, salinity is due to an undesirable increase in the concentration of cations such as K+, Mg2+, Ca2+, and Na+ and anions such as NO3−, HCO3−, SO42−, and Cl− according to Wallender and Tanji (2011). Salinity due to sodium chloride (NaCl) has been predominant (Ayers and Westcot, 1985); hence, tolerance to this type of salt was reported in this current investigation. The estimate of cropland areas facing salinity was more than 19.6 million hectares in the United States (Shannon 1997). Costs related to problems imposed by salinity on agriculture were 12 billion US dollars (Läuchli and Lüttge, 2002). Multiple factors such as rock weathering, deforestation, poor quality of irrigation water, and inadequate fertilization practices can worsen salinity on cultivated lands (Omami and Hammes, 2006).
Studies have shown that salt stress can cause serious concerns to cowpea production. Cowpea germination has been shown to be unfavorably affected by salt stress (Zahedi et al., 2012). Salt-stressed cowpea plants exhibited a reduced plant growth and vigor (Mini et al., 2015). Salt stress can impair plant physiology, photosynthesis, and absolutely important functions such as cell extension and division (Maas and Hoffman, 1977). These aforementioned factors could lead to a significant cowpea yield reduction (Dutta and Bera, 2014). Breeding for cowpea salt-tolerant cultivars is one of the most affordable solutions to tackle these issues. However, few studies have focused on addressing salt stress in cowpea in efforts to adequately providing breeders with critical information on the tolerance of cowpea genotypes to salinity.
Phenotyping is a substantial process in screening genotypes for a particular trait of interest. It is usually a labor-intensive, time-consuming, and a costly task to undertake for plant breeders. The increasing needs for accurate and less expensive phenomics require the establishment of a fast and cost-effective methodology. To the best of our knowledge, there is no reported methodology on salt tolerance phenotyping in cowpea. Salt phenotyping can be carried in fields. However, the uncontrolled factors such as differences in soil fertility, temperature, and transpiration could increase the unexplained part of the variation in salt tolerance among cowpea genotypes, thus leading to biased conclusions (Pathan et al., 2007). Hydroponic system has long been considered the ideal approach for salt tolerance phenotyping in crops. However, this requires adequate facilities and specialized skills (An et al., 2001), which could significantly increase the phenotyping cost.
Since cowpea is cultivated predominantly in developing countries, a methodology that can be applied in these areas in which funds and facilities are very limited would be most helpful. In addition, the screening methodology should allow for a rapid and accurate salt tolerance phenotyping of a large number of genotypes to be efficient. Seedling stage is one of the most vulnerable stages to salt stress in cowpea (Win and Oo, 2015). Suggesting a strategy that can help cowpea breeders select for salt-tolerant genotype at seedling stage is therefore important and can also assist with at least narrowing down the number of genotypes for salt tolerance screening at a later stage. Therefore, the objective of this study was to establish an approach that can be easily applied for salt tolerance phenotyping for cowpea at seedling stage.
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LSMeans of average number of dead plants per pot, leaf injury scores, chlorophyll content under non-salt conditions, chlorophyll content under salt stress, relative salt tolerance for chlorophyll content, plant height under non-salt conditions, plant height under salt stress, and relative salt tolerance for plant height.
LSMeans of leaf biomass under non-salt conditions, leaf biomass under salt stress, relative salt tolerance for leaf biomass, stem biomass under non-salt conditions, stem biomass under salt stress, and relative salt tolerance for stem biomass.
Pearson's correlation coefficients between trait values used for phenotyping salt tolerance at seedling stage in cowpea.