Evaluation of Salt Tolerance at Germination Stage in Cowpea [Vigna unguiculata (L.) Walp]

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  • 1 Department of Horticulture, 316 PTSC, University of Arkansas, Fayetteville, AR 72701
  • | 2 Vegetable Research Center, University of Arkansas, Alma, AR 72921
  • | 3 University of Missouri - Fisher Delta Research Center, 147 State Hwy T, Portageville, MO 63873
  • | 4 Department of Crop, Soil, and Environmental Sciences, 215 PTSC, University of Arkansas, Fayetteville, AR 72701

Cowpea is a leguminous and versatile crop which provides nutritional food for human consumption. However, salinity unfavorably reduces cowpea seed germination, thus significantly decreasing cowpea production. Little has been done for evaluating and developing salt-tolerant cowpea genotypes at germination stage. The objectives of this research were to evaluate the response of cowpea genotypes to salinity stress through seed germination rate and to select salt-tolerant cowpea genotypes. The seed germination rates under nonsalt condition and salinity stress (150 mm NaCl) were evaluated in 151 cowpea genotypes. Four parameters, absolute decrease (AD), the inhibition index (II), the relative salt tolerance (RST), and the salt tolerance index (STI) were used to measure salt tolerance in cowpea. The results showed that there were significant differences among the 151 cowpea genotypes for all parameters (P values <0.0001). The AD in germination rate was 5.8% to 94.2%; the II varied from 7.7% to 100%; the RST ranged from 0 to 0.92; and STI varied from 0 to 0.92. A high broad sense heritability (H2) was observed for all four parameters. High correlation coefficients (r) were estimated among the four parameters. PI582422, 09–529, PI293584, and PI582570 were highly salt tolerant at germination stage. In addition, genotypes from the Caribbean and Southern Asia exhibited better tolerance to salinity, whereas those from Europe and North America were the most salt-susceptible.

Abstract

Cowpea is a leguminous and versatile crop which provides nutritional food for human consumption. However, salinity unfavorably reduces cowpea seed germination, thus significantly decreasing cowpea production. Little has been done for evaluating and developing salt-tolerant cowpea genotypes at germination stage. The objectives of this research were to evaluate the response of cowpea genotypes to salinity stress through seed germination rate and to select salt-tolerant cowpea genotypes. The seed germination rates under nonsalt condition and salinity stress (150 mm NaCl) were evaluated in 151 cowpea genotypes. Four parameters, absolute decrease (AD), the inhibition index (II), the relative salt tolerance (RST), and the salt tolerance index (STI) were used to measure salt tolerance in cowpea. The results showed that there were significant differences among the 151 cowpea genotypes for all parameters (P values <0.0001). The AD in germination rate was 5.8% to 94.2%; the II varied from 7.7% to 100%; the RST ranged from 0 to 0.92; and STI varied from 0 to 0.92. A high broad sense heritability (H2) was observed for all four parameters. High correlation coefficients (r) were estimated among the four parameters. PI582422, 09–529, PI293584, and PI582570 were highly salt tolerant at germination stage. In addition, genotypes from the Caribbean and Southern Asia exhibited better tolerance to salinity, whereas those from Europe and North America were the most salt-susceptible.

Cowpea [Vigna unguiculata (L.) Walp] (2n = 2x = 22) is a legume of economic importance worldwide. It is widely grown in Central and West Africa with a production estimated at 5.4 million tons of dry seed (Olufajo, 2012) and also grown in Latin America, Southeast Asia, and in the United States (Muchero et al., 2009; Tan et al., 2012). Cowpea plays an essential role for food security and ecosystem services. Cowpea has high seed protein content and other nutritional components for human benefit. It also supplies nitrogen to soils (Duke, 1990). Mamiro et al., (2011) reported that cowpea dried seeds had a fat concentration ranging from 5.4% to 11.2% depending on cultivar. The calcium content ranged from 360 to 992.4 mg/kg. The concentration of zinc varied between 31.5 and 35.6 mg/kg. The iron content ranged from 27.6 to 28.9 mg/kg. In addition, their results suggested that cowpea leaves were also rich in micronutrients. In the United States, cowpea, also commonly referred as southernpea, blackeye pea, crowder pea, lubia, niebe, coupe, or frijole is usually grown by small-scale farmers (less than 50 ha) mainly in Southern States, and there is a significant blackeye bean industry in California. It is a profitable crop for growers (Davis et al., 2012; Okiror et al., 2008).

Germination is a pivotal step for crop propagation. Studies reported that germination is highly influenced by a large number of environmental factors. Mistura et al., (2011) stated that salinity affected germination of legumes and plant vigor. Dutta and Bera (2014) found a noticeable decrease in seed germination, plant growth, and vigor indices for mung bean supplied with salt treatment. In addition, Zhang et al., (2013) stated that low temperatures undermine cowpea germination. Their results indicated that germination rate decreased to 18% at 10 °C. Reddy and Reddy (2016) reported that excessive and incessant rainfall decreased the seed germination of soybean in India. Low germination unfavorably affects crop production. Germination ensures the propagation of plants to properly ensure food supplies for human consumption (Bewley, 2003). Moreover, seed quality is an important feature required by seed testing centers. To ensure a stable market, seed producer companies have to provide good quality seeds exhibiting high germination rate.

Saline soil is characterized by high concentration of cations (K+, Mg2+, Ca2+, and Na+) and anions (NO3, HCO3, SO42−, and Cl) (Wallender and Tanji, 2011). Sodium chloride is the most well-known salt causing damage in agriculture (Ayers and Westcot, 1985). In addition, Shannon (1997) reported that the concentration of salt in cultivated areas has been increasing because of inappropriate irrigation, fertilization, or other factors. Such increase is estimated at 1 to 60 t/ha annually. Läuchli and Lüttge (2002) pointed out that costs related to salinity issue were 12 billon US dollars per year. Rock weathering, seawater, rain, deforestation, air pollution, and contamination of river waters by chemical and domestic animals are substantial sources of salinity in agriculture (Omami and Hammes, 2006; Rengasamy et al., 2006).

However, little has been done regarding studies on seed germination in cowpea along with the factors that markedly affect germination rate. A study conducted by Zahedi et al., (2012) reported that salinity decreased germination rate in cowpea, and poor seed germination resulted in a significant reduction in yield. In addition, salinity had negative effects on plant growth, cell extension, cell division, and photosynthesis (Maas and Hoffman, 1977).

The evaluation of salt tolerance requires a controlled environment. Field screening is an available technique to screen plant genotypes for salinity. For this method, seeds are sown directly in soil with high salt concentration. However, because of the variability of salt content in the field and the considerable influence of other factors such as moisture content in soil, soil fertility, temperature, light, transpiration, and weather, the results might be highly biased and nonconclusive in a field setting (Parker et al., 1983; Pathan et al., 2007; Yang and Blanchar, 1993). Plant genotypes can be also hydroponically screened by using nutrient solution as a medium growth and adding salt solution after germination and establishment (An et al., 2001). However, such a method is expensive. Therefore, evaluating salt tolerance at germination stage would be of interest because doing so will add information to genotype response at different stages of plant growth and development.

Screening for salt-tolerant genotypes at germination stage is one of the most cost effective ways to tackle salinity-related issues. In this view, some research has dealt particularly with the effects of salinity on germination rate and emergence (Badia and Meiri, 1994; Carter et al., 2006; Kent and Lauchli, 1985; Mauromicale and Licandro, 2002). Kan et al. (2015) evaluated the effects of salt stress on a panel of 191 soybean (Glycine max L.) genotypes and identified eight SNP markers highly associated with tolerance to salinity at germination. Concerning cowpea, Wests and Francois (1982) suggested that a salt concentration higher than 120 mm would reduce cowpea germination. Salinity engenders osmotic or toxicity effects, which result in low seed germination (Waisel, 1972). In addition, Taffouo et al. (2009) reported that high sodium chloride concentration significantly affected germination rate in cowpea. Ashebir et al. (2013) studied the effects of salinity at germination in cowpea. Their results revealed that there was a significant variability in response to salt stress among cowpea genotypes. They found that the genotypes ‘211557’ and ‘Asebot’ were salt tolerant at germination stage. The objectives of this study were to evaluate the responses of cowpea genotypes to salinity (NaCl) at germination stage, to screen salt-tolerant cowpea genotypes, and to select for the most accurate parameter for assessing cowpea salt tolerance at germination stage for a salt-tolerance breeding program.

Materials and Methods

Plant materials.

A total of 151 cowpea genotypes consisting of 116 United States Department of Agriculture (USDA) Germplasm Resources Information Network (GRIN) germplasm accessions and 35 University of Arkansas lines were used in this study (Table 1). The 116 germplasm accessions were originally collected from 31 countries and classified into 12 regions depending on their origin (Caribbean, Eastern Africa, Southern Africa, Western Africa, Eastern Asia, Southeastern Asia, Southern Asia, Western Asia Europe, Latin America, and North America). All original seeds of the GRIN germplasm were obtained from USDA Plant Genetic Resources Conservation Unit at Griffin, GA, and were increased at the Research and Extension Station of University of Arkansas at Fayetteville, AR, in the summers 2014 and 2015.

Table 1.

Cowpea name (accession number), seed color, origin, and germination rates without salt treatment and under salt stress, absolute decrease (AD), inhibition index (II), relative salt tolerance (RST), and salt tolerance index (STI).

Table 1.

Determination of optimal seed germination temperature and salt concentration.

Two pilot experiments were conducted to determine the optimal temperature and salt (NaCl) concentration for cowpea seed germination. For the temperature experiment, the cowpea cultivar Early Scarlet was used to test seed germination rates under three thermogradient temperatures (25, 28, and 31 °C). The results showed that ‘Early Scarlet’ had the highest seed germination rate at 28 °C, which was the same optimum temperature for cowpea seed germination reported by Souza et al., (2004). In regard to salt concentration, five cowpea genotypes (‘Early Scarlet’, ‘07-303’, ‘09-655’, ‘PI293584’, and ‘PI527561’) were tested for seed germination under six levels of concentrations (0, 50, 100, 150, 200, and 250 mm NaCl) to determine the optimal NaCl concentration for salt stress. The concentrations were obtained by dissolving at 2.92, 5.84, 8.77, 11.69, and 14.61 g of sodium chloride powder of Science Company®, Lakewood, CO, into 1 L of distilled water, respectively. The salt concentration where germination rate between cowpea genotypes was the most significantly different was chosen as the optimal salt concentration to evaluate salt tolerance at germination stage.

Germination conditions.

The cowpea seeds used in this study were harvested from the field of University of Arkansas Research and Extension Center at Fayetteville, AR, during Summer 2015. Seeds having uniform size from each cowpea genotype were selected. To avoid any contamination, clean seeds were selected, and petri dishes used for germination were sterilized 60 s by washing with bleach 2% followed by 75% ethanol.

Forty seeds from each cowpea genotype were put on paper filter (Laboratory Nerd) (Avogadro’s Laboratory Supply, Inc, Shamong NJ 08088), which was previously placed in a petri dish of 9 cm in diameter. The treatment consisted of adding 14 mL of NaCl solution and deionized water for the control (0 mm NaCl) to each dish. After treatments were applied, the petri dishes were placed in an incubator New Brunswick Scientific Innova 4230® (Manasquan, NJ) at 28 °C for 48 h. Each genotype and salt treatment combination was placed on three different shelves in the incubator with three replicates, and each shelf was treated as a block. The experiment was run in multiple times because of space limitations. After each run, 75% ethanol solution was sprayed into the incubator to limit any microbial growth.

Measurements.

The seed germination data were gathered in 48 h after placing the petri dishes in the incubator. The seed germination rate was recorded when the radicle reached one third of the seed length. The performance of the cowpea genotype under salinity stress was evaluated by computing the values of AD due to salinity, II (González, 1996), RST, and STI (Fernandez, 1992; Saad et al., 2014). These parameters were estimated using the following formulas:
UNDE1
UNDE2
UNDE3
UNDE4
where GC = seed germination rate without salt stress, GS = seed germination under salt stress, and GCav = average of the seed germination rate of a cowpea genotype without salt stress.

Experimental design.

Regarding the preliminary test related to the determination of the optimal salt (NaCl) concentration, a two-factor factorial (genotype X salt) organized in a randomized complete block design (RCBD) with three blocks was adopted. The genotype consisted of ‘Early Scarlet’, ‘07-303’, ‘09-655’, ‘PI293584’, and ‘PI527561’, and the salt treatment levels were 0, 50, 100, 150, 200, and 250 mm. Three replications per genotype and salt treatment combination were used.

With respect to the assessment of salt tolerance among the 151 cowpea genotypes, the design was similar to that of the preliminary test using RCBD. However, the salt treatments were the optimal salt concentration from the preliminary test (150 mm NaCl) and the deionized water with 0 mm NaCl. Each salt-genotype combination was assigned to petri dishes, replicated three times, and each replication corresponded to each of the three shelves in the incubator and the shelf was used as a block.

Data analysis.

The parameters used for the analysis resulted from pairing data on a genotype under salinity treatment and without salt stress. Therefore, the statistical model for the analysis was as described below.

In the calculations, Yijk = value of the parameters for the jth cowpea genotype on the ith shelve at the kth replication, for i = 1, 2, 3; j = 1, …, 151, and k = 1, 2, 3.
UNDE5
where µ = constant (overall mean), Si = effect of the ith shelf (random effect) on the variability of the response, Gj = effect of the jth genotype (fixed effect) on the mean response, and εijk = experimental error associated with the ijkth observation. In this study, the effects of experiment runs would be assumed as negligible as the germination study was carried out in an incubator. However, the effect of shelves in the incubator should be taken into account because there could be within incubator temperature variability.

The analysis of variance (ANOVA) test was carried out using the general linear model (GLM) procedure of JMP Genomics 7 (SAS Institute, Cary, NC). The mean separation was performed using the Student’s t test at alpha = 0.05. The descriptive statistics were generated using “Tabulate”; the correlations among the parameters were analyzed using “Multivariate Methods” by “Multivariate” function; and the distributions of the data were drawn using “Distribution” in JMP Genomics 7.

The broad sense heritability (H2) was obtained from (Hosseini et al., 2012)
UNDE6
where = genotypic variance, = phenotypic variance, = variance associated with the experimental error, = variance associated with the shelf, s = number of shelves, and r = number of replications per treatment.
, , and were obtained using the following formulas:
UNDE7
where MSG = mean square genotype, MSE = mean square error, MSS = mean square shelve, r = number of replications, and n = number of genotypes.

In addition, ANOVA of the effects of the origin (region) of the cowpea genotypes on salt tolerance were analyzed using GLM of JMP Genomics. Before analysis, the genotypes without any information on their origin were discarded so that 132 genotypes were analyzed in this study.

The cluster analysis involved 151 individuals which were measured using six parameters: germination rates without salt treatment (nonstress) and under salt stress (stress), AD, II, RST, and STI. Ward’s method was used as a clustering technique (Sahu, 2013). The phylogenetic tree diagram was constructed using “Multivariate Methods” by “Cluster” in JMP Genomics 7 (SAS Institute, Cary, NC).

Results and Discussion

Optimal concentration for evaluation of cowpea salt tolerance at germination stage.

From our preliminary experiments with six salt concentrations: 0, 50, 100, 150, 200, and 250 mm of NaCl, when the NaCl concentration increased, the seed germination rate decreased either pooled across cowpea genotypes or individually (Figs. 1 and 2; Tables 2 and 3). At the salinity level at 250 mm, the seeds had minimal germination rate which made it difficult to separate the tolerant from the susceptible genotypes (Tables 2 and 3). At 150 mm, seed germination differed the most among the cowpea genotypes (F = 27.37, P value = 0.0001) (Table 3). It was reported that increasing salt concentration reduced the germination rate of cowpea genotype in other studies (Thiam et al., 2013; Wests and Francois, 1982; Zahedi et al., 2012). Lobato et al. (2009) used 150 mm NaCl to conduct their study on the effects of salinity on cowpea germination. Therefore, the salt concentration of 150 mm NaCl would be a reasonable concentration to perform salt tolerance testing in cowpea and it was used in our study.

Fig. 1.
Fig. 1.

Seed germination rates under six salt (NaCl) concentrations (0, 50, 100, 150, 200, and 250 mm): (A) five cowpea genotypes, respectively, and (B) the pooled five cowpea genotypes.

Citation: HortScience horts 52, 9; 10.21273/HORTSCI12195-17

Fig. 2.
Fig. 2.

An example of the seed germination rate in two cowpea genotypes, ‘09-655’ and ‘PI293584’ under six salt (NaCl) concentrations in the petri dishes (left) and the radicle length (right) in 48 h after NaCl treatment at 28 °C condition.

Citation: HortScience horts 52, 9; 10.21273/HORTSCI12195-17

Table 2.

ANOVA for pooled seed germination rates of five cowpea genotypes under six NaCl concentrations (0, 50, 100, 150, 200, and 250 mm).

Table 2.
Table 3.

ANOVA for seed germination rates of five cowpea genotypes under six salt concentrations of NaCl.

Table 3.

Germination at nonsalt stress and salt stress conditions.

The average germination rate among the 151 cowpea genotypes under the nonsalt stress condition varied from 60.0% to 99.2%, with a mean of 80.2% and a standard deviation of 11.0%. Incubating seeds in 150 mm NaCl resulted in a drop of germination to 0% to 77.5%, with an overall average of 38.7% and a standard deviation of 18.3%. These results suggested that salinity significantly reduced the germination rate in cowpea. For all parameters, there were no significant incubator shelf effects. The germination rate was significantly different among the cowpea genotypes under nonstress condition (F = 7.37, P value < 0.0001) (Table 4). ‘Envoy’ (99.2%), ‘PI583194’ (99.2%), ‘PI487518’ (98.3%), ‘PI582579’ (97.5%), ‘PI218123’ (97.5%), ‘PI253428’ (97.5%), ‘PI255765’ (97.5%), and ‘PI582421’ (97.5%) had the highest germination rates, and ‘PI225922’ (60.8%), ‘PI339610’ (60.8%), and ‘PI347639’ (60%) had the lowest germination rates. Under salinity condition, ‘PI201498’ performed well with a germination rate of 77.5%, indicating it was a salt-tolerant cowpea accession at germination stage; however, the lowest germination rate was recorded for ‘PI252665’ (1.7%), ‘09-393’ (0.8%), ‘PI582522’ (0.8%), and ‘PI582813’ (0.0%), indicating that they were very susceptible to salt stress at the germination stage. The germination rate among the cowpea genotypes was significantly different under salt stress (F = 16.62, P value < 0.0001) (Table 4).

Table 4.

ANOVA for six parameters related to seed germination based on cowpea genotypes.

Table 4.

Absolute decrease and inhibition index.

The AD indicated the decrease of the germination rate between the nonsalinity conditions and the salt treatment. In this study, salinity reduced the germination rate from 5.8% to 94.2%. The cowpea genotypes responded differently to salinity environment at germination stage in terms of AD (F = 10.1, P value < 0.0001) (Table 4). ‘PI582522’ (94.2%) exhibited the highest AD. ‘PI585422’ had the lowest AD, 5.8%. The higher the AD was, the more salt susceptible the genotype.

The II was a parameter which was widely used for studies related to plant stress (González, 1996). The II of the germination ranged from 7.7% to 100.0%, with an average of 51.2% and a standard deviation of 22.6%, indicating a large variability in responses to salinity among the cowpea genotypes. In addition, the inhibition of the germination because of salinity significantly differed among the cowpea genotypes (F = 11.6, P value <0.0001) (Table 4). ‘PI582813’, ‘PI582522’, and ‘09-393’ had a very high II, over 99% under salt stress. These results suggested that these lines are highly salt sensitive at germination stage. ‘PI582422’, ‘09-529’, ‘PI293584’, ‘PI582570’, and ‘PI339611’ had the lowest II, which were 7.7%, 12.2%, 13.3%, 13.6%, and 14.6%, respectively. These accessions could be excellent sources for salt tolerance at germination stage. The lower the II was, the more likely the genotype withstood salt stress (González, 1996).

Relative salt tolerance and salt-tolerance index.

The RST and STI were two parameters to measure cowpea salt tolerance. The higher the RST or STI parameter was, the more likely the genotype was salt tolerant (Fernandez, 1992; Saad et al., 2014).

The RST varied from 0 to 0.92, with a mean of 0.49 and a standard deviation of 0.23. RST was significantly different among the cowpea genotypes (F = 11.99, P value < 0.0001) (Table 4). ‘PI582422’, ‘09-529’, ‘PI293584’, and ‘PI582570’ had the highest RST scores, indicating that they were salt tolerant based on RST. ‘PI582813’, ‘09-393’, ‘PI582522’, and ‘PI582665’ exhibited the lowest RST scores, suggesting that they were salt-sensitive at germination stage.

The STI ranged from 0 to 0.92, with a mean of 0.48 and a standard deviation of 0.22. Significant differences were observed among the cowpea accessions in terms of STI (F = 13.16, P value < 0.0001) (Table 4). ‘PI582422’ (0.92), ‘09-529’ (0.87), ‘PI293584’ (0.86), and ‘PI582570’ (0.85) exhibited the highest STI, suggesting that these lines were highly tolerant to salinity at germination stage. The lowest STI was found in ‘PI582813’ (0), ‘09-393’ (0.01), ‘PI582522’ (0.01), and ‘PI582665’ (0.02) (Table 4), indicating that these cowpea genotypes are highly salt-sensitive.

The population dynamic of a crop is closely related to its seed germination (TeKrony and Egli, 1991). In this study, large variation of responses to salinity among the cowpea genotypes was observed for all parameters. As expected, there was a significant effect of salinity on the germination rate of the cowpea panel. Similar results have been found in other studies (Ashebir et al., 2013; Taffouo et al., 2009; Wests and Francois, 1982).

Analysis by geographical location.

Significant differences were observed in seed germination rate without salt treatment and the germination rate under salt stress, AD, II, RST, and STI among the 12 different regions of origin of the cowpea genotypes (P values < 0.0001) (Table 5). Genotypes from the Caribbean and Southern Asia showed better salt tolerance at the germination stage. The II was smaller than 40%, and both RST and STIs were greater than 0.6 on average. Cowpea genotypes from Europe and North America were more sensitive to salinity at the germination stage (Table 6). The results suggested that the origins of those genotypes markedly impacted their response to salinity, so the origin should be taken into a consideration when one selects cowpea genotypes for salt tolerance. Little has been done regarding the effect of geographical distribution on plant salt tolerance. However, geographical distribution proved to be a strong driving factor on plant adaptation to stress. Burke (1990) reported that crops grown in semiarid areas developed mechanisms which enabled them to overcome permanent exposure to high temperature. Those mechanisms involved both cellular adaptation and photosynthetic responses to heat stress, indicating ‘PI582813’ and ‘09-393’ were salt tolerant. In addition, Stankowski et al. (2015) showed that ecology played a pivotal role in plant adaptation. Their study on Mimulus aurantiacus C., a flower plant, revealed that floral trait had evolved according to the local conditions, suggesting that geographical location had shaped the adaptation of some traits to a specific environment. With respect to salt tolerance in cowpea, further investigation is required to unravel the mechanisms behind these significant differences in response to salinity among the genotypes from different countries.

Table 5.

ANOVA for six parameters related to seed germination based on the origin of cowpea genotypes.

Table 5.
Table 6.

Mean separation for salt tolerance parameters of different regions.

Table 6.

Broad sense heritability.

H2 associated with the seed germination without salt stress was 68%. However, it was 83.9% under saline conditions. H2 was also observed as 75.2% for the AD in germination rate due to salt stress. The results indicated a high H2 for the parameters related to the II, RST, and STI, with H2 equal to 77.9%, 78.2%, and 80.8%, respectively. Foolad and Jones (1992) found high heritability for tomato salt tolerance at seed germination stage. They estimated heritability to be 76.0% for tomato salt tolerance of an F2:3 population derived from a cross between ‘PI174263’ and ‘UCT5’, which was close to those of salt tolerance in cowpea in this current investigation.

Correlation between parameters.

The six parameters, without salt stress (nonstress), with salt stress (stress), AD, RST, and STI involved in this study showed near normal distributions (Figs. 3 and 4). The correlation coefficients among the six parameters were estimated (Table 7). There was a relatively low linear correlation between the germination rate under nonsalt stress condition and the other five parameters related to salinity stress, with r = 0.20, 0.38, 0.10, −0.10, and 0.08, respectively (Table 7), suggesting that salt tolerance at germination stage had a weak association with the germination rate in normal conditions in cowpea. However, the seed germination rate under salt stress had a very high negative linear correlation with the AD (r = −0.83) and the II (r = −0.95), but a high positive linear correlation observed between the germination rate under salt treatment and the RST (r = 0.95), and the STI (r = 0.95). These results indicate that salt tolerance at germination stage is highly associated with germination rate under salt stress.

Fig. 3.
Fig. 3.

Distribution of seed germination rates among 151 cowpea genotypes: (A) without salt stress and (B) with salt stress.

Citation: HortScience horts 52, 9; 10.21273/HORTSCI12195-17

Fig. 4.
Fig. 4.

Distributions of seed germination rates among 151 cowpea genotypes in four parameters: (A) absolute decrease, (B) inhibition index, (C) relative salt tolerance, and (D) salt tolerance index.

Citation: HortScience horts 52, 9; 10.21273/HORTSCI12195-17

Table 7.

Correlation among six parameters, without salt stress (nonstress), with salt stress (stress), absolute decrease (AD), inhibition index (II), relative salt tolerance index (RST), and salt tolerance index (STI).

Table 7.

Cluster analysis.

Five different groups were identified among the 151 cowpea genotypes involved in this study (Supplemental Fig. 1). The three cowpea accessions, ‘PI582422’, ‘PI293584’, and ‘PI582570’ having the highest STI were clustered together and located at the same cluster, and the four lowest STI genotypes, ‘PI582813’, ‘09-393’, ‘PI582522’, and ‘PI582665’ were clustered together and belonged to another cluster. These results indicated that the STI was reliable method to distinguish the high salt tolerant to high salt sensitive cowpea genotypes.

Conclusion

This study provides data on the responses of cowpea genotype to salinity, which could be used to screen for salt tolerant parents for breeding purposes. To our knowledge, this is one of the first reports dealing with cowpea salt tolerance at germination stage which involved a large number of cowpea genotypes having a wide range of variability in terms of country of origin. The results suggest that ‘PI582422’, ‘PI293584’, ‘PI582570’, and ‘09-529’ have strong salt tolerance at germination stage. In addition, the most salt-sensitive lines ‘PI582813’, ‘09-393’, ‘PI582522’, and ‘PI582665’ could be used as parents for developing populations for QTL mapping for cowpea salt tolerance at germination stage. Findings from this current report will add information on cowpea genotype responses to salinity at germination stage, which definitely helps breeders to select potential lines for further variety improvement process.

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  • Lobato, A.K.S., Santos Filho, B.G., Costa, R.C.L., Gonçalves, V., Moraes, E.C., Neto, C.F.O. & Rodrigues, V.L.F. 2009 Morphological, physiological and biochemical responses during germination of the cowpea (Vigna unguiculata Cv. Pitiuba) seeds under salt stress World J. Agr. Sci. 5 5 590 596

    • Search Google Scholar
    • Export Citation
  • Maas, E.V. & Hoffman, G.J. 1977 Crop salt tolerance-current assessment J. Irrig. Drain. Div. 103 2 115 314

  • Mamiro, P.S., Mbwaga, A.M. & Mamiro, D.P. 2011 Nutritional quality and utilization of local and improved cowpea varieties in some regions in tanzania Afr. J. Food Agr. Nutr. Dev. 11 1 4490 4506

    • Search Google Scholar
    • Export Citation
  • Mauromicale, G. & Licandro, P. 2002 Salinity and temperature effects on germination, emergence and seedling growth of globe artichoke Agronomie 22 5 443 450

    • Search Google Scholar
    • Export Citation
  • Mistura, C., dos Santos, A.E.O., Ono, E.O., Rodrigues, J.D., de Almeida, M.B. & Araújo, A.D.B. 2011 Germination and seedlings development of the butterfly pea Rev.Brasi. Sau. Prod. Ani. 12 2 306 317

    • Search Google Scholar
    • Export Citation
  • Muchero, W., Diop, N.N., Bhat, P.R., Fenton, R.D., Wanamaker, S., Pottorff, M. & Close, T.J. 2009 A consensus genetic map of cowpea [Vigna unguiculata (L) Walp.] and synteny based on EST-derived SNPs Proc. Natl. Acad. Sci. USA 106 43 18159 18164

    • Search Google Scholar
    • Export Citation
  • Okiror, S.O., Onyilagha, J.C. & Dunbar, T. 2008 Investigating the potentials of four cowpea (southern pea) cultivars for fresh seed production Intl. J. Appl. 3 1 67 74

    • Search Google Scholar
    • Export Citation
  • Olufajo, O.O. 2012 Agronomic performance of improved cowpea varieties under natural infestation with Alectra vogelii (Benth.) in the northern Guinea savannah of Nigeria Agr. Trop. Subtrop. 45 2 66 71

    • Search Google Scholar
    • Export Citation
  • Omami, E.N. & Hammes, P.S. 2006 Interactive effects of salinity and water stress on growth, leaf water relations, and gas exchange in amaranth (Amaranthus spp.) N. Z. J. Crop Hort. Sci. 34 1 33 44

    • Search Google Scholar
    • Export Citation
  • Parker, M.B., Gascho, G.J. & Gaines, T.P. 1983 Chloride toxicity of soybeans grown on atlantic coast flatwoods soils Agron. J. 75 3 439 443

  • Pathan, M.S., Lee, J.D., Shannon, J.G. & Nguyen, H.T. 2007 Advances in molecular breeding toward drought and salt tolerant crops. Springer, the Netherlands

  • Reddy, K.M.M. & Reddy, M.J.M. 2016 SWOT on existing agro-ecosystem for soybean production in the northern Telangana Zone Intl. J. Farm Sci. 6 2 268 275

    • Search Google Scholar
    • Export Citation
  • Rengasamy, P., Stevens, D., Kelly, J., McLaughlin, M. & Unkovich, M. 2006 Soil salinity and sodicity. CSIRO Publ., Collingwood, Australia.

  • Saad, F.F., Abd El-Mohsen, A.A., Abd, M.A. & Al-Soudan, I.H. 2014 Effective selection criteria for evaluating some barley crosses for water stress tolerance Adv. Agr. Biol. 1 3 112 123

    • Search Google Scholar
    • Export Citation
  • Sahu, P.K. 2013 Research methodology: A guide for researchers in agricultural science, social science and other related fields. Springer, New York, NY

  • Shannon, M.C. 1997 Adaptation of plants to salinity Adv. Agron. 60 75 120

  • Souza, R.P., Machado, E.C., Silva, J.A.B., Lagôa, A.M.M.A. & Silveira, J.A.G. 2004 Photosynthetic gas exchange, chlorophyll fluorescence and some associated metabolic changes in cowpea (Vigna unguiculata) during water stress and recovery Environ. Expt. Bot. 51 1 45 56

    • Search Google Scholar
    • Export Citation
  • Stankowski, S., Sobel, J.M. & Streisfeld, M.A. 2015 The geography of divergence with gene flow facilitates multitrait adaptation and the evolution of pollinator isolation in Mimulus aurantiacus Evolution 69 12 3054 3068

    • Search Google Scholar
    • Export Citation
  • Taffouo, V.D., Meguekam, L., Kenne, M., Magnitsop, A., Akoa, A., Ourry, A. & Tenywa, J.S. 2009 Stress effects on germination, plant growth and accumulation of metabolites in five leguminous plants. 9th African Crop Sci. Conf. Proc., Cape Town, South Africa, p. 157–161

  • Tan, H., Tie, M., Luo, Q., Zhu, Y., Lai, J. & Li, H. 2012 A review of molecular makers applied in cowpea (Vigna unguiculata L. Walp.) breeding J. Life Sci. 6 1190 1199

    • Search Google Scholar
    • Export Citation
  • TeKrony, D.M. & Egli, D.B. 1991 Relationship of seed vigor to crop yield: A review Crop Sci. 31 3 816 822

  • Thiam, M., Champion, A., Diouf, D. & Sy, M.O. 2013 NaCl effects on in vitro germination and growth of some senegalese cowpea (Vigna unguiculata (L.) Walp.) cultivars ISRN Biotechnol. 2013 1 11

    • Search Google Scholar
    • Export Citation
  • Waisel, Y. 1972 Biology of halophytes. Academic Press, New York, NY

  • Wallender, W.W. & Tanji, K.K. 2011 Agricultural salinity assessment and management. Am. Soc. Civ. Eng. Reston, VA

  • Wests, D.W. & Francois, L.E. 1982 Effects of salinity on germination, growth and yield of cowpea Irrig. Sci. 3 3 169 175

  • Yang, J. & Blanchar, R.W. 1993 Differentiating chloride susceptibility in soybean cultivars Agron. J. 85 4 880 885

  • Zahedi, S.M., Ansari, N.A. & Azizi, M. 2012 The study of the effect of salinity stress on the germination and the initial growth of cowpea (Vigna unguiculata L. Walp) J. Agr. Technol. 8 7 2353 2372

    • Search Google Scholar
    • Export Citation
  • Zhang, X., Tang, L., You, C. & Hu, F. 2013 Effects of low temperatures on seed germination and seedling emergence of cowpea (Vigna unguiculata) J. South. Agr. 11 7

    • Search Google Scholar
    • Export Citation

Supplemental Fig. 1.
Supplemental Fig. 1.

Phylogenetic tree diagram among 151 cowpea genotypes based on six salt tolerant parameters: germination rates without salt treatment (Non-stress) and under salt stress (Stress), absolute decrease (AD), inhibition index (II), relative salt tolerance (RST), and salt tolerance index (STI).

Citation: HortScience horts 52, 9; 10.21273/HORTSCI12195-17

Contributor Notes

Corresponding author. E-mail: ashi@uark.edu.

  • View in gallery

    Seed germination rates under six salt (NaCl) concentrations (0, 50, 100, 150, 200, and 250 mm): (A) five cowpea genotypes, respectively, and (B) the pooled five cowpea genotypes.

  • View in gallery

    An example of the seed germination rate in two cowpea genotypes, ‘09-655’ and ‘PI293584’ under six salt (NaCl) concentrations in the petri dishes (left) and the radicle length (right) in 48 h after NaCl treatment at 28 °C condition.

  • View in gallery

    Distribution of seed germination rates among 151 cowpea genotypes: (A) without salt stress and (B) with salt stress.

  • View in gallery

    Distributions of seed germination rates among 151 cowpea genotypes in four parameters: (A) absolute decrease, (B) inhibition index, (C) relative salt tolerance, and (D) salt tolerance index.

  • View in gallery

    Phylogenetic tree diagram among 151 cowpea genotypes based on six salt tolerant parameters: germination rates without salt treatment (Non-stress) and under salt stress (Stress), absolute decrease (AD), inhibition index (II), relative salt tolerance (RST), and salt tolerance index (STI).

  • An, P., Inanaga, S., Kafkafi, U., Lux, A. & Sugimoto, Y. 2001 Different effect of humidity on growth and salt tolerance of two soybean cultivars Biol. Plant. 44 3 405 410

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  • Ashebir, G., Mebeasilassie, A. & Manikanidan, M. 2013 The response of some cowpea (Vigna unguiculata (L.) Walp.) genotypes for salt stress during germination and seedling stage J. Stress Physiol. Biochem. 9 4 57

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  • Kent, M.L. & Lauchli, A. 1985 Germination and seedling growth of cotton: Salinity-calcium interactions Plant Cell Environ. 8 2 155 159

  • Läuchli, A. & Lüttge, U. 2002 Salinity: Environment-plants-molecules. Dordrecht, Kluwer Academic Publishers, the Netherlands

  • Lobato, A.K.S., Santos Filho, B.G., Costa, R.C.L., Gonçalves, V., Moraes, E.C., Neto, C.F.O. & Rodrigues, V.L.F. 2009 Morphological, physiological and biochemical responses during germination of the cowpea (Vigna unguiculata Cv. Pitiuba) seeds under salt stress World J. Agr. Sci. 5 5 590 596

    • Search Google Scholar
    • Export Citation
  • Maas, E.V. & Hoffman, G.J. 1977 Crop salt tolerance-current assessment J. Irrig. Drain. Div. 103 2 115 314

  • Mamiro, P.S., Mbwaga, A.M. & Mamiro, D.P. 2011 Nutritional quality and utilization of local and improved cowpea varieties in some regions in tanzania Afr. J. Food Agr. Nutr. Dev. 11 1 4490 4506

    • Search Google Scholar
    • Export Citation
  • Mauromicale, G. & Licandro, P. 2002 Salinity and temperature effects on germination, emergence and seedling growth of globe artichoke Agronomie 22 5 443 450

    • Search Google Scholar
    • Export Citation
  • Mistura, C., dos Santos, A.E.O., Ono, E.O., Rodrigues, J.D., de Almeida, M.B. & Araújo, A.D.B. 2011 Germination and seedlings development of the butterfly pea Rev.Brasi. Sau. Prod. Ani. 12 2 306 317

    • Search Google Scholar
    • Export Citation
  • Muchero, W., Diop, N.N., Bhat, P.R., Fenton, R.D., Wanamaker, S., Pottorff, M. & Close, T.J. 2009 A consensus genetic map of cowpea [Vigna unguiculata (L) Walp.] and synteny based on EST-derived SNPs Proc. Natl. Acad. Sci. USA 106 43 18159 18164

    • Search Google Scholar
    • Export Citation
  • Okiror, S.O., Onyilagha, J.C. & Dunbar, T. 2008 Investigating the potentials of four cowpea (southern pea) cultivars for fresh seed production Intl. J. Appl. 3 1 67 74

    • Search Google Scholar
    • Export Citation
  • Olufajo, O.O. 2012 Agronomic performance of improved cowpea varieties under natural infestation with Alectra vogelii (Benth.) in the northern Guinea savannah of Nigeria Agr. Trop. Subtrop. 45 2 66 71

    • Search Google Scholar
    • Export Citation
  • Omami, E.N. & Hammes, P.S. 2006 Interactive effects of salinity and water stress on growth, leaf water relations, and gas exchange in amaranth (Amaranthus spp.) N. Z. J. Crop Hort. Sci. 34 1 33 44

    • Search Google Scholar
    • Export Citation
  • Parker, M.B., Gascho, G.J. & Gaines, T.P. 1983 Chloride toxicity of soybeans grown on atlantic coast flatwoods soils Agron. J. 75 3 439 443

  • Pathan, M.S., Lee, J.D., Shannon, J.G. & Nguyen, H.T. 2007 Advances in molecular breeding toward drought and salt tolerant crops. Springer, the Netherlands

  • Reddy, K.M.M. & Reddy, M.J.M. 2016 SWOT on existing agro-ecosystem for soybean production in the northern Telangana Zone Intl. J. Farm Sci. 6 2 268 275

    • Search Google Scholar
    • Export Citation
  • Rengasamy, P., Stevens, D., Kelly, J., McLaughlin, M. & Unkovich, M. 2006 Soil salinity and sodicity. CSIRO Publ., Collingwood, Australia.

  • Saad, F.F., Abd El-Mohsen, A.A., Abd, M.A. & Al-Soudan, I.H. 2014 Effective selection criteria for evaluating some barley crosses for water stress tolerance Adv. Agr. Biol. 1 3 112 123

    • Search Google Scholar
    • Export Citation
  • Sahu, P.K. 2013 Research methodology: A guide for researchers in agricultural science, social science and other related fields. Springer, New York, NY

  • Shannon, M.C. 1997 Adaptation of plants to salinity Adv. Agron. 60 75 120

  • Souza, R.P., Machado, E.C., Silva, J.A.B., Lagôa, A.M.M.A. & Silveira, J.A.G. 2004 Photosynthetic gas exchange, chlorophyll fluorescence and some associated metabolic changes in cowpea (Vigna unguiculata) during water stress and recovery Environ. Expt. Bot. 51 1 45 56

    • Search Google Scholar
    • Export Citation
  • Stankowski, S., Sobel, J.M. & Streisfeld, M.A. 2015 The geography of divergence with gene flow facilitates multitrait adaptation and the evolution of pollinator isolation in Mimulus aurantiacus Evolution 69 12 3054 3068

    • Search Google Scholar
    • Export Citation
  • Taffouo, V.D., Meguekam, L., Kenne, M., Magnitsop, A., Akoa, A., Ourry, A. & Tenywa, J.S. 2009 Stress effects on germination, plant growth and accumulation of metabolites in five leguminous plants. 9th African Crop Sci. Conf. Proc., Cape Town, South Africa, p. 157–161

  • Tan, H., Tie, M., Luo, Q., Zhu, Y., Lai, J. & Li, H. 2012 A review of molecular makers applied in cowpea (Vigna unguiculata L. Walp.) breeding J. Life Sci. 6 1190 1199

    • Search Google Scholar
    • Export Citation
  • TeKrony, D.M. & Egli, D.B. 1991 Relationship of seed vigor to crop yield: A review Crop Sci. 31 3 816 822

  • Thiam, M., Champion, A., Diouf, D. & Sy, M.O. 2013 NaCl effects on in vitro germination and growth of some senegalese cowpea (Vigna unguiculata (L.) Walp.) cultivars ISRN Biotechnol. 2013 1 11

    • Search Google Scholar
    • Export Citation
  • Waisel, Y. 1972 Biology of halophytes. Academic Press, New York, NY

  • Wallender, W.W. & Tanji, K.K. 2011 Agricultural salinity assessment and management. Am. Soc. Civ. Eng. Reston, VA

  • Wests, D.W. & Francois, L.E. 1982 Effects of salinity on germination, growth and yield of cowpea Irrig. Sci. 3 3 169 175

  • Yang, J. & Blanchar, R.W. 1993 Differentiating chloride susceptibility in soybean cultivars Agron. J. 85 4 880 885

  • Zahedi, S.M., Ansari, N.A. & Azizi, M. 2012 The study of the effect of salinity stress on the germination and the initial growth of cowpea (Vigna unguiculata L. Walp) J. Agr. Technol. 8 7 2353 2372

    • Search Google Scholar
    • Export Citation
  • Zhang, X., Tang, L., You, C. & Hu, F. 2013 Effects of low temperatures on seed germination and seedling emergence of cowpea (Vigna unguiculata) J. South. Agr. 11 7

    • Search Google Scholar
    • Export Citation
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