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Waltram Ravelombola, Jun Qin, Yuejin Weng, Beiquan Mou, and Ainong Shi

Little has been done with respect to breeding for salt-tolerant cowpea (Vigna unguiculata) cultivars despite of salt stress being a growing threat to cowpea production. Seedling stage is one the most susceptible stages to salt stress in cowpea. Establishing a streamlined methodology for rapidly screening a large number of genotypes will significantly contribute toward enhancing cowpea breeding for salt tolerance. Therefore, the objective of this study was to establish and validate a simple approach for salt tolerance evaluation in cowpea seedlings. A total of 30 genotypes including two controls (PI582468, a salt-tolerant genotype, and PI255774, a salt-sensitive genotype) were greenhouse-grown under 0 mm and 200 mm NaCl. A total of 14 above-ground traits were evaluated. Results revealed: (1) significant differences were observed in average number of dead plants per pot, leaf injury scores, relative salt tolerance (RST) for chlorophyll, plant height, and leaf and stem biomass among the 30 genotypes; (2) all PI255774 plants were completely dead, whereas those of PI582438 were fully green after 2 weeks of salt stress, which validated this methodology; (3) RST for chlorophyll content was highly correlated with number of dead plants and leaf injury scores; (4) RST for leaf biomass was moderately correlated with number of dead plants and leaf injury scores; and (5) RST in plant height was poorly correlated with number of dead plants and leaf injury scores Therefore, less number of dead plants per pot, high chlorophyll content, and less leaf injury scores were good criteria for salt tolerance evaluation in cowpea. This study provided a simple methodology and suggested straightforward criteria to evaluate salt tolerance at seedling stage in cowpea.

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Waltram Ravelombola, Ainong Shi, Jun Qin, Yuejin Weng, Gehendra Bhattarai, Bazgha Zia, Wei Zhou, and Beiquan Mou

Impacts of drought stress on crop production can significantly impair farmer’s revenue, hence adversely impacting the gross national product growth. For cowpea [Vigna unguiculata (L.) Walp.], which is a legume of economic importance, effects of drought at early vegetative growth could lead to substantial yield losses. However, little has been done with respect to breeding for cowpea cultivars withstanding drought at early vegetative growth. In addition, previous investigations have focused on how plant morphology and root architecture can confer drought tolerance in cowpea, which is not sufficient in efforts to unravel unknown drought tolerance–related genetic mechanisms, potentially of great importance in breeding, and not pertaining to either plant morphology or root architecture. Therefore, the objective of this study was to evaluate aboveground drought-related traits of cowpea genotypes at seedling stage. A total of 30 cowpea genotypes were greenhouse grown within boxes and the experimental design was completely randomized with three replicates. Drought stress was imposed for 28 days. Data on a total of 17 aboveground-related traits were collected. Results showed the following: 1) a large variation in these traits was found among the genotypes; 2) more trifoliate wilt/chlorosis tolerance but more unifoliate wilt/chlorosis susceptible were observed; 3) delayed senescence was related to the ability of maintaining a balanced chlorophyll content in both unifoliate and trifoliate leaves; and 4) the genotypes PI293469, PI349674, and PI293568 were found to be slow wilting and drought tolerant. These results could contribute to advancing breeding programs for drought tolerance in cowpea.

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Waltram Second Ravelombola, Ainong Shi, Yuejin Weng, John Clark, Dennis Motes, Pengyin Chen, and Vibha Srivastava

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.

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Yuejin Weng, Jun Qin, Stephen Eaton, Yufeng Yang, Waltram Second Ravelombola, and Ainong Shi

Cowpea [Vigna unguiculata (L.) Walp] is an annual legume crop grown worldwide to provide protein for human consumption and animal feed. The objective of this research was to evaluate the seed protein content in U.S. Department of Agriculture (USDA) cowpea germplasm for use in cowpea breeding programs. A field experiment was conducted with a randomized complete block design (RCBD) with three duplications in two locations, Fayetteville and Alma, in Arkansas, United States. A total of 173 USDA cowpea accessions were evaluated with the Elementar Rapid N analyzer III for their seed protein contents. The results showed that there was a wide range of seed protein content among the 173 cowpea genotypes, ranging from 22.8% to 28.9% with an average of 25.6%. The broad-sense heritability for seed protein among the 173 cowpea genotypes was 50.8%, indicating that seed protein content was inheritable and can be selected in breeding processing. The top five cowpea accessions with the highest seed protein contents were USDA accession PI 662992 originally collected from Florida (28.9%), PI 601085 from Minnesota (28.5%), and PI 255765 and PI 255774 from Nigeria and PI 666253 from Arkansas (28.4% each). PI 339587 from South Africa had the lowest protein content with 21.8%. The were also significant differences in seed protein contents observed among different seedcoat colors; the accessions with cream color exhibited higher protein content (27.2%) than others. This research could provide information for breeders to develop cowpea cultivars with higher seed protein content in a cowpea breeding program.

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Lingdi Dong, Waltram Ravelombola, Yuejin Weng, Jun Qin, Wei Zhou, Gehendra Bhattarai, Bazgha Zia, Wei Yang, Linqi Shi, Beiquan Mou, and Ainong Shi

Previous investigations showed that accumulations of Na+ and Cl in leaves resulted in reductions in chlorophyll content, thereby affecting photosynthesis. Understanding how chlorophyll content evolves over time will help plant breeders to select cowpea genotypes with better tolerance to salinity by allowing them to choose those with more stable chlorophyll content under salt stress. The objective of this study was to assess how the chlorophyll content of cowpea genotypes changed over the course of 24 d of salt stress at the seedling stage. A total of 24 cowpea genotypes with different salt responses were used in this study. The experiment used a split-plot design with salt treatment as the main plot and cowpea genotypes as the subplot. In the main plot, there were two salt treatments: 0 mm (ionized water) and 200 mm NaCl. In the subplot, the cowpea genotypes were arranged as a completely randomized design with three replicates per genotype. The results revealed that: a1) the time × genotype interaction was significant under conditions with and without salt; 2) chlorophyll content slowly decreased in the salt-tolerant genotypes; 3) chlorophyll content slightly increased on day 6 and day 9 of salt stress in both moderate and sensitive genotypes, but it decreased at a faster rate than in the salt-tolerant genotypes; and 4) salt-sensitive genotypes were completely dead on day 24 of salt stress, whereas the salt-tolerant genotypes were able to maintain a significant amount of chlorophyll content. These results can be used to advance breeding programs for salt tolerance in cowpea.

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Qirui Cui, Haizheng Xiong, Yufeng Yufeng, Stephen Eaton, Sora Imamura, Jossie Santamaria, Waltram Ravelombola, Richard Esten Mason, Lisa Wood, Leandro Angel Mozzoni, and Ainong Shi

Cowpea [Vigna unguiculate (L.) Walp.] is not only a healthy, nutritious, and versatile leguminous crop; it also has a relatively high adaptation to drought. Research has shown that cowpea lines have a high tolerance to drought, and many of them can survive more than 40 days under scorching and dry conditions. The cowpea (Southern pea) breeding program at the University of Arkansas has been active for more than 50 years and has produced more than 1000 advanced breeding lines. The purpose of this study was to evaluate the drought-tolerant ability in Arkansas cowpea lines and use the drought-tolerant lines in cowpea production or as parents in cowpea breeding. A total of 36 University of Arkansas breeding lines were used to screen drought tolerance at the seedling stage in this study. The experiment was conducted in the greenhouse using a randomized complete block design (RCBD) with two replicates, organized in a split-plot manner, where the drought treatment (drought and nondrought stress) as the main plot and the cowpea genotypes as the subplot. Drought stress was applied for 4 weeks, and three drought-tolerant–related traits were collected and analyzed. Results showed that cowpea breeding lines: ‘17-61’, ‘17-86’, ‘Early Scarlet’, and ‘ARBlackeye #1’ were found to be drought tolerant.