Change in Chlorophyll Content Over Time Well-differentiated Salt-tolerant, Moderately Salt-tolerant, and Salt-susceptible Cowpea Genotypes

in HortScience

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.

Contributor Notes

These authors contributed equally to this work.

Corresponding authors. E-mail: ashi@uark.edu or Beiquan.Mou@ARS.USDA.GOV.

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    Overview of greenhouse experiments to evaluate the chlorophyll content over time in cowpea at the seedling stage.

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    Significant differences in leaf greenness between salt-treated and nonsalt-stressed plants (Tr: salt-treated plants using 200 mm NaCl; Ck: plants irrigated with deionized water).

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    Heatmap showing the change in the chlorophyll content of salt-stressed cowpea genotypes. Accessions were divided into three groups (tolerant, moderate, and susceptible). (A) Almost no differences among the three groups were reported. (B) Cowpea genotypes started differentiating from each other. (C) Clear differences were found between salt-tolerant, moderately salt-tolerant, and salt-susceptible genotypes. On the heatmap, red indicates low chlorophyll content, whereas blue indicates high chlorophyll content.

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    Curves displaying the pooled chlorophyll contents of plants under salt stress for salt-tolerant, moderately salt-tolerant, and salt-susceptible genotypes.

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    Combined violin and boxplots showing the magnitude of chlorophyll content differences over time for salt-tolerant, moderately salt-tolerant, and salt-susceptible cowpea genotypes.

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    Combined violin and boxplots showing the magnitude of relative salt tolerance differences over time for salt-tolerant, moderately salt-tolerant, and salt-susceptible cowpea genotypes.

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    Cowpea salt-tolerant and salt-susceptible genotypes on day 18 of salt stress.

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    Distributions of the leaf chlorophyll content over time. Blue represents the observations of the nonsalt-treated plants, whereas red indicates those of salt-stressed plants (200 mm NaCl).

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    Distributions of relative salt tolerance for chlorophyll content over time. Relative salt tolerance was the ratio between the chlorophyll content under salt stress conditions and without salt stress.

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