Genotypic evaluation of cowpea germplasm for salinity tolerance at germination and during seedling growth

Soil salinity represents a major constraint limiting crop production in arid and semi arid countries. The effect of salinity induced by sodium chloride (NaCl) at five levels (0, 50, 100, 150 and 200 mM) was investigated on four germination traits and thirteen seedling growth characteristics in twenty cowpea [Vigna unguiculata (L.) Walp.] genotypes (ET11, KEB CP004, KEB CP006, KEB CP009, KEB CP 010, KEB CP020, KEB CP033, KEB CP038, KEB CP039, KEB CP045, KEB CP051, KEB CP054, KEB CP057, KEB CP060, KEB CP067, KEB CP068, KEB CP118, MTA22, NO74 and NO1036). The germination tests were carried out on Petri dishes in the laboratory while seedling growth experiments continued in plastic pots in the greenhouse, both setting up using a randomised complete block design with three replications. Genotypic responses were significant for all germination traits (p < 0.001). Germination percentage, germination rate index, and coefficient of velocity of germination were all decreased by salt stress. However, the mean germination time increased with increasing saline conditions. Significant differences were found between genotypes for most growth attributes. Growth rate (centimeter increased in height per week) decreased significantly with increasing salinity, starting at 100 mM NaCl (24.20% reduction, 2.66 cm/week) with maximum reduction (38.58%) corresponding to 2.16 cm/week observed at 200 mM NaCl, compared to control (3.51 cm/week growth rate). Also, significant decline in shoot weights, number of functional leaves and dry matter production were observed under salinity. Salinity also reduced water content in shoot and root and did not affect root weights. Under salinity, significant correlations were found between all germination variables (p < 0.001). Growth rate was significantly associated with ten out of the twelve other seedling growth traits. Also, the dry matter production under salinity was significantly associated with all other seedling growth characteristics with the exception of root water content. Given the effect of salt stress, cowpea genotypes, namely NO1036, KEB CP004, KEB CP038 and KEB CP051, were the most tolerant while KEB CP068 and ET11 were the most sensitive ones. The results confirm substantial genetic variation in salt stress tolerance among the studied genotypes. The most tolerant genotypes should be further explored in genetic improvement programs and should be promoted for culture in regions affected by salinity.