Optimizing NaCl concentration for screening salt tolerance in cowpea (Vigna unguiculata L. Walp.) genotypes at germination stage using salt injury index

Abstract

Salinity is a major abiotic stress that impacts plant growth and crop yields, particularly in saline-prone regions. The main objective of this work is to determine the optimum NaCl concentration for screening salt tolerance using the Salt Injury Index (SII) by analyzing the effects of NaCl-induced salinity on germination and seedling growth in 30 cowpea (Vigna unguiculata L. Walp.) genotypes. Cowpea seeds were exposed to six NaCl concentrations (0, 75, 100, 150, 200, and 250 mM) in petri plates. Seedling growth parameters such as germination, shoot and root length, seedling length, dry weight, and vigour indices (I & II), were recorded to evaluate the effects of salinity stress. A consistent decline in all measured parameters was observed as NaCl concentration increased. Linear regression of the Salt Injury Index values revealed NaCl concentrations caused a 50% reduction in specific traits, including 215 mM affecting germination, 180 mM for shoot, 210 mM for root, 200 mM for seedling length and dry weight, and 130 and 160 mM for Vigour Index-I and -II, respectively, with an average threshold of 183.57 mM. Consequently, 200 mM NaCl was identified as the optimal concentration to evaluate salt tolerance levels, offering a standardized benchmark for assessing cowpea genotypes under saline conditions.