Contrasting Responses of Guar Genotypes Shed Light on Multiple Component Traits of Salinity Tolerance Mechanisms

Guar (Cyamopsis tetragonoloba (L.) Taub.) is a legume crop, and gum derived from its seeds has various industrial applications. Due to its tolerance to various abiotic stresses, guar can be grown under water-deficit or high-salinity conditions. In this investigation, four diverse guar genotypes that performed at a similar level in field conditions were evaluated in a salinity experiment in the greenhouse lysimeter system. Based on the salt tolerance index (STI) for shoot biomass, root biomass, shoot length, and root length, Matador and PI 268229 were classified as salt-tolerant, and PI 340261 and PI 537281 as salt-sensitive. Leaf Na concentrations were 4- to 5.5-fold higher, and leaf Cl concentrations were 1.6- to 1.9-fold higher in salt-sensitive lines than salt-tolerant lines under salinity. The strong associations between the leaf K concentrations under salinity compared to the control (K-salinity/K-control) ratio and STI for stem and root length advocate higher importance of K-salinity/K-control than total leaf K concentrations. The expression analyses of genes involved in Na+ and Cl- transport revealed the importance of different component traits of salinity tolerance mechanisms, such as the exclusion of Na+/Cl- from the root, sequestration of Cl- in root vacuoles, retrieval of Na+/Cl- from xylem during salinity stress, root-to-shoot Na+/Cl- translocation, and K+-Na+ homeostasis.