Investigation of Physiological and Biochemical Mechanism of Salt Stress Tolerance in Eruca Sativa

Abstract

Salinity is one of the most lethal abiotic stresses which affect multiple aspects of plant physiology. Natural variations in plant germplasm are a great resource which could be exploited for improvement in salt tolerance. Eruca sativa (E. sativa) is an important industrial crop and exhibits tolerance to abiotic stresses. However, thorough evaluation of its salt stress tolerance mechanism is yet to be studied. Current study was designed to characterize 25 E. sativa accessions, originating from diverse geographical regions of Pakistan, for salt stress tolerance. Salt stress (150 mM NaCl) was applied for two weeks and the data of the various morphological, physiological, and biochemical traits were collected from control and treated plants of all the accessions. These traits include root length (RL), shoot length (SL), plant height (PH), leaf number (LN), leaf area (LA), fresh weight (FW), dry weight (DW), chlorophyll content (SPAD), electrolyte leakage (EL), relative water content (RWC) and proline content. Moreover, gas exchange parameters and mineral ions content were also determined. Compared with control, salt stressed group had significantly reduced mean SL, RL, PH, LN, LA, FW, DW, and SPAD. E. sativa accumulated, although with a greatly variable extent, significant proline content in salt stress conditions. NaCl treatment triggered slight increase in EL in few accessions. Mean RWC of control and treated groups was not significantly different although few accessions exhibited variation in this trait. Salt stress caused significant reduction in photosynthesis rate (PR), transpiration rate (TR) and stomatal conductance (SC) but intercellular CO2 (Ci) was not significantly different between control and treated groups. Compared with control, salt treated plants accumulated significantly higher Na+, K+, and Ca+2 while significantly lower Mg+2. Significant inter-accession variations were found with respect to all the scored traits. Based on membership function value (MFV) of the scored traits, accessions were categorized into five standard groups. Among 25 accessions, 1 accession was ranked as highly tolerant, 4 as tolerant while 15 accessions were ranked as moderately tolerant. Of the remaining 5 accessions, 4 were ranked as sensitive while 1 accession as highly sensitive. Antioxidant enzyme activities were assayed in two accessions each from sensitive and tolerant groups. Interestingly, activities of CAT, SOD and POD were clearly higher in tolerant accessions than sensitive ones under salt stress, pointing to the enhanced reactive oxygen species (ROS) scavenging capability in tolerant accessions. To conclude, accumulation of significantly higher proline, maintenance of favorable K+/Na+ ratio, and enhanced activities of ROS scavenging enzymes under salt stress were the key determinants of salt stress tolerance in Eruca sativa. In future, tolerant accessions identified in current study could be a good resource for breeding programs aiming to improve salt stress tolerance in crop plants from Brassicaceae family. Further insights into the salt tolerance mechanism could be gained through carrying out the comparative transcriptomics in highly tolerant and sensitive accessions.