Alleviating the Injuries of NaCl Exposure on Respiratory Activities, Leaf Stomatal and Antioxidant Defense of Silybum marianum L. Seedlings by Exogenous Nitric Oxide

Abstract

AbstractNitric oxide (NO) is recognized as an endogenous signaling molecule that plays an important role in the defence responses of medicinal plants to NaCl stress. In this study, we investigated the effects of sodium nitroprusside (SNP) as an NO donor at three concentrations (0, 100, and 200 µmol l−1) to alleviate the deleterious effects of salt stress (100 mM NaCl) on leaf gas exchange and biochemical characteristics of Silybum marianum L. seedlings. This study showed that salt stress significantly decreased relative water content (RWC), chlorophyll b content, endogenous NO concentration, maximum quantum yield (Fv/Fm), leaf gas exchange, stomatal size, K+/Na+ ratio, and plant dry weight, and increased malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, proline content, stomatal density, and enzyme activities. SNP treatment increased Fv/Fm, photosynthetic pigments, K+/Na+ ratio, and dry weights of the shoots and roots of NaCl-exposed plants. The exogenous application of NO increased the proline content under salinity stress more than under stress conditions without SNP application, so that the proline content increased from 32 to 47 µmol g−1. Application of 100 µM SNP also increased endogenous NO concentration (up to 43%) and consequently protected plants against salt stress-induced damage by improving enzyme activity and reducing the H2O2 generation rate (up to 14%) and MDA content (up to 50%) compared to plants treated with NaCl alone. Foliar application of NO to salt-stressed plants increased root and shoot respiration rates from 20 and 12%, respectively, under salinity stress to 57% under the application of SNP and stress conditions, and decreased stomatal conductance by up to 70%, resulting in improved RWC. Increased internal NO generation in plants induced by 100 µM SNP application has the potential to mitigate salinity injury in Silybum marianum L. plants.