Radical scavenging and antioxidant enzyme activities in ri̇ce (Oryza sativa L.) varieties exposed to salt stress

The physiological and biochemical responses of a variety of rice (Oryza sativa L.) exposed to salt (NaCI) stress was associated with their sensitive and tolerant levels. The rice varieties involve Gala, Edirne, Şumnu, Neğiş, Tunca and Aromatik-1 and 2 local populations, named Karacadağ and Hazro. These species were grown under controlled conditions and they were exposed to the salt stress factor at three different concentrations of NaCI (100, 200 and 300 mM) for 10 days. The effects of salt on the physiological and biochemical responses to the stress factors were investigated. As a result, the species having a high level of tolerance/sensitivity to salinity were identified. In the first phase of this two-stepped study, we investigated the changes in the root-shoot length, the weight of dried fresh leaf, the level of leaf damage, the relative water content, pigment content and the level of lipid peroxidation caused by the application of the salt. It was found that the highest salt concentration mostly affects the relative water content in Gala (66%) and Karacadağ (74%) while this was the least affected in Tunca (85%) and Aromatik-1 (86%) species compared with their reference conditions. Among the parameters, lipid peroxidation produced the most significant outcomes. The application of 300 mMNaCl caused a very low malondialdehyde (MDA) increase in Tunca and Aromatik-1 while this was higher in Karacadağ, Neğiş and Gala. The results also showed that among the species Gala and Karacadağ had a higher sensitivity while Tunca and Aromatik-1 had a higher resistance against salinity compared with their respective controls. In the second phase of the study, the investigation is focused on the measurement of proline content and the content of enzymes played roles in antioxidant defense system such as SOD, CAT, APX and GR, and also reactive oxygen species (ROS) scavenging activities. These observations will provide further concrete evidence of the influence of the salt stress on these species and their sensitivity and tolerance levels. Besides, the total soluble protein profilesto in these species were measured by SDS-PAGE technique. The results indicated that SOD, CAT, APX and GR enzyme activities were higher in Tunca and Aromatik-1. In addition, the amount of H2O2 in Karacadağ and Gala at 300 mM salt was significantly higher compared to that in their controls. Changes in the enzyme activities in these two species and H2O2 levels are consistent with the observations reported in the previous stage. Alltogether, it was found that the enzyme activities were higher in thespecies with the higher salt tolerance compared with those having a higher salt sensitivity. The difference in the antioxidant enzyme activity was in paralel with that in ROS scavenging activities. In conclusion, we may suggest that the antioxidant mechanisms that provide salt tolerance change depending on the types of rice and the level of NaCI.