Sodium stress-induced oxidative damage and antioxidant responses during grain filling in Indica rice

Sodium treatment caused the sodium ion accumulation at the milk stage of immature rice grains which in turn triggered the overproduction of reactive oxygen species and oxidative damage. The tolerant cultivar showed an enhanced antioxidative response and induced expressions of OsNHX and OsHKT ion-transporters. Sodium chloride-(NaCl) induced soil salinity is a major constraint hindering global rice production. Amongst its constituent ions, sodium (Na⁺) is known to be the main driver of toxicity under salt stress. The present investigation aims to measure the impacts of excess Na⁺ during rice grain filling using two Indica rice cultivars with opposite tolerances to salt (salt tolerant: Panvel-3, salt-sensitive: Sahyadri-3) mainly via oxidative and responsive antioxidative pathways. Plants were treated with Na⁺-specific treatments and NaCl with equimolar Na⁺ levels (100 mM) at the initiation of the reproductive phase. Stressed and control plants were harvested at three different grain-filling stages- early milk, milk, and dough and assessed for ion accumulation and oxidative damage/antioxidant responses under Na⁺ stress. Na⁺ toxicity triggered reactive oxygen species (ROS) production and upregulated the responsive enzymatic antioxidants. Na⁺ stress also increased the nitric oxide (NO) levels and the activity of nitrate reductase in immature grains. Differential expression levels of OsNHX and OsHKT transporters were observed in response to Na⁺ stress. Mature grains displayed a high accumulation of Na⁺ along with reduced K⁺ content and elevated Na⁺/K⁺ under high Na⁺ availability. The alterations in mature grains' sugar, starch, and protein content were also observed in response to the Na⁺ stress. Overall, the salt-tolerant cultivar displayed higher antioxidant activities and a lower rate of ROS generation in response to the Na⁺ stress. Results suggested a link between Na⁺ accumulation, Na⁺-mediated stress responses via anti/-oxidant pathways, and the grain-filling process in both rice cultivars.