Effect of symbiotic associations with Frankia and arbuscular mycorrhizal fungi on antioxidant activity and cell ultrastructure in C. equisetifolia and C. obesa under salt stress

Plants of the Casuarinaceae family are widely known for their ability to tolerate salt stress. Casuarinaceae are able to develop symbiotic association with arbuscular mycorrhizal fungi and with the soil bacteria Frankia. The aim of this study was to evaluate adaptation mechanisms of Casuarina to tolerate salt stress when interacting Rhizophagus fasciculatus and Frankia. Two species showing different salt tolerance levels, i.e. C. equisetifolia and C. obesa, were grown in sandy sterile soil, inoculated with R. fasciculatus and/or Frankia and then watered gradually with increased concentrations of saline solutions. Total antioxidant activity, antioxidant enzyme concentration, and salt effects on cellular ultrastructure of shoots were evaluated. C. obesa has a better salt stress tolerance compared to C. equisetifolia. Co-inoculation (R. fasciculatus and Frankia) improved the performance of both plant species in saline. Higher antioxidant activity was observed in C. obesa. At 400 mM of NaCl C. obesa revealed a maintenance of cellular integrity whereas cell membrane rupture and disintegration of cellular contents were observed in C. equisetifolia tissues. Our results suggest that a selection of appropriate plant species is important to improve plant performance in saline soils.