Intraspecies variation in sodium partitioning, potassium and proline accumulation under salt stress in Casuarina equisetifolia Forst

Casuarina equisetifolia Forst., a member of the Casuarinaceae family, is widely planted in coastal areas due to its ability to function as potential barrier against wind and waves. Significant variation has been reported in the ability of C. equisetifolia to grow under salinity stress. In the present study, 82 clones of C. equisetifolia were assessed for their response to 50 mM incremental NaCl concentrations ranging from 50 mM to 550 mM in Hoagland's solution and clones with contrasted salt tolerance were identified. Several earlier reports attribute salt sensitivity in Casuarina species to the toxic effect of sodium. Intraclonal variation in the levels of sodium accumulation was therefore analysed. However, sodium content in the shoots and roots, showed little correlation (0.351 and -0.171) with salt tolerance in C. equisetifolia. Similarly, sodium to potassium ratio in the shoots and roots of NaCl treated and untreated clones also did not show correlation with mortality although certain tolerant clones exhibited selectivity of potassium over sodium under salt stress. Analysis of the shoot to root ratio of sodium however, showed better correlation (0.448) with salt tolerance, suggesting that restricted translocation of sodium to shoots and its relative retention in roots might play a crucial role in the salt tolerant clones of C. equisetifolia, and that shoot to root ratio of sodium could be a better parameter for salt tolerance in C. equisetifolia clones. The higher salt tolerance observed in certain clones despite higher sodium accumulation or shoot to root ratio of sodium suggests the presence of different multiple adaptive mechanisms that may be operating in different clones to help protect the cells from the toxic effects of sodium. The tolerant clone, TNIPT 4, which accumulated high concentrations of Na+, had low shoot to root ratio of Na+, and also a higher constitutive as well as NaCl induced accumulation of the compatible osmolyte, proline. The study thus emphasizes the need for characterising the genetic components involved in sodium transport, proline metabolism and other mechanisms contributing to salinity tolerance. The identified clones with contrasted stress tolerance mechanisms would thus be a valuable resource for transcriptomic, proteomic and metabolomic exploration in addition to their utility for field evaluation in flooded and coastal saline tracts.