Differential physiological responses to salt stress in rice landraces in Thailand

Soil salinity is a limitation factor for rice cultivation in the north-eastern Thailand. Understanding the mechanisms of salt stress tolerance is critical for maintaining or improving crop yield under salt stress. Over the past ten years, many landrace rice varieties have been examined for genetic diversity. To utilize the richness of natural resources and to explore novel genetic resources for developing a new variety, evaluating local rice varieties for salinity is still needed. This study investigated the physiological and biochemical responses of 8 landrace rice varieties in comparison to 'Pokkali', the standard salt-tolerant rice genotype, to clarify the major salt tolerance mechanisms in the landraces found in Thailand. After being exposed to 120 mM NaCl for 7 days, 'Pokkali' expressed physiological parameters in response to salt stress which are indicative of salt tolerance ability including low growth reduction, low Na+/K+ ratio, low chlorophyll degradation and low membrane integrity. In contrast, the landrace varieties displayed varying patterns of response. No landrace variety showed an outstanding ion exclusion mechanism as 'Pokkali', as evidenced by the fact that all landrace varieties showed approximately two times higher Na+/K+ ratios than 'Pokkali'. However, it was found that 'Surin' had a similar except for the markedly increased proline and sugar accumulation, suggesting an osmotic adjustment mechanism. Therefore, this variety could be a potential genetic resource to be developed as a donor for the osmotic adjustment trait to improve salt-tolerant rice in the future.