Evaluation of salt tolerance at the seedling stage in rice genotypes by growth performance, ion accumulation, proline and chlorophyll content

Aims The objectives of this study were to evaluate salt tolerance level of rice genotypes using the well-established screening criteria; the salt injury score, survival percentage and ratio between Na+ and K+, as well as the contents of proline and chlorophyll, and to identify the relationship between salt tolerance and physiological characters. Methods One hundred and six rice genotypes were grown in hydroponic solutions subjected to salt stress and evaluated for salt tolerance ability and the physiological parameters. Multivariate cluster analysis was performed based on salinity tolerance scores (ST scores; score 1 being the most tolerant, score 9 the most sensitive), survival percentage and Na+/ K+ ratio. Results ST scores based on salt injury symptoms were negatively correlated with survival percentage and chlorophyll concentration in the stressed seedlings but positively correlated with Na+/ K+ ratio and proline content. Rice genotypes were classified into five salt tolerance groups: tolerant (T), moderately tolerant (MT), moderately sensitive (MS), sensitive (S) and highly sensitive (HS). The means of ST scores were significantly different among the five tolerance groups indicating that the ST score was the most reliable index for identifying salt tolerance. The means of Na+/ K+ ratio and proline content in stressed seedlings were distinctively different between the extreme T and HS groups, but the means among the intermediate groups (MT, MS and S) were not significantly different. Chlorophyll content, on the other hand, was not related to the levels of salt tolerance. Conclusions In addition to the commonly used Na+/ K+ ratio, proline content is suggested to be another useful criterion to differentiate salt-tolerant from saltsensitive rice. This study also identified several Thai improved and local cultivars with the level of salt tolerance and physiological characters comparable to Pokkali, the standard salt-tolerant donor and may be utilized as alternative sources of salt tolerance alleles.