Physiological and Growth Responses of Thai Rice Genotypes to High Salinity Stress at the Seedling and Reproductive Stages

The objectives of this study were to select Thai rice that are tolerant at the seedling stage and investigate their growth, physiological and yield responses at the reproductive stage in comparison with the standard salt-tolerant Pokkali (PK). Twenty-two local, commercial and improved Thai rice genotypes along with PK and salt-susceptible IR29 were evaluated at the seedling stage for salt tolerance using a 15 dS m^-1 saline solution with five replications. Ten selected genotypes were grown in pots with four replications and exposed to a 15 dS m^-1 saline level from early booting to the flowering stage. During the grain-filling stage, salt treatments decreased from 15 to 12 dS m^-1 and were sustained at this level until harvest. The experimental design for both experiments was a randomized complete block design (RCBD). Based on the physiology of flag leaves, almost all genotypes exceled in the protection of chlorophyll, relative water content (RWC), membrane integrity and lipid peroxidation. In contrast, the photosynthesis, growth and grain yield of all were dramatically reduced. The rice genotypes exhibited varying degrees of osmotic adjustment (OA), ranging from 1.598 to 2.541 MPa. The cultivar RD73 and line TSKC1-144, which were genetically improved from KDML105 by the introgression of a salt-tolerant QTL/gene from PK, showed the least reduction in grain yield (60 and 53% reduction, respectively) along with PK (60%). Among the five Thai local rice varieties, Go Main Surin (GMS) showed the least reduction in grain weight (58%), total plant dry weight (28%) and green leaf dry weight (1%), while Khao Gaew (KG) and Leuang Puang Tawng (LPT) were the most reduced. PK and RD73 showed a high level of tolerance at both the seedling and reproductive stages. In contrast, KG and LPT, which exhibited high tolerance at the seedling stage, showed high susceptibility in growth, yield and most physiological traits. On the other hand, TSKC1-144 was sensitive at the seedling stage but showed increased tolerance at the reproductive stage. This result implies that suitable cultural practices should be performed to obtain the best field conditions for growing rice genotypes having different levels of salt tolerance at the seedling and reproductive stages. Future research should focus on molecular characterization of tolerance mechanisms of the promising local genotypes and the potential to use them as tolerance gene donors.