This study was designed to investigate differences in patterns of physiological responses to salinity stress among five japonica rice cultivars in comparison with FL478 as a salinity tolerance check. Five japonica cultivars were screened for salinity tolerance at seedling stage based on visual symptoms of salt injury index and physiological parameters including dry matter production, electrolyte leakage ratio and ion concentration. The results indicated that cultivars Ouukan383 and FL478 were relatively more salinity tolerant than other cultivars and that Kanniho was the most salinity‐sensitive cultivar. Ouukan383 showed higher leaf water content and lower electrolyte leakage ratio under salinity stress. Notably, under salinity stress, Na+ concentration in the leaf blades was much lower in Ouukan383 than in FL478, but was much higher in Kanniho. To understand the basis for these differences, we studied transcript levels of the genes encoding Na+ transport proteins in different tissues. In response to salinity stress, Ouukan383 had highly induced expression of the OsHKT1;4 gene in the leaf sheaths, corresponding to higher Na+ accumulation in the leaf sheaths and lower Na+ accumulation in the leaf blades. On the other hand, the sensitive cultivar, Kanniho, had highly induced expression of the OsSOS1 and OsNHX1 genes in the leaf blades, whose gene products are responsible for Na+ efflux outside cells and Na+ compartmentalization into vacuoles. Thus, the salinity‐tolerant and salinity‐sensitive cultivars differed in their responses to Na+ fluxes in plant cells using different transport systems in each tissue type. The salinity‐tolerant japonica cultivar, Ouukan383, has an effective Na+ exclusion mechanism at the leaf sheaths to prevent Na+ accumulation in the leaf blades. Such a mechanism, in combination with other genetic traits (e.g. Na+ exclusion at the roots mediated by OsHKT1;5), offers the potential to improve salinity tolerance in rice.

Breeding for consumers preferring grain qualities has become a chief goal for rice breeding programs in the world. Amylose content (AC) and aroma are important qualities for consumers' preference and market price. To introgress the alleles of waxy (Wx b) and fragrance (badh2) genes into Sinthukha, a widely adaptable high-yield rice variety in Myanmar which has intermediate AC and non-aroma, RNP20-145-1-9 line was used as a donor parent, and pseudo-backcross breeding was designed to shorten the backcross program. In this approach, only one backcross (BC 1 F 1) and one self-pollinated (BC 1 F 2) population was generated to select for the plants with donor alleles of waxy and fragrance genes in foreground selection, and the selected plants were screened for the highest percentage of recurrent genome content (%RGC) in background selection by amplified fragment length polymorphism (AFLP) analysis. The progenies with the highest %RGC, 84% and 92% were selected in BC 1 F 1 and BC 1 F 2 populations, respectively, and these selected plants possessed heterozygous alleles in fragrance and waxy genes. The agronomic and yield performance, grain size and shape of selected BC 1 F 2 plants were most similar to those of Sinthukha. Nevertheless, amylose content of the selected plants was as low as that of RNP20-145-1-9 rice line. In this study, marker-assisted pseudo-backcross approach was useful in the introgression of low amylose and aroma genes from RNP20-145-1-9 line into Sinthukha, and it could accelerate backcross breeding program through the combination of marker-assisted foreground and background selections. AFLP analysis could save not only time consumption but also the cost of analysis and workload in background selection.

This study was conducted to determine the responses to saline-alkaline (SA) stress with regard to nutrient accumulation in two rice varieties having different tolerances to salt-stress. A salinity-tolerant landrace, Pokkali, and a salinity-sensitive variety, PTT1, were exposed to three levels of SA conditions, pH 7.0 (mild), pH 8.0 (moderate), and pH 9.0 (severe), under 50 mM Na stress. The results indicated that Pokkali had comparably greater SA tolerance than PTT1 owing to its higher biomass production. The maintenance of the lower Na/K ratio in Pokkali shoots was achieved by the higher expression of OsHKT1;5 encoding a Na+ transporter in the shoots, OsNHX1 encoding a tonoplast-localized Na+/H+ antiporter in the roots, and OsHAK16 encoding a K+ transporter in the roots under SA conditions. We propose that the high expression of Fe deficiency-responsive genes, OsIRT1, OsIRO2, OsYSL15, OsNAS1, and OsNAS2, in both rice varieties under all SA conditions should contribute to Fe homeostasis in the shoots. In addition, SA treatment increased the concentrations of Ca, Mn, Zn, and Cu in the roots but decreased their concentrations in the shoots of both varieties. Overall, the results indicated that high rhizospheric pH influenced nutrient uptake and translocation from the roots to the shoots in rice.

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