Abstract
Rice is highly sensitive to salt stress especially in its early growth stage, which thus is one of the major constraints in rice production. In rice plants, salt sensitivity is associated with the accumulation of $Na^+$ in the shoots, especially in the photosynthetic tissues. High salt concentrations in soil cause high $Na^+$ and $Cl^-$ transport to the shoot and preferential accumulation of those ions in older leaves, which decreases $K^+$ in the shoot, photosynthetic activity and grain yield. Salt exclusion capacity at the leaf sheath is therefore considered to be one of the main mechanisms of salt tolerance. In addition, it is suspected that the lamina joint might be involved in the salt transport from leaf sheath to leaf blade. This research aims to determine if leaf sheaths of rice exclude a large amount of $Na^+$ only or other ions such as $K^+$, $Ca^{2+}$, $Mg^{2+}$, and $Cl^-$ as well, to identify tissues in the leaf sheath, which accumulate $Na^+$, and to examine if the lamina joint is involved in the salt exclusion by the leaf sheath. The rice seedlings of salt tolerant genotype FL478 and salt sensitive genotype IR29 were independently treated with NaCl, KCl, $MgCl_2$ and $CaCl_2$, and Taichung 65 and its near-isogenic liguleless line (T65lg) were treated with NaCl. Then, the content of $Na^+$, $K^+$, $Ca^{2+}$, $Mg^{2+}$, and $Cl^-$ ions and their specific location were determined using Atomic Absorption Spectrometer, Ion Chromatograph, and Energy Dispersive X-ray Spectroscopy. Results showed that leaf sheaths of FL478 and IR29 accumulated a large amount of $Na^+$, $K^+$, $Ca^{2+}$, $Mg^{2+}$, and $Cl^-$ ons, and thus excluded them from leaf blades when treated with high concentration of each salt. When treated with NaCl, the highest $Na^+$ concentration was found in the basal part of leaf sheaths of both cultivars. Moreover, energy-dispersive X-ray spectroscopy revealed that the central parenchyma cells of the leaf sheath were the site where most Na, Cl, and K were retained under salinity in the salt tolerant genotype FL478. Also, the concentration of $Na^+$, $K^+$ and $Cl^-$ ions in leaf sheaths and leaf blades was comparable between T65 and T65lg, indicating that the lamina joint may not be involved in the exclusion of $Na^+$, $Cl^-$ and $K^+$ by the leaf sheath from the leaf blade under salinity. Therefore, we conclude that the central parenchyma cells of basal part of leaf sheath are the site that plays a physiological role to exclude $Na^+$ in the shoots of rice without the involvement of the lamina joint.