Salt tolerance in the rice varieties Minghui63 and Nipponbare: Root growth reduction correlates with local ROS...

Rice (Oryza sativa L) cultivation is severely compromised by its high susceptibility to salt stress. Performance of two rice varieties with a high significance for agriculture in China, Minghui63 (MH63) and Nipponbare (Nip) was studied using hydroponic culture, adding 0, 50, 100 or 150 mM NaCl. Physiological parameters were assessed such as growth, concentrations of reactive oxygen species (ROS), ROS detoxification, stomatal densities, proline and soluble sugar contents, root and shoot Na⁺ and K⁺ contents, as well as steady-state net H⁺ and K⁺ fluxes at the root tip, elongation zone and early mature zone using the Microelectrode Ion Flux Estimation (MIFE) technique. Root growth reduction under salt stress was more severe in Nip than in MH63, correlating with higher ROS accumulation in the elongation zone of the former variety, and characteristic differences in apical H⁺ fluxes. After 7d of exposure to 150 mM NaCl, H⁺ efflux was reduced at the root tip and elongation zone in MH63 but even turned to influx (partly or completely) in Nip, indicating a more severe reduction of plasma membrane proton pump activity in the latter cultivar. Generally, more efficient ROS detoxification and lower Na⁺/K⁺ ratios indicated MH63 being potentially more salt tolerant than Nip.