Sodium Distribution in Salt-stressed Citrus Rootstock Seedlings

Although citrus trees are considered relatively salt-sensitive, there are consistent differences in Na + and Cl – tolerance among different citrus rootstocks. We grew uniform seedlings of rough lemon (RL) and the more Na + -tolerant Swingle citrumelo (SC) with and without 50 m m NaCl for 42 days. Salinity reduced leaf chlorophyll and plant transpiration rate (E p ) more in RL than SC. Confocal laser scanning analyses using the Na + -specific cell-permeant fluorescent probe CoroNa-Red revealed a higher capacity for Na + sequestration in root tissue vacuoles of SC than in RL roots and that cell walls within the stele acted as Na + traps. In leaves, however, RL had significantly higher Na + -dependent fluorescence than SC. Thus, the sequestration of Na + in root tissue vacuoles and its immobilization by cell walls were key contributing mechanisms enabling SC leaves to maintain lower levels of Na + than RL leaves. Examination of intracellular distribution of CoroNa-Green fluorescence in SC root protoplasts verified a vacuolar localization for Na + in addition to the presence of a 2- to 6-μm unidentified endosomal compartment containing significantly higher Na + concentrations.