Base to Tip and Long-Distance Transport of Sodium in the Root of Common Reed [Phragmites australis (Cav.) Trin. ex Steud.] at Steady State Under Constant High-Salt Conditions

We analyzed the directions and rates of translocation of sodium ions (Na+) within tissues of a salt-tolerant plant, common reed [Phragmites australis (Cav.) Trin. ex Steud.], and a salt-sensitive plant, rice (Oryza sativa L.), under constant high-salt conditions using radioactive Na-22 tracer and a positron-emitting tracer imaging system (PETIS). First, the test plants were incubated in a nutrient solution containing 50 mM NaCl and a trace level of Na-22 for 24 h (feeding step). Then the original solution was replaced with a fresh solution containing 50 mM NaCl but no Na-22, in which the test plants remained for > 48 h (chase step). Non-invasive dynamic visualization of Na-22 distribution in the test plants was conducted during feeding and chase steps with PETIS. Our results revealed that Na-22 was absorbed in the roots of common reed, but not transported to the upper shoot beyond the shoot base. During the chase step, a basal to distal movement of Na-22 was detected within the root tissue over > 5 cm with a velocity of approximately 0.5 cm h(-1). On the other hand, Na-22 that was absorbed in the roots of rice was continuously translocated to and accumulated in the whole shoot. We concluded that the basal roots and the shoot base of common reed have constitutive functions of Na+ exclusion only in the direction of root tips, even under constant high-salt conditions. This function apparently may contribute to the low Na+ concentration in the upper shoot and high salt tolerance of common reed.