Characterization of the Na+ delivery from roots to shoots in rice under saline stress: Excessive salt enhances apoplastic transport in rice plants

Sodium transport to the shoots of rice (Oryza sativa L.) plants grown under salt stress conditions was characterized. The rate of Na⁺ transport to shoots increased exponentially depending on the increase in the NaCl concentration of the rooting solution, however, the rates were independent of the plant transpiration. Excessive NaCl in the rooting solution was found to enhance apoplastic transport in rice plants by using the apoplastic dye, 3-hydroxy-5,8,10-pyrene trisulfonic acid (PTS) and Fluostain I (also known as Calcoflour White M2R New). The results suggest that excessive Na⁺ in the rooting solution enlarges the apoplastic pathways, which facilitates Na⁺ intrusion into the xylem vessels, resulting in an excessive accumulation of Na⁺ in rice shoots.