Suppression of Na+ Uptake Via Apoplastic Flow by Chitosan in Rice

Purpose: Chitosan enhances tolerance to salinity in rice. Apoplastic flow plays a crucial role in the accumulation of sodium (Na+) in rice under salinity. This study investigated the effects of exogenous chitosan on apoplastic flow and Na+ uptake in NaCl-treated rice seedlings. Methods: We employed an apoplastic tracer, trisodium salt of 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), in order to evaluate apoplastic flow in rice (Oryza sativa L., cv. Nipponbare) seedlings that were hydroponically grown in the solution containing NaCl (0 and 25 mM), and chitosan (0 mg L− 1, 10 mg L− 1, and 50 mg L− 1). Results: Application of 25 mM NaCl significantly increased PTS uptake and Na+ content in shoots but did not affect K+ content, resulting in a lower K+/Na+ ratio although 25 mM NaCl did not affect the seedling growth. The application of chitosan suppressed Na+-enhanced PTS uptake and Na+ accumulation in shoots without affecting the K+ content, which led to a higher K+/Na+ ratio. Moreover, chitosan did not affect the reducing sugar content or electrical conductivity in the solution containing NaCl. Conclusions: These results suggest that application of chitosan suppressed Na+-enhanced apoplastic flow to reduce Na+ uptake in rice seedlings.