Exogenous Proline and Glycinebetaine Suppress Apoplastic Flow to Reduce Na+Uptake in Rice Seedlings

The application of exogenous proline and glycinebetaine (betaine) confers salt tolerance on plants under salt stress. The effects of exogenous proline and betaine on apoplastic flow in rice plants under saline conditions were investigated using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer. Rice plants took up more PTS under light conditions than under dark conditions. Salt stress increased PTS uptake and Na(+) content of rice leaves, but did not affect K(+) content, resulting in a lower K(+)/Na(+) ratio. Addition of proline or betaine to the saline medium suppressed Na(+)-induced PTS uptake and Na(+) accumulation, while the K(+) content was slightly increased, which led to a high K(+)/Na(+) ratio under saline conditions. These results suggest that exogenous proline and betaine suppressed Na(+)-enhanced apoplastic flow to reduce Na(+) uptake in rice plants.