Effects of proline on copper transport in rice seedlings under excess copper stress

Copper exposure affects water flux and ion path in rice seedlings. To characterize the factors related to Cu transport in response to Cu exposure, rice (Oryza sativa cv. Taichung Native 1) seedlings grown by hyperculture were subjected to excess CuSO4 treatments, and Cu transport, water flux (transpiration rate and xylem sap extrusion), and proline level were assessed. Excess Cu decreased the water flux of rice seedlings. In spite of the decrease in water flux, Cu upward transport increased steadily with time. The absorption of Cu in roots displayed a phase lag between 3 and 12 h of Cu exposure, while proline content in the rice roots decreased dramatically. The supplement of proline in culture medium increased water flux and Cu upward transport in mild Cu-treated rice seedlings. In extreme Cu treatment, however, proline supplement decreased Cu upward transport, as the extent of water flux raised by proline supplement was diminished. Proline supplement to Cu-treated rice seedlings not only reduced the Cu absorption in the roots but also the Cu exclusion, suggesting supplement of proline accompanied by Cu exposure induce a barrier of Cu influx and efflux in rice roots. Therefore, it was concluded that excess Cu leads to inadequate proline and results in the malfunction of copper transport barrier in rice roots. (C) 2003 Elsevier Ireland Ltd. All rights reserved.