Sodium Exclusion and Potassium-Sodium Selectivity in Salt-Treated Trifoliate Orange (Poncirus trifoliata) and Cleopatra Mandarin (Citrus reticulata) Plants

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Abstract
Seedlings of trifoliate orange (Poncirus trifoliata (L.) Raf.) and Cleopatra mandarin (Citrus reticulata Blanco) were grown under glasshouse conditions and supplied with dilute nutrient solution containing 0, 25, 50 or 100 mM NaCl. Trifoliate orange plants grown at 25 or 50 mM NaCl possessed an ability to restrict the accumulation of sodium in leaves. This ability was lost at 100 mM NaCl. For plants treated with 50mM NaCl, uptake of both sodium and chloride per unit root weight was higher for trifoliate orange than for Cleopatra mandarin. Root to leaf transport of chloride was also higher for trifoliate orange, whereas root to leaf transport of sodium was significantly lower. This ability to exclude sodium was unrelated to the flux of water (water flux) per plant, which was similar between the genotypes. Trifoliate orange appeared to possess a greater ability to withdraw sodium from the xylem in the proximal root and basal stem and to sequester it in both the wood and the bark of these regions. This was accompanied by a significant reduction in the concentration of potassium in these tissues, suggesting sodium-potassium exchange. Release of potassium into the xylem in exchange for sodium is implied by the significant increase in leaf potassium concentrations. This ability for sodium exclusion at low to moderate salinities (Ͱ4 50 mM NaCl) enabled trifoliate orange to maintain a relatively constant sodium : potassium ratio in leaves. Salt-treated Cleopatra mandarin plants, on the other hand, tended to show a marginal decrease in leaf potassium concentration and a substantial increase in leaf sodium : potassium ratio. Reducing the mean water flux in trifoliate orange from 38 to 26 µmol s-1 per plant by partial defoliation did not affect root to leaf transport of chloride. However, there was a marked reduction in root to leaf transport of sodium, suggesting an enhanced withdrawal of sodium from the xylem at lower water fluxes.