Ion Regulation in the Organs of Casuarina Species Differing in Salt Tolerance

Abstract

Distribution of Na⁺, Cl^-, K⁺, Ca²⁺ and Mg²⁺ was studied in individual organs of two highly tolerant and one moderately tolerant species of Casuarina. The highly tolerant species (C. equisetifolia and C. glauca) accumulated little Na⁺ and Cl^- in their shoots and the concentrations of Na⁺ and Cl^- decreased from old to young growing needles. The concentrations of Na⁺ and Cl^- were much higher in shoots of the moderately tolerant species (C. cunninghamiana) and a concentration gradient between old and young needles was not observed. The same pattern of distribution of Cl^- in C. equisetifolia was found in seedlings exposed to both short term (13 days at 100 mol m^-3 NaCl in solution culture), and long term (6 months at 250 mol m^-3 NaCl in sand culture) salinisation. The three species showed little difference in their root ion concentrations. A time sequence experiment of Cl^- uptake indicated that the better exclusion of Cl^- from the shoots of C. equisetifolia than C. cunninghamiana was due to a lower rate of Cl^- uptake and lower net transport into the shoot rather than to its retention in the roots, or reabsorption at the proximal root or hypocotyl.