EFFECT OF ROOT-ZONE SALINITY ON GROWTH AND CHEMICAL-COMPOSITION OF ACACIA-AMPLICEPS BR-MASLIN, A-AURICULIFORMIS A-CUNN EX BENTH AND A-MANGIUM WILLD AT 2 NITROGEN LEVELS

Acacia ampliceps B. R. Maslin, A. auriculiformis A. Cunn. ex Benth. and A. mangium Willd. differ in their salt tolerance. Four-month-old seedlings were treated with 100 (S1), 200 (S2) and 400 (S3) mol m-3 NaCl in sand culture. Two months prior to NaCl application the seedlings had been pretreated with either 1 mol m-3 N (low N; LN) or 5 mol m-3 N (high N; HN) as ammonium nitrate. LN seedlings were inoculated with a suspension of Rhizobium. HN plants were considerably larger than inoculated plants at each NaCl concentration. Survival, damage symptoms and relative growth reduction under NaCl were generally similar for both N treatments. There was also no effect of N level on shoot water relations and stomatal conductance of youngest expanded phyllodes. However, at S1 and S2, HN plants had significantly lower concentrations of shoot Na+ and Cl- concentrations than LN plants. There were differences in nodule number and weight per plant between species receiving the LN treatment. Nodule dry weight and nitrogenase (acetylene reduction) activity was more sensitive than plant dry weight to NaCl imposition. Ranking of species for salt tolerance, based on relative decline in dry weight of shoot, whole plant and nodules under NaCl addition, was in the order A. ampliceps > A. auriculiformis > A. mangium. The greater salt tolerance of A. ampliceps was associated with relatively low shoot Na+ and Cl- concentrations (dry weight basis) and high phyllode succulence.