Acacia ampliceps (salt wattle), a leguminous shrub, has been introduced in salt-affected areas in the northeast of Thailand for the remediation of saline soils. However, the defense mechanisms underlying salt tolerance A. ampliceps are unknown. We investigated various physio-biochemical and morphological attributes of A. ampliceps in response to varying levels of salt treatment (200-600 mM NaCl). Seedlings of A. ampliceps (25 ± 2 cm in plant height) raised from seeds were treated with 200 mM (mild stress), 400 and 600 mM (extreme stress) of salt treatment (NaCl) under greenhouse conditions. Na(+) and Ca(2+) contents in the leaf tissues increased significantly under salt treatment, whereas K(+) content declined in salt-stressed plants. Free proline and soluble sugar contents in plants grown under extreme salt stress (600 mM NaCl) for 9 days significantly increased by 28.7 (53.33 μmol g(-1) FW) and 3.2 (42.11 mg g(-1) DW) folds, respectively over the control, thereby playing a major role as osmotic adjustment. Na(+) enrichment in the phyllode tissues of salt-stressed seedlings positively related to total chlorophyll (TC) degradation (R (2) = 0.72). Photosynthetic pigments and chlorophyll fluorescence in salt-stressed plants increased under mild salt stress (200 mM NaCl). However, these declined under high levels of salinity (400-600 mM NaCl), consequently resulting in a reduced net photosynthetic rate (R (2) = 0.81) and plant dry weight (R (2) = 0.91). The study concludes that A. ampliceps has an osmotic adjustment and Na(+) compartmentation as effective salt defense mechanisms, and thus it could be an excellent species to grow in salt-affected soils.
Keywords: Na+/K+ ratio; free proline; halophytic species; net photosynthetic rate; osmotic adjustment; soluble sugar.