Effects of sodium chloride on growth, tissue elasticity and solute adjustment in two Acacia nilotica subspecies

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Abstract
The objective of this study was to compare whole plant growth and physiological responses to salt stress of two Acacia nilotica subspecies (ssp. cupressiformis and ssp. tomentosa). Salt stress was induced by adding NaCl at different concentrations to the nutrient solution: 0, 75, 100 and 200 mM. After one month under such stress, plants were still healthy and actively growing in both subspecies up to 100 mM NaCl. Water potential (Ψ) and osmotic potential (π) decreased with salinity and the lower π enabled the plants to maintain turgor. Höfler diagrams confirmed that osmotic adjustment had occurred under all treatments. Furthermore, the point of zero turgor occurred at a higher relative water content. An increase in the elastic modulus (ɛ) was observed under stress (low elasticity of the cell wall). Both osmotic adjustment and a high ɛ modified the capacity of both subspecies to maintain a positive water balance. Accumulation of ions (Na+, K+ and Cl−) and proline could explain such osmotic adjustment. Acacia nilotica ssp. cupressiformis showed a higher absorption of K+ than ssp. tomentosa up to 100 mM NaCl treatment.