2.3 Determination of monovalent cations (Na+and K+) content
Leaves were harvested and washed with deionized water. The leaf samples were dried first at 105°C for 2 h, subsequently at 70°C for 72 h, and then weighted for dry weight. Lyophilized leaves were milled to powder for mineral nutrient analyses. Powdered samples were extracted with 10 ml of HNO3 (0.1N) for 60 min at 95 ºC. The resulting solutions were filtered through Whatman filter paper, diluted and analyzed for Na+ and K+. Cation concentrations were determined with an atomic absorption spectrophotometer (PerkinElmer, PE AAS 900 F).