Plant dry matter (500 mg) was ashed at 500 ◦C for 5 h
in a muffle oven. After cooling, the samples were extracted twice
with 2 mL of 3.4 M HNO3 until dryness to precipitate SiO2. The ash
was dissolved in 2 mL of 4 M HCl, subsequently diluted ten times
with hot deionized water, and boiled for 2 min. After addition of
0.1 mL Cs/La buffer to 4.9 mL ash solution, Fe, Mn and Zn concentrations
were measured by atomic absorption spectrometry (UNICAM
939, Offenbach/Main, Germany). Phosphate in the ash solution was
determined spectrophotometrically by the method of