Roots were separated from soil by the following procedure: rinsing with
distilled water, followed by a second rinsing with 0.02 M ethylenediaminetetraacetic
(EDTA) solution and three other rinsing steps
with distilled water, before oven drying at 80 ◦C until a constant
weight was reached. Total dry weights of the roots and shoots were
measured. All shoot and root samples were finely ground before
digesting them with concentrated HNO3 (70%) in a water bath at
95 ◦C for 6 h. Solutions were filtered on 0.22 m membrane before
analysis (modified from Keller et al. [55]). Silicon was extracted
according to Guntzer et al. [56].