Root FCR activity measurement
The root FCR (Ferric-chelate reductase) activity was quantified using a ferrozine assay (Schikora & Schmidt, 2001). The roots were initially washed in 0.5 mM CaSO4 for 5 min. Then the whole roots were excised, weighed and put into a 50 mL tube with 50 mL assay solution (pH 5.8). The assay solution contained 0.5 mM ferrozine (FRZ), 0.5 mM CaSO4, 0.5 mM Fe-EDTA (ethylenediaminetetraacetic acid). The tubes were put at room temperature (~24℃) in dark for 1 h, with a handy shake every 10 min. The absorbance of the solutions was determined by a spectrophotometer (UV 1800, Shimadzu, Japan) at 562 nm. The reduction rate was calculated after subtraction of the appropriate blanks (assay solution without roots). The rate of root FCR activity was calculated as moles of Fe2C-ferrozine per gram of fresh weight per hour. Each treatment contained at least 3 biological replicates.