2.4 Mineral and inorganic phosphorus concentrations
The mineral and Pi concentrations in the root tips (the first 10-mm) after 2, 6, 24 and 72 h of 5 μM Al treatments were determined. Al and P concentrations in whole root of 12 Tibetan wild barley accessions after 24 h of 5 μM Al treatment were also measured to determine their correlations with relative root growth under Al stress. The Al-treated roots were washed with distilled water for three times, and dried in the oven at 110 °C. Dry sample was digested with HNO3 using microwave digestion system Microwave 300 (Anton PAAR, Austria). The mineral concentrations were determined using inductively coupled plasma atomic emission spectrometer (ICP-AES) (Thermo Jarrel Ash, USA). The Pi concentration was determined according to Zhou et al (2008) with some modifications. Approximately 50 mg of fresh root sample was homogenized in 1 ml of 5% (w/v) perchloric acid and placed on ice for 30 min. After centrifugation at 10,000 g for 10 min at 4 C, the supernatant was used for Pi measurement.