2.5 Measurement of citrate in root exudates
The citrate concentration of root exudate from excised root tips of XZ29, XZ9 and Dayton was determined according to Wang et al (2007). Briefly, twenty 10-mm root tips of two-day-old seedlings were excised and washed for three times with 1 ml of control solution (200 μM CaCl2, pH 4.3). Treatment solutions (200 μM CaCl2, pH 4.3, with or without 5 μM AlCl3) were added and the tubes containing the root tips were incubated at 60 rpm for 2 h using a platform shaker. Then the solution was collected for citrate measurement according to Wang et al. (2007).