Effect of different stem branches of Bupleurum chinense on saikosaponins content in its roots and root yield Optimization of pre-column acidification conditions for HPLC determination of saikosaponins a and d Effect of combined application of zinc, boron and molybdenum on yield and saikosaponin a, saikosaponin d contents of Bupleurum chinense Optimization of pre-column acidification conditions for HPLC determinationof saikosaponins a and d