Seeds extracted from mature fruits of two sweet orange (C. sinensis (L.) Osbeck) cultivars, Valencia (Val) and Bingtangcheng (BTC), were first washed with 1 M NaOH for 15 min, then disinfected in commercial sodium hypochlorite solution (3%) for 20 min, followed by several rinses with sterilized distilled water. After removal of seed coats the seeds were sown in test tubes (150 mm 25 mm) containing 15 mL of MT (Murashige and
Tucker, 1969) basal medium and maintained in the dark for 10–
15 d at 26 1 8C before transfer to a 16-h photoperiod (40 mmol m2 s1). All of the media in the current work, unless otherwise stated, were supplemented with 3% (w/v) sucrose and 0.8% agar (Sigma), and the pH was adjusted to 5.8 prior to autoclaving at 121 8C for 15 min.