2.1. Plant material and aseptic culture establishment
Three commercial self-heading cultivars of Philodendron (i.e. ‘Imperial Green’, ‘Imperial Red’ and ‘Imperial Rainbow’) were used in the present study. The plants were purchased from a local nursery and sprayed with fungicides Ridomil MZ and Mancozeb (1000× dilution) each for one week prior to explant excision and establishment in vitro. After defoliation, shoot cuttings measuring 5–10 cm long were washed under running tap water for 15 min and divided into single nodal segments each containing a lateral bud. The nodal segments were then surface-sterilized with 70% EtOH for 1 min followed by 1% (v/v) sodium hypochlorite (NaOCl) containing several drops of Tween-20 for 20 min on a rotary shaker. After three rinses with sterile water and removal of the damaged ends, the explants were cultured on MS ( Murashige and Skoog, 1962) medium supplemented with 0.1 mg l−1 of 1-naphthaleneacetic acid (NAA), 1 mg l−1 of 6-benzyladenine (BA), 3% (w/v) of sucrose and 0.7% (w/v) of agar. The pH of the medium was adjusted to 5.7 prior to autoclaving. Cultures were checked regularly for contaminations and those presented apparent infection symptoms were immediately discarded. The outgrowth of the lateral bud (i.e. approximately 2 cm long) was excised from the nodal segments and subcultured on the same medium for further shoot multiplication. Regenerated shoot clusters were divided and subcultured every eight weeks to build up a stock shoot culture. Shoots from the fifth subculture and measuring approximately 3 cm in height were used for the subsequent experiments.