Growth and Inflorescence Initiation of Phalaenopsis Orchids by Paclobutrazol Treatment

The effects of paclobutrazol (PBZ) treatment on growth and inflorescence initiation were investigated in Phalaenopsis orchids. Clones of the Phalaenopsis cultivars Queen Beer ‘Mantefon’ and Sogo Yukidian ‘V3’ were treated with 0, 100, 200, and 400 ppm (a.i. 0, 1, 2, and 4 mg) PBZ at 20°C during the forcing period and 0, 100, and 200 ppm (a.i. 0, 1, and 2 mg) PBZ at 28°C during the vegetative period by drenching. Under forcing condition, the percent plants with visible inflorescence (VI, flower-stalk longer than 0.5 cm) was significantly reduced by 200 and 400 ppm PBZ treatments, and inflorescence emergence was delayed in all PBZ treatments. The number of inflorescences decreased in ‘Mantefon’ by PBZ treatment regardless of concentrations. The PBZ treatments shortened the inflorescence length, but there was no difference in the number of days to bloom and the number of flowers. Under vegetative condition, PBZ treatment significantly increased the stem and root diameters and leaf thickness with increasing PBZ concentration, whereas it decreased the number of new leaves and leaf length and width. Thus, the PBZ-treated plants seemed to be stunted. When the plants pre-treated with PBZ during the vegetative period were moved to forcing conditions, the percent plants with VI significantly decreased and the number of days to VI increased, implying that the effects of the PBZ treatment during the vegetative period could persist in inflorescence initiation. From these results, we conclude that PBZ treatment can inhibit the growth and inflorescence initiation in Phalaenopsis orchids.