Effects of Plant growth regulators on plant height control in purple Firespike (Odontonema callistachyum)

Firespike (Odontonema callistachyum) is a tropical ornamental shrub native to Central America. It is considered as a potential potted floriculture crop because of its beautiful crimson and brilliant red tubular flowers and shiny leaves. Five growth regulators, daminozide (Dazide), chlormequat (Cycocel), paclobutrazol (Bonzi), nniconazole (Sumagic), and flurprimidol (Topflor) at different rates and application methods (spray and drench) were tested on their efficacy on plant height control. Treatments were: daminozide spray (2000, 3000, 5000 mg.L^-1), chlormequat spray (1000, 2000, 3000 mg.L^-1), tank mix of daminozide and chlormequat spray (4500/1500, 3000/1000, 1500/500 mg.L^-1), paclobutrazol spray (15 mg.L^-1), paclobutrazol drench (10, 15, 20, 30 mg.L^-1), uniconazole drench (2.5, 5, 8 mg.L^-1), flurprimidol spray (15, 30, 45 mg.L^-1), and flurprimidol drench (5, 10, 15 mg.L^-1). Plant height, leaf area, and leaf dry weight were recored at 9 weeks after PGR application. Maximum reduction in height was obtained with uniconazole drench at 8 mg.L^-1, resulting in plants 22 cm tall, which were 60.7% shorter than untreated control (56 cm).   Plant height was reduced by 56% and 45.7% using paclobutrazol 30 mg.L^-1 and flurprimidol 15 mg.L^-1 drenchs respectively. Daminozide spray 2000 mg.L^-1 and tank mix of daminozide and chlormequat at 4500/1500 mg.L^-1, suppressed stem elongation by 20.3% and 18.6% respectively.  Paclobutrazol drenches at 30 mg. L^-1 reduced leaf area and leaf dry weight effectively compared with other PGRs. While chlormequat spray at any concentration were ineffective for controlling plant growth.  Based on the results of this experiment, highest concentration of uniconazole excessively reduced plant height. Thus, the most attractive potted plants were produced using a drench application of paclobutrazol at 10 or 15 mg.L^-1.