Differences in Inflorescence Numbers and Endogenous Gibberellic Acid Levels in ‘Afterglow’ Bougainvillea

Bougainvillea (Bougainvillea sp.) plant inflorescence number will vary in response to multiple cues such as changes in temperature, water, light intensity, pruning, and photoperiod. Previous research reports that the application of plant growth regulators (PGRs) to bougainvillea grown under varying photoperiods improved inflorescence number, probably as a result of changes in gibberellic acid (GA) levels. There are many bioactive plant GAs, but we chose to investigate differences in gibberellic acid 3 (GA₃) levels and inflorescence number in response to the application of ethephon (2-cholorethylphosponic acid) or abscisic acid (ABA) to ‘Afterglow’ bougainvillea (Bougainvillea ×buttiana) grown under 14-hour photoperiod [long-day (LD)] conditions. Plants were 5 inches tall with seven visible lateral nodes and were grown in a greenhouse in 4-inch pots filled with 5-mm coarse aquarium zeolite. Ethephon was applied as a foliar spray at 0.05, 0.07, 0.10, 0.15, or 0.20 mg/plant. ABA was applied as a soil drench at 1, 1.5, 3, 6, 8, or 10 mg/plant. Endogenous levels of GA₃ were measured 1 and 48 days after treatment to calculate the change in GA₃ (∆GA₃). A short day (SD) control of 8 hours was included to measure differences in inflorescence number and ∆GA₃ between photoperiods. ‘Afterglow’ plants grown under SD conditions had the greatest decrease in ∆GA₃ (–1.09 µg·g–¹) over 48 days and the most inflorescences (10.6) compared with LD control plants with a decrease in ∆GA₃ of –0.09 µg·g–¹ and fewer inflorescences (1.0). Plants grown under LD conditions and treated with 0.05 mg/plant ethephon had inflorescence numbers (9.6) and levels of ∆GA₃ (–0.74 µg·g–¹) similar to the SD control. As ethephon rate increased to more than 0.05 mg/plant, inflorescence number on LD plants decreased and ∆GA₃ increased. Exogenous ABA rates of 1 mg/plant produced inflorescence numbers (1.4) and ∆GA₃ (–0.10 µg·g–¹) similar to the LD control. As the rate increased, ∆GA₃ increased and inflorescence number decreased. Plants treated with ABA rates of 3 mg/plant and more were defoliated and had no inflorescences.