Bougainvillea (Bougainvillea sp.) is a popular landscape ornamental plant in tropical and subtropical zones around the world, valued for its vigor and relative resistance to pests, disease, and drought; variability in form; and bright floral display of colorful bracts. Inflorescences form in apical panicles on the current year’s wood, with number of inflorescences and timing of flowering varying based on differences in cultivars as well as in response to changes in temperature, irrigation, light intensity, pruning, and photoperiod (Hackett and Sachs, 1985; Norcini et al., 1994). ‘Afterglow’ bougainvillea (Bougainvillea ×buttiana) plants grown under SDs (8 h) had visible inflorescences within 30 d, with 8 to 10 inflorescences per plant compared with no inflorescences on LD (14-h) plants after 30 d (Chng and Moore, 2016).
Inflorescence number on bougainvillea plants grown under LD conditions has been enhanced by the use of PGRs such as chlormequat, daminozide, paclobutrazol, and dikegulac, which act by inhibiting GA levels (Karagüzel, 1999; Liu and Chang, 2011b; Norcini et al., 1994). Gibberellic acids play an important role in flowering pathways (Campos-Rivero et al., 2017; Taiz et al., 2018). They promoted flowering in LD plants such as spinach [Spinacia oleracea (Metzger and Zeevaart, 1980)], and sweet william [Silene armeria (Talon and Zeevaart, 1990)], but exogenous applications of GA to ‘San Diego Red’ bougainvillea grown under SD conditions inhibited flowering (Even-Chen et al., 1979).
Hackett and Sachs (1985) suggested that GAs stimulate sinks for available plant resources that compete with developing inflorescence primordia, and the removal of young leaves reduces GA levels, potentially shifting resources to inflorescences (Hackett and Sachs, 1985). Gibberellins are senescence-repressing hormones with active forms that decline in leaves as they age, whereas ethylene and ABA levels increase in senescing leaves and are considered senescence-promoting hormones that accelerate leaf and inflorescence senescence (Taiz et al., 2018). There are many bioactive GAs in plants, but we chose to focus on gibberellic acid 3 (GA3) because this is a commercially available plant hormone that can be applied to moderate plant growth and flowering, and it has been heavily investigated (Campos-Rivero et al., 2017; Hackett and Sachs, 1967; Taiz et al., 2018).
Leaf senescence is enhanced by the plant hormones ethylene and ABA (Taiz et al., 2018). Bending shoots of ‘Taipei Red’ bougainvillea resulted in greater ethylene production, leading to enhanced inflorescence number (Liu and Chang, 2011a). In a follow-up study, application of ethephon, a synthetic ethylene-releasing compound, to vegetative ‘Taipei Red’ bougainvillea shoots produced more inflorescences than vegetative control plants. However, foliar applications of ethephon to flowering shoots caused inflorescence and bud drop (Liu and Chang, 2011b). Unfortunately, Liu and Change (2011a, 2011b) only investigated one rate of ethephon (0.15 mg/plant) and did not measure changes in endogenous GA levels. They suggested that adequately controlling the response to ethylene during shoot development would facilitate inflorescence control.
In a preliminary study, we observed that the application of ethephon at rates of 0.07 to 0.15 mg/plant to ‘Afterglow’ bougainvillea grown under SD conditions increased the number of inflorescences after 30 d, but had limited success in improving inflorescence number under LD conditions (Chng and Moore, 2018). Ethephon rates of 0.3 mg/plant applied to plants grown under both SD and LD conditions resulted in complete defoliation 3 d after treatment (Chng and Moore, 2018). The objective of this study was to monitor differences in inflorescence number and endogenous GA3 levels in response to the application of five rates of ethephon and ABA to LD plants.
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