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Optimal Photoperiod Treatment for Flowering of Bougainvillea xbuttiana 'Afterglow'

  • University of Florida, Fort Lauderdale REC


AdditionAl index words. photoperiod, flowering Bougainvillea xbuttiana cultivars are mostly considered qualitative short-day plants that flower more readily and profusely under short day lengths. The cultivar 'Afterglow' was tested in two experiments to confirm that it is a qualitative short-day plant, and to discover the optimal photoperiod under which flowering is most profuse. Seventy plants were subjected to a range of photoperiods from 8–14 hours in 1-hour increments, to establish an optimal photoperiod for the induction of flowering in the absence of other environmental factors, namely water stress, physical wounding, or high nutrient availability. Experiment 1 confirmed that 'Afterglow' is a short-day plant, producing the most flowers between 8 and 11 hours of daylight. Plants under long-day or short-day with night interruption provided by a single white LED bulb producing 240 μmol·m-2 ·s-1 of photosynthetic photon flux did not produce flowers. Experiment 2 showed that plants grown under an 8-hour photoperiod produced the largest number of flowers.
Proc. Fla. State Hort. Soc. 129: 2016.
Optimal Photoperiod Treatment for Flowering of
Bougainvillea xbuttiana ‘Afterglow’
Mun Wye Chng* and KiMberly a. Moore
University of Florida, IFAS, Fort Lauderdale Research and Education Center
3205 College Avenue, Davie, FL 33314
AdditionAl index words. photoperiod, owering
Bougainvillea xbuttiana cultivars are mostly considered qualitative short-day plants that ower more readily and pro-
fusely under short day lengths. The cultivar ‘Afterglow’ was tested in two experiments to conrm that it is a qualitative
short-day plant, and to discover the optimal photoperiod under which owering is most profuse. Seventy plants were
subjected to a range of photoperiods from 8–14 hours in 1-hour increments, to establish an optimal photoperiod for
the induction of owering in the absence of other environmental factors, namely water stress, physical wounding, or
high nutrient availability. Experiment 1 conrmed that ‘Afterglow’ is a short-day plant, producing the most owers
between 8 and 11 hours of daylight. Plants under long-day or short-day with night interruption provided by a single
white LED bulb producing 240 μmol·m-2·s-1 of photosynthetic photon ux did not produce owers. Experiment 2 showed
that plants grown under an 8-hour photoperiod produced the largest number of owers.
*Corresponding author; email: mwchng@u.edu
Scientic studies related to the owering of horticultural plants
in tropical and subtropical zones of the world, including the United
States, have largely focused on the economically important tropi-
cal fruit crops of several key genera, namely Mangifera (mango),
Citrus (citrus) and Litchi (lychee) (Koshita and Takahara, 2004;
Menzel, 1983; Núñez-Elisea and Davenport, 1994; Ramírez
and Davenport, 2010). Less attention has been paid to woody
ornamentals in recent years, as the owering behaviors of most
landscape plants are already well understood.
Ornamental crops in the tropics can generally be classied as
foliage crops and oral crops. The difference between tropical/
sub-tropical and temperate oral crops from a landscape design
point of view is the seasonality of owering, and the respective
environmental stimuli that trigger owering. In high latitudes,
day length (photoperiod) and temperature (vernalization) are
the predominant cues that signal plants to enter reproductive
phase. At lower latitudes, especially in areas with distinct wet/
dry seasonal variations, the trigger is often water availability. In
areas without signicant variation in water availability, intensity
of solar radiation has been suggested as the trigger for some spe-
cies to ower (Yeang, 2007).
Bougainvillea (Bougainvillea spp.) is a very widespread and
common woody evergreen perennial that is used as a landscape
ornamental plant in South Florida and in tropical areas around
the world. It is greatly valued for its vigor and resistance to pests,
disease, and drought, in addition to its bright oral display of
colorful bracts, and variability in form as it can be planted as a
shrub, standard, espaliered, or trained onto a trellis (Kobayashi
et al., 2007).
It is a short day (SD) plant, with owers forming in apical
panicles on the current year wood (Ma and Gu, 2010). In sub-
tropical/tropical climates like South Florida, bougainvillea owers
in late spring to early summer, and in late summer to fall, when
the daylight is less than 12-h per day and night-time tempera-
tures are above 21 °C (Schoellhorn and Alvarez, 2002). Ramina
and Sachs (1979) hypothesized that owering in bougainvillea
is a function of nutrient diversion, and in further studies (Even-
Chen and Sachs, 1980) supported the theory that SD induction
is positively correlated to photosynthetic rates in mature bou-
gainvillea leaves. Ma and Gu, (2010) built on this theory and
conrmed earlier research by Steffen et al. (1988) that owering
in bougainvillea was controlled in some way by gibberellic acid
(GA) by diverting nutrient assimilates away from the apical
meristem. The GA levels are known to change in response to
photoperiod, or more specically, to the effect of far-red light
on phytochrome photoreceptors (Taiz and Zeiger, 2010). The SD
induction in Bougainvillea ‘San Diego Red’, and Bougainvillea
glabra ‘Sanderiana’ was thought to be the result of a complex
web of interactions between GA and other hormones, as well as
environmental factors such as photoperiod and light intensity
(Even-Chen and Sachs, 1980; Joiner et al., 1962).
We hypothesized that all other environmental factors being
equal, Bougainvillea ‘Afterglow’ would exhibit the same SD
inductive response as ‘San Diego Red’, with an inductive photo-
period of between 8 and 10 h. We also sought to clarify the most
inductive day length for this cultivar. The objective of Experiment
1 was to verify that Bougainvillea ‘Afterglow’ was a quantitative
short-day plant (SDP), and that owering can be suppressed by
night interruption or daylight extension. The objective of Experi-
ment 2 was to discover the length of photoperiod that was most
inductive to owering for this cultivar.
Materials and Methods
Experiment 1 was conducted in December 2015, and Experi-
ment 2 was conducted in March 2016. Established rooted cuttings
of Bougainvillea ‘Afterglow’ were used in both experiments. They
were transplanted into 10-cm pots lled with 100% coarse washed
aquarium sand. Plants were sprayed with 30 μL of ethephon (25 mL
of 1200 ppm concentration; Southern Agricultural Insecticides,
Proc. Fla. State Hort. Soc. 129:231–233. 2016.
Ornamental, Garden & Landscape Section
232 Proc. Fla. State Hort. Soc. 129: 2016.
Inc., Hendersonville, NC) to induce leaf and inorescence senes-
cence, then pruned to remove apical buds, and as far as possible
reduced to a length with seven visible lateral nodes. They were then
kept vegetative under a 14-h photoperiod, consisting of daylight
supplemented with white LED lights supplying 70 μmol·m-1·s-1
photosynthetic photo ux (PPF). To prevent nutrient deciencies,
plants were fertilized with Peters Professional Bloom Booster
(10N–30P2O5–20K2O; JR Peters, Allentown, PA) at 9.4 μg total
nitrogen once per week starting three weeks prior to the start of
the experiment. We continued to apply the fertilizer at the same
rate once per week through the remainder of the experiment. In
addition to fertilizer, plants were watered with 50 mL of tap water
(EC = 516 μS, pH = 8.3) every two days.
experiment 1. Thirty plants (six treatments with ve replicates
each) were arranged in a completely randomized design in a
greenhouse at the University of Florida Fort Lauderdale Research
and Education Center in Davie, Florida. Photoperiod treatments
were created using a 5-gallon black plastic pot inverted over
each plant to block out light. All plants were covered at 6:00 pM
and uncovered at 8:00 aM. A single 5-watt LED bulb providing
35 μmol·m-1·s-1 PPF suspended inside each pot and set on a timer
provided night interruption or day length extension. There were
three continuous photoperiod treatments: 1) 14 h (control); 2)
11- and 8-h; and 3) three night interruption treatments (8+3 h,
8+6 h, and 11+3 h). Night interruptions occurred after 3 h of
dark. Night interruption treatments were designed to match
the number of daylight hours of the continuous photoperiod
treatments. Prior experiments (data not shown) suggested that
short night interruptions (5–30 min. of light) were insufcient
to inhibit owering.
Root zone temperature (RZT) was monitored using two data-
loggers (HOBOWare Pro U12, Onset Computer Corporation,
Bourne, MA), with sensors inserted into seven random replicates.
Temperatures were logged in °C at 30-min. intervals.
Plant size was recorded at the start and end of the experiment
to calculate growth. Plant size was determined by the formula
Size = H × W1 × W2
where H = height, rounded to the nearest centimeter, W1 = maxi-
mum width of the plant to the nearest cm, and W2 = width of the
plant perpendicular to W1, to the nearest centimeter.
Growth was dened as the difference between the plant size
at the end of the experiment and at the start of the experiment.
Relative growth was calculated as the percentage ratio of growth
over initial size.
The number of inorescences on each plant was counted at
the end of day 30. One inorescence was dened as an individual
thorn-inorescence axil, regardless of how many orets were
attached to the peduncle.
experiment 2. . The experiment was repeated with the same
preparation but with continuous photoperiod treatments without
night interruption. The treatment levels were: 14 (control), 12-h,
11-h, 10-h, 9-h, and 8-h photoperiods, respectively.
Analysis of variance (ANOVA, α = 0.05) was performed using
R statistical analysis program, with number of inorescence as
the dependent variable and photoperiod treatment and relative
growth as the independent variables. Data for Experiment 1 and
Experiment 2 were analyzed separately. Root-zone temperature
was identical across all treatments so these data were omitted
from ANOVA. The average daily temperature range was 24 °C
to 32 °C. In Experiment 1, mean separation was conducted us-
ing paired t-tests to identify which treatments were signicantly
different. In Experiment 2, Tukey’s HSD was used to identify
which treatments were signicant.
The number of inorescences was not signicantly affected
by the growth of the plants, and there was no interaction between
relative growth and photoperiod treatment on number of ino-
rescences (Table 1).
In Experiment 1, plants grown under 8-h and 11-h photope-
riods produced signicantly more inorescences than all other
treatments (Table 2). All night interruption treatments inhibited
owering response. Although relative growth was not signicantly
different, there was some variation between treatments. Plants
grown under the 11+3-h treatment grew the most, while those
under 8+6-h treatment grew the least. Among the three continu-
ous photoperiod treatments, the control plants (14-h) grew the
most but had the fewest owers, while those under 8-h grew the
least but had the most number of owers. Since night interruption
effectively inhibited owering, the treatments for Experiment 2
omitted night interruption and focused on narrowing the range
of photoperiod treatments.
In Experiment 2, control plants remained completely vegeta-
tive under a 14-h photoperiod (Table 3). Plants grown under an
Table 1. Effect of Photoperiod and relative growth on number of ino-
rescences on Bougainvillea ‘Afterglow’ in Dec. 2015.
Variable df Sum Sq Mean Sq F-Value P-value
Photoperiod 5 755.5 151.10 3.235 0.0295z
Relative Growth 1 42.5 42.51 0.910 0.3527
Treatments x Rel Growth 5 267.4 53.48 1.145 0.3732
zSignicant difference at α = 0.05.
Table 2. Effects of Photoperiod on number of inorescences and relative
growth of Bougainvillea ‘Afterglow’ in December 2015. Mean values
in the same column followed by the same letters are not signicantly
different at α = 0.05.
Treatment Mean number Relative
(h) of inorescences growth (%)
Control (14) 0.4 c 262.8 a
11 6.2 a 253.0 a
8 10.2 a 190.7 a
8+3 0.0 bc 216.8 a
8+6 0.0 bc 144.4 a
11+3 3.0 c 346.9 a
Table 3. Effects of photoperiods on number of inorescences of Bou-
gainvillea ‘Afterglow’ grown in Feb. 2016. Mean values in the same
column followed by the same letters are not signicantly different
at α = 0.05.
Treatment (h) Mean number of inorescences
14 (control) 0.0 b
12 5.8 b
11 8.0 b
10 8.4 b
9 11.2 a
8 13.6 a
Proc. Fla. State Hort. Soc. 129: 2016.
8-h photoperiod had the highest mean number of inorescences
(13.6). Plants grown under a 9-h photoperiod had the second
highest mean number of inorescences (11.2). There was no
difference in the number of inorescences produced under 12-,
11- and 10-h photoperiods. Number of inorescences appeared to
correlate with decreasing length of photoperiod between 12–8 h.
Previous studies indicated that Bougainvillea ‘San Diego Red’
owered under SD conditions (Even-Chen et al., 1979; Even-Chen
and Sachs, 1980). The results of Experiment 1 appear to support
the hypothesis that the cultivar ‘Afterglow’ is also a SD plant that
requires photoperiods of less than 12 h to induce owering, while
the results from Experiment 2 indicate that 8 h of daylight was the
most inductive photoperiod for ‘Afterglow’. These results concur
with previous research on optimal photoperiods for owering of
bougainvillea (Schoellhorn and Alvarez, n.d.; Singh et al., 2013).
In addition, owering of ‘Afterglow’ was completely inhibited by
extending the photoperiod to 14 h either continuously or as night
interruption. This result was interesting as early studies suggested
that there was no clearly dened critical photoperiod to induce
oral initiation in bougainvillea (Joiner et al., 1962). However,
these results suggest that there may be a threshold photoperiod
to inhibit owering altogether. In addition, the inability of short
night interruptions of 5–30 min. to inhibit owering will be of
interest for further research.
Bougainvillea is an important landscape shrub mostly in the
tropics and subtropics, where annual variation of day length is at
most between 10 1/2 h and 13 1/2 h. In these areas, seasonal varia-
tion in day length creates alternating inductive and sub-inductive
photoperiods. We designate the latter sub-inductive rather than
non-inductive because the photoperiod does not reach or exceed
14 h, which would completely inhibit owering. In South Florida,
the inductive season would correspond to mid-October to mid-
February, and the sub-inductive period would be from March
through September. The tropics also encompass equatorial areas
that have a constant year-round 12-h day length. In these places,
we could consider the entire year as always sub-inductive, so
owering can take place sporadically year-round in response
to other factors such as microclimate, light intensity, nutrient
availability, and environmental stresses. In particular, the water
decit and physical stress have been found to induce owering in
bougainvillea (Fang-Yin Liu and Yu-Sen Chang, 2011; Liu and
Chang, 2011; Ma and Gu, 2010; Schoellhorn and Alvarez, n.d.).
The conrmation of inductive photoperiods for Bougainvillea
‘Afterglow’ further supports the hypothesis that oral initiation is
a function of GA. Since the stress hormones ethylene and abscisic
acid (ABA) both have complex relations with GA pathways, fur-
ther investigation of the interactions between ethylene and ABA
levels on GA in bougainvillea in relation to owering responses
under subinductive conditions should be taken.
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... 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). ...
Full-text available
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 (GA 3 ) 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 GA 3 were measured 1 and 48 days after treatment to calculate the change in GA 3 (∆GA 3 ). A short day (SD) control of 8 hours was included to measure differences in inflorescence number and ∆GA 3 between photoperiods. ‘Afterglow’ plants grown under SD conditions had the greatest decrease in ∆GA 3 (–1.09 µg·g –1 ) over 48 days and the most inflorescences (10.6) compared with LD control plants with a decrease in ∆GA 3 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 ∆GA 3 (–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 ∆GA 3 increased. Exogenous ABA rates of 1 mg/plant produced inflorescence numbers (1.4) and ∆GA 3 (–0.10 µg·g –1 ) similar to the LD control. As the rate increased, ∆GA 3 increased and inflorescence number decreased. Plants treated with ABA rates of 3 mg/plant and more were defoliated and had no inflorescences.
Full-text available
Mango flowering is an important physiological event that sets the start of fruit production. Initiation is the first event that takes place for mangoes to flower. Coincident with shoot initiation, induction occurs based on the conditions present at the time of initiation. Numerous studies with mango trees support the existence of a florigenic promoter (FP) that is continuously synthesized in mango leaves and induces flowering. Translocation experiments suggest that the FP is carried from leaves to buds in phloem. Induction appears to be governed by the interaction of the FP and a vegetative promoter (VP). The FP is translocated as far as 100cm in subtropical conditions and 52cm in tropical conditions. In the tropics, floral induction occurs in stems that have attained sufficient time in rest since the previous flush. The age of the last flush is the primary factor governing flowering in the tropics as evidenced by experiments in Colombia. Tip pruning is ideal to synchronize vegetative flush events in the canopy. Potassium nitrate (KNO3) has been shown to stimulate flowering in sufficiently mature stems. Tip pruning and foliar applied KNO3 are effective methodologies that induce synchronous flowering especially in Colombia. Cool temperatures are important for mango floral induction under subtropical conditions. Mangoes grown in the low-latitude tropics rely less on low temperature. Soil and leaf analyses should be conducted to evaluate the nutrient status of trees.
Full-text available
Benzyladenine (BA) and short day (SD) induction promote and gibberellic acid (GA) inhibits flowering in Bougainvillea "San Diego Red." GA is an overriding vegetative signal maintaining plants in a vegetative state even when BA is applied in SD conditions. SD promotes a more rapid conversion of BA to the ribotide and other "polar derivatives" (containing adenine derivatives). This effect of SD on BA metabolism is seen in root, stem, and apical bud tissues and is completely prevented by prior or simultaneous application of GA. GA treatment reduces the rate of polar derivative formation to that found in plants held in long days. The working hypothesis is that SD promotes flowering in Bougainvillea owing to reduced transport of gibberellins from leaves to roots and apical buds permitting metabolism of cytokinin, and perhaps other purine bases, to more polar forms that are more readily translocated and active in promoting reproductive development of the inflorescences axes.
Although gibberellic acid (GA3) promotes stem elongation and inhibits flowering in Bougainvillea, no inhibition of floret initiation was observed when excised meristems were cultured in vitro in media containing GA. These results are predicted by a nutrient diversion hypothesis in which GA-induced inhibition of flower initiation in intact plants is considered an indirect result of growth promotion of competing vegetative sinks with consequent diversion of nutrients away from the inflorescence meristem. The failure to observe GA-induced inhibition of floret initiation in vitro was probably the result of an excess of metabolites for growth and differentiation of the excised meristems in the media.
Bougainvillea (Bougainvillea spp.) is an economically important ornamental flower in subtropicaland tropical regions. This study examined bougainvillea shoots of different developmental stages, e.g.,vegetative shoot, flowering shoot stage 1 with fully-developed thorn-inflorescence axis (FS1), flowering shootstage 2 with visible flower bud (FS2), and flowering shoot stage 3 with blooming shoot (FS3) following theirtreatment with ethephon (2-chloroethylphosphonic acid). Experimental results indicated that ethephon treatmentof bougainvillea's vegetative shoot accelerates its shoot maturity and enhances flower formation. Thesame treatment also increases endogenous ethylene production of the vegetative shoot, subsequently facilitatingflower formation whereby the endogenous ACC content is lower than that of reproductive shoots (FS1,FS2, and FS3). Moreover, the ethephon treatment of reproductive bougainvillea shoots increases the ACCcontent beyond that of the vegetative shoot. Therefore, reproductive shoots produced more ethylene than vegetativeshoots, subsequently inhibiting the development of flowers or even causing serious abscission of flowerbuds and leafs. This reveals that the role of ethylene in regulating the flowering control of bougainvillea is bidirectional.Results of this study demonstrate the significance of shoot maturity in the growth and flowering ofthe bougainvillea in which ethylene plays a major role.
This document is ENH 874, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date October 1, 2002.
Experiments were conducted in Homestead, FL, during July (mean minimum temperatures about 20°C, non-floral-inductive), and October and November (mean minimum temperatures about 15°C, floral-inductive), to determine if water stress induces floral morphogenesis in mango (Mangifera indica L.). Suspending irrigation of container-grown cultivar ‘Tommy Atkins’ trees for 25 or 36 days in July caused water stress (ΨL of −3.55 MPa and −3.78 MPa, respectively), while irrigated trees remained non-stressed (−0.30 MPa). No flowering occurred, and all apical buds produced a vegetative flush which emerged later in water-stressed trees than in controls. In October, suspending irrigation of ‘Tommy Atkins’ trees for 35 days resulted in ΨL values similar to those obtained during July. However, all apical buds, including those of irrigated trees, initiated flowers. Water stress advanced floral budbreak by nearly 2 weeks in nearly 40% of buds. In November, trees of four mango cultivars were or were not water-stressed in a glasshouse under warm conditions (mean minimum temperatures about 20°C), and were lightly pruned to stimulate growth of dormant axillary buds. Axillary buds produced vegetative growth only. Lightly pruned trees growing outdoors in cool temperatures initiated axillary floral buds.In warm temperatures (mean minimum temperatures about 20°C), water stress delayed shoot extension, but did not induce floral morphogenesis. In cool temperatures (mean minimum temperatures about 15°C), floral buds were initiated regardless of water stress. Thus, floral morphogenesis was induced by chilling temperatures. In contrast to water stress delaying the development of vegetative buds, the growth of floral buds was stimulated by water stress. Low temperatures thus promoted floral induction of mango, whereas water stress promoted growth of florally induced buds.
The effect of water stress on plant hormones (GAs, IAA and ABA) level in the leaves and flower-bud formation of the satsuma mandarin (Citrus unshiu Marc.) trees was investigated to determine the relationship between flower-bud induction and the level of endogenous plant hormones as a result of water stress. Severe water stress (−1.5 to −2.0 MPa) in autumn, which causes heavy leaf fall, reduced the percentage of flowering nodes by one third of the moderately water-stressed ones (−0.5 to −1.0 MPa). The quantity of GA1/3 from the middle of October through early December was significantly higher in the leaves of the trees under severe water stress than in the leaves of the trees enduring moderate water stress. The content of IAA in the leaves of the trees under moderate water stress was higher in late February. These findings indicate that the levels of GA1/3 are enhanced by severe water stress, higher in the leaves from the branches that produce fewer flowers during flower-bud induction periods. The levels of IAA were higher in the leaves from the branches that produced more flowers during the season when flower-buds develop.
In lychee (Litchi chinensis Sonn.), there is a strong correlation between flowering and the degree of dormancy prior to the normal period of floral induction. Low temperatures and moisture stress seem to restrict vegetative growth and promote floral initiation, but there are many exceptions and many examples of interactions of factors (crop load, tree nutrition, etc.). Both cincturing (ringing) and exogenous auxins have been shown to reduce vegetative flushing and increase flowering, but the results have not always been consistent. The best prospect for improving flowering appears to be through the selection of genotypes (probably those with low vigour) which flower under conditions which are generally warmer and wetter than those in its native environment in China. An alternative approach involves a critical study of manipulative techniques which restrict vegetative flushing and promote vegetative dormancy under non-inductive conditions, i.e. cincturing, applying growth retardants and withholding irrigation and fertilizers.
Short day induction in Bougainvillea "San Diego Red" increases photosynthetic rates in mature leaves; gibberellic acid treatments, which inhibit flowering, cancel the short day effect. These results lend support of a nutritional hypothesis that suggests that in Bougainvillea assimilate supply to the reproductive axis increases before floral initiation and during flower development.
How tropical trees flower synchronously near the equator in the absence of significant day length variation or other meteorological cues has long been a puzzle. The rubber tree (Hevea brasiliensis) is used as a model to investigate this phenomenon. The annual cycle of solar radiation intensity is shown to correspond closely with the flowering of the rubber tree planted near the equator and in the subtropics. Unlike in temperate regions, where incoming solar radiation (insolation) is dependent on both day length and radiation intensity, insolation at the equator is due entirely to the latter. Insolation at the upper atmosphere peaks twice a year during the spring and autumn equinoxes, but the actual solar radiation that reaches the ground is attenuated to varying extents in different localities. The rubber tree shows one or two flowering seasons a year (with major and minor seasons in the latter) in accordance with the solar radiation intensity received. High solar radiation intensity, and in particular bright sunshine (as distinct from prolonged diffuse radiation), induces synchronous anthesis and blooming in Hevea around the time of the equinoxes. The same mechanism may be operational in other tropical tree species.