FEC_580.fm Page 782 Monday, November 12, 2001 6:26 PM
Functional
Ecology 2001
15, 782 – 790
Why do flowers of a hummingbird-pollinated mistletoe
face down?
Blackwell Science Ltd
M. TADEY and M. A. AIZEN†
Laboratorio Ecotono, CRUB, Universidad Nacional del Comahue, Unidad Postal Universidad, 8400 Bariloche,
Río Negro, Argentina
Summary
1. Pendant flowers are common among hummingbird-pollinated plants. A downward
orientation of the flower or inflorescence could represent an adaptation to avoid either
flower flooding or direct pollen losses from anthers or stigmas under rainy conditions.
2. We studied the adaptive significance of this trait experimentally in Tristerix corymbosus
Kuijt (Loranthaceae), a mistletoe native to the temperate forests of southern South
America. We applied three treatments: (i) natural pendant inflorescences; (ii) inflorescences tethered to face up; and (iii) inflorescences tethered to face down (as a control
for tethering). We also considered natural exposure to rain as a second factor.
3. The treatments did not differ significantly in either nectar volume or concentration.
Flowers exposed to rain for most of their lives contained more diluted nectar than
those that remained dry, but this result did not depend on either inflorescence or flower
orientation.
4. We found significantly fewer pollen tubes in styles of flowers from inflorescences
tethered to face up than in flowers receiving the other two treatments, but this could
not be attributed to a direct effect of rain exposure. Inflorescence orientation did not
affect either the number of pollen grains left in anthers or seed set. No strong evidence
was found for differential visitation by hummingbirds in relation to a flower’s angle.
5. The results of this work support neither the flower-flooding nor the pollen-protection
hypothesis. However, a flower’s orientation may affect the extent of within-flower selfpollination or the efficiency of pollen transfer from a hummingbird’s bill onto a flower’s
stigma.
Key-words: Female and male reproductive success, flower orientation, hummingbird pollination, pollen
tubes, rain
Functional Ecology (2001) 15, 782 – 790
Introduction
Flowering plants that depend on animal agents of pollination may ensure successful reproduction through
traits that increase attraction and effective visitation by
pollinators (Bertin 1982; Howe & Lynn 1988; Vaknin,
Tov & Eisikowitch 1996; Waser 1983). These traits can
affect plant fitness through pollen receipt and seed set
(female function), and pollen donation and seed siring
(male function). Several studies have revealed that slight
variations in size, shape or colour can affect visitation
to individual flowers, thus affecting female as well as
male functions (Campbell 1989; Dafni & Kevan 1997;
Meléndez-Ackerman, Campbell & Waser 1997; Murcia
1990; Podolsky 1992; Podolsky 1993; Temeles 1996).
In addition to these traits, flower angle in relation to
the horizontal may also affect pollination and plant
© 2001 British
Ecological Society
†Author to whom correspondence should be addressed.
E-mail: [email protected]
reproductive success. For example, species with hanging flowers and tubular corollas presumably avoid
flower flooding during rain, exposing the external side
of petals instead of the inner part of the corolla.
Consequently they avoid nectar dilution which would
affect visitation by discriminating pollinators (Sprengel
1793 translated in Lloyd & Barrett 1996). Another
possible advantage of down-facing flowers could be
to protect pollen from being washed from anthers or
stigmas: petals act as ‘umbrellas’, enhancing both female
and male fitness (Broyles & Wyatt 1990; Campbell 1989;
Devlin, Clegg & Ellstrand 1992; Dudash 1991; Galen
1992; Schoen & Stewart 1986).
Although facing down is a widespread flower trait
among angiosperms, it may represent an important
adaptation to vertebrate pollinators that remain active
during rainy or even snowy weather (M.A.A., unpublished results). Pendant flowers or inflorescences appear
to occur frequently among hummingbird-pollinated
plants. This trait, plus red coloration, tubular shape,
782
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783
Adaptive
significance of
flower orientation
absence of odour, and diluted nectar rich in sucrose
are commonly associated in the so-called ‘hummingbird pollination syndrome’ (Grant & Grant 1968;
van der Pijl 1961; Proctor, Yeo & Lack 1996). Despite
its potential importance for plant fitness, however,
flower orientation has received much less attention
than other morphological traits of hummingbirdpollinated plants, such as flower colour, size or
shape (Campbell 1989; Campbell, Waser & MeléndezAckerman 1997; Feinsinger & Busby 1987; Fenster
1991; Hurlbert et al. 1996; Meléndez-Ackerman 1997;
Murcia & Feinsinger 1996; Podolsky 1992; Podolsky
1993; Straw 1956; Stanton, Snow & Handel 1986;
Temeles 1996).
Here we assess the adaptive significance of a downfacing orientation by altering natural flower angle in
the hanging inflorescences of the hemiparasite Tristerix corymbosus Kuijt (Loranthaceae), a hummingbirdpollinated plant native to the southern Andes. In
particular, we tested the flower-flooding and pollenprotection hypotheses detailed above, by examining
simultaneously how flower angle and exposure to rain
affect nectar quality and quantity, pollinator visitation, pollen in anthers, pollination levels, and seed set.
Materials and methods
   
Tristerix corymbosus, the ‘quintral’, occurs along the
western rim of southern South America in Chile
and Argentina, from 32° S to 42° S. It inhabits regions
with >4000 mm of annual rainfall to the south in the
Valdivian rainforest, to <500 mm to the north in the
Mediterranean region of central Chile (Kuijt 1988).
Throughout its geographical range, it parasitizes more
than 20 host species. Tristerix corymbosus has a very
extended flowering phenology. In our study area,
flowering lasts nearly 9 months from the beginning
of March to the end of November (Ruffini 1992;
M.A.A., unpublished results). Well established plants
can produce several hundred flower buds. Flowers are
arranged in short-peduncled racemes or corymbs, with
four to 14 flowers. Inflorescences are usually pendant,
or more rarely erect with flowers spreading in different
angles. Flowers are red, symmetrical, tubular, and 4–
5 cm long × 0·3 cm wide. The four petals, not truly
fused, reflex during anthesis exposing four yellow filaments and a capitate style which protrudes about
0·5 cm beyond the anthers (Fig. 1a). The filaments
and style turn red when a flower senesces. Despite
self-compatibility, pollen transfer and seed set in T.
corymbosus are mostly pollinator-mediated (M.A.A.,
unpublished results). In our study area, the exclusive
pollinator of this plant species throughout most of its
flowering phenology (from March to October) is Sephanoides sephaniodes, a trochilid hummingbird native
to the temperate forests of southern South America
(Ruffini 1992; Smith-Ramírez 1993). Pollen of T.
corymbosus can frequently be found in both the dorsal
and ventral surface of the hummingbird’s bill and head
(Ruffini 1992). Successfully pollinated flowers mature
into green viscous berries containing one naked seed
(Amico & Aizen 2000).
The study site was in the Parque Municipal LlaoLlao, about 30 km west of San Carlos de Bariloche,
Argentina (41°8′ S, 71°19′ W). The annual precipitation
is approximately 1800 mm and the mean temperature
varies from 2 °C in winter to 13·6 °C in summer. Most
precipitation (88%) falls between March and November – the flowering period of T. corymbosus – and
snowstorms are common during winter (Barros et al.
1983). The study population of T. corymbosus was
located in a large gap (≈100 × 50 m) near the path to
Villa Tacul, amidst a mixed, old-growth forest dominated by Nothofagus dombeyi and the conifer Austrocedrus chilensis. The population consisted of about 20
large flowering individuals (60–180 inflorescences per
plant) parasitizing branches of the shrubs Aristotelia
maqui and Maytenus boaria, the two most common
hosts in the study area.
 
Fig. 1. Diagrams showing (a) the developmental stages of a flower of Tristerix
© 2001 British
corymbosus
(left to right: flower bud; 1-day-old flower; mature flower close to
Ecological Society,
senescence);
and (b) the three inflorescence treatments. In (b) a solid line indicates a
flower’s
major
axis;,α+ and α– examples of flowers showing positive and negative angles
Functional
Ecology
with
respect
15
, 782
– 790 to the horizontal (dotted line), respectively.
In April 1998, we randomly selected 10 individuals of
T. corymbosus and applied one of the following treatments to individual inflorescences: (i) natural (control)
pendant inflorescences (CO); (ii) pendant inflorescences tethered to face up (FU); and (iii) pendant
inflorescences tethered to face down, as a control for
tethering (FD). Pieces of thin copper wire tied around
an inflorescence’s peduncle and the adjacent branch
FEC_580.fm Page 784 Monday, November 12, 2001 6:26 PM
784
M. Tadey &
M. A. Aizen
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
were used to tether inflorescences in the desired position (Fig. 1b). Each of the three treatments was applied
to five randomly chosen inflorescences per plant (five
inflorescences × three treatments = 15 treated inflorescences per plant). In each inflorescence selected, we
marked five flower buds using numbered jewellery tags
tied around the pedicel. Tags were hidden behind an
inflorescence’s subtending leaves. The status of each
tagged bud was checked daily or every other day, from
April to the end of September when all marked flowers
had senesced. On each sampling day tagged buds were
classified as: bud; open flower; senescent flower; swollen
ovary; developing fruit. At the end of the flowering
season we continued sampling once every 2 weeks until
late December when all marked fruits had matured.
The fate of 750 flowers was followed.
After anthesis, the angle of the major axis of each
marked flower relative to the horizontal was recorded
using a clinometer (AcuAngle, A-300, accuracy = 0·1°;
Fig. 1b). We avoided any shaking of flowers that might
cause artificial pollen removal or deposition. Angles
ranged between –90° and +90°, with negative angles
representing down-facing flowers and positive angles
representing up-facing flowers. A horizontal flower
had an angle of 0°. We estimated flower longevity as
the number of days between the date a flower was first
observed open and the date it was first recorded as
senescent.
To estimate pollination success, we collected styles
of marked flowers 1–2 weeks after flower senescence.
Styles were placed in individual microcentrifuge tubes
and fixed in formalin : acetic acid : ethyl alcohol
5 : 5 : 90. Observations showed that style removal at
this time did not affect future fruit development
(M.A.A., unpublished results). In the laboratory, styles
were cleared overnight in 10 N NaOH and stained
with 0·1% aniline blue in 0·1  K3PO4 (Martin 1959),
squashed, and examined with an epifluorescence microscope. We counted pollen tubes just below the stigma
to estimate the number of germinated pollen grains.
To determine whether hummingbirds prefer to visit
a particular inflorescence type, we observed hummingbird visits to focal inflorescences for 2 h per sampling
date. As we could not observe all the plants at once, we
observed groups of two or three different plants for
30 min at a time. Given that we could record only a few
visits to experimental inflorescences, we supplemented
information on hummingbirds’ preferences by measuring the angles of visited flowers in non-experimental
inflorescences over the whole sampling period. Each
time we measured the angle of a visited flower, we also
measured the angle of a randomly chosen flower in
the nearest inflorescence within the same plant (cf.
Stanton & Galen 1989).
Because we could not measure nectar non-destructively,
we collected non-tagged, fresh flowers (2–4 days old)
from experimental inflorescences (n = 397 flowers). The
flower angle was measured before collection. Nectar
standing crop was extracted with 5 µl capillary tubes
and its concentration measured with a hand refractometer (Reichter, model 10431, Leica Inc., Buffalo,
NY, USA). Sugar concentration in sucrose equivalents
(100 × mg solute/ mg solution) was converted to mg
solute µl–1 based on tabulated values in Kearns &
Inouye (1993, p. 172) and combined with sample volumes
to estimate nectar sugar content in mg. We also collected
the anthers from each of these flowers before nectar
measurement, and kept them in separated 0·5 mm
microcentrifuge tubes containing 70% ethyl alcohol.
The volume of each tube was raised to 0·5 ml by adding
drops of detergent solution. After vortexing for 60 s,
the number of pollen grains remaining per anther was
estimated from two aliquots using a haemocytometer
(Aizen & Raffaele 1996; Aizen & Raffaele 1998).
To test whether inflorescence treatment had an
effect on nectar production, we applied the same treatments as above (CO, FU, FD) to inflorescences of five
plants that were not used for the other observations.
Each treatment was applied to three inflorescences per
plant (three inflorescences × three treatments = nine
treated inflorescences per plant). For a given sampling
date we collected one fresh, opened flower from each
experimental inflorescence to measure the standing
volume of nectar. We then marked another flower with
a jewellery tag and bagged the focal inflorescences with
nylon netting. Tagged flowers were collected 24 h later,
and the volume and concentration of secreted nectar
was measured as above. For each inflorescence we
estimated nectar production per flower during a 24 h
period by subtracting the standing crop of the flower
collected just before bagging from the volume measured in the tagged flower. This procedure was repeated
for each focal inflorescence at least twice during the
flowering period.
We classified each date of the flowering period
sampled (April–September 1998) as ‘rainy’ or ‘dry’
based on the occurrence of rain or snow. To document
precipitation, we used five glasses, 6 cm in diameter,
exposed to the open sky as water collectors. The glasses
were placed on the ground within the sampled gap at
the beginning of the sampling period and checked on
every sampling date. We supplemented information
collected in the field with precipitation records from
the Instituto Nacional de Technología Agropecuaria
(INTA) meteorological station, situated 30 km east of
the study site.
 
To test whether inflorescence tethering effectively
modified flower angles, we used a mixed-model 
that included inflorescence treatment as the fixed
factor and plant as the random factor. The five angle
measurements for each inflorescence were averaged
before analysis. We used the same  model to test
for the effect of treatment on 24 h nectar production
(volume and concentration) averaged over each experimental inflorescence. In this and the  model
FEC_580.fm Page 785 Monday, November 12, 2001 6:26 PM
785
Adaptive
significance of
flower orientation
factor interaction (treatment × rain × plant) from the
final model because this term was never significant (P >
0·5 in all cases). Because our design was not completely
balanced, we based significance tests on type III sums
of squares (SAS 1988). Approximated error terms for
each factor included in the model were estimated using
the RANDOM statement in   (SAS 1988).
We report visit frequencies, but did not analyse these
statistically because we recorded only 15 visits to focal
inflorescences. On the other hand, we recorded many
visits to flowers in non-experimental inflorescences
because of their comparatively greater abundance.
Comparisons of angles between visited and paired
randomly chosen flowers were analysed using a nonparametric Wilcoxon test for paired observations
because the data were not normally distributed.
described below, the sum of squares associated with
inflorescence treatment was decomposed in two a
priori, orthogonal contrasts (Sokal & Rohlf 1981).
These contrasts tested for the independent effects of
inflorescence orientation (CO + FD vs FU), and
inflorescence tethering (CO vs FD).
We used a three-factor  model to analyse
flower longevity; nectar standing crop (volume, concentration and total sugar content); number of pollen
grains remaining per anther; number of pollen tubes
per style; and fruit set (number of mature fruits/
number of flowers). This model assessed the effects of
inflorescence treatment (fixed), plant (random), and
exposure to rain (fixed). We averaged values for ‘wet’
and ‘dry’ flowers for each experimental inflorescence,
and considered these means as individual observations
(Mead 1988). For the analysis of flower longevity, pollen tubes and fruit set, a flower was classified as wet if
it was exposed to precipitation for ≥50% of the days it
stayed open, otherwise the flower was considered dry.
For nectar variables and pollen remaining per anther,
we considered a flower as wet if it was exposed to precipitation during the same day or the day before collection (and dry otherwise) because we did not know
the exact date of anthesis of these flowers. These two
classifications were comparable, because flowers collected fresh for nectar analysis were less than 2–4 days
old. In addition, standing nectar might be influenced
more strongly by precipitation occurring on the day of
measurement or the day before. We tested for an effect
of inflorescence orientation on nectar, pollination
levels (estimated as number of pollen tubes), pollen
removal and seed output through the treatment factor,
particularly through the CO + FD vs FU contrast.
In addition, we sought evidence for the flowerflooding and pollen-protection hypotheses by looking
at the significance of the treatment × rain interaction,
as the adaptive consequence of flower orientation
should be more apparent for flowers exposed to rain.
To increase statistical power, we dropped the three–
Results
Inflorescence treatment strongly and significantly
affected the orientation of individual flowers (CO + FD
vs FU contrast, Table 1). A large proportion (54·8%) of
flowers within inflorescences forced to face up exhibited positive angles, whereas in control and tethereddown inflorescences only 9·5 and 1·3% of all flowers,
respectively, exhibited angles >0° (Fig. 2). Inflorescence tethering per se did not significantly affect flower
orientation (CO vs FD contrast, Table 1). Because flower
angles varied significantly among plants, the extent to
which inflorescence treatment modified natural flower
angles was also plant-dependent (treatment–plant
interaction, Table 1). Inflorescence treatment did not
affect flower longevity. In contrast, exposure to rain
–
significantly reduced flower life span (X ± 1 SE = 7·5
± 0·2 vs 6·6 ± 0·2 days; Table 1).
We recorded 15 hummingbird visits to experimental
inflorescences. We did not conduct a formal statistical analysis, but there was no evidence of differential
visitation in relation to inflorescence position: five
visits were to control inflorescences, four visits were to
Table 1. Results of mixed-model s testing the effects of inflorescence treatment (fixed) and plant (random) on individual
flower angle, and of these two factors plus exposure to rain (fixed) on flower longevity
Flower angle (degrees)
Treatment
CO + FD vs FU
CO vs FD
Rain
Plant
Treatment × rain
Treatment × plant
Rain × plant
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
Flower longevity (days)
df†
F
df†
F
2,18
1,18
1,18
–
9,120
–
18,120
–
67·95***
134·28***
1·42
–
15·18***
–
3·41***
–
2,19
1,19
1,19
1,9·4
9,12·8
2,237
18,237
9,237
1·20
2·06
0·44
15·08**
0·60
0·02
0·55
0·62
Treatment effects were decomposed in two orthogonal contrasts: (1) control (CO) and inflorescences tethered to face down (FD)
vs inflorescences tethered to face up (FU), and (2) CO vs FD. F-tests considered type III sums of squares. Approximated
denominator sums of squares and degrees of freedom were estimated using the RANDOM statement in   (SAS 1988).
†Numerator degrees of freedom, denominator degrees of freedom.
**P < 0·01, ***P < 0·001.
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M. Tadey &
M. A. Aizen
Table 2. Results of mixed-model s testing the effects of inflorescence treatment (fixed), exposure to rain (fixed) and plant
(random) on nectar standing crop (volume, sugar concentration and total sugar content)
Treatment
CO + FD vs FU
CO vs FD
Rain
Plant
Treatment × rain
Treatment × plant
Rain × plant
Nectar volume (µl)
Sugar concentration (%)
Nectar sugar (mg)
df†
F
df†
F
df†
F
2,26·9
1,26·9
1,26·9
1,14·5
9,12·9
2,143
18,143
9,143
0·83
0·05
1·46
7·27*
2·25(*)
3·17*
1·77*
1·36
2,37·2
1,37·2
1,37·2
1,11·6
9,8·4
2,143
18,143
9,143
0·47
0·98
0·06
19·53***
1·02
0·93
0·88
2·74**
2,28·9
1,28·9
1,28·9
1,15·7
9,10·8
2,143
18,143
9,143
0·76
0·24
0·27
23·97***
3·10*
2·92(*)
1·47
1·13
Treatment effects were decomposed in two orthogonal contrasts: (1) control (CO) and inflorescences tethered to face down (FD)
vs inflorescences tethered to face up (FU), and (2) CO vs FD. F-values considered type III sums of squares. Approximated
denominator sums of squares and degrees of freedom were estimated using the RANDOM statement in   (SAS 1988).
†Numerator degrees of freedom, denominator degrees of freedom.
(*)0·05 < P < 0·10, *P < 0·05, **P < 0·01, ***P < 0·001.
Fig. 2. Frequency distributions of individual flower angles
relative to the horizontal for control inflorescences (CO, dashed
curve); tethered-down inflorescences (FD, dotted curve); and
tethered-up inflorescences (FU, solid curve). For statistical
details, see Table 1.
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
inflorescences tethered upwards, and six to inflorescences tethered downwards. Comparisons of flower
angles between visited and paired, randomly chosen
flowers did not support the hypothesis of differential
visitation to more negatively oriented flowers. Indeed,
we found a trend in the opposite direction, as the mean
angle (±1 SE) of visited flowers was –52° ± 3° whereas
that of randomly chosen flowers was –56° ± 3° (T = 2469,
Z = 2·15, n = 114, P = 0·03).
Inflorescence treatment did not significantly affect
volume, concentration or total sugar content of the
nectar standing crop (Table 2, Fig. 3). Similarly, we
found no significant association across inflorescence
treatments and plants between individual flower
angle and nectar standing volume (r = 0·003, n = 373,
P = 0·95); sugar concentration (r = – 0·051, n = 372,
P = 0·32); or total sugar content (r = – 0·019, n = 373,
P = 0·71). Flowers exposed to rain contained smaller
volumes of more diluted nectar than flowers not
exposed to rain. Thus the standing nectar of wet flowers contained significantly less sugar than that of dry
flowers (Table 2, Fig. 3). Differences in volume and
Fig. 3. Mean (± 1 SE) standing crop of nectar (volume, sugar
concentration and total sugar content) in relation to inflorescence treatment and rain exposure. For statistical details, see
Table 2.
sugar content of the standing nectar between wet and
dry flowers tended to be larger for tethered-up inflorescences than for either control or tethered-down inflorescences (treatment–rain interaction, Table 2, Fig. 3).
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Table 3. Results of mixed-model s testing the effects of inflorescence treatment (fixed), exposure to rain (fixed) and plant
(random) on number of pollen grains remaining per anther, number of pollen tubes per style and fruit set
No. grains /anther
Treatment
CO + FD vs FU
CO vs FD
Rain
Plant
Treatment × rain
Treatment × plant
Rain × plant
No. tubes /style
No. fruits/no. flowers
df†
F
df†
F
df†
F
2,42·9
1,42·9
1,42·9
1,23
9,14
2,36
18,136
9,136
1·83
0·11
2·79
1·76
2·80*
0·97
0·67
0·54
2,18·4
1,18·4
1,18·4
1,9·2
9,7·2
2,238
18,238
9,238
3·81*
6·21*
1·57
35·06***
6·56**
0·77
1·09
1·20
2,16·2
1,16·2
1,16·2
1,8·1
8,9·1
2,218
16,218
8,218
0·47
0·53
0·43
1·20
0·17
0·67
1·47
1·36
The treatment factor was decomposed in two orthogonal contrasts: (1) control (CO) and inflorescences tethered to face down
(FD) vs inflorescences tethered to face up (FU), and (2) CO vs FD. F-tests considered Type III sums of squares. Approximated
denominator sums of squares and degrees of freedom were estimated using the RANDOM statement in   (SAS 1988).
†Numerator degrees of freedom, denominator degrees of freedom.
*P < 0·05, **P < 0·01, ***P < 0·001.
Neither inflorescence treatment nor exposure to rain
affected the number of pollen grains remaining in a
flower’s anthers. In contrast, both inflorescence treatment and exposure to rain significantly influenced pollen receipt (Table 3, Fig. 4). Inflorescence orientation
significantly affected the mean number of pollen tubes
counted in a flower’s style (CO + FD vs FU contrast,
Table 3). Flowers in tethered-up inflorescences had 25
and 15% fewer pollen tubes compared to control and
tethered-down inflorescences, respectively (Fig. 4).
Similarly, pollen receipt varied negatively with individual flower angle (r = – 0·113, n = 659, P < 0·005). Tethering per se did not affect pollen receipt (CO vs FD
contrast, Table 3). More important than inflorescence
orientation was the effect of rain exposure on a flower’s
pollination. Flowers exposed to rain during more than
half their life span had 40% fewer pollen tubes in their
styles than less-exposed flowers (Table 3, Fig. 4). However, the effects of inflorescence treatment or rain exposure on pollination levels were not large enough to
influence final female reproductive success, as neither
factor significantly affected fruit set (Table 3, Fig. 4).
Discussion
Fig. 4. Mean (± 1 SE) number of pollen grains remaining per
anther, number of pollen tubes per style, and fruit set (number
flowers/number fruits) in relation to inflorescence treatment
and rain exposure. For statistical details, see Table 3.
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
Inflorescence treatment did not significantly affect
either the volume or concentration of the nectar produced after 24 h (F2,8 = 0·33, P = 0·72 for nectar volume; F2,8 = 0·42, P = 0·54 for sugar concentration).
Neither overall inflorescence orientation nor individual flower angles affected properties of the standing
nectar in Tristerix corymbosus. On the other hand,
nectar found in flowers on rainy days was, on average,
more diluted than on non-rainy days. Lower nectar
concentration did not occur because of rain pouring
into either down-facing or up-facing flowers. Contrary
to expectations, flowers open during rain had significantly smaller nectar volumes than flowers that were
not exposed to rain during or just before nectar sampling. In particular, the smallest nectar volumes were
sampled during or immediately after rain in flowers
that were in tethered-up inflorescences. In many plant
species, nectar secretion is an active metabolic process
influenced by temperature (Aizen & Basilio 1998;
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M. A. Aizen
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
Pleasants & Chaplin 1983; Rathcke 1992). The differential decrease in the volume of standing nectar
observed in flowers exposed to rain may have resulted
from a slower metabolism, perhaps in association with
cooler daytime temperatures that characterize cloudy
days. In addition, flowers in tethered-up inflorescences
spread in a wider range of angles than in either control
or down-facing inflorescences (Fig. 2). This greater
spread, combined with an up-facing orientation, may
relate to less flower insulation and thus to more
exposure of flowers to temperature fluctuations. This
may also explain why we sampled the highest nectar
volumes in flowers that were open during mostly dry,
sunny weather in tethered-up inflorescences (Fig. 3).
Taken together, these results refute the flowerflooding hypothesis. This conclusion was confirmed by
a straightforward experiment. Tristerix corymbosus
flowers are not flooded even when submerged under
running tap water (M.T. and C. Brewer, unpublished
results). The narrow, tubular shape, combined with the
hydrophobic and hydrophilic properties of the inner
and outer petals’ surfaces, may be associated with this
strong water repellence (C. Brewer, personal communication). The importance of surface properties for
water retention has been studied in leaves (Brewer &
Smith 1995; Brewer & Smith 1997), but we do not
know of comparable studies conducted with flowers.
Hummingbirds are highly discriminating pollinators that can distinguish between plants, between
inflorescences within plants and even between flowers
within inflorescences, based on nectar content (Campbell et al. 1997; Meléndez-Ackerman 1997; Sutherland
& Gass 1995). As nectar quality was not affected by
either inflorescence orientation or individual flower
angle in our study, it is not surprising that we found no
evidence of differential visitation of down-facing inflorescences or individual flowers. Indeed, we recorded a
slight tendency for hummingbirds to visit less pendant
flowers than those we chose at random. This might
relate to a stronger visual attraction and greater accessibility of less pendant flowers, which usually occur in
the outer part of T. corymbosus inflorescences.
Even though hummingbirds did not discriminate
against up-facing inflorescences, we recorded fewer
pollen tubes in flowers collected from those inflorescences than from flowers in pendant or tethered-down
inflorescences. As T. corymbosus plants present several
open flowers at the same time, and stigma receptivity
overlaps anther dehiscence within flowers (M.A.A.,
personal observation), many of the grains deposited
on the stigmas could come from the same plant or even
the same flower. For instance, Bertin (1982) found that
around 50% of the pollen grains deposited on the
stigma of hummingbird-pollinated Campsis radicans
came from the same plant due to a combination of
geitonogamous (between flowers of the same plant)
and autogamous (within-flower) pollination (see also
de Jong et al. 1992). A bagging experiment conducted
in T. corymbosus showed that about 20 – 30% of the
pollen tubes growing in the style came from pollen of
the same flower (M.A.A., unpublished results). Chances
for autogamous pollination may vary with flower angle,
and may explain why we found less pollination in upfacing inflorescences. In erect flowers of T. corymbosus,
reduced chances for self-pollen deposition may be
related to the fact that, unlike pendant flowers, the
style protrudes upwards beyond the anthers. Hence
the pollen that falls down due to gravity cannot reach
the stigma (Motten & Antonovics 1992).
An alternative but non-exclusive explanation for
reduced pollination of up-facing inflorescences involves
the way hummingbirds visit flowers (Campbell, Waser
& Price 1994; Hurlbert et al. 1996; Murcia & Feinsinger
1996). When a hummingbird visits a pendant flower,
it may make a more consistent contact between its
head and either anthers or style, due to its vertical
movement when hovering while feeding on nectar. We
observed that hummingbirds approached pendant
flowers of T. corymbosus from below and contacted
sexual parts frequently while pushing through a
flower’s entrance in search of nectar. On the few occasions we could observe hummingbirds visiting erect
flowers, they approached flowers from above or the
side, sometimes missing or reducing contact with
either anthers or stigma. In an experimental study
of Impatiens capensis flowers, Hurlbert et al. (1996)
found that floral mobility, a trait often associated with
facing down, increased pollen deposition on a hummingbird’s bill and crown by increasing handling
times. Although we did not measure the cost of flower
manipulation imposed on the hummingbird S. sephaniodes, a down-facing orientation of T. corymbosus
flowers might increase handling times, resulting in
greater stigmatic pollen deposition.
In addition to an effect of inflorescence orientation, we found that rain strongly affected pollination.
Flowers open during rainy weather for most of their
life had fewer pollen tubes growing in the style than
flowers that were open during mostly dry conditions.
The reduced longevity or lower nectar volumes and
concentrations found in T. corymbosus flowers exposed
to rain might have diminished the number of visits that
a flower received over its lifespan. Rain might also
enhance pollen detachment from the stigma, or from
a hummingbird’s bill, or may simply make pollen
deposition more difficult (Bertin & Sholes 1993;
Corbet 1990). In any event, flower orientation does
not modify the reduction in pollen loads by rainfall,
as this factor affected flowers similarly in the three
types of inflorescences.
Although flower angle affected pollen deposition,
perhaps by changing the efficiency of pollen transfer, it
did not affect the number of pollen grains remaining in
flowers of T. corymbosus. A caveat should be introduced here. Given that flower age strongly determines
the amount of pollen remaining in the anthers (Ashman & Schoen 1994; Galen 1992; Murcia 1990), the
capacity to detect a significant effect of inflorescence
FEC_580.fm Page 789 Monday, November 12, 2001 6:26 PM
789
Adaptive
significance of
flower orientation
treatment on this variable might be impaired by not
controlling for flower age either experimentally or statistically. Although we did not precisely control the age
of the flowers collected for counting pollen (2–4 days
old), Fig. 4 reveals no trend for up-facing inflorescences to lose more pollen than the other two types of
control inflorescences. We did not find any trend for
flowers exposed to rain to retain fewer pollen grains in
the anthers than flowers that were open during mostly
dry conditions. Independent of rain, flower orientation could affect male function if this trait influences
the positioning of pollen removed by a hummingbird
and thus its ability to ‘father’ seeds on the same or
other plants (cf. Murcia & Feinsinger 1996; Waser
1983). This possibility remains to be tested.
Taking these results together, we found no support
for the hypothesis that a down-facing flower protects
sexual parts better against direct pollen losses from
the anthers or stigma under rainy conditions. The
pendant flower of T. corymbosus is not an efficient
umbrella.
We also found no convincing evidence that inflorescence orientation affects fruit production. The fact
that more pollination in pendant inflorescences did
not translate into greater fruit set suggests that this
effect was not strong enough to limit pollination. In a
parallel study we found that natural pollination levels
would have to be reduced by >50% to affect fruit set
(M.A.A., unpublished results). However, we cannot
rule out the possibility that changes in pollen loads of
10 – 30%, such as we found, might affect seed quality
rather than seed quantity (Lee 1984; Lee 1988; Marshall & Folsom, 1991; Mulcahy 1979), although these
effects have been rarely recorded in nature.
Even though many hummingbird-pollinated species
produce pendant flowers and are very abundant in wet
and cloudy habitats throughout the Neotropics (Bawa
1990; Sazima, Buzato & Sazima 1996; Stiles 1981), we
cannot conclude from our experimental study that facing down represents an adaptation to vertebrate pollinators, such as hummingbirds, that remain active
under rainy conditions. More experimental and comparative research in a variety of systems is needed to
determine whether this widespread floral trait plays
any role in protecting flowers from rain.
Acknowledgements
© 2001 British
Ecological Society,
Functional Ecology,
15, 782 – 790
We are particularly grateful to Lawrence D. Harder for
his careful editing and useful suggestions, and Carole
Brewer and Thomas Thompson for fruitful discussions and for helping us to set the experiment. We also
thank Diego P. Vázquez, Javier Puntieri, Nickolas M.
Waser, and an anonymous reviewer for their valuable
comments on an earlier version of this manuscript.
This research was supported by the National Geographic Society (Grant no. 6192-98) and by CONICET, the National Research Council of Argentina
(PEI no. 0018/97).
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Received 22 January 2001; revised 6 July 2001; accepted
10 July 2001