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Pollination and the Function of Floral Parts in Chamaecrista fasciculata (Fabaceae)

Authors:

Abstract

Chamaecrista fasciculata has a rigid upper petal that curves over nine of the ten anthers. The single anther is deflexed. as is the style. An experimental pollination study was designed to test the hypotheses that: 1) the rigid petal acts as flight guide to ensure pollination; and 2) the nine grouped anthers serve as fodder anthers, whereas the single deflexed anther functions in pollination. The rigid petal was removed from 97 flowers. Only 5% of the manipulated flowers set fruit in comparison to a fruit set of 47% for the control group. The results of the study support the flight guide hypothesis. Pollen from both sets of anthers is viable and germinates on receptive stigmas. A pollen flow experiment using powdered metals, backscatter scanning electron microscopy, and X-ray microanalysis revealed that there is no difference in the frequency of pollen distribution from either set of anthers. Therefore, this study refutes the fodder pollinating anther hypothesis.
American Journal of Botany
79(3): 314-317. 1992.
POLLINATION
AND THE
FUNCTION OF FLORAL PARTS IN
CHAMAECRISTA
FASCICULATA
(FABACEAE)1
ANDREA
D.
WOLFE2,4
AND
JAMES
R. ESTES3
2Department of Botany and Microbiology and 3Bebb
Herbarium,
University
of
Oklahoma, Norman,
Oklahoma 73109
Chamaecristafasciculata
has
a rigid upper petal that curves
over nine of
the ten anthers. The
single
anther
is
deflexed,
as is
the style.
An
experimental pollination study
was
designed
to
test the
hypotheses
that:
1)
the
rigid petal
acts as
flight
guide
to
ensure
pollination; and 2) the
nine
grouped
anthers
serve as fodder
anthers,
whereas the
single deflexed
anther
functions
in
pollination.
The
rigid petal
was removed from 97 flowers.
Only
5%
of
the
manipulated
flowers set fruit
in
comparison to
a fruit set of 47% for
the control
group.
The
results
of
the
study support the flight guide hypothesis.
Pollen
from both sets of
anthers is viable and germinates on receptive stigmas. A
pollen flow experiment using powdered metals,
backscatter scanning electron
microscopy, and X-ray microanalysis revealed
that there
is no
difference
in
the frequency
of
pollen distribution from either set
of
anthers.
Therefore,
this
study refutes
the
fodder/pollinating anther hypothesis.
Chamaecrista
(= Cassia)
fasciculata
(Michx.)
Greene
(Fabaceae,
tribe
Cassieae,
subtribe
Cassiinae)
is an
obli-
gately
outcrossing,
bee-pollinated
species
(Thorp
and
Estes,
1975).
Petals
of this
species
are
bright
yellow
with a
pur-
plish-red
spot at
the base,
and
its nectarless
flowers
are
zygomorphic.
One of
the five
petals,
the
cucullus, is
rigid
and
hooded
(Fig.
1).
It encloses
nine
of the
ten purple-
red
anthers,
which
open
by apical
pores.
Hardin
et al.
(1972)
reported
differences
in lengths
among
the anthers.
Thorp
and
Estes
(1975)
also
noted that
there
were
two
distinct
groups
of anthers:
a
single
deflexed anther arising
at
the base of
the
ovary,
opposite
the cucullus,
and
par-
alleling
the style;
and
a
group
of
nine
anthers
arising
be-
tween
the
ovary
and the cucullus
(Fig.
1).
Chamaecristafasciculata
flowers
are visited by pollen-
collecting
bees of three
families and
three size-classes
(Thorp
and
Estes,
1975).
Pollen
is collected
by
these
bees
in
two
ways: 1)
The anther
is milked as the
bee
grasps
the base
of the anther and pulls
toward
the
apical
pore
in
a series
of stroking
motions;
and/or 2)
The
anther
is
vibrated
as the bee
shivers the
indirect flight
muscles
of
the
thorax
with its
wings
in
repose.
Thorp
and Estes
(1975)
postulated
that
the cucullus,
the
positions
of the
two sets of anthers,
and
style position
combine
to ensure
pollination.
In this
hypothesis,
access
to the
greatest
concentration
of
pollen
is restricted
by
the
cucullus. Therefore,
to
forage
the
pollen
from
the
nine
anthers
enclosed
by
the
cucullus, large
foraging
bees
must
move across
the deflexed
anther and
upturned
style
with
their
thorax.
They,
thereby,
contact
both
the
stigma
and
apical
pore
of
the
deflexed
anther
effecting
pollination.
Previous
workers (Hardin
et
al.,
1972;
Thorp
and
Estes,
1975) hypothesized
that
the
dimorphic
stamens
may
serve
'
Received for
publication
24
June
1991; revision
accepted
22 No-
vember 1991.
The authors thank
William F.
Chissoe
III
and Scott
D.
Russell,
Samuel
Noble Electron Microscopy
Laboratory,
for
assistance
using
the scanning
electron
and fluorescence microscopes;
R. Thompson
for technical as-
sistance;
Wayne J.
Elisens
for
helpful
comments
on
an
earlier
draft
of
the
manuscript;
and Matt
Lavin and an
anonymous
reviewer
for critical
review
of this manuscript.
4
Author for correspondence.
as feeding
and
pollinating anthers as
has been found
in
related
genera of subtribe
Cassiinae,
and other
enantiosty-
lous
(with
styles right or
left of the floral
axis and
opposing
the stamens)
and/or
heterandrous species
(van
der
Pijl,
1954;
Percival, 1965;
Bowers,
1975; Vogel,
1978;
McCollum,
Estes, and
Sullivan, 1984).
Hardin et al.
(1972)
proposed
that the longest
two anthers of
the
grouped sta-
mens
function as
pollinating anthers
and
the
remaining
eight
as
fodder
anthers.
In
contrast,
Thorp
and Estes
(1975)
proposed
that the
deflexed anther
functions as a
polli-
nating anther
and the
grouped ones are
fodder
anthers.
In
order to
test the
Thorp
and Estes
(1975)
hypotheses,
we
conducted
experiments to address the
role of
the cu-
cullus
in
pollination of
C. fasciculata and
purported
di-
vision of
labor between
the deflexed
(putative
pollinating
anther)
and the
grouped anthers
(hypothesized fodder
anthers).
MATERIALS
AND
METHODS
Role of
the cucullus-We
removed
the
cucullus from
97
flowers
1 hr
before
sunrise
(ca.
30 min before the first
floral
visitors),
and the
manipulated
flowers were
labeled.
Behavior of
the floral
visitors
was
observed for
3 hr
to
determine
whether the labels and
removal
of
the cucullus
affected
visitation.
A
control
group
of
61
flowers
was not
manipulated,
but was
also
labeled. At
the end
of
the flow-
ering
season
fruits
were
recovered
or
their absence
noted.
To
determine whether
this
manipulation itself
affects
fruit
set,
an
experiment
was
also conducted
in
the
green-
house. The cucullus from
25 flowers were removed
and
then
each flower was hand
pollinated.
Fruits were col-
lected
or
their absence
noted at the end of
4
wk.
Role of
the deflexed vs.
the grouped
anthers-Experi-
mental
manipulation
of
Chamaecrista
fasciculata
pollen
flow, pollen
germination,
and
pollen
tube
growth
have
concentrated on
interfloral studies
(Fenster
and
Sork,
1988;
Fenster,
1
99
1
a, b). Equivalent
intrafloral studies of C.
fasciculata
would
allow
testing
the
hypothesis
ofa division
of labor between
the functions of the
single
deflexed
and
nine
grouped
anthers.
314
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March
1992]
WOLFE AND ESTES-CHAMAECRISTA FASCICULATA
315
Pollen viability-To
determine
whether
pollen from
both sets of
anthers is viable,
we noted
whether pollen
germinated
on the stigma and
whether pollen
tube growth
occurred
in
the style by
conducting a semivivo
pollen
germination
experiment (Mulcahy
and Mulcahy,
1985).
Hand pollinations
of 18 bagged
flowers were
performed
in the field.
The flowers were
divided into two treatments:
1) cross-pollination
where
the stigma of a
flower was in-
serted into
the apical pore
of the single deflexed
anther;
and 2) cross-pollination
using
one of the grouped
anthers.
The blossoms
were rebagged,
labeled, and left
on the plant
for 6
hr, at
which time the
pistils were removed
from the
flowers. The styles
were
cut
three-fourths
of the distance
from the stigma
to the ovary
with a razor
in order to
facilitate determination of
pollen tube growth
without
tissue clearing
of the style.
The cut ends of
the styles were
immediately
placed in a
modified Brewbaker's
medium
(10% sucrose,
100 ppm
H3B03, 300 ppm Ca[NO3]2)
(Brewbaker
and Kwack, 1963;
Mulcahy
and Mulcahy,
1985) and
incubated at 25
C for 24 hr.
The styles were
divided into three
groups
and stained
with either 0.01%
Calcofluor,
0.0025% aniline
blue,
or
DAPI
(4',6-diami-
dino-2-phenylindole).
Calcofluor and
aniline blue
stains
were used to
visualize pollen
tubes, whereas
DAPI was
used to visualize the
generative
and
sperm
cell nuclei.
Observation of
pollen grains
and
pollen
tubes
by
fluo-
rescence
microscopy
occurred using
a
Leitz
Dialux 20
Photomicroscope.
Intrafloralpollen
tracking-We
conducted
an intraflo-
ral pollen
tracking experiment
using
backscatter
scanning
electron
microscopy (SEM)
and
X-ray
microanalysis
(Wolfe,
Estes,
and Chissoe, 1991).
To
differentiate
be-
tween
pollen
originating
from
the deflexed vs. the
grouped
anthers, pollen
from
each
set of anthers
was
differentially
labeled using fine metal-powders.
Pollen
from
the deflexed
anther
was labeled with
tin
(Sn),
and
pollen
from one of
the
grouped
stamens was
labeled
with
zinc
(Zn). Labeling
of each metal occurred
early
in
the
morning
on
separate
days.
All
of the
flowers of a
single plant
were
marked
each
day.
Flowers
from
surrounding
unlabeled
plants
were col-
lected
during
the
late
afternoon.
The
study
area
for each
labeling experiment
was of
equal
size.
A
total of
50
flowers
was collected.
The
styles
of the collected
blossoms were
air dried for
24
hr.
Stigmas
were dissected
and mounted with
double-
sided
tape
onto
copper
sample
boats.
The
samples
were
gold-palladium
coated
in
a Hummer
VI
sputter-coater
for
3
min.
The
prepared specimens
were
examined with
a
JSM
880
scanning
electron
microscope
with a
LaB6
emit-
ter at
an
accelerating voltage
of
20
kV. Backscatter
imaging
and
X-ray
microanalysis
was used
for identification of
any
metal
particles
found on the
stigmas.
RESULTS
Role
of
the cucullus -Observation
of
pollinator
activity
on the
manipulated
and control
flowers revealed no
ap-
parent
discrimination,
with bees
visiting
both blossom-
types
and labeled and unlabeled
flowers. Fruits
from all
97
of the
manipulated
flowers
and
45 of 61 of the control
group
were recovered or their absence
noted. The loss
of
some controls was a result of mowing
by highway
crews.
Fig. 1. Floral
morphology
of
Chamaecristafasciculata.
C, cucullus;
D, deflexed
anther; G, grouped
anthers; S, style.
Fruit
set for the control
group was 47.2%,
whereas that
of the
manipulated group
was 5.2%. In the
greenhouse
experiment
in
which
the cucullus
was
removed from flow-
ers
that
were
then
hand
pollinated, fruit set
was 84%.
Role
of the deflexed vs.
the grouped
anthers-Pollen
viability-Pollen grains, pollen
tubes, and
pollen nuclei
were
easily observed
using each staining
protocol. Each
stigma
had an overload of
pollen (compared
to number
of
ovules) as
a
result of
hand pollination.
There were no
apparent differences
in
pollen
germination and
pollen tube
growth
from
the two sources of
pollen. The
number of
germinated
pollen grains
and
number of
pollen tubes
reaching
the cut
end
of
the
style observed for
both pollen
sources
were
comparable
to
results obtained
for interfloral
studies
(Fenster
and
Sork,
1988).
Intrafloral pollen
tracking-Some
particles
of
each of
the
metal labels adhered to
individual
pollen
grains
and
others were
scattered on the
stigma (Figs. 2,
3). Stigmas
collected the
day
Zn
was
applied
to the
grouped
anthers
(putative
fodder
anthers)
had a
positive assay
for Zn
par-
ticles
in
SEM
X-ray
microanalysis
on 78%
of the
samples.
Stigmas collected when Sn
was applied to the
single de-
flexed
anther
(purported
pollinating anther)
also
had a
positive
assay
for Sn
particles
on
78%
of the
samples.
Additionally,
there
was
no
observed
carryover
of
Zn
and
Sn
metal
powders
from one
day
to
the
next:
Zn
and
Sn
were found
on
stigmas
only
on their
day
of
application
to
the
grouped
and
deflexed
anthers,
respectively.
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316
AMERICAN
JOURNAL
OF
BOTANY
[Vol.
79
Figs. 2, 3.
Backscatter
electron SEM
images of
Chamaecristafasciculata penicillate
stigmas.
Carbon
background
appears
uniformly
gray.
Brighter
spots on
images are a result of
elements
present
having
an atomic
number
higher
than carbon
(Z
=
12).
Bars
=
10
,um.
1.
Stigma labeled
with zinc
(Zn)
particles.
Stigma
was collected the
day
Zn
was
applied to one of the
grouped
anthers
(purported
fodder
anthers).
Each peak
(p)
corresponds
to a
positive
identification of
Zn
on
line scan
(ls).
2.
Stigma
labeled with tin
(Sn)
particles.
Stigma
was
collected the
day
Sn
was
applied
to the
single
deflexed
anther
(putative
pollinating
anther). Line
profile
analysis
identifies Sn where
indicated
by peak
(p)
on line
scan
(ls).
DISCUSSION
Fruit set of
flowers in the field that have the
cucullus
removed is only
5.2% compared to 47.2% fruit
set for an
unmanipulated
control group. Removal of the
cucullus
does not
directly
affect fruit
set,
as
demonstrated
by
the
greenhouse study.
Therefore, either removal of
the cu-
cullus
changed
the
floral morphology
in
a way that
reduced
pollinator attraction,
or the cucullus does
act as a
guide.
We observed
Bombus pennsylvanicus Deeger
and
Xylo-
copa virginica virginica
L.
forage partially
closed blossoms
and those
damaged
by herbivory,
as well
as the
manip-
ulated
flowers.
Therefore,
it
seems
unlikely
that the
change
in
floral morphology reduced
pollinator
attraction.
For intact flowers, the
approach flight
of
large
bees is
from the side opposite
the cucullus.
In
order to
forage the
nine grouped
anthers,
and
consequently the
greatest con-
centration
of pollen, bees move across the deflected anther
and
upturned style.
With the
cucullus
removed,
the
flight
approach
is from a forward
position,
and
the bee is not
forced
to
crawl across the
style.
The reduced
fruit set for
the
field-manipulated
flowers
suggests
that
pollination
was
reduced. Therefore, we infer that the cucullus
does func-
tion as a
flight guide
to force
incoming large
bees to contact
the
stigma.
For some heterandrous
species, fertility of pollen from
fodder anthers is reduced
compared
to
pollinating
anthers
(Vogel, 1978).
For
Chamaecristafasciculata, pollen from
both sets of anthers is
viable,
will
germinate
on
receptive
stigmas,
and
pollen
tube
growth
is similar
in
a semivivo
environment. Our results of intrafloral
pollen germination
are similar to those for
interfloral
studies of
C.
fasciculata
(Fenster
and
Sork,
1988).
Intrafloral behavior of the
pollinators,
the
two
largest
classes of
bees,
and
floral
morphology
seem
to
preclude
the Hardin
et al.
(1972) hypothesis
in
which the
longest
two anthers of the
grouped
stamens function as
pollinating
anthers and the
remaining eight
as fodder anthers. Both
the
long
and
short
grouped
anthers are
foraged
as a unit
with the pollen
primarily deposited
sternotribically (on
the
underside of
the
bee's
thorax).
However,
the
stigma
contacts
the bee's
side.
Thus
pollination
in
Chamaecrista
fasciculata
would seem
to be
more
similar to
that of
Senna
(=
Cassia)
corymobosa
Lam.
and S.
bacillaris L.
(subtribe
Cassiinae)
in
which
the
deflexed
anthers
function as
pol-
linating
anthers,
and
pollination
is
pleurotribic
(from
pol-
len
deposited on
the
bee's
side),
whereas
pollen
deposition
from
the
grouped
anthers
is
sternotribic
(Percival,
1965).
Our
intrafloral
pollen
tracking
experiment
was
designed
to
determine
the
origin
of
pollen
and
pollen
analogues
found on
stigmas.
Previous
studies have
shown
that
metal
dusts
can
function
as
pollen
analogues
similar to
mi-
cronized
fluorescent
powders,
and
transfer
of
pollen an-
alogues
from
other
floral parts
has not
been
reported
(Was-
er
and
Price,
1982;
Thomson
et
al.,
1986;
Campbell
and
Waser,
1989;
Wolfe,
Estes,
and
Chissoe,
1991).
There
was
no
difference
in
the
percent of
metal
label
on
the
collected
stigmas
when
Sn
was
applied
to the
deflexed
anther
vs.
Zn
to one
of the
grouped
anthers.
Consequently, pollen
from
either
set
of
anthers
was
equally
likely
to
land
on
a
receptive
stigma.
We,
therefore,
rejected
the
hypothesis
that an
exclusive
or
near-exclusive
fodder/pollinating
an-
ther
relationship
exists
in
Chamaecrista
fasciculata.
LITERATURE
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... Chamaecrista brachyblepharis and C. ciliolata, here excluded from series Rigidulae, possess flowers with concentric petals. The exact function and pollination efficiency of these various floral types is still to be fully explored, although Wolfe & Estes (1992) and Costa & al. (2012) have already commented on the different pollination systems related to differing floral morphologies in Chamaecrista. ...
Article
Chamaecrista is a monophyletic genus, but most of its infrageneric categories have been shown to be paraphyletic, including series Rigidulae which currently comprises 30 species, all endemic to Brazil and most occurring in the cerrado vegetation of the central highlands of the country. This molecular phylogenetic study tests the monophyly of C. ser. Rigidulae based on a broad sampling, considers its relationship with members of C. sect. Absus subsect. Absus, and estimates its divergence time in relation to the age of the genus. For that, individual and combined analyses were performed by the parsimony and Bayesian methods using chloroplast (trnL‐F) and nuclear (ITS1‐5.8S‐ITS2) markers and a matrix containing 75 taxa of Chamaecrista (29 belonging to series Rigidulae), 6 of Senna and 1 of Cassia. The analyses showed that series Rigidulae, as traditionally circumscribed, is polyphyletic. When C. brachyblepharis and C. ciliolata are excluded and C. botryoides and C. sincorana included, the series is resolved as monophyletic, comprising 30 species here designated as the Rigidulae clade. This clade is further subdivided into two geographically and genetically structured subclades, the first containing 23 species, mostly from the highlands of Goiás State, and the second with 6 species from the Espinhaço Range, running north to south through central Bahia and Minas Gerais States. Divergence time analyses suggest that the Rigidulae clade originated about 5 million years ago. The recent radiation of the series repeats that seen in other species‐rich genera in the Cerrado Biome, thus corroborating previous hypotheses about the recent age of the biome. The Rigidulae clade, as here circumscribed, has the following morphological synapomorphies: asymmetric flowers with their posterior petals similar to a typical papilionoid standard petal, and leaflets divaricate along the leaf rachis.
... Senna seems not to be the only genus presenting ricochet pollination or similar mechanisms. In Chamaecrista, a genus related to Senna (Bruneau et al. 2008), many types of asymmetrical petals also deflect or guide pollen jets to the bee body (Todd 1882;Gottsberger & Silberbauer-Gottsberger 1988;Wolfe & Estes 1992;Almeida et al. 2013Almeida et al. , 2015a, b). In buzz-pollinated species outside the legume group, floral organs such as bracteoles, sepals, petals, connectives and even the pistil can assume the role of guiding the pollen jets to the pollinator's body (seeMacior 1983;Kubitzki & Amaral 1991;Bittrich et al. 1993;Endress 1994;Hardy et al. 2000;Houston & Ladd 2002;Huang & Shi 2013). ...
Article
Naturalists Fritz and Hermann Müller hypothesised that heteranthery often leads to a division of labour into ‘feeding’ and ‘pollinating’ stamens; the latter often being as long as the pistil so as to promote successful pollination on the bees’ back. In many buzz-pollinated species of Senna, however, the so-called pollinating stamens are short and do not level with the stigma, raising the question of how pollen is shed on the bees’ back. Here we explore a mechanism called “ricochet pollination”. We test whether division of labour is achieved through the interaction between short lower stamens and strongly concave “deflector petals”. We studied the arrangement and morphology of the floral organs involved in the ricochet pollination, the functioning of the flowers through artificial sonication, and observed the interactions between bees and flowers in the field. The middle stamens are adapted to eject pollen downwardly, which can be readily collected on the bees’ ventral parts. Most part of the pollen is ejected towards the deflector petal(s), The short lower stamens that stay below large Xylocopa bees. Pollen from this set of stamens is more likely to contribute to pollination. The pollen grains seem to ricochet multiple times against the deflector petals to eventually reach the bee's back. The pollen ricochet mechanism promotes a division of labour by involving additional floral organs, such as petals, reinforcing Müllers’ division-of-labour hypothesis. However, alternative, non-multiexclusive hypotheses could be explored in genus Senna and other angiosperm species. This article is protected by copyright. All rights reserved.
... These pollinators may thus exert similar impacts on floral diversity in the context of pollen collection, particularly as regards the form and arrangement of anthers and associated structures. For instance, the strongly curved petals of many species of Senna enhance pollination by sonicating bees via deflection of expelled pollen onto pollinating spots (e.g.,Wolfe and Estes 1992;Westerkamp 2004). Similarly, bee-pollinated plant species often present their pollen more gradually and/or divide it among " feeding " and " pollinating " anther sets and thereby optimize pollen delivery (Castellanos et al. 2006;Vallejo-Marín et al. 2010). ...
Article
Full-text available
Bees foraging for floral rewards are one of our most thoroughly studied examples of generalist foraging ecology. Generalist bees rely considerably on instrumental (associative) learning to acquire routines that allow them to collect nectar efficiently from diverse plant species. Although such bees must also collect pollen from diverse species, few studies have examined if and how high efficiency is achieved. We characterized how generalist bumble bees (Bombus impatiens) foraged effectively for pollen from diverse floral resources, by manipulating the presence of pollen and anther cues, in a series of experiments using pollen-bearing live flowers, flowers of a sterile pollen-less horticultural hybrid, and artificial flowers. We show that generalist bumble bees exhibit flexible and effective pollen collection by switching between 2 routines: " scrabbling " when pollen is abundant and " sonicating " when pollen is scarce. Efficient switching between these behaviors is regulated by the interplay of 2 ubiquitous floral cues: chemical anther cues stimulating pollen collection behavior and mechanical pollen cues suppressing sonication (and eliciting scrabbling). Flexible pollen collection behavior is functional: When pollen on anthers was scarce, bees collected it at a greater rate by sonicating than scrabbling. This mechanism of behavioral flexibility likely allows generalist bees to handle diverse anther morphologies efficiently and may have facilitated the recurrent evolution of plant species that conceal pollen rewards via pored floral morphology. Whereas effective nectar foraging relies heavily on associative learning of unique routines for each flower type, a weighing of 2 types of cues regulates the flexible pollen collection mechanism we describe.
... This plant is a monomorphic, enantiostylous species (with both left-and right-styled flowers on the same individual) having the buzz-pollination syndrome. The selfcompatible (Arceo-Gó mez G., unpublished data), large yellow flowers (Fig. 1) provide no nectar reward, and the pollen is released through apical pores in the anthers (poricidal anther dehiscence) following vibration by large bees, as reported for other buzz-pollinated species (Thorp & Estes 1975; Wolfe & Estes 1992; Fenster 1995). C. chamaecristoides usually flowers between July and October (rainy season) and flowering lasts for ca. ...
Article
Chamaecrista chamaecristoides is an enantiostylous shrub endemic to the coast of Mexico that is being heavily impacted by human activities along the Gulf of Mexico. Chamaecrista chamaecristoides is considered a key species of the sand dune communities, particularly for its role as a nurse species and in dune stabilization. While certain aspects of its population biology have been well studied, others, such as its reproductive biology, have been overlooked. Understanding the factors that contribute to the successful reproduction of this species is crucial for its maintenance in natural populations. In this study, we examine the floral and reproductive biology of C. chamaecristoides. Specifically, we evaluate flower morphology and phenology, the breeding system and the identity and foraging patterns of floral visitors. Our results show that flower morphs are morphologically identical; however, position, size, and pollen production vary between stamens within each flower, suggesting functional differentiation. Furthermore, plants differ in their proportion of right-and left-styled flowers over the flowering season. While this could promote cross-fertilization by favoring pollen transfer among individuals with opposite floral-morph ratios, the presence of a stamen adjacent to the style, its selfcompatibility and the absence of inbreeding depression all compromise the role of enantiostyly as an outcrossing strategy in this species. Finally, even though flowers are visited by at least ten different floral visitors, reproduction of C. chamaecristoides depends heavily on only three species that together account for more than 70% of the total number of visits. Overall, our results suggest that C. chamaecristoides is highly dependent on a small subset of the community of floral visitors and that it has evolved floral strategies to reduce pollen loss and enhance the efficiency of pollen transfer. For these reasons, future conservation strategies must also consider the preservation of suitable habitat for the somewhat limited pollinator community of this species.
Article
The influence of enantiostyly (reciprocal segregation of anthers and stigmas to different sides of the flower) on outcrossing rate was examined in Chamaecrista fasciculata (Leguminosae). I hypothesized that enantiostyly has not evolved to increase the female component of outcrossing and actually acts to increase the selling rate through geitonogamy. To quantify the role of enantiostyly to outcrossing, plants of known isozyme genotype were manipulated to be either completely left- or right-styled (nonenantiostylous) or to have equal numbers of left- and right-styled flowers (enantiostylous). Flower number was varied to quantify any interaction between floral display size and enantiostyly on outcrossing rate. These “target” plants were surrounded by unmanipulated plants homozygous for the alternative allele. Outcrossing rates of the target plants were determined by scoring the presence or absence of heterozygotes. The contribution of enantiostyly to geitonogamy may be reduced if pollinators discriminate among the floral types. Thus, observations of pollinator movement between flowers on the same plant were made to determine if pollinators discriminate between the floral types. Although pollinators moved randomly between flower types, outcrossing rate was only marginally effected by the presence of enantiostyly. Enantiostylous plants outcrossed at a slightly lower rate than nonenantiostylous plants only when the opportunity for geitonogamy was great. These results suggest that the contribution of enantiostyly to selling is minimal.
Article
Although intraspecific variation in plant floral traits has been documented for a number of plant species, the causes of such variation are largely unknown. We first quantified floral trait variation in an Illinois prairie population of Chamaecrista fasciculata Michx. We then used a field experiment to determine the contribution of leaf herbivory to this variation and a greenhouse experiment to determine the contribution of leaf herbivory, and variable soil nutrient and water content to floral trait variation. Variation in environmental factors explained a significant portion of the naturally occurring variation in corolla width, ovule number, ovule size, and anther length. In the field, manual removal of 25% or more leaf area reduced ovule size and anther length (and by inference, pollen production), and delayed flowering. In the greenhouse, plants from which we removed 25% or more of their leaf area or which were given limited water produced fewer ovules than control plants. Addition of nutrients interacted with soil moisture to affect corolla diameter and ovule number. Despite our demonstration of significant environmental impacts on reproductive traits, these impacts were relatively much smaller than those on plant size, suggesting that floral traits are buffered against variable resource availability.
Article
Pollen viability is known to decline, sometimes rapidly, with age and exposure to environmental stresses. Because of the potential impact of nongenetic factors on the ability of pollen to fertilize ovules, researchers conducting hand-pollinations should attempt to control the freshness or viability of pollen used. We surveyed hand-pollination experiments published in seven major journals from 1980 until mid-1994, collecting data on the purpose of the experiment, the degree of care taken to ensure pollen viability, and the degree of care taken to ensure stigmatic receptivity. Fewer than one-third of the papers reported any consideration of pollen freshness or viability, whereas over one-half made some mention of stigmatic receptivity. Pollen freshness or age was mentioned more frequently for some types of experiments than for others. Experiments attempting to compare performance of different donors are especially susceptible to error when donor pollen is not treated equally or otherwise controlled for viability. We discuss strengths and weaknesses of tests to measure pollen viability, and experimental protocols to reduce differences in pollen condition across donors.
Chapter
Der Besprechung der einzelnen Taxa seien wenige allgemeine Bemerkungen vorabgeschickt.
Article
The mericarpids ("nutlets") of the four Boraginaceae subfam. Boraginoideae tribes, the Anchuseae. Cynoglosseae. Eritrichieae, and Lithospermeae are known to possess anatomical differences aside from the morphological ones. They differ in the shape of sclerified outer and inner epidennis (exocarp and endocarp) and the presence or absence of a mesocarpal stone cell layer. With its three-layered mesocarp, the Lithospermeae exhibit the most elaborate pericarp anatomy. The stability of the fruit wall is increased by additional deposition of calcium and/or silicon compounds. The exact location of deposition has not yet been identified.
Article
1.In plants, the temporal pattern of floral displays, or display schedules, delimits an individual's mating opportunities. Thus, variation in the shape of display schedules can affect the degree of population synchrony and the strength of phenological assortative mating by flowering onset date. A good understanding of the mechanisms regulating the timing of flowering onset has been developed, but we know less about factors influencing subsequent patterns of floral display.2.We observed unusual multimodal display schedules in temperate populations of the annual legume Chamaecrista fasciculata. Here we ask if ‘flowering pulses’ are simultaneous among individuals and populations and explore potential underlying mechanisms and consequences of pulsed flowering.3.We monitored daily flower production for individual plants from genetically divergent populations during a series of field experiments that manipulated three potential influencers of display schedule shape: average daily temperature, pollinator availability, and watering schedules. We measured floral longevity to isolate the contributions of flower retention and flower deployment to display schedules. We assessed relationships between flowering and environmental variables and compared estimates of population synchrony, individual synchrony, and the strength of assortative mating with those of 29 unimodally-flowering species from the area.4.We observed simultaneous flowering pulses in all experiments, with peaks aligned among individuals and populations despite variation in flowering onset and/or duration. Pulses were not the result of increases in average temperature, pollinator availability, or variation in watering schedules. Seasonal fluctuations in temperature correlated with floral longevity and flower deployment, suggesting that the shape of display schedules may be plastic in response to temperature. Average population and individual synchrony differed only slightly from those of the species with unimodal schedules, while the average strength of assortative mating for flowering onset date was strongly reduced (0.21 in C. fasciculata vs. 0.35 for the 29 other species).5.Synthesis. Researchers should take caution in assuming that components of display schedules are genetically or developmentally correlated with flowering onset. Variation in the shape of display schedules can influence patterns of gene-flow within or between populations, with potential effects on the strength of phenological assortative mating and subsequent responses to selection.This article is protected by copyright. All rights reserved.
Article
The pollination mechanisms and pollen vectors of Solarium rostratum have been examined by greenhouse experiments and field studies. Although the capacity for autogamy exists in this weedy annual, it rarely occurs because of two factors: (1) the morphology of the flower and (2) the foraging behavior of the various species of Bombus, the primary pollen vector in the regions studied. The percentages of geitonogamy and xenogamy are dependent on the flight pattern of the bees and the number of open flowers on a plant.
Article
Brewbaker, James L., and Beyoung H. Kwack. (U. Hawaii, Honolulu.) The essential role of calcium ion in pollen germination and pollen tube growth. Amer. Jour. Bot. 50(9): 859–865. Illus. 1963.—A pollen population effect occurs whenever pollen grains are grown in vitro. Small pollen populations germinate and grow poorly if at all, under conditions which support excellent growth of large pollen populations. The pollen population effect is overcome completely by a growth factor obtained in water extracts of many plant tissues. This factor is shown to be the calcium ion, and its action confirmed in 86 species representing 39 plant families. Other ions (K⁺, Mg⁺⁺, Na⁺) serve in supporting roles to the uptake or binding of calcium. The high requirement of calcium (300–5000 ppm, as Ca (NO3)2·4H2O, for optimum growth) and low calcium content of most pollen may conspire to give calcium a governing role in the growth of pollen tubes both in vitro and in situ. It is suspected that ramifications of this role extend to the self-incompatibilities of plants and to the curious types of arrested tube growth distinguishing, for example, the orchids. A culture medium which proved its merit in a wide variety of pollen growth studies included, in distilled water, 10% sucrose, 100 ppm H3BO3, 300 ppm Ca (NO3)2·4H2O, 200 ppm MgSO4·7H2O and 100 ppm KNO3.
Article
With the semivivo technique, pollen tubes grow from the bases of cut styles. The length of pollen tubes protruding from such styles, or growing through intact styles, is greatly enhanced by allowing initial pollen tube growth to occur before the ovary is removed. The presence of other floral parts has no apparent effect in this study. This improved semivivo method provides an abundant supply of pollen tubes that may show fewer artifacts than do those produced with in vitro growth. It also suggests that the ovary plays an important role in pollen-style interactions.
Article
Pollen dispersal is a major component of gene flow in plant populations. It can influence microevolution within and among populations as well as the evolution of floral characters that affect dispersal. Most previous studies have relied on point estimates to characterize dispersal distances, even though there is likely to be substantial intrapopulational and interpopulational variation. We measured variation in pollen dispersal for the hummingbird-pollinated herb Ipomopsis aggregata (Polemoniaceae), using powdered fluorescent dyes to estimate pollen movement. Analysis of 5-6 natural populations in each of three years indicated that mean and mean squared distances of pollen dispersal, measured over the reproductive lifespan of individual plants, varied more than threefold among populations and years. Dispersal distances also shifted over the season within a given population. Unlike the variation among populations, these seasonal changes were associated in part with changes in flower density. The mean distance of pollen dispersal from an individual plant was unrelated to the date of first flowering, but did reflect two floral characters. Plants with higher variance in stamen length across flowers delivered pollen farther on average, as predicted by computer simulations of pollen carryover. Plants with lower mean stamen lengths also delivered pollen farther. Such effects of plant characters on pollen dispersal are a critical prerequisite for dispersal to evolve in response to its effects on fitness.
Article
Pollination ecology and floral biology were studied in Passiflora incarnata, Cassia fasciculata, Campsis radicans, and four Solanum species, all growing as weeds in Marshall County, Oklahoma. Passiflora and Cassia are facultatively autogamous, the others obligately xenogamous. All are entomophilous and Campsis also ornithophilous. Solanum rostratum is reproductively isolated from S. dimidiatum (S. torreyi), carolinense, and elaeagnifolium. Crosses between the last three yielded normally developed fruit but questionably viable seeds. Intrinsic barriers among the four Solanums are discussed. A new chromosome count for S. dimidiatum, n=18, 2n=36, is reported.
Article
Both pollen and seed dispersal components of gene flow were examined in the annual plant Chamaecrista fasciculata (Leguminosae) and quantified in terms of Wright's neighborhood area. Pollen dispersal was estimated by measuring pollinator flight movement throughout the flowering season and the contribution of pollen carryover to pollen dispersal was determined by comparing pollinator flight movement with dispersal of electrophoretic markers in an experimental transect. Phenological effects on the probability of fruit set were measured to determine whether pollinations should be weighted differentially across the flowering season. The outcrossing rate, a major determinant of the role of pollen dispersal in gene flow, was estimated from electrophoretic analysis of progeny arrays and by measuring the proportion of nongeitonogamous pollinator flight movements. Seed dispersal was measured in a prairie habitat and in experimental plots without surrounding vegetation. Seed dispersal was small in comparison to pollen dispersal in both environments. Fruit set was low at the beginning and end of the flowering season, periods when flower density is low and pollinator flight distances are large. Although the outcrossing rate was high (t = 80%) and pollen carryover substantial, pollen dispersal was limited. Averaged over 4 years, neighborhood area, based on both seed and pollen dispersal, was 17.6 m2, and corresponds to a circle of radius 2.4 m. The observed limited gene dispersal suggests the population of C. fasciculata is genetically subdivided into small breeding units of related individuals.
Article
Pollen dispersal is a major component of gene flow in plant populations. It can influence microevolution within and among populations as well as the evolution of floral characters that affect dispersal. Most previous studies have relied on point estimates to characterize dispersal distances, even though there is likely to be substantial intrapopulational and interpopulational variation. We measured variation in pollen dispersal for the hummingbird-pollinated herb Ipomopsis aggregata (Polemoniaceae), using powdered fluorescent dyes to estimate pollen movement. Analysis of 5-6 natural populations in each of three years indicated that mean and mean squared distances of pollen dispersal, measured over the reproductive lifespan of individual plants, varied more than threefold among populations and years. Dispersal distances also shifted over the season within a given population. Unlike the variation among populations, these seasonal changes were associated in part with changes in flower density. The mean distance of pollen dispersal from an individual plant was unrelated to the date of first flowering, but did reflect two floral characters. Plants with higher variance in stamen length across flowers delivered pollen farther on average, as predicted by computer simulations of pollen carryover. Plants with lower mean stamen lengths also delivered pollen farther. Such effects of plant characters on pollen dispersal are a critical prerequisite for dispersal to evolve in response to its effects on fitness.