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Floral nectaries from Limodorum abortivum (L.) Sw and Epipactis atropurpurea Rafin (Orchidaceae): Ultrastructural changes in plastids during the secretory process

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Maria Pais at University of Lisbon
Maria Pais
A. C. S. Figueiredo
A. C. S. Figueiredo
A. C. S. Figueiredo
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Abstract

Ultrastructural features of the floral nectaries of 2 orchid species, Limodorum abortivum (L) Sw and Epipactis atropurpurea Rafin, were compared. In particular, ultrastructural changes were followed in the plastids throughout the process of nectar secretion. There is evidence that plastids play an important role in this process. Before secretion begins, plastids of epidermal and subepidermal cells of the nectaries of both species, accumulated large quantities of starch. However, during the phase of nectar secretion, the plastids were found to contain little if any, starch, suggesting that starch degradation had augmented nectar sugar production. Plastoglobuli were evident during all developmental stages. The tubular reticulum found in plastids of pre-secretory nectaries was still evident during secretion, but was then found to contain an unknown, dense osmiophilic material. The floral nectary of L abortivum is sucrose dominant, whereas that of E atropurpurea is hexose rich. Therefore, the differences in nectar-carbohydrate composition are apparently not explainable by the similar ultrastructural changes of their floral nectaries. Further investigation is required to determine the biochemical basis for the disparity in nectar sugar composition between these 2 species.
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Original
article
Floral
nectaries
from
Limodorum
abortivum
(L)
Sw
and
Epipactis
atropurpurea
Rafin
(Orchidaceae):
ultrastructural
changes
in
plastids
during
the
secretory
process
MS
Pais,
ACS
Figueiredo
Departamento
de
Biologia
Vegetal,
Faculdade
de
Ciências
de
Lisboa,
Bioco
C2,
Campo
Grande,
1700
Lisbon,
Portugal
(Received
9
May
1994;
accepted
2
September
1994)
Summary —
Ultrastructural
features
of
the
floral
nectaries
of
2
orchid
species,
Limodorum
abortivum
(L)
Sw
and
Epipactis
atropurpurea
Rafin,
were
compared.
In
particular,
ultrastructural
changes
were
followed
in
the
plastids
throughout
the
process
of
nectar
secretion.
There
is
evidence
that
plastids
play
an
important
role
in
this
process.
Before
secretion
begins,
plastids
of
epidermal
and
subepidermal
cells
of
the
nectaries
of
both
species,
accumulated
large
quantities
of
starch.
However,
during
the
phase
of
nectar
secretion,
the
plastids
were
found
to
contain
little
if
any,
starch,
suggesting
that
starch
degradation
had
augmented
nectar
sugar
production.
Plastoglobuli
were
evident
during
all
developmental
stages.
The
tubular
reticulum
found
in
plastids
of
pre-secretory
nectaries
was
still
evident
during
secre-
tion,
but
was
then
found
to
contain
an
unknown,
dense
osmiophilic
material.
The
floral
nectary
of
L
abortivum
is
sucrose
dominant,
whereas
that of
Eatropurpurea
is
hexose
rich.
Therefore,
the
differences
in
nectar-carbohydrate
composition
are
apparently
not
explainable
by
the
similar
ultrastructural
changes
of their
floral
nectaries.
Further
investigation
is
required
to
determine
the
biochemical
basis
for
the
disparity
in
nectar
sugar
composition
between
these
2
species.
Epipactis
atropurpurea
Rafin
/
Limodorum
abortivum
(L)
Sw
/
nectary
/
orchid
/
plastid
/
starch
Abbreviations
used
in
the
figures:
CW
=
cell
wall;
CWp
=
cell-wall
pit;
CWi
=
cell-wall
ingrowths;
D
=
dic-
tyosome;
ER
=
endoplasmic
reticulum;
M
=
mitochondrion;
N
=
nucleus;
NC
=
nectariferous
cells;
OD
=
osmiophilic
droplets;
P
=
plastid;
PR
=
plastid
reticulum;
S
=
secretion;
St
=
starch;
V
=
vacuole;
VB
=
vascular
bundles.
INTRODUCTION
The
Orchidaceae
is
one
of
the
largest
and
most
evolved
families
in
the
angiosperms.
A
large
number
of
genera
within
this
family
possess
both
floral
and
extra-floral
nectaries.
Extra-floral
nectaries
secrete
nectar
on
the
outside
of
buds
or
inflorescences
when
the
flowers
are
developing
(van
der
Pijl
and
Dod-
son,
1966).
Floral
nectaries
are
particularly
important
due
to
their
adaptative
significance
related
with
the
ability
to
attract
pollinators.
The
morphology
and
location
of
floral
nec-
taries
in
the
Orchidaceae
are
variable:
a)
shallow
and
cup-like,
at
the
base
of
the
label-
lum;
b)
in
long
spurs,
produced
either
from
the
fused
sepals
or
from
the
base
of
the
labellum;
c)
long,
tubular
and
embedded
in
the
base
of
the
flower
alongside
the
ovary;
and
d)
on
the
side-lobes
or
along
the
central
groove
of
the
labellum
(see
van
der
Pijl
and
Dodson, 1966).
Epipactis
atropurpurea
and
Limodorum
abortivum
are
2
examples
of
this
diversity.
The
labellum
of
E
atropurpurea
is
divided
into
2
parts:
the
outer
part
(epichile)
is
tongue-shaped,
whereas
the
proximal
part
(hypochile)
has
the
shape
of
a
concave
gut-
ter.
On
the
other
hand,
in
L
abortivum
each
flower
has
a
long,
thin
and
curved
nectary
spur
produced
from
the
internal
portion
of
the
labellum.
In
continuation
of
our
work
on
these
orchids
(Pais
and
Chaves
das
Neves,
1980;
Pais
et
al,
1986;
Pais,
1987;
Figueiredo
and
Pais,
1992)
we
present,
in
this
paper,
a
study
comparing
the
ultra-
structural
aspects
of
the
plastids
at
differ-
ent
stages
during
the
nectar
secretion
pro-
cess.
MATERIALS
AND
METHODS
Plant
material
Flowers
of
L
abortivum
and
E
atropurpurea,
of
different
developmental
stages,
were
collected
from
plants
growing
in
the
field,
at
Cotovia
(Ses-
imbra,
Portugal).
Transmission
electron
microscopy
Cross-sections
of
the
floral
nectaries
at
differ-
ent
stages
of
development
(pre-secretory,
secre-
tory
and
post-secretory
stages),
were
fixed
overnight
in
2%
glutaraldehyde
(GA)
in
0.1
M
sodium
cacodylate
buffer,
pH
7.2,
at
4°C.
The
material
was
rinsed
thoroughly
in
the
same
buffer
and
postfixed
with
2%
OSO4
(aqueous
solution)
for
2
h.
After
dehydration
in
a
graded
acetone
series,
the
material
was
embedded
in
Epon-Araldite
according
to
Mollenhauer
(1964).
Ultrathin
sections
were
stained
with
uranyl
acetate/lead
citrate
(Reynolds,
1963),
and
observed
with
a
Jeol
100C
electron
microscope
at
80
kV.
Light
microscopy
Semi-thin
sections
from
material
fixed
within
GA/OsO
4
as
above
were
stained
with
Methy-
lene
Blue-Azure
A-Safranin
(Warmke
and
Lee,
1976).
Scanning
electron
microscopy
The
material
was
fixed
with
GA/OsO
4
as
above,
rinsed
in
water
and
plunged
into
liquid
propane,
subcooled
in
liquid
nitrogen.
Specimens
were
freeze-dried
at
-65°C
for
4
d
in
a
Freeze-dryer
Polaron
E
5300.
Dried
specimens
were
sputter-
coated
with
gold
in
a
Polaron
E
5350
and
obser-
vations
were
made
at
15
kV
using
a
Jeol
JSM
T220
scanning
electron
microscope.
RESULTS
Epipactis
atropurpurea
The
internal
portion
of
the
floral
nectary
of
E
atropurpurea
has
the
shape
of
a
concave
gutter
(plate
I,
fig
1b,
c).
In
longitudinal
sec-
tion
a
nectariferous
layer
and
several
lay-
ers
of
parenchyma
cells
can
be
seen
(plate
I, fig
1a).
Both
the
nectariferous
and
parenchyma
cells
of
E
atropurpurea
contained,
in
the
pre-secretory
stage,
numerous
amyloplasts
filled
with
starch
grains
(plate
II,
fig
1-3).
Plastoglobuli
and
a
peripheral
reticulum
were
also
evident
in
these
plastids.
Dic-
tyosomes
and
endoplasmic
reticulum
occurred
in
the
cytoplasm
(plate
II,
fig
1).
At the
secretory
stage,
a
reticulum
with
osmiophilic
content
was
common
in
the
plas-
tids
(plate
III,
figs
1, 2).
At
this
stage,
little
or
no
starch
was
present
inside
such
plastids.
Dictyosomes
and
ER
profiles
were
abun-
dant
both
in
nectariferous
(plate
III,
fig
1)
and
parenchyma
cells.
Cell-wall
ingrowths
were
profuse,
but
were
present
only
along
the
external
walls
of
the
nectariferous
cells
(plate III,
fig
3).
Limodorum
abortivum
Like
in
E
atropurpurea,
when
observed
in
cross-section,
the
floral
nectary
from
L
abortivum
consists
of
an
internal
epidermal
cell
layer
of
nectariferous
cells
surround-
ing
the
secretory
cavity
and
various
layers
of
parenchyma
cells
(plate
I,
fig
2a,b).
At
the
pre-secretory
stage
the
nectarif-
erous
cells
of
L
abortivum
showed
numer-
ous
polymorphic
plastids
with
a
dense
stroma,
an
osmiophilic
reticulum,
some
plastoglobuli
and
starch
grains
(plate
IV,
figs
1, 2).
During
this
stage,
the
parenchyma
cells
contained
numerous
amyloplasts
with
several
starch
strains
filling
almost
the
entire
organelle
(plate
IV,
fig
3).
During
the
secretory
stage
starch
dis-
appeared
from
the
plastids
of
nectariferous
cells
and
the
plastid
reticulum
appeared
filled
with
osmiophilic
contents
(plate
IV,
fig
4;
plate
V,
fig
1).
In
the
cytoplasm
of
these
cells
numerous
osmiophilic
droplets
could
be
seen.
Cell-wall
pits
were
also
frequently
observed
between
adjacent
nectariferous
and
parenchyma
cells
(plate
V,
fig
2).
In
the
nectariferous
cells,
ER
was
profuse
(plate
V,
fig
3)
with
numerous
profiles
adja-
cent
to
plastids
(plate
V,
fig 1).
Dictyosomes
were
also
abundant
(plate
V,
fig
3).
Later
in
the
secretory
stage,
the
apparent
exuda-
tion
from
nectariferous
cells
of
some
osmio-
philic
droplets
was
observed
(plate
V,
fig
4).
The
exudation
of
the
nectar
components
into
the
secretory
cavity
occurred
by
dis-
ruption
of
the
cuticle
(plate
V,
fig
5).
DISCUSSION
Previous
studies
on
the
chemical
composi-
tion
of
the
floral
nectars
of
E
atropurpurea
and
L
abortivum
have
indicated
clear
dif-
ferences.
L
abortivum
has
been
classified
as
belonging
to
a
fairly
primitive
group
within
the
evolutionary
sequence
of
orchids,
since
its
nectar
contains
mainly
sucrose
and
a
low
total
amino-acid
concentration
com-
pared
to
the
open
E
atropurpurea
nectary
(Pais
and
Chaves
das
Neves,
1980;
Pais
et al,
1986).
From
the
ultrastructural
point
of
view
the
main
feature
of
the
nectariferous
and
parenchyma
cells
of
these
orchids
were
the
changes
observed
in
the
plastids
dur-
ing
the
secretion
process
(Pais,
1987;
Figueirido
and
Pais,
1992).
In
the
pre-
secretory
stage,
both
the
nectariferous
and
parenchyma
cells
of
E
atropurpurea
showed
numerous
starch
grains.
This
type
of
plastid
could
only
be
seen
in
the
parenchyma
cells
of
L
abortivum.
In
the
nectariferous
cells
of
L
abortivum,
plastids
with
an
osmiophilic
reticulum
and
small
starch
grains
were
found.
This
type
of
poly-
morphic
plastids
with
dense
stroma
and
osmiophilic
reticulum
are
uncommon
in
nectariferous
tissues
but
have
been
described
in
many
other
secretory
struc-
tures,
especially
in
those
secreting
monoterpenes
(Fahn,
1988).
The
accumulation
of
nectar
both
in
the
nectary
of
E
atropurpurea
and
in
that
of
L
abortivum
occurred
simultaneously
with
starch
degradation,
which
might
indicate
that
the
starch
accumulated
in
both
parenchyma
and
nectariferous
cells
is
the
source
of
some
of
the
secreted
sugars.
Also
common
to
both
glands
is
the
presence
of
a
plastid
reticulum.
Nevertheless,
these
sim-
ilar
ultrastructural
features
are
not
in
agree-
ment
with
the
nectar/sugar
ratios
of
both
glands:
the
floral
nectar
of
L
abortivum
is
sucrose
dominant
(Pais
et
al,
1986),
whereas
that
of
E
atropurpurea
is
hexose
rich
(Pais
and
Chaves
das
Neves,
1980).
Therefore,
further
investigation
is
required
to
determine
the
basis
for
the
differences
in
the
nectar/sugar
composition.
Probably
these
differences
will
reside
in
the
type
and
amount
of
enzymes
involved
in
sugar
trans-
formations
in
both
glands.
Numerous
osmiophilic
droplets
could
be
seen
in
the
plastids
and
cytoplasm
of
the
nectariferous
cells
in
both
nectaries.
Although
the
chemical
nature
of
this
com-
ponent
is
not
clear,
the
appearance
of
sim-
ilar
osmiophilic
droplets
has
been
inter-
preted
as
corresponding
to
the
presence
of
lipids
or
phenolic
material
in
different
types
of
floral
(Peterson
et al,
1979;
Mey-
berg
and
Kristen,
1981;
Kronestedt
et al,
1986,
Sawidis
et al,
1989)
and
extra-floral
nectaries
(Clair-Maczylajtys
and
Bory,
1983).
In
the
outer
walls
of
the
nectariferous
cells of
E atropurpurea cell-wall
ingrowths
could
be
found,
forming
a
thick
labyrinthine
layer.
These
wall
ingrowths
are
common
in
other
outer
walls
of
the
secretory
cells
of
nectaries
(Durkee
et al,
1981;
Fahn,
1988),
nevertheless
they
were
not
observed
in
the
L
abortivum
nectary.
Résumé —
Nectaires
floraux
de
Limo-
dorum
arbortivum
(L)
Sw
et
d’Epipactis
atropurpurea
Rafin
(Orchidaceae) :
modifications
de
l’ultrastructure
des
plastides
au
cours
du
processus
de
sécrétion.
Nous
avons
comparé
les
carac-
téristiques
ultrastructurales
de
la
sécrétion
nectarifère
de
2
espèces
d’orchidées.
Des
coupes
dans
les
nectaires
floraux
ont
été
réalisées
à
divers
stades
de
développe-
ment
(avant,
pendant
et
après
la
sécré-
tion),
fixées
avec
2%
de
glutaraldéhyde,
postfixées
durant
2
h
avec
une
solution
aqueuse
de
tétroxyde
d’osmium
(OsO
4)
à
2%
et
étudiées
en
microscopie
électro-
nique
à
transmission
et
à
balayage.
La
par-
tie
interne
du
nectaire
floral
d’E
atropur-
purea
a
la
forme
d’une
gouttière
concave
(planche
1,
fig
1 a,b,c).
Le
nectaire
floral
de
L
abortivum
montre
une
couche
cellulaire
épidermique
interne
de
cellules
nectari-
fères
parenchymateuses
(planche
I,
fig
2a,b).
Avant
la
sécrétion,
les
cellules
nec-
tarifères
et
parenchymateuses
d’E
atro-
purpurea
présentent
de
nombreux
amylo-
plastes
remplis
de
grains
d’amidon
(planche
II,
figs
1,
2,
3).
Pendant
la
sécré-
tion,
des
structures
tubulaires
avec
un
contenu
osmiophile
se
rencontrent
fré-
quemment
dans
les
plastides
(planche
III,
figs
1, 2).
À
ce
stade
on
trouve
peu
ou
pas
d’amidon
à
l’intérieur
de
ce
type
de
plas-
tides.
La
paroi
des
cellules
nectarifères
présentent
de
nombreuses
invaginations
(plante
III,
fig
3).
Avant
la
sécrétion,
les
cellules
nectarifères
de
L
abortivum
pré-
sentent
de
nombreux
plastides
poly-
morphes
avec
des
structures
tubulaires
osmiophiles,
des
stroma
denses,
quelques
plastoglobules
et
de
petits
grains
d’ami-
don
(planche
IV,
figs
1, 2).
Au
cours
de
ce
stade,
les
cellules
parenchymateuses
mon-
trent
de
nombreux
amyloplastes
avec
plu-
sieurs
grains
d’amidon
remplissant
presque
entièrement
l’organite
(planche
IV,
fig
3).
Durant
la
sécrétion
l’amidon
disparaît
des
plastides
des
cellules
nectarifères
et
les
structures
tubulaires
apparaissent
emplies
d’un
contenu
osmiophile
(planche
IV,
fig
4 ;
planche
V,
fig
1
). On
observe
fréquem-
ment
des
renfoncements
dans
la
paroi
cel-
lulaire
entre
les
cellules
nectarifères
et
parenchymateuses
voisines
(planche
V,
fig
2).
Dans
les
cellules
nectarifères,
le
reti-
culum
endoplasmique
est
abondant
(planche
V,
fig
3)
avec
de
nombreux
diver-
ticules
contigüs
aux
plastides
(planche
V,
fig
1).
Plus
tard
on
peut
observer
l’exuda-
tion
du
nectar
dans
la
cavité
sécrétrice,
formée
par
déchirement
de
la
couche
externe
de
la
cuticule
(planche
V,
fig
5).
L’ultrastructure
des
cellules
nectarifères
et
parenchymateuses
suggère
que
les
plas-
tides
jouent
un
rôle
important
dans
la
sécrétion
du
nectar.
Chez
L
abortivum
le
nectar
accumulé dans
l’espace
sous-cuti-
culaire
est
déversé
dans
la
cavité
nectari-
fère
par
la
rupture
de
la
couche
externe
de
la
cuticule.
Chez
E atropurpurea
l’exu-
dation
du
nectar
se
fait
à
travers
les
pores
dation
du
nectar
se
fait
à
travers
les
pores
de la cuticule.
Orchidaceae
/
Epipactis
atropurpurea
/
Limodorum
abortivum
/
nectaire
/
plas-
tide / amidon
Zusammenfassung —
Florale
Nektarien
bei
Limodorum
abortivum
(L)
Sw
und
Epipactis
atropurpurea
Rafin
(Orchida-
ceae):
Ultrastrukturelle
Veränderungen
in
den
Plastiden
während
des
Sekre-
tionsprozesses.
Wir
führten
eine
verglei-
chende
Untersuchung
über
die
Ultrastruk-
tur
und
die
chemische
Zusammensetzung
der Nektarsekretion
in
Blüten
bei
zwei
Orchideenarten,
Limodorum
abortivum
und
Epipactis
atropurpurea
durch.
Querschnitte
der
Nektarien
wurden
in
verschiedenen
Entwicklungsstadien
(vor,
während
und
nach
der
Sekretion)
für
die
Transmission-
und
Rasterelektronenmikroskopie
in
2%
Glutaraldehyde
fixiert
und
2
Stunden
mit
2%
wässeriger
Osmiumtetroxidlösung
nachfixiert.
Der
innere
Teil
der
Nektarien
von
E atropurpurea
hat
die
Form
einer
kon-
kaven
Rinne
(Tafel
I, Abb
1a,b,c).
Die
Nek-
tarien
von
L
abortivum
weisen
eine
innere
epidermale
Zellschicht
aus
nektarerzeu-
genden
Drüsenzellen
auf,
die mit
ver-
schiedenen
Lagen
von
Parenchymzellen
das
Sekretreservoir
umgibt
(Tafel
I,
Abb
2a,b).
Sowohl
die
Drüsen-
als
auch
die
Parenchymzellen
von
E atropurpurea
zeig-
ten
im
Stadium
vor
der
Sekretion
viele
mit
Stärkekörnern
gefüllte
Amyloplasten
(Tafel
II,
Abb
1, 2, 3).
Im
sekretorischen
Stadium
traten
vermehrt
tubuläre
Strukturen
mit
osmiophilem
Inhalt
in
den
Plastiden
auf
(Tafel
III,
Abb
1, 2).
In
diesem
Stadium
ent-
hielten
die
Plastiden
nur
noch
wenig
oder
keine
Stärke
mehr.
Die
Zellwand
der
Drü-
senzellen
wiesen
zahlreiche
Einstülpungen
auf
(Tafel
III,
Abb
3).
Vor
Beginn
der
Sekre-
tion
enthielten
die
nektarerzeugenden
Zel-
len
von
L
abortivum
zahlreiche
polymorphe
Plastiden
mit tubulären
osmiophilen
Struk-
turen,
dichtem
Stroma,
einigen
Plastoglobuli
und
kleinen
Stärkekörnern
(Tafel
IV,
Abb
1,
2).
Gleichzeitig
wiesen
die
Parenchym-
zellen
zahlreiche
Amyloplasten
mit
einigen
Stärkekörnern
auf,
die
fast
die
ganze
Orga-
nelle
ausfüllen
(Tafel
IV,
Abb
3).
Während
der
Sekretion
verschwand
die
Stärke
aus
den
Plastiden
der
Drüsenzellen
und
die
mit
osmiophilen
Inhalt
gefüllten
tubulären
Struk-
turen
traten
auf
(Tafel
IV,
Abb
4;
Tafel
V,
Abb
1).
Häufig
wurden
Vertiefungen
der
Zellwand
zwischen
aneinander
grenzen-
den
Drüsen-
und
Parenchymzellen
beob-
achtet
(Tafel
V,
Abb
2).
In
den
nektarer-
zeugenden
Zellen
war
das
Endoplasmatische
Reticulum
sehr
gut
ent-
wickelt
(Tafel
V,
Abb
3),
zahlreiche
Aus-
läufer
grenzten
an
die
Plastiden.
Später
konnte
die
Ausscheidung
einiger
Nektar-
produkte
in
der
Zelle
beobachtet
werden
(Tafel
V,
Abb
4).
Die
Ausscheidung
der
Nektarkomponenten
in
das
Sekretreservoir
erfolgte
durch
Aufreißen
der
äußeren
Kuti-
kulaschicht
(Tafel
V,
Abb
5).
Die
Ultra-
struktur
der
Drüsen-
und
Parenchymzellen
läßt
vermuten,
daß
die
Plastiden
eine
wich-
tige
Rolle
bei
der
Nektarsekretion
spielen.
Bei
L
abortivum
sammelt
sich
der
Nektar
in
einem
subkutikularen
Raum
und
wird
durch
Aufreißen
der
oberen
Kutikulaschicht
in
das
Nektarreservoir
abgegeben.
In
E
atropurpurea
erfolgt
die
Nektarausschei-
dung
durch
Poren
in
der
Kutikula.
Orchidaceae
/
Epipactis
atropurpurea
/
Limodorum
abortivum
/
Nektarium
/
Pla-
stide
/
Stärke
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... Nectar is secreted along the central groove of the labellum of another mycotrophic orchid, European Epipactis atropurpurea Raf. (Pais and Figueiredo 1994), tribe Neottiae, subfamily Epidendroideae (Chase et al. 2015), which has, however, epidermal nectariferous tissue along the groove and not papillae. ...
... Nectariferous tissue is composed of epidermal, specialized parenchymatic cells present on the surfaces of plant tissue that are usually either elevated or sunken (Fahn 1988;Galetto et al. 1997). In many orchids, the nectariferous tissue contains starch grains in the pre-secretory stage, which is the energy source to produce the nectar (Pais and Figueiredo 1994;Stpiczyńska and Matusiewicz 2001). Starch grains are frequent in angiosperm nectaries, and vascular bundles also can occur, although not necessarily (Fahn 1988(Fahn , 1989). ...
... The cuticle allows both scent diffusion in osmophores and secretion releasing in nectaries; the cuticle expands and there is secretion accumulation in the periplasmic space; this phase was clearly recorded in the nectariferous papillae of Cyclopogon guayanensis in our study ( Fig. 4M-N, arrows). In the secretory labellum tissue of Epipactis atropurpurea, the nectar is released by disruption of the epidermal cell cuticle in the secretory stage (Pais and Figueiredo 1994). In the nectariferous papillae of cotton (Gossypium hirsutum L., Malvaceae), however, a two-layered cuticle is detached from the wall in the secretory papillae and the nectar crosses this structure (Eleftheriou and Hall 1983); further studies are needed to clarify the nectar releasing mechanism in C. guayanensis. ...
Cyclopogon Guayanensis is an Unusual Orchid With a Generalistic Pollination System and Hexose Dominant Nectar
Preprint
Full-text available
  • Aug 2024
Cyclopogon is a large Neotropical orchid genus pollinated by halictid bees that offers nectar as reward. In a recent phylogenetic tree, Brachystele guayanensis emerged nested within Cyclopogon and was transferred to that genus. The hypothesis for this study was that C. guayanensis would show a similar floral biology to Cyclopogon , although distinctive in its small, congested white flowers. Data on floral biology, pollinators, micromorphology, histochemistry, and nectar sugar composition of C. guayanensis in the Distrito Federal, Brazil were gathered. C. guayanensis is pollinated by at least four species of bees belonging to genera Exomalopsis , Nomada , Tetrapedia (Apidae) and Dialictus (Halictidae) foraging for nectar. Nectar is produced in visually imperceptible quantities by papillae on the inner surface of the labellum; similar papillae occur in other species of Cyclopogon but nectar class is unknown. C. guayanensis nectar is hexose dominant (< 10% sucrose) in the Baker and Baker system and is the second record of this nectar class in the Orchidaceae. Pollinia are dorsally adhesive and probably attach to the underside of the bee labrum, as in other Cyclopogon . The inflorescence rachis, bracteoles, and outer surfaces of the base of the sepals are covered with lipid-secreting glandular trichomes; sepals and petals have numerous raphid-rich idioblasts. This is the first record of papillae on a spurless labellum shown to produce nectar in the Orchidoideae. We suggest that hexose dominant nectars in the Orchidaceae are associated with a geophytic habit, small pale flowers, exposed nectaries, visually imperceptible quantities of nectar, and a generalistic pollination system, and coin the term ‘modest pollination strategy’ for this syndrome.
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... The secretory epidermal cells of O. fulgens contained plastids with starch grains (amyloplasts), similarly as in H. imbricata 33 and S. sanguineum 34 , however not present in O. coccineum 23 . The starch grains commonly occur in plastids of nectariferous cells 39,40,[44][45][46][47] . Starch is utilised as a source of sugar and energy for metabolic processes during nectar secretion 48 , so the presence/absence as well as number and volume of amyloplasts can describe the suitable stadium of anthesis: pre-or post-secretory 24,39,44 . ...
... During starch depletion, the plastids became elongated, irregular in shape and more plastoglobuli (described as lipid droplets within the plastids) occurred. In cytoplasm, the lipid droplets (sometimes described as osmiophilic content) were accumulated 33,34,39,40,45 . They also were present in O. fulgens. ...
Ornithophily in the subtribe Maxillariinae (Orchidaceae) proven with a case study of Ornithidium fulgens in Guatemala
Article
Full-text available
  • Apr 2022
Ornithophily has been long speculated to occur in the subtribe Maxillariinae (Orchidaceae), relying either solely on micromorphological analyses or scarce field observations of undefined species. In Guatemala we were able to observe regular visits of the azure-crowned hummingbirds feeding on flowers of Ornithidium fulgens. These observations have led us to investigation of floral attractants by means of scanning and transmission microscopy, histochemical and chemical analyses (GC–MS). Conducted investigation revealed that the epidermis of basal protuberance of column-foot has features proving the secretory activity and that secreted nectar is sucrose-dominant. Slight secretion on the middle part of the lip is puzzling. The presence of other potential pollinators has not been reported. Based on the results of this study, we confirmed that the flowers of O. fulgens meet all criteria of ornithophily and thus that the hypothesis about bird pollination in the subtribe Maxillariinae is proven. The presented results confirm that the previously described floral features predicting the bird pollination in this group are justified. This strengthens the theory about floral adaptations to different pollinators and gives valid reasons to consider species with flowers with a certain set of traits as ornithophilous, even in the absence of the pollination observation.
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... Wall ingrowths indicative of transfer cells found in the labellar nectary of Epipactis atropurpurea (Pais and Figueiredo 1994) were not encountered in floral nectaries of these two species. Thus, the presence of wall ingrowths within nectariferous tissue is not a necessary condition for nectar reabsorption, but their full potential role in the nectarreabsorption process awaits investigation. ...
... Besides dictyosomes, the protoplasts of the labellum's nectariferous cells of A. obovata and O. crassifolia contained many plastids with lipid droplets. Like in floral nectaries of the orchid Epipactis atropurpurea (Pais and Figueiredo 1994), abundant osmiophilic bodies (plastoglobuli) but a sparse thylakoid reticulum were detected in the stroma of plastids studied here. These lipophilic components within the nectaries are suggestive that, besides nectar carbohydrates, these cells are also involved in the production and release of nectar volatiles. ...
Floral glands in myophilous and sapromyophilous species of Pleurothallidinae (Epidendroideae, Orchidaceae)—osmophores, nectaries, and a unique sticky gland
Article
Full-text available
  • Sep 2021
  • PROTOPLASMA
Pleurothallidinae orchids have been the focus of many multidisciplinary studies due to their challenging systematics and taxonomy. The synapomorphies already recognized in the group are mostly related to floral characters, the last proposed being the occurrence of alkanes in the floral fragrance. The composition of the floral bouquet varied significantly among the studied species, leading us to hypothesize that the variations in volatiles emitted could be linked to the structure of osmophores, especially when comparing the myophilous and sapromyophilous pollination syndromes. Sepals and labellum at different developmental stages of seven Brazilian Pleurothallidinae species were examined using light, scanning, and transmission electron microscopy. Nectar reabsorption was assessed by Lucifer Yellow CH tracer and imaged under confocal microscopy. Nectaries were restricted to the labellum of the myophilous species, whereas osmophores occurred in the dorsal and/or lateral sepals, varying according to species. In the sapromyophilous species, floral nectaries were not detected and osmophores were restricted to the labellum. Osmophore structure was correlated with the volatiles emitted, being the trichome osmophores notably present on the sepals of both myophilous species that possess nectaries. For the first time, we demonstrated reabsorption of the released nectar in Pleurothallidinae and the occurrence of a unique gland named sticky-exudate glands, which occurred in the lateral sepals and labellum of Echinosepala aspasicensis, a sapromyophilous species, that released a heterogeneous exudate composed of polysaccharides and lipids. Similar glands have been reported in Bulbophyllum, highlighting the convergence between both groups.
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... Different types of endoplasmic reticulum membranes in nectary cell protoplasts have been described. Tubular reticulum was observed in Epipactis atropurpurea, Limodorum abortivum, Polemonium caeruleum, Prunus laurocerasus, and Salvia farinacea [20,53,68,108], whereas a peripheral reticulum was reported in Salvia farinacea, Citharexylum myrianthum, Geranium macrorrhizum, and Geranium phaeum [53,95,96]. As suggested by Chatt et al. [29], the dense distribution of the endoplasmic reticulum with the characteristic arrangement parallel to the cell walls, the large number of vesicles merging with plasma membranes, and the progressive disintegration of starch grains and sucrose synthesis mentioned above confirm the assumption of the merocrine model of nectar biosynthesis. ...
Nectar Secretion, Morphology, Anatomy and Ultrastructure of Floral Nectary in Selected Rubus idaeus L. Varieties
Article
Full-text available
  • Jul 2022
The distinctive features of floral nectaries facilitate identification of ecological and phylogenetic links between related taxa. The structure and functioning of nectaries determine the relationships between plants, pollinators, and the environment. The aim of the study was to determine and compare the micromorphology of the epidermis in the floral nectaries of six Rubus idaeus cultivars belonging to biennial (‘Glen Ample’, ‘Laszka’, ‘Radziejowa’) and repeated fruiting (‘Pokusa’, ‘Polana’, ‘Polka’) groups. Another objective was to characterize the cuticle ornamentation and stomatal morphology, the anatomy of the nectary epidermis, parenchyma, and sub-nectary parenchyma in the initial nectar secretion phase, as well as the ultrastructure of the nectary epidermis and parenchyma cells in the initial and full nectar secretion phases. The study was carried out using light, fluorescence, scanning and transmission-electron microscopy techniques. Semi-thin and ultrathin sections were used for the microscopic analyses. The cuticular ornamentation and stomatal morphology may be helpful elements in the identification of relatedness between Rubus species. The interaction of the extensive system of endoplasmic reticulum membranes, mitochondria, and Golgi apparatus indicates high metabolic activity, and the fusion of transport vesicles with the membrane suggests granulocrine nectar secretion. The results bring new data to the biology of plants.
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... Floral nectar is synthesized and produced in glands called nectaries and collected by different group of pollinators such as fruit-eating bats, hummingbirds, sunbirds, and insects and other animals [9]. In orchids nectaries are usually located at the base of the lip, in the concave basal part of labellum called hypochile or at the base of the flower alongside the ovary and on the side lobes, or along the central groove of the labellum, as well as in long spurs [7,8,10,11]. ...
Why does an obligate autogamous orchid produce insect attractants in nectar? – a case study on Epipactis albensis (Orchidaceae)
Article
Full-text available
  • Apr 2022
  • BMC PLANT BIOL
Background: The flowers of some species of orchids produce nectar as a reward for pollination, the process of transferring pollen from flower to flower. Epipactis albensis is an obligatory autogamous species, does not require the presence of insects for pollination, nevertheless, it has not lost the ability to produce nectar, the chemical composition of which we examined by gas chromatography-mass spectrometry (GC–MS) method for identification of potential insect attractants. Results: During five years of field research, we did not observe any true pollinating insects visiting the flowers of this species, only accidental insects as ants and aphids. As a result of our studies, we find that this self-pollinating orchid produces in nectar inter alia aliphatic saturated and unsaturated aldehydes such as nonanal (pelargonal) and 2 pentenal as well as aromatic ones (i.e., syringaldehyde, hyacinthin). The nectar is low in alkenes, which may explain the absence of pollinating insects. Moreover, vanillin and eugenol derivatives, well-known as important scent compounds were also identified, but the list of chemical compounds is much poorer compared with a closely related species, insect-pollinating E. helleborine. Conclusion: Autogamy is a reproductive mechanism employed by many flowering plants, including the orchid genus Epipactis, as an adaptation to growing in habitats where pollinating insects are rarely observed due to the lack of nectar-producing plants they feed on. The production of numerous chemical attractants by self-pollinated E. albensis confirms the evolutionary secondary process, i.e., transition from ancestral insect-pollinating species to obligatory autogamous.
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... The hydrolysis of starch results in a source of energy for fragrance and nectar production [11,12]. The volatile components of fragrances are synthesized in plastoglobuli and are probably transferred from them to the intraplastidal membranes, before traversing the plastid envelope, passing to the endoplasmic reticulum, and finally traversing the plasmalemma [13,14]. In the labellum, electron-dense material was observed to accumulate in close proximity to the irregular plasmalemma and structurally resembled that of tannin-like material. ...
Crepidium sect. Crepidium (Orchidaceae, Malaxidinae)—Chemical and Morphological Study of Flower Structures in the Context of Pollination Processes
Article
Full-text available
  • Nov 2021
Crepidium is a large genus of mainly pantropical orchids. The lips of its flowers are upwardly directed and do not serve as landing platforms for pollinators. This role is assumed by the dorsal sepal and/or gynostemium. Information about the pollination and floral morphology of this genus is scarce. To date, no papers have been published on these topics. Field observations have revealed that the flowers are visited by small flies, midges, fruit flies, other small dipterans, ants, spiders, and mites. Preliminary observations revealed at least two forms of small liquid droplets secreted on the lip surface of Crepidium species: simple secretions from epidermal cells, and cell sap released upon the rupturing of raphide-producing cells. Further research revealed that this was the first time liquid secretion was recorded in this genus. Floral secretions were subjected to sequential organic solvent extraction and gas chromatography–mass spectrometry (GC–MS). Floral parts were investigated by means of scanning (SEM) and transmission electron microscopy (TEM), and histochemical tests. The presence of liquid droplets on the lip of Crepidium, the presence of a food reward, and the sequence of raphide development are reported here for the first time.
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... As stated by Figueiredo and Pais [74], Stpiczyńska and Matusiewicz [75] and Naczk et al. [70], components being part of the floral fragrance belong to plastoglobuli which can be present in osmophores. The transportation of these products outside secretory tissues can occur through cuticular microchannels [70,[76][77][78][79][80]. A high presence of cytoplasmatic globules and secondary metabolites has been noticed in spur papillae of O. patens, but especially in O. × fallax. ...
Floral Trait and Mycorrhizal Similarity between an Endangered Orchid and Its Natural Hybrid
Article
Full-text available
Hybridization can often lead to the formation of novel taxa which can have traits that resemble either or both parental species. Determining the similarity of hybrid traits to parental taxa is particularly important in plant conservation, as hybrids that form between rare and common taxa may more closely resemble a rare parental species, thereby putting the rare parental taxon at further risk of extinction via increased backcrossing and introgression. We investigated the floral (morphological and chemical) traits and orchid mycorrhizal (OrM) fungal associations of the endangered orchid Orchis patens, its more common sister species O. provincialis, and their natural hybrid O. × fallax in natural sympatric populations. We found that both morphological and chemical floral traits of O. × fallax are shared by the parents but are more similar to O. patens than O. provincialis. OrM fungi were shared among all three taxa, indicating that the availability of OrM fungi should not represent a barrier to establishment of individuals of any of these taxa. These results suggest that O. × fallax may be able to expand its distribution within a similar niche to O. patens. This highlights the importance of quantifying differences between hybrids and parental taxon in species conservation planning.
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Nectar Chemistry or Flower Morphology—What Is More Important for the Reproductive Success of Generalist Orchid Epipactis palustris in Natural and Anthropogenic Populations?
Article
Full-text available
The aim of this study was to determine the level of reproductive success (RS) in natural and anthropogenic populations of generalist orchid Epipactis palustris and its dependence on flower structure and nectar composition, i.e., amino acids and sugars. We found that both pollinaria removal and female reproductive success were high and similar in all populations, despite differences in flower traits and nectar chemistry. Flower structures were weakly correlated with parameters of RS. Nectar traits were more important in shaping RS; although, we noted differentiated selection on nectar components in distinct populations. Individuals in natural populations produced nectar with a larger amount of sugars and amino acids. The sucrose to (fructose and glucose) ratio in natural populations was close to 1, while in anthropogenic ones, a clear domination of fructose and glucose was noted. Our results indicate that the flower traits and nectar composition of E. palustris reflect its generalist character and meet the requirements of a wide range of pollinators, differing according to body sizes, mouth apparatus, and dietary needs. Simultaneously, differentiation of nectar chemistry suggests a variation of pollinator assemblages in particular populations or domination of their some groups. To our knowledge, a comparison of nectar chemistry between natural and anthropogenic populations of orchids is reported for the first time in this paper.
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Floral Nectar Chemistry in Orchids: A Short Review and Meta-Analysis
Article
Full-text available
  • Oct 2021
Nectar is one of the most important flower traits, shaping plant–pollinator interactions and reproductive success. Despite Orchidaceae including numerous nectariferous species, nectar chemistry in this family has been infrequently studied. Therefore, the aim of this study is to compile data about nectar attributes in different orchid species. The scarcity of data restricted analyses to sugar concentration and composition. Our results suggest that the most important factor shaping nectar traits in orchids is the pollinator type, although we also found differentiation of nectar traits according to geographical regions. In spurred orchids, the length of the spur impacted nectar traits. We recommend the development of studies on nectar chemistry in orchids, including a wider range of species (both in taxonomic and geographical contexts), as well as extending the analyses to other nectar components (such as amino acids and secondary metabolites). The nectar biome would be also worth investigating, since it could affect the chemical composition of nectar. This will enrich the understanding of the mechanisms of plants–pollinators interactions.
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Amino acid and sugar content of the nectar exudate from Limodorum abortivum (Orchidaceae). Comparison with Epipactis atropurpurea nectar composition
Article
Full-text available
  • Jan 1986
SUMMARY The amino acid content of nectary exudates from two orchids, L. nbortivum (spur nectary) and E. ati!optirpiti-ea (open nectary) were studied by means of capillary gas chromatography. Amino acids were analyzed as the corresponding N-heptafluorobutyryl isopropyl esters. For each amino acid, enantiomeric composition was achieved by gas chromatography of the N-pentafluoropropionyl isopropyl esters on a capillary coated with the chiral liquid phase Chirasil-Val. Comparison of the amino acid contents of both nectars, as well as the enantiomeric composition of the particular amino acids, indicate evolutionary differences of both nectaries. Gas chromatographic analysis of the nectar sugars, supports the conclusion that the open nectary of Epipacti,r atropbelongs to a more advanced evolutionary type compared to the spur nectary of Gimoclonun o6ot!)t<m. The possible influence of the nectar composition on pollinators is discussed.
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SOME ASPECTS OF CARPEL STRUCTURE IN CALTHA PALUSTRIS L. (RANUNCULACEAE)
Article
  • Mar 1979
Caltha palustris L. carpels obtained from closed flower buds, from flowers that had just reached anthesis, and from older flowers were examined by light and electron microscopy. Trichomes located on either side of the cleft towards the base of each carpel, cells along the margins of the carpel cleft, and transfer cells along the locule lining immediately beneath the micropyle of the anatropous ovule were examined. Numerous, smooth endoplasmic reticulum cisternae and dictyosomes, the presence of material between the cell wall and cuticle, and droplets of material in the region of trichomes is evidence that the trichomes are nectaries. The cells lining the cleft and the transfer cells which have wall ingrowths along the tangential wall facing the locule may be involved in the secretion of substances for pollen tube growth.
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The Nectaries of Aptenia cordifolia Ultrastructure, Translocation of 14C-labelled Sugars, and a Possible Pathway of Secretion
Article
  • Oct 1981
In Aptenia cordifolia the nectar is secreted into four cavities which are located between the stamens and the ovary wall. The secretory tissue forms a relatively small region of the peripherical cell layers of each cavity. The Golgi apparatus and the ER of the secretory cells are only slightly developed. For these reasons we conclude the presence of an eccrine secretion mechanism. The nectar is composed of sucrose, glucose and fructose and, in contrast to the nectary squash, free of ninhydrin positive substances. This demonstrates the selective ability of the secretional mechanism. Sucrose, glucose or fructose labelled with 14C were applied to the excised flowering shoot tips. After incubation with any one of the labelled sugars, 14C-label was found in all three sugars secreted by the nectary. Therefore, it is suggested that the nectary acts as a sink for sugars. Hydrolyzation of sucrose into its monomers could be observed in the nectary squashes and the stem squashes, but not in the nectar. Thus, hydrolyzation might play a role in maintaining such a sink.
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Some Aspects of Carpel Structure in Caltha palustris L. (Ranunculaceae)
Article
  • Mar 1979
Caltha palustris L. carpels obtained from closed flower buds, from flowers that had just reached anthesis, and from older flowers were examined by light and electron microscopy. Trichomes located on either side of the cleft towards the base of each carpel, cells along the margins of the carpel cleft, and transfer cells along the locule lining immediately beneath the micropyle of the anatropous ovule were examined. Numerous, smooth endoplasmic reticulum cisternae and dictyosomes, the presence of material between the cell wall and cuticle, and droplets of material in the region of trichomes is evidence that the trichomes are nectaries. The cells lining the cleft and the transfer cells which have wall ingrowths along the tangential wall facing the locule may be involved in the secretion of substances for pollen tube growth.
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Les nectaires extrafloraux pédicellés chez l’Ailanthus glandulosa
Article
  • Jan 2011
  • CAN J BOT
Extrafloral nectaries are present on the first leaves appearing when the buds open. Located at the base of the petiole, they are stalked and have an apical pore. They are initiated early on the margins of leaf primordia. These organs are interpreted as reduced stipules consisting of one axis with its vascular system with the extremity occupied by a nectary. A viscous and clear nectar is extruded from the pore. Without vascularization, there is never an apical gland. Stalked nectaries are also observed on the adaxial surface of the lamina. Described as teratological events, they correspond to the deviation of a vein perpendicular to the plane of the leaf. For aerial organs these nectaries show clearly the close connection between the vascular system and the glandular extremity. The tissue structure of these stalked nectaries is similar to that of the marginal nectaries. The morphological and physiological signification of these structures is discussed.
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Secretory tissues in vascular plants
Article
Secretory tissues occur in most vascular plants. Some of these tissues, such as hydathodes, salt glands and nectaries, secrete unmodified or only slightly modified substances supplied directly or indirectly by the vascular tissues. Other tissues secreting, for instance, polysaccharides, proteins and lipophilic material, produce these substances in their cells. The cells of secretory tissues usually contain numerous mitochondria. The frequency of other cell organelles varies according to the material secreted. In most glandular trichomes the side wall of the lowest stalk cell is completely cutinized. This prevents the secreted material from flowing back into the plant. The salt glands in Atriplex eliminate salt into the central vacuole of the bladder cell but, in other plants, the glands secrete salt to the outside. Different views exist as to the manner in which salt is eliminated from the cytoplasm. According to some authors, the mode of elimination is an eccrine one, while others suggest the involvement of membrane‐bound vesicles. Nectar is of phloem origin. The pre‐nectar moves to the secretory cells through numerous plasmodesmata present in the nectariferous tissue. Nectar is eliminated from the secretory cells by vesicles of either KR or dictyosomal origin. In some cases, both organelles may be involved but an eccrine mode of nectar secretion has also been suggested by some authors. Carbohydrate mucilages and gums are synthesized by dictyosomes but virtually every cell compartment has been suggested as having a role on the secretion of lipophilic substances. Most commonly, plastids are implicated in the synthesis of lipophilic materials but KR may also play a part. In some cases lipophilic materials may be transported towards the plasmalemma in the KR. Resin and gum ducts of some plants develop normally or in response to external stimuli, such as microorganisms or growth substances. Among the latter, ethylene is the most effective. During the course of evolution, secretory tissues seem to have developed from secretory idioblasts scattered among the cells of the ordinary tissues. Subsequently ducts and cavities developed and finally secretory trichomes. CONTENTS Summary 229 I. Introduction 230 II. Salt glands 231 III. Nectaries 236 IV. Mucilages and gums 241 V. Tissues secreting lipophilic material 242 VI. Factors influencing the development of certain secretory tissues 246 VII. Evolutionary considerations 248 References 250
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The floral nectaries of Hibiscus rosa-sinensis III. A morphometric and ultrastructural approach
Article
The secretory hairs of Hibiscus rosa-sinensis nectaries have been studied ultrastructurally, particularly at stages before, during and after secretion. Morphometric cytology revealed considerable changes in the volume of endoplasmic reticulum (ER) and the vacuoles. At least three different forms of ER have been noticed, cisternal, tubular and vesicular. The intermediate cells of the hairs are ultrastructurally and morphometrically similar to the tip cell, suggesting their probable involvement both in a symplastic prenectar transport via the plasmodesmata and in nectar release into an extracellular space provided by the lateral cell walls. Nectar would then be apoplastically moving towards the tip cell, where it is forcibly expelled to the outside via transient pores of the cuticle. Ultrastructural evidence indicates that ER is the cell compartment principally involved both in prenectar transport and nectar elimination. In the hair cells it provides a “secretory reticulum” mediating between prenectar accumulation and nectar release. Vacuoles are voluminous in the basal cells, and in all cell types of old nectaries. The results are discussed in relation with other plant glands, especially with the closely related Abutilon nectaries.
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