The reproductive biology of Magnolia grandiflora

20 Pages·1999·8.2 MB·English

by  L K Allain

RHODORA
THE
REPRODUCTIVE
BIOLOGY
OF
MAGNOLIA
GRANDIFLORA
Larry
K. Allain
Wetlands
LA
Lafayette, 70504
Michael Zavada
and Douglas Matthews
S. G.
1
Department
of Biology, Providence
College, Providence, RI 02
abstract. The
reproductive biology of Magnolia was
grandiflora inves-
tigated at three localities in south Louisiana. Over 3-4 day
the flowering
period, the flowers of M.
grandiflora exhibited changes in sex expression
(protogyny), stigmatic and
receptivity (self- cross-compatibility to self-incom-
UV
patibility), reflectance (strong reflectance of the stigmas to strong
reflec-
tance of the androphore), and pollinator reward hexose-dominated
(a
stig-
matic nectar to pollen). Although beetles were occasional and
floral visitors
carried pollen, bees (non-native Apis and
mellifera indigenous Lasioglossum
bruneri) were frequent and were
floral visitors the only whose
floral visitors
showed
behavior any correlation with the array of changes
floral that occurred
3-4
over the day flowering
period.
Words
A
Magnolia among
considered be
to primitive angiosperms.
is
widely accepted view of the evolution of the flower Euan-
the
is
thial or Anthostrobilus theory. This hypothesis
asserts that the
primitive flower is large, actinomorphic, solitary, white (some-
times pink or yellow), and borne terminally. The primitive flower
may
have showy, and numerous
a undifferentiated perianth
floral
parts arranged spirally on an elongated axis. The stamens are
broad, three veined, and each has four elongate microsporangia
on The gynoecium
adaxial surface. apocarpous, with con-
its is
few
duplicate carpels that enclose a ovules (Arber and Parkin
1907; Bessey 1897, 1915; Canright 1952, 1960; Maneval
1914).
This widely held view of the magnolian flower has been consid-
ered a theoretical starting point for understanding angiosperm
evolution Cronquist 1981). However, recent phylogenetic
(e.g.,
Donoghue
analyses (Crane 1985; and Doyle 1989; Loconte and
Nixon
Stevenson and
1990, 1991; et al. 1994), fossil evidence of
MSZ
Reprint requests should be addressed to
i
143
44
Rhodora
1
[Vol. 101
(Crane
1994, 1995;
;t al. Friis et 1994,
al.
992) suggest
that the morphological
angiosperm
flower
are unresolved.
There
sperms
associated with
is the
diversification of Apidae
the (Mich-
ener and
Grimaldi
1988a, Crepet
b; 1984, 1996; Crepet
et
al.
99 The
occurrence of
1 first angiosperms, however,
1 ).
significantly
predates
the occurrence of
first bees
in the fossil record (Laban-
The
(Kearns
Kearns
their origin
in the Late
Triassic-Early
Jurassic (Rohdendorf
1 974), prior to the origin and
the
diversification of angiosperms.
Recent
studies have shown
that the
floral foraging behavior of
some
groups of
flies is similar to bees, and
that certain groups of
flies are
faithful
pollinators (Kearns
1992; Kearns and Inouye
Ren
1993;
1998). Beetles known
are well from
the Mesozoic and
occurred
contemporaneously
with known
the
earliest
angio-
sperms. The
large, unspecialized
flowers of Magnolia
are be-
lieved be
to associated with
the
unspecialized "mess
and
soil"
pollination syndrome
of
beetles (Faegri and van
der
1979)
Pijl
Other
investigators view
beetle
pollination
in the Magnoliidae
(Armstrong
and
Irvine 1990; Bernhardt and
Thien
1987) and
var-
ious
other
related groups
(e.g. Cyclanthaceae; Beach
1982)
as a
specialized
interaction
involving
floral modifications
that are ben-
eficial to the beetle
pollinators. For
example,
in Cyclanthus
bi-
partitus
the bracts of
the
inflorescence
produce
a
specialized high
lipid (50%
tissue by dry
weight)
that the consume
beetles
along
with
pollen (Beach
Taxa
1982).
that are unequivocally
beetle pol
hnated
frequently have
floral modifications
that
specifically
influ-
ence
beetle behavior
in the flower
or
inflorescence
Despite
a
diversity of
insect groups
reported
to Magnolia,
visit
most
investigators have
focused on
the occurrence
of
beetles
in
the flowers
Z^™of^Magn^olia?, ^
and on
their
role
in pollen
transfer (Bak-
:*™^
-d
75
Espinosa
1994;
Kikuzawa
and Mizu
M.
grandiflora have been
demonstrated
be
to
specifically
correlated
with
the floral behavior of
the
beetles.
The
purposes
of
this study are
as
follows:
a) to examine
the
may
floral characteristics
that
function
as
insect
attractants and/
Allain
1999]
et al. Magnolia
45
1
part
and
composition,
pollen and
availability, the origin and longevity
determine
this
to
floral characteristics
that are pollinator and
attractants rewards;
and
determine
c) to the kinds, numbers, and
pollen loads of
the
various
floral visitors, their behavior in the flowers of Magnolia
how
and may
grandiftora, this behavior be
related
to floral char-
acteristics that function as pollinator and
attractants rewards.
MATERIALS AND METHODS
The
was
investigation
carried out three
at localities in south
Louisiana. Site #1 consisted of 59
cultivated
trees located
throughout
the city of Lafayette, Louisiana, a suburban
habitat.
Arboretum
The
Parish, Louisiana.
The
#2 was
tree at Site used for observing
pollinators in the forest
m
30-40
canopy. Access to the forest canopy was achieved using
mountain
climbing techniques (Perry #3
1978). Site consisted of
a monoculture of over 30 trees of varying ages at the Louisiana
Nursery, Opelousas, Louisiana. The nursery
located a
is in rural
area and the trees were grown under horticultural conditions.
and
Pollinator attractants rewards. To determine
mm
camera
UV
Wratten Filter No. 8 A. This
1
UV
wave
(320-400 High (ASA
radiation nanometers). speed
Max
reflectance patterns
determined
the flower into separate parts (gynoecium, androphore, upper
pet-
and lower and
als, petals) placing the separated floral organs in
Thirty
parts
and 5 beine the most fraeran
The
flowers used for hand pollinations (see below) were mon-
itored for the presence of a stigmatic exudate (nectar) over the
The
four-day flowering period. nectar used in the sugar analysis
was
from from
collected first-day flowers a variety of
plants
at
146
Rhodora
[Vol. 101
#1 by
Site using capillary tubes. (Nectar production ceased
after
day The
the of was
first anthesis.) nectar immediately
placed
in
a cooler with
ice packs, transported to the and
lab, refrigerated
EM
The was X cm
at 1°C. nectar run on 20 20
Reagent,
Silica
Gel 60 Type The
plates. gel thickness was mm.
0.25 After the
run
various
The
fructose
ose, and mannitol. The
solvent used was
a
9:6:3:1 solution of
n-
The
—
— — —— — — — -
oped
100-1
at 10°C for 10-20 minutes
The
methanol.
i:
determined bv
th
Thin
Chromoto
The mean
number
of
pollen
grains per flower was
estimated
three
three
Each
of
the stamens was
thirty
known
The
number
of
pollen
grains per stamen
and was
flower
calculated.
In addition, number
the
of
ovules per flower was
recorded
to
denve
an
estimate of
the pollen-ovule
ratio (P/O).
Determination
of and
self-
cross-compatibility. At #
Site
1
flowers were hand
pollinated
to determine
stigmatic
receptivity
and
cross- and
self-compatibility. The
pollinations were
divided
among
ten
treatments, 14-39
plants per
treatment. Four
of
the
treatments
consisted
of
cross-pollinations on
days 1-4
of
anthesis
four
consisted of
self-pollinations on
days
1-4, one
treatment was
an
unpollinatcd bagged
control
for days
1-4, and one
treatment
was
an unbagged
control
for days
1-4,
to determine
the seed
set
under
natural
conditions.
Prior to
anthesis, the gynoecia
of hand
the
pollinated flowers
were bagged
with
tubular nylon
hosiery.
After hand
the
pollina-
tions, the flowers were
tagged.
Pollen was
collected
for various
pollinations in paper
envelopes. The
pollen was
stored -10°C
at
with
a desiccator
(Williams
To
1980).
insure
that
all pollinations
from
the stored pollen had
a similar
level of
viability, the duration
of
pollen was
viability
tested. Flowers
were
collected from
three
Allain
1999]
et Magnolia
al. 147
different trees during the morning of
first anthesis, the petals were
removed, and column
the floral with the stamens was placed on
a sheet of paper. Within 24
hr., the anthers dehisced, the
floral
column was
removed, one
aliquot of pollen was room
stored
at
temperature (21°C), and one was
aliquot stored -10°C.
Pollen
at
was
from
viability tested each of
the aliquots daily for five con-
secutive days following method
anthesis using the of Alexander
(1969).
The
gynoecia were
collected as they ripened and number
the
of number
stigmas, the of carpels and number
setting seed, the
two
setting seeds were recorded. The percentage of ovules
fertil-
was
ized by
calculated dividing the seed by
total set the
total
number
of ovules (2/carpel) and multiplying by The
100. per-
centage of ovules fertilized for the ten treatments was compared
ANOVA
A
using
(analysis of variance). general model
linear for
unbalanced was
designs chosen and
a two-factor general
linear
ANOVA
model was
run on Minitab (Cruze and Weldon
1989).
The model was
fitted with two main effects, crossed and selfed
The
treatments. four days over which were made
pollinations
were
an
treated as interactive factor with four were
levels. F-ratios
compared
at the 0.01 level of significance to detect differences
among
treatments. Significant differences between treatments
were then tested using Tukey's multiple-comparison (W)
proce-
=
dure with Alpha 0.05 (Cruze and Weldon 1989; Ott 1988).
Observations of insects. The types of insects and the number
of visits by each taxon at Sites #1, #2, and #3 were recorded by
observation by Each was
direct or videotape. of
the three
sites
May
monitored
14 times equally spaced between
at intervals 19
and
were
July 29. Insect visitor data recorded for an average of
three hours per observation time, yielding a of 42 hours of
total
observations per Unfamiliar were
insects collected for iden-
site.
tification and five individuals of each species were collected as
voucher specimens. All insects except thysanopterans were placed
in a kill jar containing ethyl acetate. Thysanopterans were fixed
(AGA)
two
in alcohol-glycerine-acetic acid killing solution. After
AGA,
weeks were
in the the thysanopterans dehydrated in an
and mounted on microscope
alcohol series slides for identification
number
(Borror 1989). For each the of by each
et al. site visits
was The
type of insect recorded. duration of the was
visits re-
corded by observation and from time and
direct lapse
real-time
Rhodora
148 [Vol. 101
videotaping of the flowers. The pollen load per individual of each
mean number
insect species was calculated as the of grains car-
carried
mellift
position and so was not included in the pollen load of this species.
However, the pollen carried on the legs of halictid bees dislodged
easily and was available for stigmatic deposition; thus these pol-
len grains were included in the pollen load for this species.
The importance (RI) of various insect species as pol-
relative
was
calculated using the following equation:
linators
—
— X
V:
P:
=
x
100
RI;
x
V)
(P
where
=
Index of importance of
Rli relative pollinator
i,
=
Mean
Pj pollen load of pollinator
i,
= Sum
P mean
of pollen loads of
insects,
all
=
Number
Vj of of
pollinator
visits
i,
V
=
number
Total of
insect
visits.
RI was
Overall or total calculated for each taxon by using the
mean
pollen load of the taxon over three and mean
all sites the
three
on
run Minitab
three
sites.
RESULTS
and On
Pollinator attractants rewards.
day
the of an-
first
UV
thesis the stigmas strongly On
reflected light (Figure
1, 2).
second day
the of anthesis the stigmatic was
reflectance evident,
had
but significantly faded in comparison day
to the of an-
first
By UV
thesis (Figure 3). the third day of anthesis the reflectance
of the stigmas was no longer evident. As
the stigmatic
reflectance
faded on the second day of
anthesis, the androphore, from which
UV
stamens had
the abscised, reflected
light in a checkered
pat-
The
tern (Figure 3, 4). reflectance of the androphore was evident
on the third day of anthesis, but was not on
detectable
the fourth
—
1999] Allain et Reproductive Biology of Magnolia
al. 149
Figures 1-4. The two Magnolia
stages in grandiflora floral phenology.
Ultraviolet photograph of a day flower showing the reflective stigmatic
1 first
.
UV
crests. 2. Photograph of a day flower taken without the
first filter. 3.
Ultraviolet photograph of a second day flower showing the faded stigmas and
"checkered pattern" of the reflective androphore (arrowhead). Photograph
4.
UV
of a second day flower without the
filter.
day of anthesis. Throughout the four days of anthesis the corolla
UV
absorbed
strongly
light.
In the determination of the origin of the fragrance, 29 of
floral
the 30 volunteers rated the petals most fragrant, and the andro-
phore
least fragrant.
Nectar secretion associated with the stigmas occurred only on
day of Nectar was not evident on days 2-4.
the anthesis.
first
Using thin layer chromotography (TLC), three sugars were iden-
tified in the stigmatic exudate collected soon after the flowers
opened on the day of anthesis; these were glucose, fructose,
first
TLC
when
and sucrose. Glucose and fructose, visualized on the
15%
showed
plates, a similar intensity to the (w/v) standard sugar
although
Sucrose, easily detectable, consistently exhib-
solutions.
50
Rhodora
1
[Vol. 101
ited less
intensity, suggesting
that lesser amounts of
sucrose were
fructose
3-6
collected
hr.
breaks
known
er 1975,
Magnolia
grandifl<
positive
for
protein.
On
the second day
of
anthesis the stamens
dehisced and
sub-
sequently
abscised from
the androphore,
falling into the cup-
shaped
was
petals.
It in the cup-shaped
petals magnolia
that
pol-
The
pollen
grains per flower was
58,000,000. The
pollen-ovule
ratio
was
estimated
to range from
207,000:
to 520,000:
1 with an av-
1
erage of 4
2,000:
1
1
.
and
Self-
cross-compatibility. To
insure
that differences
in
seed
were
set not due
to differences
in pollen
from
viability day
to day, pollen was
On
tested
for
viability. day
the of
anther
de-
hiscence
(day
of
2 anthesis) a mean of 94.5%
of
the pollen
grains
were
viable. There was no
significant decrease
in pollen
viability
lor the
five consecutive
days
following
when
anthesis
was
pollen
stored 2l°C
at or -10°C.
Results of the hand
pollinations showed
on
that the da
first V
of
anthesis
the stigmas were
receptive
to geitonogamous
and
xe-
nogamous
On
pollinations.
the day
of
first anthesis
the average
seed
set for xenogamous
pollinations was 55%.
The
average
seed
set for geitonogamous
pollinations was
38%,
which was
signifi-
cantly lower
than
the
cross-pollinated
flowers On
(Table
1) the
second
day
of
anthesis
cross-pollinated
flowers 27%
averaged
seed on
set the day 13%
third
seed
and
on
set,
the fourth day
2%
seed
set which was
not
significantly
different from
the
bagged,
unpolhnated
control
(Table
This
1). indicates
that recep!
tivity to xenogamous
pollinations was
gradually
reduced
over
the
3-4 day
flowering
period.
In
flowers
were
that
self-pollinated
was
there
a
significant
decrease
SuriTS
in percent
seed
™™ "img
set
uic
Second nf
nntlwcic ~ ^
ftav
anthesis _
4 -
and
Seed
on
set days
1).
and
2, 3,
vas not
significantly
different
ft
The unbaked
m******^
titrol.
)
>
5
Table seed
1.
d = 7
%
c, are statistically different at Alpha 0.05. n number of flowers/treatment; x mean
seed each
set for treatment; sd
p
standard
deviation.
Treatments Day Day
2 Day
Day
3 4
1
Cross
n 14
n 16 n 18 n 14
x 55 b o
27
13 x 2 d
Jc :
sd 24 20 c
sd
sd 17 sd 3
'
Self n 20
n 20 n
12 n
15
38 b
x d
x
Jc : 5 5 x 3 d
(I
24
sd
sd 11 sd 15 sd 4
Control
39
n
cT
(bagged, unpollinated d
5
O
sd
11
'
Control n 20
I
(unbagged, untreated) x 34
sd 26
&
u.
152
Rhodora
[Vol. 101
34%
aged seed which assumed
approximate
set, is to seed
set
under
natural conditions.
The
results of the hand
pollinations
in the cross-pollinated
showed
flowers that from day day 4 was
to there a stepwise
1
reduction in receptivity. Each day was
significantly less receptive
than the previous day, culminating on day which was
4, not sig-
from
nificantly different the unpollinated, bagged The
control.
flowers were
self-compatible on the day of and
first anthesis seed
set in the self-pollinated flowers was
similar
to the percent seed
set in the unbagged, untreated However, on
control.
the second,
and
third, fourth days of
anthesis, self-pollinated seed was
set not
significantly from
different the unpollinated, bagged
control,
in-
dicating was
that the flower
essentially
self-incompatible
after the
day
of
first anthesis.
Observations
of Seven
insects.
orders of
insects accounted
more 99%
for than of
the insect Magnolia
visits to flowers
at
all
The
three
sites. correlation
coefficient for comparisons
of
all the
insects recorded
at the three was
sites greater than
0.93, indicating
that the types of
insects each and
at
site, the relative proportions
of
the insects Magnolia
visiting
each
at were
site similar. Fre-
quencies of by
visits the various among
insects varied
the
sites,
with #2
Site having almost many
twice
as
insect
visitors as Sites
similar
Thysanopt
Hemipt
Plecoptera
for the remaining 13%
of
total insect
visits (Table
2).
sven Mag
orders ot
insects
visiting
98.9%
carried
of
the pollen
(exclud
The
counted
for only 1.0%
of
the pollen
carried (Table The
3). relative
j*^
•
w w" w Tl rf" "V W f\ 'W^ ^"^ J-*. m m \. t *.
earners
i
three
lo-
lon-native honeybee
(Apis
mellift
Magnolia
are excluded from
the
calculation,
native
halictid bees have
an
three
Magnolia
and
diflora carried
ants
The