Annals of Botany 81 : 697–703, 1998
Pollen Tube Distribution in the Kiwifruit (Actinidia deliciosa A. Chev. C. F. Liang)
Pistil in Relation to its Reproductive Process
D. H O W P A GE*, V. V I T H A N A GE† and R. S P O O N E R-H A R T*
* School of Horticulture, UniŠersity of Western Sydney (Hawkesbury), Bourke Street, Richmond, NSW 2753,
Australia and † CSIRO Plant Industry, Horticulture Research Unit, 306 Carmody Road, St Lucia,
QLD 4067, Australia
Received : 7 October 1997
Returned for revision : 14 November 1997
Accepted : 30 January 1998
High resolution light and fluorescence microscopy were used to investigate the structural and cytochemical features
of the kiwifruit (Actinidia deliciosa) pistil and to follow the pollen tube pathway after pollination. The multicarpellary
ovary is syncarpous only at the ovary level thus leaving 30–40 free styles on top. The fusion of the longitudinallyfolded carpels to form the syncarpous ovary forms a central parenchymatous axis, the columella, from which the
ovules radiate outwards into the ovary cavity. A prominent cup shaped depression on the columella at the top end
of the ovary, termed the pollen tube distributor cup (PTDC) was detected. Pollen tubes from the stigma travel through
the transmitting tract and enter the PTDC from where they are distributed towards the ovary. Even when pollination
is restricted to two stigmas, the PTDC seems to ensure that the pollen tubes are evenly distributed around the ovary
resulting in an even distribution of seeds. This suggested role of the PTDC which could compensate for over and
under pollination of individual stigmatic arms is another adaptive feature which plays a crucial role in the
reproductive process of kiwifruit. The significance of this structure for pollination by insects is discussed.
# 1998 Annals of Botany Company
Key words : Actinidia deliciosa, kiwifruit, pollination, floral anatomy, reproductive success, pollen tube distribution.
INTRODUCTION
Kiwifruit, Actinidia deliciosa (A. Chev.) C. F. Liang and
A. R. Ferguson is a dioecious fruit species native to China
(Ferguson, 1983, 1984) and grown in many parts of the world
for its edible fruit.
In kiwifruit, staminate (male) vines produce male flowers
that contain viable pollen, but they have no functional
ovaries. Pistillate (female) vines produce female flowers with
b1
a1
a2
st
b2
F. 1. A female (pistillate) kiwifruit flower. Sections were taken along
the symmetrical line (a a ) of the flower where styles (st) are arranged
" #
linearly, and at right angles (b b ) to a a . Bar ¯ 1 cm.
" #
0305-7364}98}060697­07 $25.00}0
" #
a functional ovary and non-viable pollen (Schmid, 1978).
Pistillate vines bear fruit. In pistillate flowers, the ovary is
formed by the fusion of 26–41 carpels with each housing up
to 40 ovules with an estimated total of 1400–1500 ovules per
pistil (Hopping, 1976 ; Gue! de! s and Schmid, 1978). Thus,
there is a need to transfer an exceptionally large amount of
pollen to fertilize all ovules. Fertilization of a large
proportion of ovules results in bigger fruit (Hopping, 1976),
and fruit size and seed numbers are positively correlated
(Pyke and Alspach, 1986 ; Testolin, Vizzotto and Costa,
1991).
Honey bees (Apis mellifera L.) are regarded as essential
and true pollinators of kiwifruit (Palmer-Jones and Clinch,
1974 ; Howpage, Spooner-Hart and Vithanage, 1996 ;
Vaissie' re et al., 1996). The wet stigma (Harvey et al., 1987)
and the short pollination period measured in terms of
stigma receptivity lasts for 4 d (Gonza! lez, Coque and
Herrero, 1995 a, b) and is considered an adaptation for
insect pollination. However some floral characteristics of
kiwifruit, such as the pendulous nature of flowers, absence
of pollenkit, high ovule : pollen ratio and large multibranched stigmatic surfaces suggested that wind may be an
equally important agent of pollen transfer as honey bees
(Craig and Stewart, 1988 ; Costa, Testolin and Vizzotto,
1993).
The pistil is well adapted to nourish a large number of
pollen tubes as demonstrated by the copious secretions
found in the pollen tube pathway, including the placental
surface (Gonza! lez, Coque and Herrero, 1996), an attribute
linked to its high reproductive success. In this paper, we
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# 1998 Annals of Botany Company
698
Howpage et al.—Pollen Tube Distribution in Kiwifruit
trace the path of pollen tube growth and report some
structural features of the pistil which play an additional,
crucial role in the reproductive process of kiwifruit ; their
significance in crop production is discussed.
MATERIALS AND METHODS
Plant material
Flowers used in this study were from a 5 hectare commercial
orchard in Agnes Banks (33° 37« S, 150° 41« E) NSW,
Australia. The vines were 8-year-old ‘ Hayward ’ cultivar
planted in 4¬5 m spacing. Male vines (four–five unknown
cultivars) were planted in every third position of every third
row. Flowering commenced on 1 Nov. 1996, and 40 honey
bee colonies of Apis mellifera ligustica were introduced to
the orchard on 3 Nov. 1996 at approx. 5 % flowering level
(female flowers), which is normal management practice.
Controlled pollination
Over 100 flower buds from a single vine were bagged
1–2 d before anthesis, using brown paper bags. Ten flowers
were subjected to each of the six pollination treatments
described below. All pollinations were carried out with a
fine camel hair paint brush using freshly collected pollen
from a single male vine. The application was limited to a
single dab of the paint brush. The pollination treatments
were : (a) hand pollinated on two adjacent stigmas leaving
the rest of the stigmas unpollinated ; (b) hand pollinated as
above but on two stigmas at opposite ends of the flower ; the
rest of the stigmas were left unpollinated ; (c) hand pollinated
on two stigmas at opposite ends ; unpollinated styles were
removed from the base of the ovary using a pair of sharp
entomological scissors ; (d) hand pollinated on all stigmas ;
(e) open pollinated control ; (f) unpollinated control. All
pollinations were carried out on the day of bagging at
anthesis or 1 d after anthesis. Flowers were re-bagged
immediately after hand pollination and five flowers from
each treatment were sampled 3 d after pollination (3 d after
anthesis for open pollinated and unpollinated controls) for
processing and the rest were re-bagged. The bags were
removed after 15 d to assess fruit set. Fruit was harvested on
3 Apr. 1997 and fruit weight and seed distribution in cross
section were assessed. Photographs of fruit cross sections
taken at three levels were divided into four quarters and the
seeds in each quarter were counted to estimate the
distribution of seeds within the fruits produced by each
pollination treatment. These were statistically compared
pairwise using Student’s t-test.
In addition to the above flowers, ten more open pollinated
flowers from ten randomly selected vines were sampled 3 d
after anthesis (3 DAA) for processing to estimate pollen
germination on stigmas under field conditions.
Tissue preparation
Specimens were fixed in 3 % glutaraldehyde in 0±1 
phosphate buffer (pH 7±5) for 12 h at room temperature, or
in acetic : alcohol (acetic acid : ethanol, 1 : 3). After washing
the pistils in 0±1  phosphate buffer (pH 7±5), they were
dehydrated in an acetone series and embedded in Historesin2
(Reichert-Jung, Heidelberg).
Flowers used for sectioning were cut into convenient sizes
for ease of handling during fixation and embedding. Initially,
all anthers, petals and sepals were removed from flowers
and styles were cut from the base. Longitudinal sections of
the ovary were made in two planes at right angles to each
other as shown in Fig. 1 : (a) symmetrically along the mid
line of the ovary where all styles were arranged linearly (Fig.
1, a a ) ; (b) at right angles to the above (Fig. 1, b b ). Serial
" #
" #
sections, 2–3 µm thick, were taken throughout each ovary
and style and stained with periodic acid–Schiff’s reagent
(PAS) and toluidine blue O (TBO) (O’Brian and McCully,
1981) and observed using light microscopy.
Pistils used for fluorescence microscopy were cut into
approx. 1±0 mm thick sections along the same planes a a
" #
and b b (Fig. 1) prior to fixing in acetic : alcohol (1 : 3 v}v).
" #
The fixed material was then softened in 10 % sodium
sulphite for 40–60 min in a water bath at 60 °C, stained in
decolorized aniline blue (Dumas and Knox, 1983), squashed
gently on a glass slide and observed using fluorescence
microscopy (Olympus BX 60).
RESULTS
Anatomical features of the pistil
The free styles in female kiwifruit flowers are arranged in
two rows on the ovary (Figs 1, 3 A and B). The papillate
F. 2. Sections of the style 3 d after anthesis stained by PAS and
counterstained with toluidine blue, showing papillae (p), and transmitting tissues (t). Bars ¯ 150 µm. A, Transverse section of the style at
the distal end ; B, transverse section of the style at the base, showing the
closed transmitting channel.
Howpage et al.—Pollen Tube Distribution in Kiwifruit
699
F. 3. Light micrographs showing sections of the kiwifruit pistil at the ovary-stylar junction, stained with PAS. Transverse sections taken at the
base of the style and moving progressively towards the ovary (A–E). Longitudinal sections taken along two planes at right angles to each other
(F–G). The cup (c) is described in the text as the pollen tube distributor cup (PTDC). A, Transverse section of kiwifruit pistil at the ovary–stylar
junction where the base of the style (st) is closed and heavily stained with PAS. Bar ¯ 350 µm ; B, transverse section of kiwifruit pistil taken from
just below the ovary–stylar junction where the base of the style (st) opens into the free space. Bar ¯ 350 µm ; C, transverse section of kiwifruit
pistil taken from above the upper carpellary level showing the widening free space to form the cup (c) and carpels (ca). Bar ¯ 350 µm. s,
Secretions. D, Transverse section of kiwifruit pistil taken at the cup and carpellary level, showing the cup (c) with secretions (s) and ovules (ov).
Bar ¯ 350 µm ; E, transverse section of kiwifruit pistil taken from a lower carpellary level ; note the reduced cup size (c) and ovules (ov).
Bar ¯ 350 µm. F, Longitudinal section taken at the axis, a a of the pistil (Fig. 1), showing cup (c) and its association with a carpel. Bar ¯ 350 µm.
" #
S, Secretions. G, Longitudinal section taken at the axis, b b of the pistil (Fig. 1), showing the cup (c) with secretions (s) and its association with
" #
half of the styles. Bar ¯ 350 µm.
700
Howpage et al.—Pollen Tube Distribution in Kiwifruit
stigmas are seen at the free distal ends of the style. Further
towards the base of the styles, the carpels are longitudinally
fused to enclose the transmitting tract for pollen tubes. This
is seen in the transverse sections at the distal and the
proximal ends of the style (Fig. 2 A and B).
The multicarpellary ovary in kiwifruit is syncarpous only
at the ovarian level, hence the free styles at the distal end.
Each carpel bears two rows of ten–20 anatropous ovules
along either side of the carpellary sutures. The fusion of the
longitudinally-folded carpel bases to form the multicarpellary ovary has left a central column of parenchymatous tissue, termed the ‘ columella ’ (Schmid, 1978). In
the ovary, the ovules radiate outwards from the columella
into the carpellary cavity of the ovary. At the top of the
columella, just below the free stylar zone, there is a
prominent cup shaped depression which forms a cavity (Fig.
3 A, F and G).
The transmitting tract, which is enclosed by carpellary
tissue near the base of the styles (Fig. 3 B–D), opens into this
cavity which we describe as the pollen tube distributor cup
(PTDC) (Figs 3 C, D, F, G, 4 A, C, D, and 5).
Pollen tube pathway
The pollen tube pathway is clearly supplied with copious
amounts of secretions containing pectinaceous polysaccharides as revealed by PAS and toluidine blue staining.
Upon pollination, the pollen tubes grow through the
transmitting tract as seen in TS of styles (Figs 3 A, B, C and
4 B). In the upper region of the ovary, the individual
transmitting tracts open into one large space which is
continuous with the ovary cavity (Fig. 3 C, F and G). Once
the tubes leave the individual transmitting tracts, they then
enter this PTDC (Fig. 4 A, C and D) before growing over
the rim to enter the carpellary cavity, towards the ovules.
This is seen clearly in hand pollination experiments with
limited amounts of pollen (Fig. 5). In normal open pollinated
situations, the entire PTDC is filled with growing pollen
tubes (Fig. 4 A, C and D).
No matter whether adjacent or opposite stigmas were
pollinated, pollen tubes reaching the ovary remained evenly
distributed around the ovary. In the treatment in which
unpollinated stigmas were removed (treatment C), pollen
tubes entered the PTDC (Fig. 5) and from there they were
distributed evenly around the ovary and entered individual
ovules effecting fertilization (Fig. 4 D and E). The PTDC is
also heavily lined with secretory material containing
pectinaceous polysaccharides as revealed by PAS and TBO
staining (Fig. 3 C, D and G). This suggested functional
feature of the PTDC is further confirmed by the seed
distribution of fruits produced in our hand pollination
experiments. Student’s t-test applied to seed data from
different fruit quarters showed no significant departure
(P " 0±31) from those of any other quarter (Table 3).
Pollen germination and fruit}seed set
Squashed preparations of open pollinated styles collected
3 DAA showed that the deposition and subsequent ger-
mination of pollen between stigmatic surfaces within the
same flower was uneven (Table 1). Some stigmas had high
numbers of pollen grains germinating—up to 300—while
others had none.
All pollination treatments, including open pollination,
produced 100 % fruit set except in the unpollinated controls
where there was no fruit set. Although seed distribution was
unaffected by a low level of pollination, fruit size and weight
was affected (Table 2).
DISCUSSION
Our results show the existence of a special cup-shaped
structure on the central axis of the ovary, the pollen tube
distributor cup, which appears to be involved in distributing
pollen tubes evenly around the ovary, even when pollination
is restricted to two stigmas. The significance of this cupshaped structure and its suggested function seems to be in
the even distribution of seeds around the fruit core. Whilst
observations of in ŠiŠo pollen tube growth and seed
distribution data seem to support this hypothesis, other
factors such as ovule signals and nutrient gradients may also
have an influence on the random seed distribution in
kiwifruit.
Pollination is an important pre-requisite for fertilization
and is also influential in maintaining fruit shape and
uniformity via its effect on seed production not only in
kiwifruit fruit (Lawes, Wooley and Lai, 1990) but also in
other fruits such as feijoa (Patterson, 1990), nashi fruit
(Rohitha and Klinac, 1990) and strawberry (Svensson,
1991). Whilst the biological significance of an evenly shaped
fruit is speculation at this stage, it has commercial
significance in that it guarantees a quality product in terms
of shape, even when pollination conditions are limiting.
This contrasts with observations made in some multi-seeded
fruit, e.g. Annona (Sanewski, 1988), where inadequate
pollination results in misshapen fruit.
Honey bees play a major role in kiwifruit pollination.
Under orchard conditions, the low attractiveness of the
flowers to bees (Jay and Jay, 1984), low levels of pollen
production in the afternoon (Goodwin, 1995), low levels of
bee cross over (Goodwin and Steven, 1993), less pollen
carryover in bee corbiculae (Goodwin and Perry, 1992) and
the release of pollen in clumps (McKay, 1976) have
contributed to uneven and inadequate pollination of
stigmatic surfaces. Theoretically, a kiwifruit flower needs at
least 50 pollen grains per style to produce a fruit with 1400
seeds, provided each stigma receives an equal amount of
pollen. However, this is not always the case. We have
observed hundreds of pollen tubes growing through the
transmitting tract within a single style (Fig. 4 B), while other
styles had none. In such cases, seed distribution is even and
the fruit shape regular indicating that there is a mechanism
to compensate for any uneven pollination by ensuring an
even distribution of seeds. This mechanism is also likely to
be advantageous to the developing seed ensuring maximum
spacing is available to each. The kiwifruit system contrasts
with that described for maize by Heslop-Harrison, HeslopHarrison and Reger (1985) in an elegant series of experiments in which pollination of a sector of the maize silk
Howpage et al.—Pollen Tube Distribution in Kiwifruit
701
F. 4. Fluorescence micrographs of open pollinated kiwifruit pistils showing the pollen tube pathway (stained with decolorized aniline blue). A,
LS view at the a a (Fig. 1) axis of the pistil ; pollen tubes compacted in the cup (c) ; the cup is described in the text as the pollen tube distributor
" #
cup (PTDC). Bar ¯ 150 µm ; B, hundreds of pollen tubes (pt) passing through the transmitting tissue of the style (st). Bar ¯ 150 µm ; C,
longitudinal section along the symmetrical axis b b (Fig. 1), showing pollen tubes compacted within the cup (c). Bar ¯ 150 µm ; D, pollen tubes
" #
that come out of the cup have reached the ovules. Bar ¯ 150 µm ; E, a pollen tube (pt) entering an ovule (ov) through the micropyle (m) ; note
the funicle (f). Bar ¯ 70 µm.
702
Howpage et al.—Pollen Tube Distribution in Kiwifruit
F. 5. Fluorescence micrograph of the ovary–stylar region of a kiwifruit pistil where only two stigmas at opposite ends were pollinated ; the few
pollen tubes have reached the cup (c) and from there are distributed towards the ovary ; some pollen tubes (pt) have reached the ovules (ov) ; the
cup (c) is described in the text as the pollen tube distributor cup (PTDC). Bar ¯ 150 µm.
T     1. Number of pollen grains germinating on stigmas of
open pollinated kiwifruit flowers
Flower
no.
Mean³s.e.
(n ¯ 10)
Minimum
Maximum
1
2
3
4
5
6
7
8
9
10
28±1³9±9
42±7³21±5
4±7³0±8
47±7³6±7
66±6³27±1
12±8³3±7
88±1³29±2
35±4³17±7
10±2³5±3
15±9³6±4
0
3
2
9
2
2
1
0
0
0
90
230
11
81
250
38
300
180
55
64
T     3. Mean seed number counted in each quarter of fruit
TS taken from the top, middle and bottom of each fruit
obtained from the different pollination treatments
Mean seed number counted in each
quarter of the fruit from three layers
of the cross section
Pollination
treatment
Adjacent stigma
Opposite stigma
Pruned stigma
Open pollination
1st
quarter
2nd
quarter
3rd
quarter
4th
quarter
6±6
4±6
4±6
6±6
4±3
3±0
3±6
6±3
5±3
2±6
2±3
5±0
7±0
3±6
3±3
4±6
Pairwise comparison of seeds in each quarter for each treatment
using Student’s t-test did not show any significant deviation from those
of any other quarter (P " 0±31).
T     2. Mean fruit weight (g) resulting from different
pollination treatments
Pollination
treatment
Mean fruit
weight³s.e.
1. Adjacent stigmas
2. Opposite stigmas
3. Opposite stigmas
others removed
4. Open pollination
51±2³11±7
53±25³10±0
43±2³8±0
91±6³4±7
Treatment 1, two adjacent stigmas were pollinated while the rest of
the stigmas received no pollen ; treatment 2, two stigmas at the opposite
ends of the flower were pollinated while the rest of the stigmas received
no pollen ; treatment 3, as for treatment 2 except that the unpollinated
styles were cut off ; treatment 4, all stigmas were pollinated ; an
unpollinated control (data not shown) had no fruits.
(stigma) led to fertilization of only that sector of the cob. In
kiwifruit, this may have a special adaptive significance not
only towards pollinating agents but also towards the seed
dispersing agents in its native habitat, China.
This special structure, the PTDC, therefore, adds to the
other floral features such as abundant flowering, multibranched stigmatic surfaces and copious secretions in the
ovary system (Gonza! lez et al., 1996) that contribute towards
the reproductive success of kiwifruit. Our study also
confirms the presence of copious, pectinaceous secretions in
the ovary system as localized by PAS and TBO staining in
the stylar transmitting tract, the PTDC and the base of the
ovules. Such secretions are found along the pollen tube
Howpage et al.—Pollen Tube Distribution in Kiwifruit
pathway of most angiosperms (Knox, 1984) in varying
amounts, as revealed by staining.
Low levels of pollination followed by fertilization usually
produce smaller fruit, a result confirmed by our study as
well as those of others (McKay, 1976 ; Pyke and Alspach,
1986). In our experiments hand pollinations were carried
out with only a single dab of the paint brush to ensure
uniform transfer of a small quantity of pollen. However,
this did not restrict the seed development to only one sector
as observed in other fruit, thus confirming our earlier
hypothesis. Whilst wind pollination would allow an even
distribution of pollen on the stigmas, which in turn would
lead to an even fruit, the mechanism in kiwifruit seems to
achieve the same result even if an insect pollinates only one
part of the style. This can be considered a true adaptation of
this flower for pollination by insects such as honey bees.
A C K N O W L E D G E M E N TS
We thank Ms Arkey James for providing access to the study
property. We also thank the University of Western Sydney
(Hawkesbury) for funding this research and Mr Greg
Turnbull for taking photographs of plates. We also thank
Mr V. Premajayanthe for statistical advice and Ms Barbara
May for drawing Fig. 1. D. Howpage was funded by a
Hawkesbury Postgraduate Award.
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