Real-time Analysis of Photoassimilate Translocation in Intact Eggplant Fruit using 11CO2 and a Positron-emitting Tracer Imaging System
- ISSN: 1882336X
- DOI: 10.2503/jjshs1.77.199
A recently developed positron-emitting tracer imaging system (PETIS) noninvasively produces quantitative real-time images of the movement of various compounds in plants. To clarify the mechanism of the growth and development of the fruit of eggplant (Solanum melongena L.), we fed 11 CO2 to a leaf and monitored the translocation of 11C-labeled photoassimilate into the fruit by PETIS. Continuous images of the translocation of (11Cphotoassimilate from the leaf to the fruit and inside the fruit were produced, with a shorter time resolution than reported previously. 11C signal intensity in the fruit increased gradually, and its distribution was not uniform. The fruits were sliced after PETIS measurement and exposed to the imaging plates of a bio-imaging analyzer system. The resultant images showed the localization of11C activity inside the fruit, and indicated that translocation of photo assimilate within the fruit was not uniform. The velocity of photo assimilate translocation in the peduncle and changes in the rate of translocation of photo assimilate in the fruit were estimated by analysis of PET IS data. The velocity of photo assimilate translocation through the peduncle was estimated as 1.17 cm'min -1. About 60 min after the start of 11CO2 feeding, the 11C activity ofthe fruit began to increase, and by 120 min it had reached about 8% of feeding 11 CO2. These results indicate that it took about 60 min for the first (11Cphotoassimilates to reach the fruit. Real time observation of photo assimilate translocation within a fruit has never been reported. PETIS may be a powerful tool for revealing the mechanisms of fruit development and maturity.
Real-time Analysis of Photoassimi...
J. Japan. Soc. Hort. Sci. 77 (2): 199–205. 2008.
Available online at www.jstage.jst.go.jp/browse/jjshs1
JSHS © 2008
Real-time Analysis of Photoassimilate Translocation in Intact Eggplant Fruit
and a Positron-emitting Tracer Imaging System
Kaori Kikuchi1*, Satomi Ishii2, Shu Fujimaki2, Nobuo Suzui2, Shinpei Matsuhashi2,
Ichiro Honda1, Yoshihiro Shishido3 and Naoki Kawachi2
1National Institute of Vegetable and Tea Science, National Agriculture and Food Research Organization, Kusawa, Ano, Tsu 514–
2Japan Atomic Energy Agency, Watanuki, Takasaki 370–1292, Japan
3MKV Platech Co. Ltd., Shiba, Minato-ku, Tokyo 108–0014 Japan
A recently developed positron-emitting tracer imaging system (PETIS) noninvasively produces quantitative real-
time images of the movement of various compounds in plants. To clarify the mechanism of the growth and
development of the fruit of eggplant (Solanum melongena L.), we fed 11CO
to a leaf and monitored the
translocation of 11C-labeled photoassimilate into the fruit by PETIS. Continuous images of the translocation of
[11C]photoassimilate from the leaf to the fruit and inside the fruit were produced, with a shorter time resolution
than reported previously. 11C signal intensity in the fruit increased gradually, and its distribution was not uniform.
The fruits were sliced after PETIS measurement and exposed to the imaging plates of a bio-imaging analyzer
system. The resultant images showed the localization of 11C activity inside the fruit, and indicated that translocation
of photoassimilate within the fruit was not uniform. The velocity of photoassimilate translocation in the peduncle
and changes in the rate of translocation of photoassimilate in the fruit were estimated by analysis of PETIS data.
The velocity of photoassimilate translocation through the peduncle was estimated as 1.17 cm·min−1. About 60 min
after the start of 11CO
feeding, the 11C activity of the fruit began to increase, and by 120 min it had reached about
8% of feeding 11CO
. These results indicate that it took about 60 min for the first [11C]photoassimilates to reach
the fruit. Real time observation of photoassimilate translocation within a fruit has never been reported. PETIS
may be a powerful tool for revealing the mechanisms of fruit development and maturity.
Key Words: carbon-11, fruit, imaging, PETIS, photoassimilate.
To clarify the mechanism of the growth and
development of fruits such as eggplant and tomato in
relation to plant growth and yield, examination of the
import of dry matter into fruits from other organs such
as leaves is essential. To investigate this important
physiological event, which is associated with the so-
called source-sink relationship, various techniques that
use radio-tracers to measure photosynthesis and the
translocation and distribution of photosynthates have
Radioactive compounds containing 14C have often
been used for such studies. Shishido et al. (1988)
investigated the relationship between vascular architec-
ture and photosynthate translocation in tomato by using
14C. Furthermore, the carbon balance of a single leaf can
be obtained by 14CO2 steady-state feeding methods,
which enable the rates of photosynthesis, translocation,
and respiration to be quantified (Shishido et al., 1987,
1989). The stable isotope 13C has been used similarly
(Bledsoe and Orians, 2006). As 14C has a long half-life
(ca. 5700 years), and 13C is stable, both of these methods
are suitable for experiments in which there are long
periods of observation, but they are invasive and require
the breakdown or extraction of plant tissues.
Troughton et al. (1974) and Moorby et al. (1974)
established a basic system to observe carbon transport
in intact plants by using 11C as another radioactive
nuclide. Radioactive 11C, generally produced by a
cyclotron, decays quickly and emits positrons, which are
bombarded with electrons to produce high-energy
annihilation γ-rays that can easily penetrate intact tissues.
Received; July 23, 2007. Accepted; October 11, 2007.
* Corresponding author (E-mail: firstname.lastname@example.org).
Radiation from 11C can be detected quantitatively in
intact plants without the need to break up or cut plant
tissues. Moreover, repeated analyses of one plant are
possible, as 11C has a short half-life (20 min). By using
this system, the above-mentioned authors estimated the
translocation velocity of 11C in various plants; however,
there are still a limited number of points at which γ-rays
could be detected at one time, and translocation and
distribution in each tissue could not be monitored in
Recently, a positron-emitting tracer imaging system
(PETIS) that observes annihilation γ-rays produced from
positron-emitting radionuclides was developed (Uchida
et al., 2004). PETIS can obtain two-dimensional serial
images of the distribution of tracers in real time with a
wide view-range and high resolution. As PETIS can
observe the dynamics of tracers which emit γ-rays from
positron-emitting radionuclides, it can be useful in
various plant physiological and agricultural studies for
examining the distribution and translocation of nutrients
and metabolites. Various important compounds that
contain positron-emitting nuclides (e.g., major nutrients,
water, amino acids, glucose, minor elements, and
environmental pollutants) can be synthesized routinely
and safely after bombardment of the target elements with
the accelerated beam from a cyclotron (Ishioka et al.,
1999; Kume et al., 1997). By using PETIS and 11C, it
recently became possible to observe the translocation of
photosynthate in intact plants quantitatively and
noninvasively (Kawachi et al., 2006; Matsuhashi et al.,
Here, to clarify the enlargement mechanism of
eggplant fruit, we used 11CO2 and PETIS to visualize
photoassimilate translocation to, and distribution in the
fruit. After completion of the PETIS measurements, the
fruit was sliced and the distribution of radioactivity
analyzed by autoradiography to fill in the gaps on the
two-dimensional PETIS images.
Materials and Methods
Seeds of eggplant (Solanum melongena L.) ‘Nakate
Shin Kuro’ and the line AE-P03, which were bred at the
National Institute of Vegetable and Tea Science, Japan,
were sown in 14 cell plug trays (5 cm × 5 cm; Takii,
Kyoto, Japan) in a commercial soil mixture (Kureha
Chemicals, Tokyo, Japan). Several of the seedlings were
subsequently transplanted into 10.5-cm-diameter plastic
pots filled with a standard growth medium. They were
then grown in growth chambers under a controlled
temperature (28/18°C, day/night) and a 14-h light
photoperiod. All plants were pruned to one main stem
and staked, and then the first flower was removed. The
second flower was hand pollinated. About 2 weeks later,
fruit of average growth were selected and subjected to
the PETIS experiment.
Production of the 11CO
11CO2 (half-life 20.39 min) was produced from the
(p,α) 11C reaction by bombarding nitrogen gas with an
energetic proton beam delivered from a cyclotron,
located at Takasaki Ion Accelerators for Advanced
Radiation Application, Japan Atomic Energy Agency
(Ishioka et al., 1999). The 11CO2 gas intensity was
100 MBq when PETIS began acquiring images.
Synthesized 11CO2 was collected in a CO2 trap and
transferred to a gas circulation system.
Outline of PETIS
PETIS is a system for obtaining images of the
distribution and long-distance transport of radioactivity
both quantitatively and non-invasively by using
radionuclide-labeled tracers. The principle of the system
is described below.
The positron-emitting nuclide decays and emits
positrons (β+ decay). The positron undergoes annihila-
tion immediately after collision with an electron, and
two γ-rays, each with an energy of 511 keV, are emitted
in opposite directions. We used the positron-emitting
nuclide 11CO2. When absorbed by the test plant body,
11C begins β+ decay, and soon after, the positrons undergo
annihilation through collision with abundant electrons
in the plant tissue. As a result, a set of two γ-rays is
emitted in opposite directions from the tissue, passing
through the tissue, organs, and plant body so it can be
detected outside the plant.
The PETIS scanner has two opposing planar detectors
consisting of Bi4Ge3O12 scintillator arrays that detect
γ-rays (Nagai et al., 1996; Uchida et al., 2004). When
each head of the detector detects an annihilation γ-ray
at the same moment (within 20 ns), the emission point
is determined as the point mid-way between the two
detected points. The coincidence detection is consecu-
tively repeated and the plot of the determined emission
points reconstructs a static image of the tracer
distribution. These events are recorded on a personal
computer and converted to coordinate data in the focal
plane to produce a two-dimensional image. In PETIS
experiments with plants, it usually takes 10 s to obtain
one static image of sufficient quality. This unit period
for imaging is called a “frame”, and imaging is continued
to make a serial image. The typical size of the field of
view is 12 cm wide by 19 cm high, and the spatial
resolution is approximately 0.2 cm.
The major advantages of using this system are that
high-energy annihilation γ-rays from the positron-
emitting nuclide can be quantified non-invasively in vivo.
Setting and procedure of PETIS measurement
The test plant was placed so that the fruit was located
in the center, between the PETIS detectors (center of
the field of view) (Fig. 1a). The PETIS was installed in
a growth chamber with a controlled environment
(temperature 28°C; relative humidity of 65%). Light
intensity was measured with a light meter (model
LI-189, LI-COR, Inc., Nebraska, USA) and maintained
at a photon flux density of approximately 150
The study protocol was as follows (Fig. 1b). The
seventh leaf of the eggplant was covered with a gas cell
made from transparent acrylic plates 20 cm (inside
length) × 15 cm (width) × 1 cm (depth). 11CO2 gas was
fed in by a gas-circulation system consisting of pumps,
mass flow controllers, gas valves, the gas cell, and a
CO2 trap. First, room air was fed into the cell. Soon
after the PETIS imaging was started, 11CO2 was
introduced from the CO2 trap into the gas cell, along
with room air. The 11CO2 gas and ambient air
were introduced into the cell and maintained at 200
mL·min−1. PETIS images were acquired every 10 s for
180 min. The decay of radioactivity within the field of
view was calibrated automatically for the half-life of 11C
(20.39 min) and the image was displayed on a monitor.
PETIS images were constructed by positioning the
positron annihilation data on the focal plane with
coincidence lines. Sensitivity corrections were per-
formed for all constructed images by using data from a
uniform plane phantom filled with 18F− as a calibrator.
The images were transferred to a second personal
computer for analysis with NIH Image J 1.37 software
(http://rsb.info.nih.gov/ij/, June 30, 2007). To analyze
the dynamics of photoassimilate translocation through
the peduncle and into the fruit, we drew three circular
regions of interest (ROIs) on the integrated PETIS image
(Fig. 4a). Two time-activity curves (TACs: C(t)) were
generated against ROI1 and ROI2, which were defined
in the peduncle. Transfer function analysis of both TACs
was performed according to Minchin and Troughton
(1980) and Minchin and Thorpe (1989). In this approach,
the following model was used for TACs of ROI2 [Cd(t)]
and ROI1 [Cu(t)]:
Cd(t) = a·Cd(t − ∆t) + b·Cu(t − d), (1)
Where ∆t is the time to image one frame, and a, b,
and d are parameters (0 < a < 1, 0 < b < 1; d is the delay
factor, a positive integer). The velocity of [11C]photo-
assimilate translocation was calculated with the distance
of the two ROIs and d estimated in equation (1). The
details of this method are described in Matsuhashi et al.
The accumulation of [11C]photoassimilates in the fruit
was analyzed with a TAC generated against ROI3, which
was defined in the fruit. As almost all 11CO2 activity
was absorbed by the leaf (as confirmed by a preliminary
experiment), the rate of export of [11C]photoassimilate
from the leaf was calculated as:
Percentage distribution of 11C to fruit =
(11C activity in fruit/total 11C activity in the supplied gas)
To obtain autoradiographic images, the fruits were cut
transversely in slices about 1.5 mm thick after the PETIS
measurement. The sliced fruits were exposed to the
imaging plates of a bio-imaging analyzer system (BAS-
MP2040S, Fujifilm, Tokyo, Japan) for 12 h. After
exposure, the plates were scanned with a bio-imaging
analyzer system (BAS-1500, Fujifilm).
Result and Discussion
Figure 2 gives examples of the test plants (a) and serial
PETIS images (b) every 12 min after 11CO2 feeding;
these images were generated from integration of the
corresponding images collected every 10 s. The
Fig. 1. Set-up of the PETIS experiment (a). The fruit was placed at
the center of the imaging region, between the two detectors. The
cell for feeding 11CO
to the leaf was set outside the region.
Study protocol (b).
advantage of PETIS measurement is that it enables the
visualization of translocation in the intact plant body.
[11C]photoassimilates began to be translocated down-
ward to the stem via the leaf petiole about 40 min after
feeding (Fig. 2b). The photoassimilates then moved
upward, reaching the edge of the fruit by about 70 min,
but were not translocated to the upper stem and leaves
above the fruit, probably because the sink represented
by the shoot above the fruit had been removed.
Furthermore, we successfully observed the movement
of [11C]photoassimilates inside the fruit. 11C signal
intensity in the fruit increased gradually, and its
distribution was not uniform. In one report of the14CO2
analysis of photoassimilate translocation in tomato, 90%
of 14C was transformed to sugar (Ho et al., 1983; Shishido
et al., 1990). Our results also suggested that 11CO2 is
converted into [11C]sugar and is translocated to fruits.
Hori and Shishido (1980) reported that the lower stem
and roots seemed to store assimilates temporarily and
then retranslocate them to the upper parts. We observed
a similar translocation pattern in eggplant. Although a
number of experiments have reported the translocation
of photoassimilates from source leaf to sink fruit
(Moorby et al., 1974; Rao, 1988), the translocation of
photoassimilates within the fruit has never been reported.
We examined photographs of sliced eggplant fruit
after PETIS measurement (Fig. 3a), together with BAS
images (Fig. 3b), and superimposed images (Fig. 3c) of
the photographs and BAS images. Traces of translocated
[11C]photoassimilates inside the fruit were clearly shown
on the BAS image. The images showed that the
translocation of [11C]photoassimilates was localized.
Fig. 2. A plant photograph in the experiment. In the PETIS experiment, the upper leaf attached to the same node as the leaf that received 11CO
was set outside the imaging region but the petiole was inside (a). Serial PETIS images of translocation of [11C]photoassimilates. Images
were continuously acquired every 10 s for 3 h. Each image represents the integration of 72 serial images (b).
Similar PETIS and BAS images were obtained in other
independent experiments using other plants (data not
Shishido et al. (1988, 1999) reported in tomato that
leaf position determines which source leaf will contribute
to the growth of each particular sink. They suggested
that phyllotaxis and the arrangement of the vascular
system were related to the distribution of photoassimi-
lates. This finding has also been reported in beans
(Biddulph et al., 1958; Nelson and Gorham, 1957),
tobacco (Shiroya et al., 1961; Jones et al., 1959), beets
(Joy, 1964), and osier (Ho and Peel, 1969). Data
indicating photoassimilate localization inside the
eggplant fruit clearly demonstrated that photoassimilate
translocation was related to phyllotaxis and the
arrangement of the vascular system. Knowledge of the
distribution pattern of photoassimilates inside the fruit
is important for our understanding of the morphogenesis,
development, and maturation of fruit. The production of
malformed fruits, such as those that are puffy or curved
or that show defective development, is a very serious
problem that reduces fruit yield and quality (Hosoki et
al., 1985; Northmann and Koller, 1975; Northmann et
al., 1979). Thus, PETIS may be a powerful tool for
revealing the mechanisms of fruit malformation in future
Furthermore, PETIS images enable us to conduct
kinetic analyses of [11C]photoassimilate translocation in
the peduncle and export into the fruit. Figure 4b shows
the TACs of ROI1 and ROI2, set for photoassimilate
transfer analysis through the peduncle. Using both TACs,
the velocity of photoassimilate translocation through the
peduncle of the eggplant was estimated to be
approximately 1.17 cm·min−1. Moorby et al. (1974)
estimated the velocity of translocation through the petiole
of tomato plants by using 11CO2 detected with a
scintillation counter; their value was 0.95 to
1.13 cm·min−1. Our estimated value was therefore almost
We determined the rate of export of [11C]photoassim-
ilates from the fed leaf to the fruit (Fig. 4c). About 60 min
after 11CO2 feeding, the
11C activity of the fruit began
to increase, and by 120 min after feeding it had reached
about 8%. These results indicated that it took about
60 min for the first [11C]photoassimilates to reach the
fruit. Walker and Ho (1977a, b) estimated the ratio of
carbon export from leaf to fruit in tomato, expressed as
mg14C·h−1 over a 48-h period. As they did not use real-
time measurement, the time of arrival of assimilates from
the leaf to the fruit was not estimated. Rao (1988)
reported that when [14C]sucrose was supplied to the fifth
leaf of eggplant, about 31.6% of the radiocarbon was
present in the fruit 48 h after feeding. Three weeks after
feeding, 48.5% of the activity present at 48 h remained
in the fruit. The arrival times and initial export rates of
photoassimilates estimated here have never been
reported. As they were based on only one experiment,
repetitive analyses will be needed to determine precise
values. Nevertheless, we have demonstrated the
advantage of the PETIS experiment: if we can obtain
data successfully, then we can set ROIs to any preferred
position and can calculate TACs and various values
Many researchers have suggested that photoassimilate
translocation and accumulation are readily affected by
environmental conditions, injury, stress, and growth
regulators (Shishido and Hori, 1989; Suwa et al., 2006;
Walker and Ho, 1978). Also, virtually all aspects of
photosynthesis-related changes in metabolite levels are
affected by light or the diurnal regulation of respiratory
metabolism (Kumudini, 2004; Scheidegger and
Nosberge, 1984). Shishido et al. (1990) reported that the
rate of starch accumulation varied inconsistently with
the light period, and starch and sugars accumulated in
leaves at the end of the night with longer periods of
light. In one experiment using 14CO2, [
ilate translocation was generally measured over one day,
and diurnal change was not examined (Walker and Ho,
1977a, b). With PETIS measurements, we can observe
short-term phenomena such as translocation and the
diurnal patterns of photoassimilate movement, as well
as the effects of external stimuli.
PETIS measurement enables us to clarify the precise
timing of individual events visually. Here, we
demonstrated that; 1, it was possible to visualize the
translocation of photoassimilate inside fruit; 2, the
Fig. 3. Photographs (a) and BAS (bio-imaging analyzer system) images (b) of sliced eggplant fruit after PETIS measurement. Images (a) and
(b) were superimposed using an image analyzer (c).
distribution of photoassimilates inside fruit was not
uniform; and 3, the translocation velocity of photoas-
similates through the peduncle and their export rate to
fruit, as well as their arrival time, were easily calculable.
These results clearly showed the considerable advantages
of PETIS analysis in increasing our understanding of
many of the physiological processes involved in fruit
We gratefully acknowledge Mr. H. Suto and all the
staff at Takasaki Ion Accelerators for Advanced
Radiation Application, Japan Atomic Energy Agency,
for providing the cyclotron and the radiotracer
synthesizer. We also thank the staff of the plant positron
imaging group and Dr. Ishida of the National Food
Research Institute for their fruitful discussions.
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