Plant Physiol. (1989) 91, 291-297
0032-0889/89/91/0291 /07/$01 .00/0
Received for publication January 20, 1989
and in revised form April 13, 1989
Carbon Partitioning among Leaves, Fruits, and Seeds
during Development of Phaseolus vulgaris L.'
Donald R. Geiger*, Wen-Jang Shieh, and Rose M. Saluke
Department of Biology, University of Dayton, Dayton, Ohio 45469-0001
ABSTRACT
ance along the entry-path sieve tubes, and in physiological
processes, such as sucrose uptake, conversion of sucrose, and
use of the products in synthesis (1 1, 21).
A basic premise of this study is that changes in rates of
accumulation of carbon imported by individual fruits often
are caused by changes in rates of processes that sustain import,
such as uptake, degradation, and metabolism of the imported
sucrose. At the plant level, a new pattern of carbon partitioning contributed to the marked increase in overall fruit growth
that was observed about 10 d AA2 in Phaseolus vulgaris L.
(8). The growth rate of individual seeds followed a predictable
pattern during its development (3, 14). These times of faster
or slower import and growth provide opportunities for identifying mechanisms that sustain import into developing fruits
Development of vegetative and floral buds was found to be a
key factor in establishing the way carbon is distributed among
growing leaves and fruits in Phaseolus vulgaris L. plants. Leaves
emerged principally during a period 14 to 32 days after planting
while flowers were produced during a 10- to 12-day period near
the end of leaf emergence. Timing of anthesis established the
sigmoidal time course for dry weight accumulated by the composite of all fruits on the plant. During the first 12 days following
anthesis, fruit growth mainly consisted of elongation and dry
weight accumulation by the pod wall. Thereafter, seed dry weight
increased for about 1 week, decreased markedly for several
days, and then increased again over the next 2 weeks. Accumulation of imported carbon in individual seeds, measured by
steady-state labeling, confirmed the time course for dry weight
accumulation observed during seed development. Seed respiration rate initially increased rapidly along with dry weight and then
remained nearly steady until seed maturation. A number of developmental events described in the literature coincided with the
different phases of diauxic growth. The results demonstrated the
feasibility of relating current rates of carbon import in individual
seeds measured with tracer 14C to the rates of conversion of
imported sucrose and use of the products for specific developmental processes. The resulting data are useful for evaluating
the roles of conversion and utilization of imported sucrose in
regulating import by developing seeds.
(5, 6).
A major goal of the present study was to identify times
during individual fruit development for studying how gene
activation and changes in activity of certain enzymes may
sustain import and regulate partitioning of carbon. Time
courses for rates of carbon import, dry weight accumulation,
and oxygen uptake in individual seeds showed the importance
of developmental events for overall control of carbon partitioning in the plant. The sum of the import rate for a specific
seed obtained from radiotracer data (8) and the respiration
rate provided a measure of current carbon import rate for a
specific seed. Joined with data on physiology or developmental processes, the import rate promises to provide a means
for studying the molecular basis for carbon partitioning in
individual organs within the context of the whole plant (5, 6).
Translocation and partitioning of assimilated carbon are
major determinants of plant productivity and crop yield (9).
Productivity is affected both by distribution of translocated
carbon to new leaf growth and by accumulation of carbon in
the harvestable organs. The former affects light interception
for biomass production while the latter affects harvest index.
As a consequence, increased plant yield requires a balanced
increase in the source carbon that is partitioned to these two
sites (7). In practice, higher yields of economically important
crops are the result of both greater dry-matter production in
leaves as well as increased carbon accumulation in harvestable
organs ( 11).
Understanding regulation of carbon partitioning among
sinks and its relation to plant growth and development is
important for establishing strategies for increasing plant productivity and yield. Regulation of import and partitioning
occurs by changes both in physical factors, such as conduct'Supported by grant DMB-8303957 from National Science Foun-
MATERIALS AND METHODS
Plant Material
Bean plants, Phaseolus vulgaris L., cv Black Valentine, a
determinate variety, were grown hydroponically in 3 x 103
cm3 containers under a 14-h photoperiod as described previously (8). Irradiance was 0.75 mmol quanta m-2 s-' at canopy
level. Initial anthesis occurred approximately 28 d AP.
Measuring Leaf and Fruit Development for an Individual
Plant
Time of flower production was calculated from the day of
anthesis. Leaf production was based on leaf emergence reported as days AP when the terminal leaflet of the leaf reached
a length of 3 cm. Areas of leaves and dry weight of fruits and
seeds were measured nondestructively for four plants at 2- to
4-d intervals for over 2 months.
dation and grant USDA 87-CRCR- 1-2486 from the U.S. Department
of Agriculture.
2
Abbreviations: AA, after anthesis; AP, after planting.
291
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292
GEIGER ET AL.
Plant leaf area was determined at intervals by multiplying
the area of the three circumscribing rectangles for the leaflets
of each leaf by an empirically determined factor. The latter
was calculated from data that were obtained by sampling a
large number of leaves of different sizes from other plants
growing with the four used for continuous growth measurements. The factor was computed as the ratio of the area of an
individual leaf, measured by optical planimeter, and the sum
of the areas of the rectangles circumscribing the three leaflets.
Because leaf shape in the various plants was quite similar, the
value of 0.57 was able to be used for all the conversions.
For calculating overall accumulation of dry weight by the
fruits on a plant, the length, width, and thickness of individual
fruits were measured and plotted against dry weight. Empirical
formulas for computing fruit dry weight from fruit dimensions
and final weight were based on a linear regression model
established by sampling several hundred fruits of different
sizes. Measured dimensions of the fruit were used to calculate
estimated fruit dry weight for days 1 to 15 after anthesis. For
days 16 to 35 AA, the value for the final dry weight of a fruit
was included with the three dimensions to calculate estimated
dry weight. Data from other plants, harvested at four times
including at the start of flowering, and early and late in fruit
development, were found to substantiate the accuracy of the
leaf area and fruit dry weight curves. While the ranges of
values for cumulative leaf area and for fruit dry weight were
up to 25% of the average, the rank order of the values from
the four plants was the same for each measurement time. This
uniformity resulted in a family of four parallel time course
curves of similar shape.
Carbon content was calculated from dry weight measurements on the assumption that 40% of the dry weight is carbon.
Measurements of carbon in representative samples of fruits
and leaves gave values of 36% to 42% (data not shown).
Measuring Development of Individual Fruits
The time courses for pod wall, seed, and total fruit growth
in individual bean fruits were obtained by collecting fruits
from plants at 10-d intervals starting near the end of the
flowering period, 40 d AA, when fruits 0 to 10 d AA were
present. Starting at about 10 d AA, fruits were able to be
separated into seeds and pods, and fresh and dry weight
were obtained for the fruit and its individual seeds. Fruits
with fewer than five seeds were excluded from the time
course because their fruits were smaller throughout their
development.
Measuring Carbon Accumulation by Steady-State
Labeling
Accumulation of recently assimilated carbon imported by
parts of the plant during the 24-h experimental period was
determined by measuring the tracer carbon present in these
parts following steady-state labeling (8). Tracer carbon is the
14C and associated 2C that were assimilated during the 14-h
labeling period. To make these measurements, the entire plant
shoot was placed in an enclosure in which the atmospheric
CO2 was maintained at a constant concentration throughout
the light period by supplying '4C02 with a specific radioactivity of approximately 20 kBq mg-' C. Flux of CO2 into the
Plant Physiol. Vol. 91,1989
atmosphere of the labeling apparatus was measured by a mass
flow controller (model FC 260, Tylan, Carson, CA) that also
regulated the level of CO2 within a set range, based on data
from a nondispersive infrared CO2 analyzer (LIRA 3000,
MSA, Pittsburgh, PA). Relative humidity in the enclosed
atmosphere was regulated to maintain the desired dew point,
while fans provided mixing and reduced boundary layer resistance over the leaf surfaces.
For plant growth analysis, plants that had been kept in
labeled CO2 through the light period were freeze-dried,
weighed, and reduced to a fine powder. Weighed aliquots
were oxidized to measure radioactivity and carbon content
(8).
Respiration Rate of Seeds
Oxygen uptake was measured with a YSI model 53 oxygen
analyzer (Yellow Springs, OH). Seeds were collected just prior
to the measurement and placed in 4 mL of 200 mm sucrose
solution at 25C for the determination of oxygen uptake.
Tabulated values for oxygen solubility in water at the assay
temperature were used for calculating oxygen consumption
rates.
RESULTS
Plant Leaf Growth and Photosynthesis
Cumulative leaf area (Fig. 1B) and area growth rate (Fig.
lC) increased during the first 6 weeks of vegetative growth as
a result of continuing leaf initiation (Figs. IA and 2). Emergence of individual leaves followed a similar pattern in each
of the plants. Primary leaves emerged about 4 d AP and
trifoliate leaves emerged principally between 16 and 36 d AP,
with the highest frequency in the middle of this period (Fig.
1A). The enlargement rates of successive trifoliate leaves were
similar for the majority of leaves on a plant (Fig. 2), but the
rates slowed progressively for the last several leaves. There
was some variation in the total number of leaves and the time
of their emergence, and this contributed to the variation in
total leaf area among the four plants. From 12 to 19 trifoliate
leaves were present on a mature plant (Fig. 1 A), corresponding
to the low and the high range of leaf area values in Figure l B.
Leaf area increased until about 45 d AP and then decreased
as leaves began to senesce (Fig. 1 B).
Photosynthesis rate per unit leaf area, measured for the
entire canopy, was highest during the first 3 weeks AP and
gradually decreased as leaves aged, and mutual shading increased with increasing leaf area index (data not shown).
Plant Reproductive Growth
Anthesis started on day 26 to 30 in the four plants for
which data are presented (Fig. IA). The plants produced from
21 to 32 flowers, mostly over a 10- to 12-d period and had an
abortion rate of 20% to 30% (data not shown).
Total dry weight accumulation in the fruits began to increase markedly near the middle of the main flowering period
and reached its highest rate about 5 d later, that is, 2 weeks
after first anthesis (Fig. 1, B and C). The rate held nearly
steady for about 10 d and thereafter decreased gradually over
the next 3 weeks.
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CARBON PARTITIONING DURING DEVELOPMENT
293
E
F-
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C-
TIME (d)
Figure 1. Times of leaf and flower initiation and growth in overall leaf area and leaf and fruit dry weight in bean plants. A, Time of leaf emergence
(-) and anthesis (*) in days after planting for four plants. B, Dry weight accumulation in fruits (0, left ordinate) and growth in overall leaf area (A,
right ordinate). C, Rates of dry weight accumulation (left ordinate) for leaves (solid line) and for fruits (dashed line) obtained from the smoothed
curves in B. Fruit dry weight and leaf area were calculated from measurements of leaf and fruit dimensions at the times indicated by data points.
Averages and ranges are shown for data obtained from four plants. The decrease in area after 45 d was determined from the area lost when
leaves senesced. Leaf dry weight values were derived from leaf area values by means of specific leaf area measurements taken at the sampling
times. Carbon accumulation rates (right ordinate) were calculated from leaf dry weight.
Growth of Individual Fruits and Seeds
Accumulation of dry weight by individual bean fruits, their
pod walls, and enclosed seeds is shown in Figure 3. Fruits
began to grow in length soon after anthesis (Fig. 3B), and
appreciable dry weight began to accumulate several days
thereafter. Fruits with fewer than five seeds grew more slowly,
had a shorter final length (Fig. 3B), and accumulated less dry
weight. Nearly all fruits on the plants had five to seven seeds,
and these showed no clear relationship between seed number
and final length or dry weight accumulation. For this reason,
analysis of growth of individual fruits, pods, and seeds generally was limited to data from fruits with five or more seeds.
Although seed volume increased as fruits grew in length,
appreciable dry weight accumulation by individual seeds occurred only after the fruits stopped elongating, about 12 d AA
(Fig. 3, B and C). From about 19 to 22 d AA, there was a
temporary decrease in growth in dry weight accumulation by
the seeds, followed by an increase in the rate of dry weight
accumulation by fruits. Both pod walls and seeds lost dry
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GEIGER ET AL.
294
Plant Physiol. Vol. 91,1989
the transient decline in dry weight accumulation midway in
seed development.
E
(d
L6
L5L
L 12
4
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-j
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Fiu.e2 ntarwhi araofsceivlaesttemrd
L 13
o
20
30
40
50
60
70
TIME (d)
Figure
2.
Initial
growth
in area of successive leaves that
emerged
during the life of a bean plant. Individual data points are not shown.
The first line (Li) shows growth in area throughout the life of the leaf.
Subsequent lines are drawn through the initial two or three values
for leaf area taken at 12-h intervals. Measurement of each leaf was
started when it exceeded 3 cm. Data are from one of the plants from
Figure 1. Time shown is days after planting.
weight during the final period of maturation starting about
30 d AA. The timing of developmental events in individual
fruits is summarized in Figure 4.
Dry Weight Accumulation by Individual Seeds
Accumulation of tracer carbon by individual seeds (Fig. 5)
confirmed the pattern for the rate of dry weight accumulation
in seeds during their growth (Fig. 3C). Variation in import
rate among seeds of a given age, as measured by tracer carbon
accumulation, became quite large during the pause in growth
(Fig. 5).
Tracer carbon accumulation among the seeds in an individual fruit varied with seed dry weight (Fig. 6). The degree of
dependence, slope of the regression line, was similar for fruits
of the same age but changed with age. The correlation coefficients for the regression lines were in the range of 0.74 to
0.84.
Respiration of Individual Seeds during Development
Respiration per unit of dry weight, measured as oxygen
consumption, was highest in young seeds and decreased rapidly as their dry weight increased (Fig. 7). As a result, total
oxygen uptake per seed increased rapidly with initial dry
weight accumulation up to about 10% of final weight (Fig.
5). For the remainder of seed development, respiration in
individual seeds remained relatively steady. Respiratory carbon loss was calculated from oxygen consumption rate on the
assumption that the respiratory quotient was 1, in the absence
of measured values. It is unlikely that all ofthe carbon respired
was from the newly imported carbon and, for this reason, the
calculated rates of import likely are overestimates. There was
no marked change in respiration rate that corresponded to
DISCUSSION
Partitioning of Carbon between Vegetative and
Reproductive Growth
The relative frequency of development of vegetative and
floral buds during plant growth (Fig. IA) was consistent with
the pattern of partitioning ofcarbon between leaves and fruits
expected for a determinate variety of bean. Leaf initiation
and subsequent growth formed a positive feedback loop with
total leaf photosynthesis and resulted in a 6-week period of
rapid leaf growth (Fig. IC) and increasing canopy photosynthesis. The timing of anthesis of individual flowers (Fig. IA),
along with the relatively uniform time course for dry weight
accumulation in individual fruits (Fig. 3), produced the sigmoid time course for overall fruit dry weight accumulation
(Fig. 1 B). As a result, there was a marked increase in the
proportion of newly assimilated carbon partitioned to reproductive growth, beginning about 1 week after the start of
anthesis (8). The diversion of carbon to development ofyoung
fruits (Fig. 1 B) likely was a dominant factor in the cessation
of flowering and increased abortion of flowers, which were
observed late in the anthesis period in this determinate variety
(data not shown). It appears that growth of the individual
developing fruits was not altered to a noticeable extent by the
overall determinate development pattern of the plant.
Growth and Development of Individual Fruits and Seeds
Three periods of fruit growth were observed during the
development of individual fruits. During the first 12 d following anthesis, fruit grew rapidly, primarily by elongation and
by dry weight accumulation in the pod wall (Fig. 3, B and C).
Thereafter, fruit growth consisted mainly of the two phases of
diauxic seed growth (Fig. 3, A and C; ref. 3). During the first
period, seed growth appears to result mostly from growth and
dry weight accumulation by the seed coat (10, 12, 13, and
refs. in Fig. 4). Although the volume of a seed increases
rapidly soon after anthesis (12, 13, 22), the enclosed ovule
remains small during this time (17). The second phase of
rapid seed growth appears to occur mainly from accumulation
of dry weight in the cotyledons as starch and seed proteins
( 17). The cause for the period of slow growth that separated
the two periods of rapid growth is not known, although there
are a number of possibilities, such as transition from a period
of rapid cell division to a period of synthesis of compounds
that are stored in the developing cotyledons (4, 22, and refs.
in Fig. 4). It is likely that termination of the expression of
certain genes also is involved (15).
Import of Carbon by Seeds
The rate of tracer carbon gain by individual seeds over a
24-h period (Fig. 5) generally increased with seed age until
maturation and showed a time course similar to that for
gravimetrically measured dry weight accumulation in the
seeds contained in a single fruit (Fig. 3C). Import during the
same period is the sum of net carbon accumulated by a seed
plus its respiratory loss of carbon imported during the period.
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CARBON PARTITIONING DURING DEVELOPMENT
L
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Figure 3. Accumulation of dry weight by bean fruits, pod walls, and seeds following anthesis. A, Total dry weight accumulated by the five to
seven seeds contained in a fruit. B, Dry weight accumulation by individual fruits (U) and their pod walls (0) and growth in length (three solid lines)
of fruits with two, four, and five to seven seeds per fruit. C, Rate of dry weight (left ordinate) and carbon accumulation (right ordinate) in individual
pod walls (dashed line) and seeds (solid line) derived from smoothed curves in A and B. Data are from 165 fruits collected during the period from
shortly after anthesis to maturity. The average and range that are given for each point are generally derived from three to five fruits that contained
at least five seeds per fruit. Data were hand-fitted with a smooth curve. Time shown is days after anthesis.
The latter is not measured readily, but was estimated from
the rates of oxygen uptake (Fig. 7), which was relatively
constant throughout most of seed development. As a consequence of the latter, the ratio of carbon lost by respiration:imported carbon accumulated decreased with age. For
seeds 8 d AA, the ratio was about 2:3, and this value decreased
to less than 1:9 by 30 d AA. The pause in dry weight
accumulation midway in seed development appeared to result
from a marked slowing of import rather than a high rate of
carbon loss by respiration at that time (Fig. 7).
The rate of import into individual seeds in the same pod
varied, and these differences appeared to be a function of
their dry weight (Fig. 6). For reasons not known, the degree
of this dependence changed at different stages of seed development (Fig. 6). As seeds filled, specific import, the ratio of
carbon imported per day to the standing dry weight, decreased
from 0.075 per day at 8 d AA to 0.0045 per day near end of
seed growth (Fig. 5).
Studying Potential Mechanisms That Regulate Carbon
Import during Seed Growth
The contribution of a sink to keeping itself supplied with
organic solutes, sometimes termed potential sink strength
(1 1), includes physical factors and physiological processes that
are involved in import of assimilated carbon in the phloem
and its subsequent unloading from the sieve elements (5).
During at least part of bean seed development, solutes that
are imported through sieve elements are thought to enter the
reticulate phloem of the seed coat and exit symplastically into
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Plant Physiol. Vol. 91,1989
GEIGER ET AL.
296
TIME (day)
2
4
6
10
8
12
18 20 22 24 26 28 30 32 References
16
14
*-Cotyledon IL-Proembryo- -Globular- ,Heart. .Cotyledon I Embryo completely,*MaturationI. .Maturation II
STAGE
embryo
fills
POD
5
LENGTH (mm)
SEED
LENGTH (mm)
-f35-*70*
sac
*130*
-75-*1059
*.-10--19-*35o
SEED DRY
WEIGHT (mg)
SEED_
SEED
.90.
CELL
DIVISION
~
~
PROTEIN
SYNTHESIS
~
9320
*180a
*-Pale Green *-
Green
COLOR
22
Dormancy3
Black
White----
4
Cessation of
--Rapid
-Rapid Cell Division ~~~~ Cell Division
*-Seed Coat Protein Synthesis-.
STARCH
omRNA . for Globulins*
Maximum Storage-,
Protein Synthesis
--Maximum
SYNTHESIS
PLANT GROWTH
.GA
10,15
to
~
Synthesis-.
Starch
17
*Free ABA Peake
Peake
12
-GA Peak-
REGULATOR
18
Figure 4. Timing of developmental events in the growth of bean fruits under controlled conditions. Seed dry weight is for a single seed. Time
shown is days after anthesis.
0
12
6
~~~
~~~~23
I
1
8.0
S 8 -/
0'
a
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.sA
20
I5
TIME (d)
25
~~~~/A,29
I
t
60
30
Figure 5. Rates of tracer carbon accumulation and respiratory loss
by individual seeds over 24-h periods throughout the course of fruit
development. The sum (*) of tracer carbon accumulation rate (0)
and the loss rate for respired carbon derived from Figure 7 likely
gives an overestimate of import rate. The standard deviation bars for
carbon accumulation data are not shown if they fall within the symbol.
The data are from 145 seeds from 32 fruits. Time shown is days after
anthesis.
the parenchyma of the seed coat (21). Solutes and water exit
through the membranes of the seed coat parenchyma and
form the apoplast solution that bathes the developing embryos
(16, 21). Under these circumstances, increases in membrane
permeability and in the rates of metabolic processes that
prevent solute build-up in developing cells would promote
import of solutes.
In many plants, sucrose is a major component of the
imported solutes and a common starting point of sink carbon
16
/A
,E
-1.2
ma
~~~A
/
I
o
4
2
4
A
in
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.
t6.
8
4z
0
:n
cr.
0
~0
60
60
120
120
18
24
180
240
DRY WT (mg)
30
300
360
360-
Figure 6. Relationship between seed dry weight and rate of tracer
carbon accumulation for seeds of a given age. Scales for the bottom
abscissa and left ordinate refer to data from days 16, 23, and 29
after anthesis, and the top abscissa and right ordinate refer to the
data for 12 d after anthesis only. Dashed lines are for the overall
regression data for seeds from three fruits. Regression lines (solid
lines) for seeds from individual fruit and for individual fruit are shown
for each set of seeds for three fruits (*, *, ) analyzed at 12 and 29
d after anthesis. Two regression lines coincide for data from individual
fruits for day 12. To avoid crowding, data and regression lines for
individual fruits are not shown for values from 16 and 23 d. Correlation
coefficients for regression lines fell between 0.77 and 0.84.
metabolism (1). In view of this, it has been proposed that, in
plants, glycolysis should be more correctly termed sucrolysis
(20). Activity levels of the enzymes of sucrolysis, which convert and metabolize sucrose for respiration and synthesis (2,
19, 20), likely are important in sustaining the supply of carbon
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CARBON PARTITIONING DURING DEVELOPMENT
297
for careful preparation of tissues for analysis, and to Kimberly Boller
for her skillful preparation of the figures.
LITERATURE CITED
Ji
IE a
100
Ioo0
0
E
o5o
50
0i
2
40
80
120
r0
200
240
280
320
DRY WT (mg)
Figure 7. Oxygen uptake by individual seeds sampled throughout
the course of development. A, Oxygen uptake per unit of seed dry
weight (@) for young seeds. B, Oxygen uptake per unit of seed dry
weight for older seeds (0) and uptake per seed throughout
development (U).
to sink tissues. Consequently, it is likely that regulation of the
enzymes responsible for sucrolysis (2, 19, 20)
various processes that utilize imported sucrose
and for the
(Fig. 4) play
an important part in the changes of import into seeds observed
in the present study.
Times during seed development when there are changes in
rates of carbon accumulation or shifts from one dominant
carbon-consuming process to the another are times when the
processes that sustain import and regulate partitioning among
sinks likely are most in evidence. Knowing the time course
for dry weight accumulation in individual fruits and seeds is
helpful for scheduling sampling for studying these processes.
Because of variability among seeds and fruits measurements
of rates and current import by steady state labeling prior to
sampling allows us to evaluate the roles of molecular events
related to sucrolysis and syntheses in regulating import into
developing seeds and fruits. Currently, we are studying the
involvement of gene activation and the activity of key enzymes of sucrolysis and molecular synthesis in regulation of
carbon import by individual seeds and carbon partitioning
among
seeds and fruits.
ACKNOWLEDGMENTS
We are grateful to Chris Stander for her dedication and skill in
raising the plants, to Sarah Scott, Carolyn Steele, and Jude Stratford
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