Plant Physiol. (1986) 80, 211-215
0032-0889/86/80/0211/05/$0 1.00/0
Relationship between Photosynthesis and Protein Synthesis in
Maize
I. KINETICS OF TRANSLOCATION OF THE PHOTOASSIMILATED CARBON FROM THE EAR LEAF
TO THE SEED
Received for publication May 6, 1985 and in revised form September 17, 1985
FRANgOIS MOUTOT, JEAN-CLAUDE
HUET, JEAN-FRAN(OIS MOROT-GAUDRY, AND JEAN-
CLAUDE PERNOLLET*
Laboratoire du Metabolisme et de la Nutrition des Plantes (F.M., J-F.M-G.) and Laboratoire d'Etude des
Proteines (J-C.H., J.-C.P.), Departement de Physiologie et Biochimie Vegetales, Centre LN.R.A, route de
St-Cyr, 78000 Versailles, France
ABSTRACT
To gain a better understanding of the biochemical basis for partitioning
of photosynthetically fixed carbon between leaf and grain, a '4C02
labeling study was conducted with field-grown maize plants 4 weeks after
flowering. The carbon flow was monitored by separation and identification
of '4C assimilates and '4C storage components within each tissue during
the chase period (from 4 to 96 hours) following a 5 minute '4CO2 pulse.
In the labeled ear leaf, the radioactivity strongly decreased to reach, at
the end of the experiment, about 12% of the total incorporated radioactivity, mostly associated with sucrose and proteins. Nevertheless, an
unexpected reincorporation of radioactivity was observed either in leaf
starch or proteins, the day following the pulse. Conversely, the radioactivity in the grain increased to attain 66% of the total incorporated '4C
after a 96 hour chase. The photosynthates, mostly sucrose, organic and
free amino acids, rapidly translocated towards the developing seeds,
served as precursors for the synthesis of seed storage compounds, starch,
and proteins. They accumulate in free form for 24 hours before being
incorporated within polymerized storage components. This delay is interpreted as a necessary prerequisite for interconversions prior to the
polycondensations. In the grain, the labeling of the storage molecules,
either in starch or in storage protein groups (salt-soluble proteins, zein,
and glutelin subgroups), was independent of their chemical nature but
dependent on their pool size.
Maize is known for its high capacity for dry matter production
associated with a high potential photosynthesis (10). Whereas
elementary processes of carbon assimilation (7, 9), sucrose synthesis (5), phloem loading and unloading (3, 6, 27) have been
extensively described, little information is available regarding the
relationships between leaf and seed during grain filling, except
that the developing ear is rapidly supplied with the assimilates
originating from the ear leaf (4, 11, 17, 28). Before being used in
the synthesis of grain compounds, photosynthetic intermediates,
especially sugars, may be temporarily stored in the stem tissue
(4, 26). The effect of sink strength on the partitioning of assimilates in source leaves and their subsequent distribution in the
plant has been approached by Koch et al. (13). These studies,
focused on the export of carbon from the source leaf, do not
assess the contribution of current photosynthates to the nutritional demand of seed formation. Only Tsai et al (29) have
suggested that seed storage proteins serve as a sink to regulate
the movement of photosynthates into the grain. To gain a better
understanding of the biochemical basis for partitioning of photosynthetically fixed carbon between leaf and grain a '4C02
labeling kinetic study was carefully conducted with field-grown
plants at mid-development stage of the grain (milky stage).
Beginning with early translocation steps, carbon flow was followed by separation and identification of 14C assimilates and 14C
storage components within the tissues involved in this process,
during short and longer chase times (4, 10, 19, 24, 30, 48, 72,
and 96 h) following a 5 min 14C02 pulse.
MATERIALS AND METHODS
Plant Material. Zea mays L. var INRA 180 (Brulouis) was
grown at INRA, Versailles, between May and August 1982. At
28 ± 3 d after flowering, 36 plants were labeled with 14C02
directly in the field.
'4C02 Labeling. Between 10 AM and 2 PM, the median part
(200 cm2, i.e. 2-3 g of dry matter) of the attached ear leaf was
sealed into a 600-ml Plexiglas '4C02 feeding chamber connected
to a closed gas circuit. From a controlled "4C02 air mixture
reserve, a cylinder was filled by pressure adjustment. The 14C02
cylinder was connected to the gas flow circuit at the commencement of pulse time. This device permitted a very reproducible
delivery of 1.1 MBq 14C02 for each experiment.
The chamber was displayed perpendicular to incident sunlight
(irradiance around 700 to 1200 uE m-2 s-'). The leaf was
maintained at a mean temperature of 27°C in the chamber until
total incorporation of the CO2 (i.e. a 5 min pulse), monitored
with an ADC IR CO2 analyzer to verify the efficiency and
reproducibility of ear leafphotosynthesis. The chamber was then
removed and the plant left under field conditions until sampling:
the chase time varied from 5 min (assumed zero time) to 96 h.
Immediately after harvest, plant tissues were separated and
frozen in liquid N2. Tissue samples were defined as follows:
14C02 fed leaf area (L), blade below the fed area and sheath (S),
node at the leaf and shank (N), husk (H), cob (C), and grains
(G). Chase experiments were done in triplicate and the presented
results correspond to mean values.
Extraction and Separation of 'T( Labeled Products. The liquidN2 frozen samples were first lyophilized and weighed before
being pulverized in a Cyclotec 1092 sample mill at liquid N2
temperature. Their radioactivity was determined with a SL4000
Intertechnique liquid scintillation counter following both the use
of a thixotropic scintillation mixture and combustion with an
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212
MOUTOT ET AL.
Intertechnique Oxymat IN 4101. For leaf and intermediary organ
samples, the powders were extracted successively in 95, 80, 60%
(v/v) ethanol-water and finally with water. Aliquots of extracts
were evaporated and dissolved in 10 N H2SO4 for organic acid
analysis and in HCI N/100 for amino acid analysis. For sugar
determinations, the extracts were separated into basic, acidic,
and neutral fractions on cation and anion exchange resins. The
radioactivity of each fraction was measured by liquid scintillation
to verify all subsequent analyses of "1C labeled compounds. The
remaining insoluble material was first treated by a and ,3 amylases at 37°C for 12 h to estimate the radioactivity incorporated
in starch. Then, it was treated with pronase in 20 mM Tris HCI
(pH 7.4) at 30°C for 24 h to determine the radioactivity incorporated into proteins. The protein radioactivity was also measured by summation of the radioactivity of the protein amino
acids determined with a Kontron Liquimat III analyzer equipped
with a continuous flow Berthold LB 504 monitor (18).
For the grains, the flours were submitted to a sequential
extraction of proteins (14, 18), after defatting in acetone at -10°C
to obtain the lipid fraction. The salt-soluble extract, obtained
with a 0.5 M NaCl solution, was separated into salt-soluble
proteins by TCA precipitation (10% w/v final concentration)
and the supernatant split into free amino acids and sugars plus
organic acids by ion exchange chromatography (Biorad
AG50WX8). The storage proteins were then separated into zein
(soluble in 55% v/v isopropanol), G, (soluble in 55% v/v propanol with 0.6% 2-mercaptoethanol), G2 (soluble in borate
NaOH, 0.5 M [pH 10] in presence of 0.6% 2-mercaptoethanol)
and G3 (soluble in the previous buffer added with 0.5% SDS)
glutelins. G2 and G3 fractions were dialyzed against 1% acetic
acid and evaporated before hydrolysis. The residue was assumed
to be starch. The radioactivity of samples was measured by liquid
and/or solid scintillation. As for leaf proteins, it was verified by
summation of amino acid radioactivity (18). The nitrogen
amounts of all nitrogenous samples were determined by the
micro-Kjeldahl method. All the results were corrected on an
analytical yield basis and fitted to a standard initial leaf incorporation of 100 MBq.
Plant Physiol. Vol. 80, 1986
the organic and amino acids strongly decreased. The radioactivity
of the free amino acids dropped to a negligible value after 10 h
of chase, while the radioactivity of the organic acids dropped to
1.5% of the total incorporated "'C at time zero, i.e. 8% of the
remaining radioactivity in the leaf. Nevertheless the organic acid
radioactivity showed two maxima (about 4% ofthe total initially
incorporated radioactivity, i.e. 25% of the remaining radioactivity in the leaf) at 24 and 48 h of chase. Likewise, the radioactivity
of the free sugars decreased quickly during the first 10 h. From
10 to 30 h, the radioactivity of the free sugars decreased more
slowly and remained steady during the rest of the experiment.
Sucrose represented the most labeled compound (over 70%)
among the free sugars (8).
The insoluble material was rapidly labeled during the chase.
The radioactivity recovered in starch and proteins just after a 5
min pulse accounted for 2% ofthe total incorporated radioactivity (Table I). In the course of the chase, the proportion of 14C in
insoluble fraction varied from 3 to 10% ofthe total initially fixed
14C. Its distribution between starch and proteins varied considerably as the chase proceeded. After 96 h, almost all the radioactivity of the leaf remained, equally distributed, in sucrose and
in proteins with a lesser amount in organic acids.
"'C Distribution in Different Compounds of the Intermediate
Plant Organs. The radioactivity of the intermediate plant organs
accounted for one-fifth to one-fourth of the total assimilated 14C
and remained nearly constant during the chase period (Fig. 1).
In the node and shank (Table II), the soluble sugars were the
most labeled components, in which sucrose represented nearly
90% of the radioactivity of this compartment. Basic and acidic
fractions comprised only 10% ofthe total radioactivity recovered
in node and shank. The radioactivity found in all the free
substances slightly varied all along the chase, reaching a maximum between 24 and 72 h, and was slowly decreasing at the end
of the experiment. The insoluble compounds represented 3% of
the radioactivity at 24 h of chase, 1 1% at 48 h, 15% at 72 h, and
then decreased to 10% at the end of the experiment.
"'C Distribution in Different Compounds of the Grain. The
kinetics of distribution of radioactivity in the main compounds
of the grain are shown in Figure 2. During the first 24 h of the
chase period, the radioactivity recovered in the free soluble
RESULTS
rapidly increased to comprise half of the finally
"1C Incorporation and Partitioning in the Plant Organs. At the compounds
incorporated
radioactivity in the grain. During the rest of the
considered stage of plant development, the apparent photosyn- experiment, the
radioactivity of the soluble compounds of the
thesis was maximal (30-40 ,umol m-2 s-' of C02) and in agree- grain drastically decreased
while starch became the major labeled
ment with previously observed values (1, 10). After "'CO2 incormore than two-thirds of the grain radiocompound,
representing
poration, about 90% of the "1C was recovered in the ear leaf
blade, cob, and intermediate organs between the leaf source and activity.
The
recovered in proteins and lipids increased
grains. Losses by respiration and translocation into other parts regularlyradioactivity
to reach an equal value (10% of the radioactivity
of the plant (unloaded leaves, sheaths, and roots) were negligible recovered in the grain) at the end of the experiment. As illustrated
during the experiment and discrepancies in radioactivity recovery in Figure 3, among
seed storage proteins, zein, the major storage
lower than 5%.
was the most quickly labeled compound and accounted
protein
The kinetics of distribution of radioactivity are reported in for the most labeled protein. GI, G2, G3 glutelins and salt-soluble
Figure 1. During the chase period, the initial radioactivity proteins were more slowly
labeled during the chase period. The
strongly decreased in the "'CO2 fed leaf area. Half of the total 14C amount in seed storage compounds
appears to be proporincorporated "1C was exported in 5 h and only 10 to 12% tional to the pool size of each compound, as
through
remained after 96 h. Whereas no significant radioactivity was nitrogen measurements for proteins (results determined
not shown).
found in the distal part of the ear leaf, appreciable "1C amounts
(20-25%) were recovered in the intermediate organs and stayed
DISCUSSION
roughly constant during the chase period. Conversely the radio14C losses during the experiment were negligible. They are
activity increased rapidly in the grains and reached two-thirds of
the total incorporated "'C at the end of the experiment.
attributed to technical difficulties in measuring radioactivity by
"1C Distribution in Different Compounds of the Assimilating liquid scintillation and to respiration processes. After 48 h, a
Ear-leaf. The kinetics of distribution of radioactivity in the main major part of 14C was recovered in grain starch which is considcompounds of the leaf are reported in Table I. After a 5 min ered as an important quenching factor of the ,B emission of 14C,
14CO2 feeding, the major part of radioactivity (98%) was re- as we found by comparing radioactive measurements using thixcovered in the water-ethanol extracts. The labeling of organic, otropic scintillation mixture to combustion. Maize is also known
amino acids, and neutral sugars of the leaf is roughly similar. to exhibit a very limited carbon loss by respiration throughout
During the first hours of the chase period, the radioactivity of growth (I17) when compared with wheat, a C3 plant that respires
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Copyright © 1986 American Society of Plant Biologists. All rights reserved.
PHOTOSYNTHETIC SUPPLY OF MAIZE SEED
213
FIG. 1. Kinetics of partitioning of 14CO2 radioactivity from the ear leaf to the grain. Values (mean of 3
samples) have been calculated for a 100 MBq standard
intital incorporation in the plant. L(A), 4CO2 fed leaf
area; S(A), sheath and proximal leaf moiety; N(E),
node and shank; C(O), cob; H(O), husk; G(-), grains.
G
72
CHASE (h)
Table I. Kinetics of Partitioning of Radioactivity in the Ear Leaf Fed Area
Radioactivity. expressed in kBq per g of dry matter, has been calculated for a 100 MBq standard 4C02
incorporation and corrected for extraction yields.
Compounds
Free amino acids
Organic acids
Free sugars
Phosphorylated com-
pounds
Total of free compounds
Starch
Proteins
Starch + proteins
Chase (h)
24
19
185
80
655
1910
5550 4610
30
120
1040
2070
48
55
1720
2250
72
42
598
1840
96
70
770
1760
0
14150
11050
17410
380
1300
15480
10
168
848
6904
7740
690
0
0
0
0
0
0
0
50350
17850
7920
6390
6600
3230
4025
2480
2600
600
550
3490
510
1820
1040
1615
595
400
950
3260
1100
1670
960
430
920
260
1550
1150
4000
2860
2210
1350
4360
2630
1350
1810
51500
21850
10780
8600
7950
7590
6655
3830
4410
4
Total of free + polymerized com-
pounds
40 to 50% of the photoassimilated carbon (16).
The translocation from the leaf to the intermediate and storage
plant organs of recently assimilated "1C occurred very soon after
the pulse. Troughton et al. (27) have demonstrated that the
photosynthates moved very rapidly with water by the mass flow
in sieve tubes. The resulting osmotically generated pressure might
be the driving force for movement of substances in the plant.The
blade below the fed area, node, shank, and cob are considered as
conduits and as temporary storage reservoirs for carbon skeletons
used thereafter as precursors in the synthesis of starch, proteins,
and lipids for the developing grain (2, 4, 17, 26). The accumulation of a limited but significant fraction of radioactivity in the
insoluble components of the intermediary organs is in accordance with the results showing that photosynthates temporarily
accumulate in the cell wall of vascular bundles as structural
carbohydrates and proteins (2, 8). Besides, we confirm here the
large size of the conduit compartment which retains 20% of the
photoassimilated carbon for at least 4 d.
The patterns of "4C distribution between plant organs reveal
that developing ear and specifically grains were major sinks for
14C photoassimilated by ear leaf blade, 28 d after silk emergence.
Other previous works have shown that the development of grain
was the main process determining the fate of photosynthetic
carbon after flowering in maize (4, 7, 17). Nevertheless we show
that only two-thirds of the photosynthetic carbon is trapped
within the grain. This is explained not only by the size of the
conduits but also by the relatively high amount of radioactivity
remaining in the leaf.
14C free sugars (mainly sucrose) are exported rapidly during
the first 10 h of the chase as previously observed by Hofstra and
Nelson ( 11) and Prioul and Rocher (20). Exported sucrose provides carbon skeletons and energy for organic synthesis in the
dark; this movement persists as long as reserves are available,
but after 30 h there is no significant export of 14C sucrose. We
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214
MOUTOT ET AL.
Plant Physiol. Vol. 80, 1986
Table II. Kinetics of Partitioning ofthe Radioactivity in the Node and Shank
Radioactivity, expressed in kBq per g of dry matter, has been calculated for a 100 MBq standard 4C02
incorporation.
Chase (h)
Compounds
4
10
19
24
30
48
72
96
Free amino acids
6.3
17.4
12.1
12.7
11.3
12.9
6.0
3.4
Organic acids
32.1
26.0
32.0
35.4
33.8
38.3
62.7
13.2
Free sugars
512
538
456
601
588
684
549
476
Total solubles
Insoluble compounds
Total recovered radioactivity
550
581
500
649
633
735
618
493
11
33
70
19
60
93
112
53
561
614
570
668
693
828
730
546
ZEIN
8
2
0
0
FREE AMINO ACIDS
C)
5
_
4
2
C)
4
0
a
4r
SALT
cc
sdLUBLE PROTEINS
a
4c
0
(A),
443
glutelins.
o
0
z O1.58
a
4C
0
o
72
9
C)
z
-J
I-.C
0
0
24
48
96
72
CHASE (h)
FIG. 2. Kinetics of "4CO2 radioactivity of the major biochemical
components of the grain. Values have been calculated for a 100 MBq
standard initial incorporation in the plant. (*), Total radioactivity of the
grain; (0), free sugars (and organic acids); (0), starch; (U), proteins; (A),
lipids; (E), free amino acids.
24
48
72
96
CHASE (h)
FIG. 3. Kinetics of rC02
radioactivity accumulation in the nitrogenous compounds of the grain. Values have been calculated for a 100
MBq standard initial incorporation in the plant. (0), Free amino acids;
(J), salt-soluble proteins; (0), zein; (U) GI glutelins; (A), G2 glutelins;
(A), G3 glutelins.
that the second labeling of starch occurring after 24 h of chase,
resulting from the incorporation of the '4C02released during the
show that, in the leaf at the end of the chase, free sugars (mainly first night and early in the next mor ing. This original result
sucrose) still represent more than 3% of the initially incorporated supports the contention that the maize leaf is able to reincorpo4C02 issued from general metabolism to syntheradioactivity, i.e. 40% of the remaining radioactivity. They form rate secondary
m
a storage pool not directly accessible to translocation, in agree- size starch again.
ment with the results of Prioul and Rocher (20) who have
Leaf proteins are also quickly labeled after a briefcaCO2apulse.
provided evidence of such a sucrose pool in the leaf vacuoles.
Their radioactivity increases during the chase to reach nearly 3%
The rate of sucrose synthesis and translocation indirectly reg- of the initially incorporated radioactivity at the end of the
ulates the rate of starch formation which is associated with the experiment while the labeling of starch dropped to zero. Photophotosynthate demand. The partitioning of carbon between su- synthetic carbon is stored temporarily in leaf proteins which are
crose and starch is controlled by fructose 2,6-bisP, phosphate also considered as the main reservoir of nitrogen for the maize
translocator, Pi, and triose-P concentrations and an important grain from pollination to maturity (2). As for starch, the radiodeterminant of carbon partitioning into starch may be the rate activity in leaf proteins obeys a nyctemeal period, characterized
of sucrose synthesis and the related generation of Pi into cytosol by a depletion during the first night and a secondary increase in
(12, 19, 21, 24, 25). Transient starch in the leaf is well known to the course of the 2nd d. This can only be explained by a relatively
be a response to a temporary oversupply of carbohydrates from rapid turnover of at least half of the leaf proteins, involving
photosynthesis. In fact, after a rapid depletion from 4 to 10 h of proteins different from ribulose- l,5-bisP carboxylase, which is
chase, we have observed only a slight decrease of the starch known to have a 7-d half-life (23).
Another new observation is that the photoassimilated carbon,
radioactivity during the night, the rest of the degradation taking
place early in the morning. Nevertheless this is in agreement with translocated to developing grains, appears temporarily in free
starch hydrolysis in chloroplast into exportable triose-P by the molecules, mainly sucrose, amino and organic acids. These molreactions of the glycolytic sequence (15). It is worth emphasizing ecules set up a large transient reservoir of precursors used thereDownloaded from on July 31, 2017 - Published by www.plantphysiol.org
Copyright © 1986 American Society of Plant Biologists. All rights reserved.
PHOTOSYNTHETIC SUPPLY OF MAIZE SEED
after in the synthesis of starch, proteins, and lipids of the grain.
This delay may be explained by the well known mechanisms of
unloading and transfer of sucrose from the phloem of intermediate organs to the endosperm (22), mechanisms involving the
passage through specialized basal endosperm cells prior to movement into the starchy endosperm and embryo. The transient
storage of precursors could also be attributed to slow biochemical
conversions of molecules prior to their polymerization into storage macromolecules.
Although produced since the first hours of chase, the final
storage molecules are more intensively synthesized from the
incoming precursors after a 24 h chase. Starch appears as the
principal carbon sink of the grain, whereas the radioactivity
associated with storage proteins accounts for only 10% of the
radioactivity accumulated in the grain. The same level is observed in the lipids. In the grain, the labeling of the storage
molecules is independent of their chemical nature but dependent
on their pool size.
The patterns of 14C distribution reveal that, at mid-development stage of the seed, grains are the major, but not the only,
sink for the carbon photoassimilated by ear leaf blade. The
photosynthates, mostly sucrose, organic and amino acids, rapidly
translocated to the developing seeds, serve as elementary components for the synthesis of seed storage compounds, starch, and
proteins. In the grain, the final storage molecules, either starch
or proteins, are not immediately synthesized from the incoming
precursors: a 24 h delay is a necessary prerequisite for their
polycondensation. On the other hand, about 12% of the photoassimilated carbon remained trapped in the assimilation area
of the leaf, mostly in sucrose and proteins.
Reliable information about the translocation processes involved in the seed protein synthesis from photosynthates in maize
is presented. Further studies are in progress on the biochemical
interconversions necessary for the photosynthetic carbon to accumulate in the grain under suitable forms. Only appropriate
techniques of labeling and analysis and a judicious choice of the
sampling time in the course of chase after a short "CO2 pulse
allow one to properly examine the fate of photoassimilated
carbon in the source leaf along the translocation path and in
assimilation into grain components of maize.
Acknowledgments-We particularly want to thank Dr. J. Mosse for critical
reading of the manuscript and Dr. J. Baudet for helpful advice. We are grateful to
M. C. Aubriere, G. Colliere, J. C. Lescure, and S. Wuilleme for their skillful
technical assistance, and to all colleagues who kindly participated in "CO2 incorporations.
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