Copyright ß Physiologia Plantarum 2006, ISSN 0031-9317
Physiologia Plantarum 127: 701–709. 2006
Awns play a dominant role in carbohydrate production
during the grain-filling stages in wheat (Triticum aestivum)
Xiaojuan Lia,b, Honggang Wangc, Hanbing Lia, Lingyun Zhanga, Nianjun Tenga,b, Qingqing Lina,d, Jian
Wanga,d, Tingyun Kuanga, Zhensheng Lie, Bin Lie, Aimin Zhange and Jinxing Lina,*
a
Key Laboratory of Photosynthesis and Molecular Environment Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100 093, China
Graduate School of the Chinese Academy of Sciences, Beijing 100 049, China
c
College of Agronomy, Shandong Agriculture University, Tai’an, Shandong, 271 018, China
d
College of Bioengineering, Fujian Normal University, Fuzhou, Fujian, 350 007, China
e
The State Key Laboratory of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of
Sciences, Beijing 100 101, China
b
Correspondence
*Corresponding author,
e-mail: [email protected]
Received 27 October 2005; revised 14
December 2005
doi: 10.1111/j.1399-3054.2006.00679.x
In wheat (Triticum aestivum L), the leaves particularly flag leaves have been
considered to be the key organs contributing to higher yields, whereas awns
have been considered subsidiary organs. Compared with extensive investigations on the assimilation contribution of leaves, the photosynthetic characteristics of awns have not been well studied. In this study, we investigated the
ultrastructure of chloroplasts, oxygen evolution, and phosphoenolpyruvate
carboxylase [phosphoenolpyruvate carboxylase (PEPCase) EC 4.1.1.31)]
activity in both flag leaves and awns during the ontogenesis of wheat.
Transmission electron microscope observations showed initial increases in
the sizes of grana and the degree of granum stacks from the florescenceemergence stage both in flag leaves and in awns, followed by the breakdown
of membrane systems after the milk-development stage. The results of oxygen
evolution assays revealed that in both organs, the rate of photosynthesis
increased in the first few stages and then decreased, but the decrease
occurred much earlier in flag leaves than in awns. A PEPCase activity assay
demonstrated that the activity of PEPCase was much higher in awns than in
flag leaves throughout ontogeny; the value was particularly high at the late
stages of grain filling. Our results suggest that awns play a dominant role in
contributing to large grains and a high grain yield in awned wheat cultivars,
particularly during the grain-filling stages.
Introduction
For most terrestrial plants, the leaves are believed to be
the key organs for photosynthesis and carbohydrate
production. However, non-leaf organs may also play
an important role in grain and fruit production, because
some organs possessing chlorophyll, such as stems,
branches, leaf sheaths, floral parts, and fruits, have
distinct photosynthetic functions (Blum 1985,
Hetherington et al. 1998, Tambussi et al. 2005). In
different wheat cultivars, the total contribution of nonleaf green organs, including ears and peduncles, accounts
for about 40–50% of grain mass per ear, which is higher
than the total contribution of the flag leaves and penultimate leaf blades (Thorne 1963, Araus et al. 1993, Wang
Abbreviations – OAA, oxaloacetate; PEP, phosphoenolpyruvate; PEPCase, phosphoenolpyruvate carboxylase; PSII, photosystem II; TEM, transmission electron microscope.
Physiol. Plant. 127, 2006
701
et al. 2001). In wheat, all parts of the ear, such as the awn,
glume, lemma, palea, pericarp, and even peduncle, are
capable of photosynthetic CO2 fixation, and a considerable portion of grain mass derives from the photosynthesis
of these organs (Evans and Rawson 1970, Ram and Singh
1982, Wang et al. 2001). Kriedemann (1966) reported that
the contribution to assimilation made by ear photosynthesis ranged from 10 to 44%, depending on environmental
conditions and genotypes. However, the mechanism of
ear contribution to a higher yield is still not clear and
remains to be further explored.
The awn, i.e. the terminal part of the bearded lemma,
can increase the amount of light energy captured by the
plant and facilitate more CO2 flux. Awns increase the
surface area of the ear from 36 to 59%, resulting in an
average of 4% more radiation intercepted by awned ears
(Motzo and Giunta 2002). Thus, awns contribute about
40–80% of the total spike carbon exchange rate, depending on the species (Blum 1985). Consequently, awned
genotypes of wheat have attracted considerable attention
from breeders, particularly when lodging resistance is not
a problem in low-yield fields. To understand the role of
awns in yield production, Imaizumi et al. (1990) have
investigated the general mode of growth, morphological
description, and physiological comparison of awns. Few
studies, however, have focused on the sequential
changes in chloroplast ultrastructure and photosynthetic
activity of the awns, and to our knowledge, no report has
compared the phosphoenolpyruvate carboxylase
(PEPCase) activity in flag leaves to that in awns.
The purpose of this study was to examine the variability in chloroplast ultrastructure of flag leaves and
awns during different developmental stages, with a particular focus on membrane systems, including the integrity of envelope and thylakoid membranes as well as the
thylakoid organization. The dynamic changes in the rate
of oxygen evolution and PEPCase activity of awns compared with flag leaves were also examined. In addition,
the role of awns in transporting assimilates to filling
grain is discussed in an attempt to reveal the potential
contribution to photosynthesis of the whole plant.
Materials and methods
Plant materials
In this investigation, we used wheat (Triticum aestivum L)
Jing 411, an awned cultivar that is widely cultivated in
northern China, grown at the experimental farm of the
Institute of Genetics, Chinese Academy of Sciences.
Samples of flag leaves and awns were collected every 6
days from 13 May to 12 June 2005. All samples were
taken from the mid-portions of flag leaves and awns to
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ensure uniformity of sample material. Based on the
developmental conditions, six growth stages were designated for measurement, i.e. stage 1 (florescenceemergence stage) on 13 May; stage 2 (anthesis stage) on
19 May; stage 3 (milk-development stage) on 25 May;
stage 4 (dough-development stage) on 31 May; stage 5
(ripening stage) on 6 July; and stage 6 (harvest stage) on 12
July. Stage 1 was defined by the florescence-emergence
time, and stage 6 was the time when wheat was harvested.
Scanning electron microscopy
Different portions of awns were cut into 5-mm-length
sections, fixed in 5% formaldehyde, 90% ethanol, 5%
glacial acetic (FAA) for 2 days, dehydrated in a graded
ascending series of ethanol, and critical-dried in a CO2
atmosphere. The samples were then mounted on aluminium stubs using double-sided adhesive tape, sputtercoated with gold, and observed on a Hitachi Hvs-5GB
(Hitachi, Tokyo, Japan) scanning electron microscope.
Stomatal density
Stomatal density was measured on the flag leaves and
awns. The organs were coated with a thick layer of nail
polish, and the dried replicas were carefully peeled off
the organs and placed on microscope slides. Stomatal
density was counted under the microscope, and each
value represents the mean of five replicates.
Transmission electron microscopy
Samples of flag leaves and awns were collected and
immediately fixed in 2.5% glutaraldehyde solution in
0.1 M sodium phosphate buffer (pH 7.0) overnight at
room temperature, post-fixed with 1% (w/v) osmium
tetroxide in phosphate buffer at 4 C, and then
embedded in Epon812 (Shell Chemical, Houston, TX,
USA) following a standard dehydration procedure.
Semi-thin transverse sections were cut with a LKB-V
microtome and then observed under an optical microscope (Zeiss Axioskop 40, Leica, Germany). Thin transverse sections were cut with a LKB-V microtome and
then mounted on formvar-coated brass grids. The sections were stained with 1% uranyl acetate and lead
citrate for 10 min, respectively and were examined
and photographed under a JEM-1230 transmission electron microscope (TEM) (JEOL Ltd, Tokyo, Japan).
Rate of oxygen evolution
The samples of flag leaves and awns were cut into small
sections. The reaction mixture (per ml) was composed
Physiol. Plant. 127, 2006
of 20 mM NaHCO3 and 60 mM Tris–HCl (pH 7.5). The
oxygen evolution rates of the samples were measured at
25 C using a Clark-type O2 electrode (Hansatech,
Cambridge, UK). Irradiation was provided by a cold
light source at a photon flux density 1000 mE m2 s1.
The reaction was started by giving the irradiation and
lasted for 5 min with an interval of 30 s (Tang et al.
2002). The unit of the rate of oxygen evolution is
mmol mg1 chl h1 (Ségui et al. 2000, Leu et al.
2002). Data were averaged from five replications.
the addition of phosphoenolpyruvate (PEP). Data were
averaged from five replications.
Results
Morpho-anatomical description of the awn
The awn, i.e. the terminus of the bearded lemma,
tapered from base to tip and was subtriangular in transaction. Its epidermis comprised elongated cells with
sinuous walls, as well as oval cells that were often
papillate (Fig. 1A). The fine-pointed, thick-walled hairs
were directed toward the apex of the awn, giving it a
scabrous character. Stomata frequently occurred in rows
on the dorsal faces of the awns predominantly while few
stomata or none were observed on the ventral face
(Fig. 1B). Although the stomatal density decreased
from the basal regions to the tip of the awns, the
stomatal density on the dorsal faces may reach
82.93 7.41 mm2 in the basal regions in comparison
with 62.71 5.54 mm2, 46.09 4.83 mm2 on
adaxial face and abaxial face in the flag leaves, respectively. The cross section of a wheat awn appeared to be
acutely triangular, while the angles were reinforced by
bands of mechanical tissue. Under the stomatic band
were green tissues that were differentiated from the
parenchyma, and the green cells were rich in chloroplasts. One large and two small vascular bundles were
present; the former was continuous with the midrib of
the glume (Figs 1C–G).
PEPCase extraction and activity assays
Flag leaves and awns were illuminated outdoors under
direct sunlight for several hours for full activation of
PEPCase. About 4 g of the samples were frozen in liquid
nitrogen and pulverized with a mortar and 4 ml of grinding
medium consisting of 1 mM Tris–H2SO4 (pH 8.2), 7 mM
mercaptoethanol, 1 mM EDTA, 5% (v/v) glycerol, and 1%
(w/v) insoluble polyvinylpyrrolidone. The homogenates
were filtered through cheesecloth, and the filtrates were
centrifuged at 20 000 g for 15 min at 4 C. The supernatants obtained were used for assays of enzymatic activity.
Activity of PEPCase was determined spectrophotometrically at 340 nm by coupling the reaction to the
oxidation of NADH in the presence of MDH. Each 3-ml
aliquot of standard assay medium contained 100 mmol
Tris–H2SO4 (pH 9.2), 10 mmol MgSO4, 10 mmol
NaHCO3, 0.5 mmol NADH, superfluous MDH, and
some diluent crude extract. Reactions were initiated by
C
A
E
Ap
Sc
St
Vb
Pa
F
Sc
D
B
H
St
G
H
Vb
Vb
Physiol. Plant. 127, 2006
Fig. 1. Scanning electron microscope (SEM) images and semithin
sections show morpho-anatomical
structure of awns. (A) SEM image of
the awn apex (Bars 5 60 mm); (B),
SEM image of the middle part of the
awn (Bars 5 60 mm); (C–G), semithin
sections of the awn at various distances from the basis to the tip
(respectively correspond to 10, 30, 50,
70, 90% of total awn length)
(Bars 5 60 mm). Ap, apex; H, hair; Pa,
parenchyma; Sc, sclerenchyma; St,
stomata; Vb, vascular bundle.
703
Chloroplast ultrastructure of flag leaves and awns
at different stages
Flag leaves at the florescence-emergence stage possessed well-differentiated chloroplasts, which contained
grana of numerous layers and well-developed stroma
lamellae with dense internal contents and small starch
grains (Fig. 2A). From the commencement of the
anthesis stage until the milk-development stage, the
chloroplasts gradually expanded in size and reached
their maximum volume at the milk-development stage.
During these stages, the system of granal and intergranal
thylakoids was fully developed, and the grana reached
their largest volume accompanied by an increasing
number and size of starch granules (Figs 2B, C). From
A
the dough-development stage onward, the shape of the
chloroplasts changed from lenslike to round. Although
the granule-filled matrix was still dense in most chloroplasts, granal thylakoids began to dilate slightly; this
process was accompanied by an apparent diminishing
inclusion of starch and an obvious increase in the
number of plastoglobuli of approximately the same size
(Fig. 2D). At the ripening stage, the envelope membrane
invariably ruptured, and the vast stacks of thylakoids
disappeared with only a few small grana remaining
(Fig. 2E). By the harvest stage, the matrix had almost
disappeared, and the whole structure was ruptured,
with small granular patches remaining (Fig. 2F).
In the awns, the chloroplasts were rather flat in
appearance; some of them were even considered pro-
G
CW
Th
P1
S
B
H
CW
S
G
G
Mt
C
I
S
G
Fig. 2. continued
704
Physiol. Plant. 127, 2006
D
J
En
S
Pg
E
K
Th
Pg
G
Pg
F
L
Th
Pg
Fig. 2. Transmission electron microscope images show the ultrastructure of chloroplast at different stages in flag leaves and awns. (A–F), chloroplasts
in flag leaves. (G–L), chloroplasts in awns. (A–F) and (G–L) correspond to the florescence-emergence stage, the anthesis stage, the milk-development
stage, the dough-development stage, the ripening stage, the harvest stage, respectively. Bars 5 2 mm. CW, cell wall; En, envelop; G, granum; Mt,
mitochondrium; Pg, plastoglobuli; Pl, prolamellar body; S, starch; Th, thylakoid.
plastids at the florescence stage. Although they were not
well differentiated, and some may have a prolamella
body, they possessed a thylakoid system (Fig. 2G). At
the anthesis stage, the volume of chloroplasts increased
slightly, and their shape was longer and narrower than
before. The stroma was filled with numerous electrondense granules and small grana, with several parallel
layers of thylakoids emerging at this stage (Fig. 2H). At
the milk-development stage, the chloroplast volume
enlarged dramatically, and well-developed grana consisting of a large number of thylakoids were observed in
mature awns. In addition, small amounts of starch and
Physiol. Plant. 127, 2006
plastoglobuli were found in the chloroplasts (Fig. 2I). At
the dough-development stage, chloroplast volume
achieved its maximum, and the matrix was so dense
that the membranes constituting the thylakoids were
not very distinct, while the contents of starch grains
and platoglobuli increased noticeably, and the starch
inclusions reached their largest dimensions (Fig. 2J).
From the dough-development stage to the ripening
stage, the volume of chloroplasts decreased gradually,
accompanied by an apparent decrease in starch content. The most striking changes occurred in the structure
of the thylakoids, i.e. granules became suspended in the
705
Oxygen evolution (µmol mg–1 chl h–1)
FI
Aw
50
40
30
Rate of oxygen evolution
20
10
1
2
3
4
Stages
5
6
Fig. 3. Rate curve for oxygen evolution from flag leaves (&) and awns
(&) at different stages. Although the oxygen evolution in awns was
lower than that in flag leaves, it increased until the dough-development
stage and remained a rather high value afterward when the value in
flag leaves began to decrease sharply. Therefore, the value in awns was
higher than that in flag leaves during the ripening stage and the harvest
stage. Each value of the oxygen evolution represented the average of
five experiments. Stage 1–6 correspond to the florescence-emergence
stage, the anthesis stage, the milk-development stage, the doughdevelopment stage, the ripening stage, the harvest stage, respectively.
Aw, awns; Fl, flag leaves.
FI
Aw
Activity (µmol mg–1 protein min–1)
2.0
1.8
1.6
The patterns of variation in photosynthetic oxygen evolution differed between flag leaves and awns at the six
different stages (Fig. 3). In flag leaves, the rate of oxygen
evolution showed a pronounced linear increase beginning at the florescence-emergence stage and reached the
highest value (51.38 mmol mg1 chl h1) at the milkdevelopment stage; this value was about two times
higher than in awns. Oxygen evolution then decreased
sharply during the dough-development and the ripening
stages and declined by about 70% of the maximum rate.
Afterwards, the rate continued to decline until the harvest
stage, but the rate of decrease was slower than before. In
awns, the rate of oxygen evolution increased slightly
from the florescence-emergence stage to the anthesis
stage and rapidly reached its maximum (29.92 mmol mg
1
chl h1) at the dough-development stage. Although
the maximum value in awns was much lower than in
flag leaves until the dough-development stage, the rate in
awns decreased only slightly compared with that in flag
leaves, and the rate in awns was nearly twice higher than
in flag leaves at the ripening stage. From the ripening
stage to the harvest stage, the rate in awns declined
dramatically but remained markedly higher than in flag
leaves during this stage.
1.4
1.2
PEPCase activity
1.0
The value of PEPCase activity in awns was significantly
higher than that in flag leaves throughout all stages (Fig. 4).
In flag leaves, we observed no major change in the
PEPCase activity from the florescence-emergence stage
to the milk-development stage. After the milkdevelopment stage, PEPCase activity rose steadily and
then reached its maximum value (1.013 mmol mg1
pro min–1) at the ripening stage. Thereafter, the value
declined to 0.8918 mmol mg1 pro min1 at the harvest
stage. The tendency of PEPCase activity in awns, however, was very different from that in flag leaves. The
activity of PEPCase in awns increased gradually from
the florescence-emergence stage until the milkdevelopment stage and then increased sharply and
reached the highest value of 1.6466 mmol mg1
pro min1 at the dough-development stage, which was
nearly twice that in flag leaves at the same stage. After
0.8
0.6
0.4
0.2
0.0
1
2
3
4
Stages
5
6
Fig. 4. Changes in the phosphoenolpyruvate carboxylase (PEPCase)
activity in flag leaves and awns at different stages. The value of
PEPCase activity in awns was higher than that in flag leaves throughout
all the stages. The enzyme activity in awns increased since florescenceemergence stage and reached the highest value at the doughdevelopment stage. Each value of the PEPCase activity represented the
average of five experiments. Stage 1–6 correspond to the florescenceemergence stage, the anthesis stage, the milk-development stage,
the dough-development stage, the ripening stage, the harvest
stage, respectively. Aw, awns; Fl, flag leaves.
706
lumen of the thylakoid, loss of the parallel arrangement
of the thylakoids became evident in some chloroplasts,
and some of the thylakoids became swollen (Fig. 2K). At
the harvest stage, the shape of the chloroplasts became
spherical, and the envelope as well as the thylakoids of
grana was indistinguishable (Fig. 2L).
Physiol. Plant. 127, 2006
that, the value decreased noticeably from the doughdevelopment stage to the ripening stage and then
dropped slightly until the harvest stage.
Discussion
Chloroplasts are sites of photosynthesis, and their ultrastructural development during leaf ontogeny is strongly
associated with changes in photochemical activity. High
photosynthetic activity is positively correlated with
chloroplasts possessing a high proportion of stacked
thylakoids (Kutı́k et al. 1999). In an investigation of the
changes in the number and size of chloroplasts during
senescence of primary leaves of wheat, Ono et al. 1995)
concluded that photosynthetic activity decreased gradually with the degradation of chloroplasts that contained
all of the photosynthetic pigments and 70–80% of the
total protein present in a green leaf. Because the granum
is the site of photosynthesis and chlorophyll, a close
relationship is expected to exist between photosynthesis
and membrane system (Bondada and Oosterhuis 1998).
In our study, we found that the chloroplasts in flag
leaves possessed well-organized thylakoids and a wellorganized structure from the florescence-emergence
stage to the milk-development stage. Nevertheless, the
granal thylakoids degraded, and the content of starch
grains diminished, in concomitance with an increase in
plastoglobuli from the dough-development stage, when
leaf senescence commenced. In awns, chloroplasts
developed much later, and they also contained a large
number of grana per chloroplast with a high degree of
granal stacks. Moreover, the chloroplasts of awns
remained intact in structure and active in function,
while those in flag leaves were almost degraded at the
dough-development stage and the ripening stage.
Deducing from the high number of granal stacks in
flag leaves, the results described here confirm that the
flag leaves are the principal functional organs for photosynthesis in wheat, and thus their photosynthetic production is the main source of assimilates for grain filling
at the early stages. Based on the intact chloroplasts in
awns compared with the ruptured ones in flag leaves
between the dough-development stage and the ripening
stage, it is reasonable to propose that awns remain
functionally active during the grain-filling period and
make additional contributions to assimilation production, when leaves senesce quickly, and their photosynthetic activity declines. Because the awns also have the
advantage of being located near the grain, their translocation path is short, and their assimilates are mostly
stored in the grain; therefore, the existence of awn
photosynthesis is perhaps more meaningful given the
photosynthetic potential of the awn.
Physiol. Plant. 127, 2006
The oxygen evolution rate is an indication of the
photosynthetic activity, and net oxygen evolution indicates that the production of oxygen by photosynthesis
exceeds its use in respiration (Caley et al. 1990). In
oxygenic photosynthesis, water is oxidized and cleaved
to four protons and molecular oxygen; this reaction is
accomplished by the photosystem II (PSII) proteincofactor complex embedded in the thylakoid membrane
(Pospı́šil et al. 2003). Because stacked thylakoid membranes are the main sites of PSII, they are connected
with oxygen evolution. In an investigation of the development of chloroplast ultrastructure during leaf ontogeny
in maize, Kutı́k et al. 1999) revealed that the increase in
oxygen evolution is positively correlated with increasing
thylakoid compartments in developing leaves. The
results of oxygen evolution in our study demonstrated
that the rate was substantially higher in flag leaves than
in awns before the dough-development stage, and the
highest value at the milk-development stage was almost
twice higher in flag leaves than in awns. However, the
rate in awns continued to increase until the doughdevelopment stage, in contrast to the dramatic decrease
in the flag leaves. Afterward, there was no substantial
decline in awns while the rate in flag leaves dropped by
almost 70% of the maximum. As a result, the rate in
awns was about twice that in flag leaves at the ripening
stage, and it remained much higher in awns than in flag
leaves until the harvest stage, indicating that awns possessed higher photosynthetic activity during the final
stages of grain filling. Because the enzyme responsible
for water oxidation and oxygen evolution is referred to
as PSII on the granula thylakoids of chloroplasts
(Blankenship and Hartman 1998), the data here confirmed the results obtained under TEM for the ultrastructure of chloroplasts described above. In general,
photosynthesis is mainly limited by light harvesting
under low light and by carboxylation and photorespiration under low CO2. Awns were fully exposed to light
and atmosphere, ensuring a minimum amount of shading and abundant CO2 exchange, which caused the
relatively low limitation of photosynthesis in awns. As
a result, awns were recognized as being actively photosynthetic, and they made a considerable contribution to
photosynthetic assimilation.
PEPCase catalyzes the carboxylation of PEP with
HCO3– to produce oxaloacetate in the presence of
Mg2þ under physiological conditions (Ting and
Osmond 1973). PEPCase is present in all fruit tissues
examined to date (Blanke and Lenz 1989). In C3 cereals,
the presence of PEPCase in ears has been reported or
suggested in several studies (Nutbeam and Duffus 1976,
Wirth et al. 1976, Singal et al. 1986). As far as wheat is
concerned, the activity of ribulose bisphosphate
707
carboxylase (RuBP carboxylase, EC 4.1.1.39), a key
enzyme of the Calvin cycle, was greater in the flag
leaves than in the awns at any stage of grain development, the enzyme activity was generally higher in
younger stages and decreased as they matured. In contrast to RuBP carboxylase, PEPCase was more active in
ear parts than in the flag leaves (Singal et al. 1986).
PEPCase in ears of wheat can fix CO2 under light and
refix CO2 released from respiration under light or dark
conditions. The re-assimilation of CO2 is independent of
gas exchange with the external environment, which
increases the overall water-use efficiency and provides
ecological advantages under conditions of warm temperature and water shortage (Araus et al. 1993). Apart
from the physiological advantages, awns showed distinct xeromorphic features, such as a thick epidermis
and cuticle, and a predominance of sclerophylous and
conductive tissues. The observation results here supported a conclusion that the transpiration ratio (carbon
exchange rate/transpiration) in awns was higher than in
flag leaves, by several orders of magnitude (Blum 1985).
In our investigation, we found evidence of PEPCase
activity in both flag leaves and awns, and this activity
was higher in awns than in flag leaves in all stages.
Because PEPCase could supply substrates for carbohydrate synthesis, and the activity in awns is high during
all stages, it is believed that awns could play an important part in assimilation production, i.e. the accumulation of starch, lipid, and protein, during seed
development, and may contribute greatly to grain filling. Furthermore, it is of interest to note that the PEPCase
activity in awns was particularly high, i.e. nearly twice
that of flag leaves, at the dough-development stage.
Considering the great demands of carbohydrate and
protein for the formation of grain mass, we conclude
that activities of PEPCase and corresponding metabolite
transporters are induced during the period of the headfilling stage, because more photosynthate is required to
satisfy the active pool at this stage. Moreover, recycling
of respired CO2 may provide an ecological advantage to
awns under conditions of warm temperature and water
shortage; thus, photosynthesis by awns may contribute
more to the final grain yield than photosynthesis by flag
leaves.
In summary, our work has revealed the details of
sequential changes in the ultrastructure of chloroplasts,
oxygen evolution, and PEPCase activity in both flag
leaves and awns during the ontogenesis of wheat. The
data presented here suggest that awns are superior to flag
leaves on a cellular and physiological level throughout
the grain-filling period. Although awns of wheat cannot
fully replace flag leaves as the source for photosynthates,
the awns, particularly in awned cultivars, possess a strong
708
capacity to photosynthesize and provide assimilation
products to grain mass during the grain-filling stage.
Acknowledgements – This work was supported by National
Science Foundation of China (30330390) and National
Science Fund of China for Distinguished Young Scholars
(30225005). We thank Professor Yuxi Hu for valuable
discussion at the early stages of these experiments and
Dr Richard Turner for his valuable comments on an early
draft of this manuscript.
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