全 文 ：Received 19 May 2003 Accepted 2 Aug. 2003
Supported by the National Natural Science Foundation of China (30170544) and Natural Science Foundation of Jiangsu Province (BK2002205,
BK2001063).
* Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Diurnal Changes in Activities of Related Enzymes to Starch Synthesis
in Grains of Winter Wheat
JIANG Dong, CAO Wei-Xing* , DAI Ting-Bo, JING Qi
(Key Laboratory of Crop Growth Regulation, Ministry of Agriculture, Nanjing Agricultural University, Nanjing 210095, China)
Abstract: The conversion from sucrose to starch in grains is a key process during yield formation of
wheat. In a field study, diurnal changes in ATP content and activities of enzymes catalyzing starch
synthesis in grains of two winter wheat (Triticum aestivum L.) cv. Lumai 22 and Lumai 14 were investigated.
The enzymes included sucrose synthase (SS; EC 2.4.1.13), ADP-glucose pyrophosphorylase (ADPGPPase;
EC 2.7.7.27), soluble starch synthase (SSS), and starch granule-bound synthase (GBSS; EC 2.4.1.21).
Activities of these enzymes and ATP content showed obvious diurnal patterns. The enzyme activities
were generally higher during the nighttime than the daytime, but ATP content showed an opposite pattern.
It was found that the super-optimum air temperature could be partly responsible for the low enzyme
activities during the daytime. The possible factors influenced the diurnal changes in activities of the
enzymes for starch synthesis in wheat grain were discussed.
Key words: winter wheat (Triticum aestivum); diurnal change; starch synthesis; enzyme activity
It is well accepted that photosynthate supplying activ-
ity in source and photosynthate utilizing activity in sink
determine food productivity in crop plants (Zamski and
Schaffer, 1996). The former is controlled by photosynthe-
sis and dark respiration in a whole day. In wheat, the diur-
nal changes of photosynthesis and dark respiration have
been reported extensively (Ghildiyal, 1991; Singh et al., 1993;
Deng et al., 2000; Srivastava et al., 2002). The record of
high yields of wheat in Qinghai and rice in Yunnan of China
were found to be associated with the large differences in air
temperature between daytime and nighttime, which caused
high photoassimilate accumulation in daytime and low as-
similate loss in nighttime. The photosynthate utilizing ac-
tivity is determined by the conversion capacity from su-
crose to starch in sink. In wheat, this process is controlled
by five enzymes (Keeling, 1988), i.e. sucrose synthase (SS;
EC 2.4 .1.13), UDP-glucose pyrophosphorylase
(UDPGPPase; EC 2.7.7.9), ADP-glucose pyrophosphorylase
(ADPGPPase; EC 2.7.7.27), and starch synthase. There are
two forms of starch synthase, soluble starch synthase (SSS;
EC 2.4.1.21) and starch granule-bound synthase (GBSS; EC
2.4.1.21), which is in charge of amylopectin and amylose
synthesis, respectively. The SS, ADPGPPase and the two
forms of starch synthases were closely related to starch
synthesis and grain yield in wheat under different environ-
mental conditions (Hawker and Jenner, 1993; Jenner et al.,
1993; Li et al., 2001; Ahmadi and Baker, 2001; Jiang et al.,
2002; Xie et al., 2003). However, the diurnal kinetics of ac-
tivities of these enzymes during grain filling in wheat is not
yet to be reported. Therefore, the present study is attempted
to determine the diurnal patterns of the activities of key
enzymes involved in starch synthesis in grains of two high-
yield winter wheat varieties. The expected results will help
to understand the dynamic mechanism of starch synthesis
in grains and the regulation principle for yield formation in
wheat.
1 Materials and Methods
1.1 Experimental description
The field experiment was carried out on the experimental
farm of Shandong Agricultural University, Taian, Shandong
Province. Cement boxes (4.5 m × 2.0 m) without enveloped
bottom were used to grow wheat plants. The soil is brown
earth, and the 0-20 cm soil layer contained 1.59 % total
organic matter, 0.097% total nitrogen (N), 48.6 mg/kg avail-
able phosphate (P) and 103.9 mg/kg available potassium (K).
Two high-yield cultivars were used as Lumai 22 and Lumai
14. The experiment was a randomized block design, with
three replications for each variety. The total N rate was 21
g/m2, with split application at pre-planting and at jointing.
In addition, 7.5 kg/m2 of swine waste manure, 17 g/m2 of
P2O5 and 11 g/m2 of K2O were applied as basal fertilizer.
Eight rows of seeds per cement box were sown on October
9 with density of 210 seedlings/m2. The other cultural
Acta Botanica Sinica
植 物 学 报 2004, 46 (1): 51-57
Acta Botanica Sinica植物学报 Vol.46 No.1 200452
practices followed the local standard for high yield wheat
production.
1.2 Plant sampling
After full heading, heads flowered on the same date
were labeled with red thread. In the clear days, 5 of the
labeled heads for each cultivar were sampled with three-
hour intervals from 10:00 on May 25 (20 d after anthesis) till
07:00 on May 26. The samples were frozen immediately in
liquid N for at least 2 h and then stored at -40 ℃ till en-
zymes assay.
1.3 Enzyme assay
Preparation of enzyme extraction The preparation
procedure was similar to the description by Nakamura et al.
(1989). Five to 10 frozen grains were weighed and homog-
enized with a pestle in an ice-cold motor, which contained
10 mL of 50 mmol/L HEPES-NaOH (pH 7.5), 10 mmol/L
MgCl2, 2 mmol/L EDTA, 50 mmol/L 2-mercaptoethanol,
12.5% (V/V) glycerol, and 5% (W/V) insoluble PVP (polyv-
inylpyrrolidone-40). Thirty mL of the homogenate was
added to 1.8 mL of the buffer solution, which was then
centrifuged at 2 000g at 0-4 ℃ for 20 min. The sediment
was then suspended in 2 mL of the buffer solution for GBSS
activity assay. The rest of the homogenate was centri-
fuged at 10 000g at 0-4 ℃ for 10 min for assay of ATP
content and activities of SS, ADPGPPase, UDPGPPase, and
SSS. All enzymes were not purified to avoid enzyme activ-
ity loss. During assay, the background value of each sample
was determined by adding the same volume of denatured
enzyme extraction by heating in boiling water to correct the
possible substrate in the crude enzyme extraction.
SS (EC 2.4.1.13) The assay was carried out following
the method of Wardlaw and Willenbrink (1994). The reac-
tion mixture contained 50 mmol/L HEPES-NaOH (pH 7.5),
50 mmol/L fructose, 50 mmol/L UDPglucose, 15 mmol/L
MgCl2 and made up a volume of 3.5 mL. The reaction was
started by adding 0.2 mL of enzyme crude extraction. After
30 min, the reaction was terminated in boiling water for 1
min. Then 0.1 mL of 2 mol/L NaOH was added to the
solution, which was heated for 10 min in boiling water. After
cooling, 3.5 mL of 30% HCl and 1 mL of 0.1% resorcin were
added to the solution, which was then heated for 10 min in
80 ºC water. Finally, the formation of UDPglucose-depen-
dent sucrose catalyzed by SS was monitored at 480 nm with
a UV-160A Shimadzu spectrophotometer.
ADPGPPase (EC 2.7.7.27) The assay protocol was
according to the method of Preiss (1974). The reaction so-
lution contained 100 mL of 14 mmol/L ADPG, 50 mL of 50
mmol/L MgCl2, 700 mL of 50 mmol/L HEPES-NaOH (pH
7.5), and 50 mL of enzyme crude extraction. After incubated
at 30 ℃ for 10 min, the reaction was initiated by adding 100
mL of 20 mmol/L PPi and stopped after 15 min by heating at
boiling water. After cooling, 100 mL of 20 mmol/L NADP+,
1.5 IU phosphoglucomutase (EC 5.4.2.2), 1 IU 6-
phosphogluconate dehydrogenase (EC 1.1.1.44), 5 IU glu-
cose-6-phosphate dehydrogenase (EC 1.1.1.49), 50 mL of
50 mmol/L MgCl2, and 200 mL of 50 mmol/L HEPES-NaOH
(pH 7.5) were added to the reaction solution. The mixture
was incubated at 30 ºC for 10 min, and then monitored at
340 nm.
SSS and GBSS (EC 2.4.1.21) The assay was similar to
the procedure of Nakamura et al. (1989). The reaction solution
contained 100 mL of 14 mmol/L ADPG and 700 mL of 50 mg/
mL amylopectin. After incubated at 30 ℃ for 5 min, the rea-
ction was initiated by adding 50 mL enzyme extraction and
stopped after 20 min by heating in boiling water. The ADP
produced by the SSS or GBSS was converted to ATP by
adding 100 mL of 40 mmol/L PEP, 50 mL of 50 mmol/L MgCl2
and 1 IU pyruvate kinase (EC 2.7.1.40) and then incubating
at 30 ℃ for 30 min. The resultant ATP was determined by
adding 5 mL of luciferin-luciferase reagent (Li and Sun,
1980). Five mL of luciferin-luciferase reagent for mea-
surement of ATP content was added to 50 mL enzyme
extraction.
1.4 Photosynthesis and respiration rates
Just before each sampling, photosynthetic rates of five
flag leaves on the plants to be sampled were measured with
a LI-COR 6200 portable photosynthesis system (LI-COR
Inc., USA). Relative humidity, temperature and CO2 con-
centration of the air were simultaneously logged with the
same instrument. After each measurement of photosyn-
thetic rate, the leaf chamber was immediately darkened by
closely covered with opaque black and red clothes for 3
min. The dark respiration rate was then measured.
2 Results
2.1 Diurnal changes in ATP content and enzyme activi-
ties in wheat grains
ATP content in wheat grains showed obvious diurnal
pattern, and was highest at 10:00 and lowest at 16:00 in
both varieties (Fig.1). The ATP content at 10:00 was 3.8
and 2.6 times higher than that at 16:00 in Lumai 22 and in
Lumai 14, respectively. Then from 16:00 to 01:00, ATP con-
tent in grains slightly increased.
The activities of SS, ADPGPPase, SSS and GBSS in
wheat grains also exhibited diurnal patterns in both varieties.
SS activities were very low from 10:00 to 19:00, then in-
creased quickly and maintained very high from 22:00 to
07:00 (Fig.2, solid lines). Mean SS activity between 10:00
53JIANG Dong et al.: Diurnal Changes in Activities of Related Enzymes to Starch Synthesis in Grains of Winter Wheat
and 19:00 was 36% and 60% lower compared with that be-
tween 22:00 and 7:00 in Lumai 22 and Lumai 14, respectively.
ADPGPPase activities showed typical “V” patterns in both
varieties (Fig.2, dot lines) over the whole day, which were
lowest at 10:00, and maintained higher from 19:00 to 04:00.
Diurnal changes of SSS and GBSS activities showed similar
patterns in both varieties (Fig.3). SSS activity increased
straightly from 10:00 to 04:00, while GBSS activity increased
from 10:00 to 19:00 and maintained high levels between
19:00 and 07:00. SSS activity at 04:00 was 3.5 and 4.5 times
higher than that at 10:00 in Lumai 22 and Lumai 14,
respectively, while GBSS activity at 04:00 was 2.6 and 3.2
times higher than that at 10:00 in Lumai 22 and Lumai 14,
respectively.
2.2 Diurnal changes in rates of photosynthesis and res-
piration in flag leaf and relative humidity and temperature
in air
Photosynthetic rates of flag leaf showed a single-peak
curve from 07:00 to 16:00, with the highest rates occurred at
10:00 in both varieties (Fig.4). Respiration rates of flag leaf
increased quickly from 07:00 and reached the highest at
16:00, then decreased straightly and maintained very low
over the night.
Air temperature showed a single-peak curve with the
highest temperature of 32.2 ℃ at 13:00 and the lowest of
20.3 ℃ at 04:00. Relative humidity (RH) in air showed a “V”
type pattern with the lowest RH of 18% at 16:00.
2.3 Relationships between activities of enzymes for starch
synthesis in grain and respiration rate in flag leaf, and
RH and temperature of air
Over the whole day, ATP content was negatively re-
lated to activities of ADPGPPase (r = -0.487 6) and GBSS (r
= -0.541 8) in wheat grains, and irrelated to activities of SS
and GBSS. Activity of SS was not related to activities of
ADPGPPase, SSS and GBSS, indicating that the rate of
Fig.1. Diurnal changes of ATP content in wheat grains of two
cultivars.
Fig.2. Diurnal changes in activities of SS (solid lines, left y-axis)
and ADPGPPase (dot lines, right y-axis) in wheat grains of two
cultivars.
Fig.3. Diurnal changes in SSS (solid lines, left y-axis) and GBSS
(dot lines, right y-axis) activity in wheat grains of two cultivars.
Fig.4. Diurnal changes in respiration rate (solid lines, left y-axis)
and photosynthetic rate (dot lines, right y-axis) of flag leaves in
two wheat cultivars.
Fig.5. Diurnal changes in relative humidity and temperature of
air.
Acta Botanica Sinica植物学报 Vol.46 No.1 200454
sucrose degradation by SS is very fast and thus does not
limit the subsequent reaction in the pathway of starch
synthesis. Activities of SSS and GBSS were significantly
related to activity of ADPGPPase (Fig.6). Because ADPG is
the direct precursor for the final synthesis of starch and the
formation of ADPG is catalyzed by ADPGPPase, it can be
inferred that the final synthesis controlled by SSS and GBSS
could be limited by the process of ADPG formation. GBSS
activity was signifcantly related to SSS activity (Fig.7), in-
dicating the close relationship between synthesis of amy-
lose and amylopectin.
Flag leaf respiration rate was negatively related to
activities of SS (r = -0.746 3), ADPGPPase (r = -0.614 4)
and SSS (r = -0.696 8), indicating that a competitive rela-
tionship existed between physiological activity in flag leaf
and starch synthesis in wheat grain.
The activities of SS, ADPGPPase and SSS were posi-
tively related to air relative humidity, but negatively related
to air temperature (Fig.8). ATP content and GBSS activity
in grains were not related to either relative humidity or tem-
perature in air. These results infer that SS, ADPGPPase and
SSS are easily fluctuating with environmental factors.
3 Discussion
Diurnal fluctuations of photosynthetic and respiration
rates in source leaves have been reported in wheat (Ghildiyal,
1991; Singh et al., 1993; Deng et al., 2000; Srivastava et al.,
2002). The activities of sucrose phosphate synthetase (SPS,
EC 2.4.2.14), the key enzyme controlling sucrose synthesis
from triose-P produced during photosynthesis, also showed
significant diurnal patterns in the leaves of several plants
(Cheikh and Brenner, 1992; Mitra and Srivastava, 1992;
Misra et al., 1995; Pattanayak, 1998). This suggests that
there is a diurnal pattern for photoassimilate supply in
source leaves of plants. However, it is unknown if there is
a diurnal pattern for starch synthesis in sink. Under drought
(Ahmadi and Baker, 2001; Xie et al., 2003) and high tem-
perature (Hawker and Jenner, 1993; Jenner et al., 1993)
conditions, activities of some or all of the enzymes involved
in starch synthesis in wheat grains decreased. This implies
that activities of these enzymes might show fluctuating
patterns in response to the variation in environmental
factors, such as air temperature during a day. In the present
study, we have proved that activities of SS, ADPGPPase,
SSS and GBSS in wheat grains expressed distinct diurnal
patterns in both wheat varieties. Whether these diurnal
patterns belong to circadian rhythms in nature remains to
be elucidated.
In contrast to leaf photosynthetic rate and respiration
Fig.6. Relationship between ADPGPPase activity and activi-
ties of SSS (l) and GBSS (m).
Fig.7. Relationship between activity of SSS and GBSS.
Fig.8. Relationships between air temperature and activities of SS (m), ADPGPPase (l), SSS (r) and GBSS (p).
55JIANG Dong et al.: Diurnal Changes in Activities of Related Enzymes to Starch Synthesis in Grains of Winter Wheat
rate (Fig.4), activities of the enzymes in grains were
generally lower during the daytime than nighttime (Figs.2,
3). This may be associated with the relative lower air tem-
perature at night, because at the range of 21.3 to 33.2 ℃,
activities of SS, ADPGPPase and SSS negatively are corre-
lated to air temperature (Table 1). At the present, there is
limited evidence for the optimum temperatures of these
enzymes in sink organs. However, it has been reported
that activities of SSS and ADPGPPase in wheat grains de-
creased at high temperatures between 31 ℃ and 40 ℃
(Jenner et al., 1993), especially when temperature exceeded
35 ℃ (Hawker and Jenner, 1993). In barley, the conversion
of sucrose to starch was reduced under high temperature
due to the effects of diminished catalytic activity of en-
zymes by 11%-75% in the committed pathway of starch
synthesis, and activities of ADPGPPase and GBSS increased
at moderate temperatures between 28 ℃ and 32 ℃, but
decreased at high temperature (35 ℃) while SSS immedi-
ately lost activity (Wallwork et al., 1998). In cultured maize
kernels, activity of ADPGPPase was reduced by heat stress
at temperatures between 30 ℃ and 35 ℃, resulting from the
declines at transcript level (Duke and Doehlert, 1996). Ac-
tivities of SS, ADPGPPase and SSS in maize endosperm
were very sensitive to high temperatures between 25.0 ℃
and 33.5 ℃ (Wilhelm et al., 1999). In potato, activities of SS
and ADPGPPase in tubers were reduced when plants were
transferred from day/night temperatures of 19/17 ℃ to 31/
29 ℃. In a japonica rice variety, activities of SS,
ADPGPPase and SSS in grains increased at temperatures
between 15 ℃ and 25 ℃ (Umemoto et al., 1995). Thus,
from the above reports and the present study, it can be
concluded that the optimum temperatures for most enzymes
involved in starch synthesis in grains of cereal crops should
be between 20 ℃ and 30 ℃, varying with species. These
also indicated that controlled study is needed to accurately
determine the maximum temperatures of enzymes catalyzed
starch synthesis in wheat grains.
In this study, ATP content in grains reached the maxi-
mum at 10:00, while leaf photosynthetic rate was also the
highest, indicating that the highest ATP content in grains
may be synchronized with the highest energy production
in leaves. However, activities of enzymes involved in starch
synthesis in grains were very low at the same time. This
may suggest that energy supply (ATP) in grains is not a
limiting factor in the pathway of starch synthesis. This is
consistent with our previous conclusion that no relation-
ship existed between ATP content and activities of enzymes
for starch synthesis in wheat grains (Jiang et al., 2002).
Since net photosynthate is only produced during the
daytime, while assimilate consumption via dark respiration
happens during the whole day, grains cannot receive cur-
rent photosynthates during the nighttime and daytime when
net photosynthesis is below zero. Thus sucrose in grains,
the substrate for starch synthesis, could be exhausted be-
fore the re-supply of current photosynthates. This implies
that the sucrose levels in grains could show diurnal
fluctuation, which is probably another reason for the low
activities of the enzymes for starch synthesis at 10:00 found
in this study. Ugalde and Jenner (1990) also reported that
the turnover of sucrose in the wheat grain was rapid and
sucrose in the endosperm and endosperm cavity was ad-
equate for starch deposition of only 3.1 h and 1 h,
respectively. In ear cultured experiments, when sucrose
concentrations in the culture solutions were below 40 g/L,
both the concentrations of endosperm sucrose and the rate
of starch synthesis were lower than the values observed in
vivo, while higher sucrose concentrations than 40 g/L had
little promotive effects (Jenner, 1970; Jenner and Rathjen,
1978; Armstrong et al., 1987). On the other hand, it should
be noted that great bulk of temporary carbohydrates is
stored in vegetative organs such as internodes of wheat,
which can be mobilized to sustain grain growth during lim-
ited photosynthate supply under drought (Spiertz and van
de Haar, 1978) or leaf senescence (Blacklow et al., 1984).
The leaves, glumes and exposed portion of the peduncle
contained diurnal carbohydrate storage pools with diurnal
storage functions (Schnyder, 1993). These diurnal storage
pools, of course, will play important roles in buffering and
diminishing the possible diurnal fluctuation of sucrose lev-
els in grains. Thus, it is clear that more evidence is needed
to illustrate if the sucrose levels indeed exhibit a diurnal
patter in wheat grains, and if so, how this diurnal fluctua-
tion is related to the diurnal pattern of activities of the en-
zymes involved in starch synthesis.
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