Introduction
Buckwheat belongs to the genus Fagopyrum
Mill., and is an annual herbaceous plant widely distributed worldwide,
mainly in Russia, China, Ukraine, France, Kazakhstan, Poland and Japan. Tartary buckwheat (Fagopyrum tataricum
Gaertn)
is a food crop of great health value and has remarkable functions of lowering
blood sugar, blood pressure, blood lipid, and antitumor risk due to its rich
flavonoid and D-chiral inositol contents (Steadman et
al. 2001a; Choi et al. 2015; Giménez-Bastida and Zieliński
2015). However, the yield of Tartary buckwheat is currently low at
approximately 1,500–2,400 kg·hm-2 (Song et al.
2014). Therefore, the production of high
and stable yield of Tartary buckwheat is of great importance to develop the
buckwheat industry (Huang et al.
2019).
The yield of crops depends on the size of grain storage and the degree of
grain filling. The grain weight of cereal crops, such as rice, wheat, and corn
varies greatly according to the position of grain formation on panicle. In
general, the grain with good grain filling and high grain weight is called
superior spikelet, whereas that with slow filling, poor filling, and low grain
weight is called inferior spikelet (Mohapatra et al. 1993; Yang et al.
2000; Ali et al. 2010). Poor grain
filling and low grain weight of inferior spikelet limit the yield potential of
crops and seriously affect grain quality. Inferior spikelet needs to consume
many nutrients and water during differentiation and growth. Poor filling of
inferior spikelet also greatly influences the efficient use of nutrients and
water in crops. Therefore, the mechanism of superior spikelet formation should
be clarified for the realization of high yield and good quality of crops.
In our previous study, the grains on the upper part and main stem of
buckwheat were regarded as superior spikelet, whereas those on the lower part
and branches were regarded as inferior spikelet (Wang et al.
2016).
However, the mechanism of poor filling of inferior spikelet that affects the
reasonable regulation and control of buckwheat is poorly studied. We predict
that grain filling characteristics and starch synthesis are important physiological conditions for the low grain
filling rate and the light grain weight of inferior spikelet. Thus, Tartary buckwheat “cv. Jinqiao
2” (JQ2) was used to investigate the differences in grain filling
characteristics, starch content, and enzyme activities related to starch
synthesis between superior and inferior spikelet. The results have important theoretical and
practical importance for defining the formation mechanism of Tartary buckwheat
inferior spikelet and provide some theoretical basis for high-yield cultivation
of Tartary buckwheat.
Materials and Methods
Plant materials and growth
Tartary buckwheat “cv. Jinqiao 2” (JQ2) with high yield was used. The experiment was conducted in cement pools at Huangnitang’s Cultivation Experiment Station of Guizhou Normal University (Bijie City, Guizhou Province,
922 m, 27°05′ N, and 105°71′ E) on March 9, 2018 and March 1, 2019. The soil in the pool is yellow loam with 31.37 g·kg-1 soil organic
matter, 1.06 g·kg-1 total
nitrogen, 111 mg·kg-1 hydrolyzed
nitrogen, available phosphorus, 112.72
mg·kg-1 available potassium, and 1.34 g·cm-3 soil bulk density. Furthermore, the soil pH was 5.76.
Tartary buckwheat was cultivated in cement pools with an
area for each test plot measuring 2 m × 10 m × 0.3 m. The optimum application
rates of nitrogenous, phosphate, and potassium fertilizers were 100 (urea), 69
(calcium superphosphate), and 5.1 kg·hm-2 (potassium chloride),
respectively (Song et al.
2014). The three fertilizers were mixed and applied as base fertilizer at one
time, and no fertilizer was applied throughout the growth period. The row
spacing was 0.33 m, the sowing rate was 52.5 g per plot, and the basic seedling
per plot was 900–1000 plants. Tartary buckwheat seeds were harvested on
June 18, 2018 and June 12, 2019. Normal
agricultural practices were implemented.
Sample preparation
At the beginning of the flowering
period, approximately 1,000–1,500 flowers that bloom on the same day when
Tartary buckwheat plants were marked in each pool, and the marked flowers
were sampled every 7 days from flowering to maturation to determine. The
superior spikelet (SS) was the grain of 1–3 nodes at the top of the main stem
of Tartary buckwheat, and the inferior spikelet (IS) was the grain on the
secondary branch at the base of Tartary buckwheat.
Growth and physiological determinations
Simulation of grain filling: The dried
grains were weighed to calculate the average dry weight of 100 grains. Richards’ equation (Richards 1959) was used to describe
the grain filling of superior and inferior spikelet Zhu et al. (1988).
W = A/(1 + Be−Kt)1/N
Divided grain-filling stage: The contribution rates of the grain-filling period, including the
prophase of filling stage (RGC1), the middle of filling stage (RGC2), and the anaphase of filling stage (RGC3) for grain weight, were calculated as described by Yang et al. (2013).
RGC1=W1/A×100%
RGC2=(W2－W1)/A×100%
RGC3=(W3－W2)/A×100%
Starch synthase enzyme activity: The grain was ground in a mortar
with 3–5 mL Tricine-NaOH (100 mmol·L-1) extract containing MgCl2 (10 mmol L-1), EDTA (2 mmol·L-1), 2-mercaptoethanol (50 mmol·L-1), glycerol (12%, v/v), and PVP40 (5%, w/v)
at pH 8.0, and the
temperature was kept at 0şC. It was centrifuged at 15,000 ×g for 10 min (4şC),
and the supernatant (crude enzyme solution) was used for the determination of
enzyme activity. Referring to Yang et al (2003), the activities of adenosine
diphosphate glucose pyrophosphate (AGPase), soluble
starch polymerase (SSS), and starch branching enzyme (SBE) were determined.
Photosynthetic characteristics: LI-COR-6400 portable photosynthetic meter (Li-Cor 6400 portable photosynthesis
measurement system (Li-Cor, Lincoln, NE, USA) was used to determine the net photosynthetic rate, stomatal
conductivity, and transpiration rate of the leaves, where the superior and
inferior spikelets were located. The measurements
were obtained at 10:00–11:00 in the morning, and 10 leaves were measured in
each treatment.
Determination of agronomic
characters and yield: Agronomic
characters items were measured as described by Zhang and Lin (2007). The yield
was determined at maturity and converted per ha yield.
Statistical analysis
The collected data were
statistically analyzed through SPSS analysis of variance from CR design.
Treatment means were compared using the least significant difference at the 5% probability
level” as “Excel 2003 and SPSS 22.0” were used for processing and one-way
analysis of variance was performed.
Results
Simulation of
grain-filling process
A 100-grain dry
weight of JQ2 increased rapidly at the early filling stage, and extent of
increase decreased significantly after 28 days after anthesis (Table 1). The
dry weight of 100-grain superior spikelet was significantly higher than that of
inferior spikelet. The laws of change in 2018 and 2019 were similar, and the dry weight of 100-grains in each period in 2019 was
lower than that in 2018 of JQ2.
Table 1: The hundred-grain weight of superior and inferior
spikelet of Tartary buckwheat (g/100 grains DW)
|
Year |
Grain position |
period |
||||
|
7d |
14d |
21d |
28d |
35d |
||
|
2018 |
SS |
0.086a |
0.370a |
0.708a |
0.877a |
0.901a |
|
IS |
0.062b |
0.269b |
0.513b |
0.622b |
0.655b |
|
|
2019 |
SS |
0.056a |
0.224a |
0.508a |
0.686a |
0.712a |
|
IS |
0.034b |
0.198b |
0.437b |
0.553b |
0.591b |
|
Note: P<0.05. The same
below
Table 2: Parameters of the Richards equation for evaluating the grain-filling
process of Tartary buckwheat
|
Year |
Grain position |
A |
B |
K |
N |
R2 |
R0 |
Tmax.G /d |
Gmax (g/100·d) |
Gmean g/100 |
I/% |
D/d |
|
2018 |
SS |
0.9198 |
0.9920 |
0.1835 |
0.0827 |
0.9958 |
2.2174 |
13.5400 |
0.0596 |
0.0405 |
38.2594 |
22.7051 |
|
IS |
0.6693 |
1.2935 |
0.1431 |
0.0850 |
0.9972 |
1.6831 |
19.0275 |
0.0244 |
0.0230 |
38.2985 |
29.1456 |
|
|
2019 |
SS |
0.7371 |
1.1364 |
0.1512 |
0.0738 |
0.9938 |
2.0483 |
18.0861 |
0.0360 |
0.0269 |
38.1052 |
27.4352 |
|
IS |
0.6252 |
1.2089 |
0.1231 |
0.1053 |
0.9947 |
1.1693 |
19.8284 |
0.0229 |
0.0183 |
38.6437 |
34.2045 |
Table 3: The divided
grain-filling stage of Tartary buckwheat
|
Year |
Grain position |
Early filling stage |
Middle filling stage |
Later filling stage |
||||||
|
Duration/d |
Average rate (g/100·d) |
Contribution% |
Duration/d |
Average rate (g/100·d) |
Contribution (%) |
Duration/d |
Average rate (g/100·d) |
Contribution (%) |
||
|
2018 |
SS |
8.0972 |
0.0094 |
8.2629 |
18.9828 |
0.0510 |
60.3937 |
38.6121 |
0.0142 |
30.2765 |
|
IS |
12.0416 |
0.0155 |
27.9057 |
26.0134 |
0.0296 |
61.7964 |
51.1765 |
0.0027 |
10.1798 |
|
|
2019 |
SS |
11.5063 |
0.0108 |
16.8268 |
24.6659 |
0.0364 |
65.0059 |
48.5123 |
0.0055 |
17.8216 |
|
IS |
11.6392 |
0.0110 |
20.4556 |
28.0176 |
0.0249 |
65.1344 |
57.1934 |
0.0030 |
14.1916 |
|
The determination coefficient R2 of each curve equation
ranged from 0.9938 to 0.9972, indicating that fitting the
grouting process of Tartary buckwheat with Richards’s equation is feasible (Table
2). A value was the maximum value for
100-grain weight in theory during simulated grain filling. The A value of four treatments was very
close to the final actual 100-grain weight, and the A value of superior
spikelet was higher than that of inferior spikelet. The N values of superior and inferior spikelet were both less than 1, and the N value of inferior spikelet was higher than that of superior spikelet. The initial
growth power (R0), maximum grain filling rate (Gmax), and average grain filling rate (Gmean) of superior spikelet were significantly higher than
those of inferior spikelet, while the time to reach the maximum grain filling rate (Tmax.G)
and active filling growth period (approximately 90% of total growth completed)
(D) was lower than that of inferior
spikelet. The ratio of the growth of maximum grain filling rate to final value
of grain (I) for superior and inferior spikelet did not differ. The results in 2019 were similar
to those in 2018, and A
value, R0, Gmax and Gmean of 2018 were higher than those in 2019.
Divided grain-filling stage of superior and inferior
spikelet
In comparison with the superior spikelet, the days of reaching the early filling stage and
the average filling rate of the inferior spikelet were longer and larger (Table 3). In comparison with the superior spikelet, the time of reaching the middle
filling stage and the late filling stage of the inferior spikelet was longer,
but the average filling rate was smaller. The contribution rate to grain weight
in the middle filling stage was the largest, followed by the later filling
stage, and then the early filling stage of superior spikelet. The contribution
rate of the middle filling stage was the largest, followed by the early filling
stage, and then the late filling stage of inferior spikelet. There was no significant (P>0.05)
difference in the 2018 and 2019 results.
Starch accumulation and starch synthase enzyme activity
The starch content of JQ2 increased rapidly from 7 days to 14 days, and
then the extent of the increase decreased slightly from 14 days to 21 days
after anthesis and almost stopped decreasing 21 days after anthesis (Table 4).
The starch content of superior spikelet was higher than that of inferior
spikelet. The AGPase activity in the grains of JQ2
initially increased and then decreased with the increase in growth stage. The
highest AGPase activity of superior spikelet was
reached on the 14th day after anthesis, while the AGPase
activity of inferior spikelet reached the maximum on the 21st day after
anthesis. In the early filling stage (7–14 days), the AGPase
activity of superior spikelet was higher, while that of the inferior spikelet
was higher in the middle and late filling stage (21–35 days). The highest SSS
activity in the grains of JQ2 was reached on the 14th day after anthesis, which
then decreased rapidly. Before 14 days after anthesis, the SSS activity of
superior spikelet was higher than that of inferior spikelet, and became lower
thereafter. The SBE activity in the grains of JQ2 initially increased and then
decreased with the advancement of growth stage, reaching the maximum at 14 days
after anthesis. The SBE activity of superior spikelet was higher than that of
inferior spikelet. The starch content
and starch synthase enzyme activity of 2018 were higher than those in 2019.
Table 4: The starch accumulation and starch
synthase activity of superior and inferior spikelet of Tartary buckwheat
|
Year |
Item |
Grain position |
Period |
||||
|
7d |
14d |
21d |
28d |
35d |
|||
|
2018 |
Starch (%) |
SS |
27.80a |
67.70a |
76.34a |
77.84a |
79.17a |
|
IS |
15.05b |
56.79b |
69.98b |
72.41b |
73.52b |
||
|
AGPase (U g-1 min-1) |
SS |
0.280a |
0.493a |
0.416b |
0.320b |
0.244b |
|
|
IS |
0.181b |
0.328b |
0.437a |
0.385a |
0.294a |
||
|
SSS (U mg-1 min-1) |
SS |
5.560a |
8.502a |
4.216b |
2.018b |
0.870b |
|
|
IS |
4.167b |
7.247b |
5.070a |
2.864a |
1.290a |
||
|
SBE (U g-1 min-1) |
SS |
2.099a |
4.024a |
3.891a |
3.517a |
3.054a |
|
|
IS |
1.170b |
2.928b |
3.601b |
3.275b |
2.855b |
||
|
2019 |
starch（%） |
SS |
26.32a |
61.57a |
73.81a |
76.03a |
78.86a |
|
IS |
13.22b |
50.38b |
66.51b |
70.83b |
71.79b |
||
|
AGPase (U g-1 min-1) |
SS |
0.235a |
0.417a |
0.319b |
0.243b |
0.186b |
|
|
IS |
0.136b |
0.283b |
0.372a |
0.288a |
0.210a |
||
|
SSS (U ·mg-1 min-1) |
SS |
3.149a |
7.861a |
3.582a |
1.832b |
0.827b |
|
|
IS |
2.887b |
6.394b |
4.018b |
2.016a |
1.126a |
||
|
SBE (U ·g-1 min-1) |
SS |
2.023a |
3.933a |
3.823a |
3.425a |
2.869a |
|
|
IS |
1.054b |
2.760b |
3.507b |
3.081b |
2.249b |
||
Table 5: Photosynthetic characteristics of superior and inferior spikelet of Tartary buckwheat
|
Year |
Item |
Grain position |
Period |
||||
|
7d |
14d |
21d |
28d |
35d |
|||
|
2018 |
Net photosynthetic rate (μmolCO2/m2/s) |
SS |
12.891a |
15.072a |
9.545a |
6.214a |
5.150a |
|
IS |
9.245b |
13.034b |
8.805a |
5.629a |
3.913b |
||
|
Stomatal conductivity (mmol H2O/m2/s) |
SS |
0.060a |
0.071a |
0.055a |
0.044a |
0.021a |
|
|
IS |
0.051a |
0.056b |
0.059a |
0.028b |
0.018a |
||
|
transpiration rate (mmol H2O/m2/s) |
SS |
1.869b |
2.955b |
1.908b |
1.534b |
0.732b |
|
|
IS |
2.561a |
3.672a |
2.732a |
2.651a |
0.940a |
||
|
2019 |
Net photosynthetic rate (μmol
CO2/m2/s) |
SS |
11.064a |
14.885a |
9.048a |
5.923a |
4.404a |
|
IS |
8.728b |
12.284b |
8.031b |
5.177a |
3.267b |
||
|
Stomatal conductivity (mmol H2O/m2/s) |
SS |
0.055a |
0.058a |
0.054a |
0.021a |
0.017a |
|
|
IS |
0.048a |
0.049a |
0.049a |
0.026a |
0.016a |
||
|
Transpiration rate (mmol H2O/m2/s) |
SS |
1.835a |
2.501a |
1.173b |
1.311a |
0.685a |
|
|
IS |
1.957a |
2.550a |
2.127a |
1.523a |
0.693a |
||
Table 6: Agronomic traits and
yield of Tartary buckwheat
|
Year |
Plant
height (cm) |
Number of main
stem nodes (individual) |
Number of
branches of main stem (individual) |
Grain number per
plant (grain) |
Grain weight per
plant (g) |
1000-grain
weight(g) |
Yield (kg·ha-1) |
|
2018 |
120.92a |
9.1b |
8.6a |
501.3a |
12.63a |
25.07a |
2103.6a |
|
2019 |
72.22b |
10.3a |
5.7b |
474.6b |
11.52b |
22.10b |
1858.2b |
Photosynthetic
characteristics
The net photosynthetic rate in the leaves of JQ2 initially increased and
then decreased with the advancement of growth period and reached the maximum at
14 days after anthesis (Table 5). The net photosynthetic rate of superior
spikelet was significantly higher than that of inferior spikelet. Stomatal conductivity initially
increased and then decreased with the advancement of growth period. The stomatal conductivity of superior spikelet reached the
maximum on the 14th day after anthesis, while that of inferior
spikelet reached the maximum on the 21st day after anthesis. The
stomatal conductivity of superior spikelet was higher than that of inferior
spikelet, and their difference reached a significant level at 14 and 28 days
after anthesis. The transpiration rate initially increased and then decreased
with the advancement of growth period and reached its maximum at 14 days after
anthesis. The transpiration rate of inferior spikelet was higher than that of
superior spikelet. The net
photosynthetic rate, stomatal conductivity, and transpiration rate of 2018 were
higher than those in 2019.
Agronomic traits
and yield
Differences were observed in the agronomic characters and yield between
2018 and 2019 (Table 6). The plant height, branch number of main stem, grain
weight per plant, 1000-grain weight, and yield in 2018 were significantly
higher than those in 2019, while the number of main stem nodes in 2019 was
significantly higher than that in 2018.
Discussion
The Richards equation (Richards 1959) growth curve was used to fit the
filling process of superior and inferior spikelet, and the fitting degree
exceeded 0.9938, which indicated that the Richards growth curve could
quantitatively express the continuous process of the quality change of superior
and inferior spikelet. The growth curve of
Richards equation presents a cluster of curves
determined by the size of N value. Results showed that the N values of superior and inferior spikelet were less than 1, which
indicated that the filling rate curve leans to the left. This further indicated
that the grain filling was limited by reservoir capacity (Zhu et al. 1988) and that the grain filling
material was more sufficient (Gu et al.
2001), witnessing that the grain filling material grows rapidly in the early
stage of filling and then gradually weakened. Grain filling initiation
potential (R0) reflects
the growth potential of ovary. The R0
value was large, the endosperm cell division cycle was short, the division was
fast, and grain filling started early. In this study, the grain filling
initiation potential of superior spikelet was significantly higher than that of
inferior spikelet, which indicated that the grain filling of superior spikelet
starts early. The photosynthetic products were obtained first, and the maximum
grain filling rate was reached in a short time after anthesis, which supports
that the time for superior spikelet to reach the maximum filling rate (Tmax.G) was less than that of inferior spikelet.
Grain weight is a function of filling rate and filling
duration (Zhai et
al. 2017). Some studies show that grain weight is positively correlated
with filling rate (Li et al. 2019),
but not with grouting duration, while some studies show that grain filling duration
is closely related to grain weight (Lu et
al. 2002). Results showed that the grain weight of Tartary buckwheat was
affected by filling rate more than filling time, which was consistent with the
results of Peng and Xiao (2012) on wheat and those of Wang et al. (2017). The grain weight of the Tartary buckwheat might be
mainly affected by the grain filling rate, especially the grain filling rate in
the middle of the grain filling, because the contribution rate of the medium
term to the grain weight of grain exceeded 60%. Besides being affected by the
filling rate in the middle stage of grain filling, the superior spikelet was
mainly affected by the late stage of grain filling, while the inferior spikelet
was affected by the filling rate in the early filling stage, that is, the rate
of late grain filling has more influence on its particle weight.
Generally, the accumulation of starch in grains is the
result of plant photosynthesis, and the net photosynthetic rate is the most
direct manifestation of the utilization of light energy by crops (Ju et al. 2018; Ba et al. 2019). Our results showed that the photosynthetic rate of
superior spikelet was higher than that of inferior spikelet, which indicates
that the utilization of light energy of strong grain is better than that of
inferior spikelet, which indicates that the superior spikelet has more
“source”, thus providing the large grain filling initiation potential of
superior spikelet. The starch content in Tartary buckwheat grain is
approximately 75% (Steadman et al. 2001b). The process of grain filling in Tartary buckwheat is
the same as the process of starch accumulation. The light contract compounds of
source organ are transported to the grains in the form of sucrose through
phloem, and then starch is formed under the action of a series of enzymes (Nakamura and Yuki 1992; Jeng et
al. 2003; Yang et al. 2003). AGPase, SSS, and SBE are the key enzymes during starch synthesis and metabolism
(Fu et al. 2012). These enzymes play
an important role in regulating the synthesis and accumulation of grain starch.
Our results showed that the AGPase and SSS activities of superior spikelet in the early
filling stage were higher than those in inferior spikelet, while those in the
middle and late filling stages were lower than those in inferior spikelet,
which is consistent with the results of Yang et al. (2001). Considering that the superior spikelet has a high
filling rate, the low starch biosynthesis activity in the early filling stage
explains the low filling rate and light grain weight of inferior spikelet (Fu et al. 2012).
Conclusion
The low light energy utilization and resource
assimilation efficiency in the early filling stage are important physiological
factors for the low grain filling rate and the light grain weight of inferior
spikelet.
Acknowledgments
We acknowledge the support of the
National Science Foundation of China (31560358), Joint Project of Natural
Science Foundation of China and Guizhou Provincial Government Karst Science
Research Center (U1812401), the Science and Technology Support Plan of Guizhou
province, China (QianKeHe ZhiCheng
[2018]2297 and [2019]2297), the Major Research Projects of Innovation Groups of
Guizhou province, China (QianJiaoHe KY Zi [2017]033
and [2018]015), and the science and technology projects of Guiyang, China (Zhuke Hetong [2019]11-6).
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