Study on the Growth Dynamics of Tartary Buckwheat Flowers and Grains, as Well as Material Basis and Physiological Changes of Their Seed-Setting Differences

Abstract

Tartary buckwheat is a cereal crop that has both medicinal and food origins. However, the underlying factors that contribute to Tartary buckwheat’s flowering and seed-setting characteristics, effective grain formation, and physiological changes are still not well understood. This study aimed to investigate the flowering and seed-setting characteristics of different parts of Tartary buckwheat, as well as the grain-filling characteristics after flowering. To achieve this, Tartary buckwheat cultivars with high (QK3) and low (XQ2) seed-setting rates were selected for pot and field experiments. The study found that Tartary buckwheat undergoes flowering and seed setting simultaneously. Many wilted flowers and grains were observed 45 and 51 d after flowering. Compared to XQ2, QK3 exhibited a higher grain formation rate and seed-setting rate by 7.42% and 26.16%, respectively. Additionally, QK3 had a significantly lower grain abortion rate by 12.03%. The 1000-grain weight and average grain-filling rate of QK3 were 11.10% and 14.81% higher than those of XQ2, respectively. QK3 exhibited a faster maximum grain-filling rate (R_max), reaching 18.38% faster than XQ2. Additionally, the dry matter average distribution rate in the main stem and branched grains of QK3 was 13.26% and 23.07% higher than that of XQ2, respectively. The sucrose concentration, SS, and SSS enzyme activities of QK3 were all higher than those of XQ2, by 0.29–25.99%, 5.22–11.62%, and 6.64–12.47%, respectively. Correlation analysis revealed a significant positive correlation between sucrose, soluble sugar, and starch concentration during the grain formation process and SS and SSS activities. This suggests that the levels of SS, SSS, soluble sugar, and sucrose in the grain play a crucial role in grain filling.

1. Introduction

Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) is an annual herbaceous plant of the genus Buckwheat of the Polygonaceae family [1]. It is commonly cultivated in high, cold mountainous regions such as Yunnan, Guizhou, and Sichuan in China, with an annual planting area of approximately 300,000 ha [2,3]. Since Tartary buckwheat flowers grow very few seeds and the fruiting process is affected by many climatic factors, the seed-setting rate of Tartary buckwheat is generally below 35% [4]. The flowering and seed-setting stages play crucial roles in determining the yield of Tartary buckwheat. Although Tartary buckwheat plants produce numerous flowers, the number of effective grains is small at maturity, and there is a high occurrence of wilted flowers and grains. Tartary buckwheat flowers exhibit a yellow–green color and typically bloom between 5 and 9 a.m., with the highest number of open flowers observed at 9 a.m. This flowering phase lasts approximately 40 to 70 d, constituting around two-thirds of the growth period [5]. Notably, the spatial distribution of flower quantity within the plant exhibits distinct differences, with proximal flowers receiving more resources than distal ones [6]. Previous research has indicated a gradual decline in the seed-setting rate of Tartary buckwheat as it progresses in growth and development [7]. Despite the significant increase in yield achieved through the selection of high-quality germplasm and effective cultivation and management practices in recent years, there is still a need for further measures to meet the growing demand in both domestic and international markets [4]. Therefore, studying the growth changes of Tartary buckwheat flowers and grains is crucial to improving Tartary buckwheat cultivation.

The accumulation and distribution of dry matter are closely linked to economic yield. Dry matter accumulation is influenced by traits of yield components, particularly the transfer of biomass to flowers and grains after anthesis [8]. Studies have shown that approximately one-third of the dry matter in wheat grains is transported by vegetative organs before flowering. Meanwhile, the remaining portion comprises photosynthetic dry matter accumulated in functional leaves after flowering [9]. Grain filling, a crucial physiological process for transporting photosynthetic products to grains [10], is closely associated with crop yield [11]. According to research, Tartary buckwheat grain filling exhibits a growth pattern known as ‘slow–fast–slow’ [12]. The contribution rate of grain-filling period to grain weight follows the order of mid-stage > late-stage > early-stage. It has been reported that a crop’s grain-filling process can be analyzed using fitting equations. The most-used simulation models for the fitting analysis of the grain-filling process include the Richards growth equation [13], the logistic growth equation [14], and the cubic polynomial growth equation [15]. Li et al. [16] used the Richards equation to fit the wheat grain-filling process and found that exogenous spermidine significantly increased the grain-filling rate of wheat under water-shortage conditions. The grain-filling process involves starch accumulation, which is achieved through the degradation and transformation of sucrose. A higher sucrose concentration in the grain leads to a faster grain-filling rate and increased dry matter accumulation. Fahy et al. [17] identified the activity of adenosine glucose diphosphate pyrophosphorylase (ADGPase), sucrose synthase (SSase), and soluble starch synthase (SSS) as key enzymes that impact starch synthesis, thereby influencing starch accumulation, grain filling rate, and ultimately the grain yield of wheat [18].

In recent years, research on Tartary buckwheat has primarily focused on physiological functions and nutritional quality. However, there is still a lack of information regarding the characteristics of Tartary buckwheat flowering and seed setting, as well as post-flowering grain-filling characteristics and the study of dry matter accumulation and distribution. This study investigated different cultivars of Tartary buckwheat with varying seed-setting rates as test materials. Outdoor potting experiments were conducted to examine the development of flowers and grains in Tartary buckwheat and its accumulation characteristics. The objective was to gain insights into the developmental dynamics of flowering and seed setting during the flowering period of Tartary buckwheat, as well as the characteristics of flowering and seed setting in different parts of the plant. A further objective was to explore the main factors affecting the Tartary buckwheat seed-setting rate and its grain-filling characteristics through the field trial, for further investigation of Tartary buckwheat flowering and seed setting of the regulatory mechanism and Tartary buckwheat high-yield cultivation and selection of high-yielding cultivars to provide a theoretical basis.

2. Materials and Methods

2.1. Experimental Materials

The experiment was conducted at Chengdu University’s experimental base (104°19′ E, 30°65′ N, 495 m above sea level) from 2018 to 2019. The average rainfall was 873 to 1265 mm (temperature variations during the experiment are shown in Appendix A, Figure A1). The experimental materials used were the Tartary buckwheat high seed-setting cultivar Qianku No.3 (QK3) and the low seed-setting cultivar Xiqiao No.2 (XQ2), as previously screened and identified. These materials were selected and provided by the Guizhou Weining Academy of Agricultural Sciences and Xichang College. All chemicals used in the experiment were of analytical purity (AR).

2.2. Experiment 1: Tartary Buckwheat Flower and Grain Development and Its Dynamic Changes

The experiment utilized a fully randomized block design with three blocks. The pots used in the experiment had dimensions of 26.5 cm in diameter on the outside, 23 cm on the inside, and 19.5 cm in height. These pots were placed in the field to mimic natural growth conditions. Each pot was filled with five kilograms of natural and nutrient soil in a 3:1 ratio. In the field, a trench was dug for the pots, with approximately two-thirds of each pot’s volume buried underground. The pots were arranged in rows, each consisting of thirty pots. The rows were spaced sixty centimeters apart (refer to Figure 1). Once the pots were placed in order, the soil was watered. After soaking in distilled water for eight hours to allow the seeds to germinate, each cultivar’s sixty pots, each containing eight seeds, were planted. When the seedlings reached two true leaves, they were thinned, keeping only five plants in each pot. Once the seedlings grew five true leaves, two seedlings were retained in each pot. When the first Tartary buckwheat plant in the group developed flower buds, 15 pots were randomly selected for tagging.

2.2.1. Flower and Grain Accumulation Observation Statistics

A manual counting method was used to determine the flower and grain-related indices. The day when the labeled plants first displayed buds was day 0. Subsequently, the total number of flowers, total number of grains, number of wilted flowers, number of wilted grains, and number of effective grains of the marked plants were recorded at 2-day intervals. Moreover, the main stems were recorded separately from the branches. Any plants that broke during the counting process or exhibited significant differences in reproductive characteristics compared to other plants, such as malformed and disease-damaged buckwheat, were excluded from the count. When the majority of the Tartary buckwheat in the population reached the harvest standard of 70% maturity, the harvest occurred and the counting stopped. The specific division of flowers and grains is as follows, along with the indicators of measurement:

Total number of flowers: the sum of all flowers and grains, where flowers include buds, opened flowers, and wilted buds and grains include filling grains, effective grains, and wilted grains.

The number of grains: when the buds were visible with young grain tips, they were categorized as grains, including grains in filling, effective grains, and grains that had already been wilted.

The number of wilted flowers: wilted buds and green florets, along with the wilted flowers when wilting.

The number of wilted grains: the wilted grains were harvested, and the unfilled grains were counted as the wilted grain count.

The number of effective grains: full and plump grains.

Yield per plant (g): the main stem and branch effective grains were weighed separately after harvest, and the sum of the two was the yield per plant.

Grain formation rate (%) = (number of grains/total number of flowers) × 100%.

Seed-setting rate (%) = (number of effective grains/total number of flowers) × 100%.

Grain abortion rate (%) = (number of wilted grains/number of grains) × 100%.

Superior grain: the grains with early flowering, fast grain filling, good filling, and high grain weight.

Inferior grain: the grains with late flowering, slow grain filling, poor filling, and low grain weight.

2.2.2. Bud Development Marker

Ten days after buds appeared in the two cultivars, 50 buds of uniform size and specifications were randomly selected and marked with thin lines of different colors. Then, marked buds were checked at the same time every day until the bud appeared as the seed. The labeled flower buds were sampled according to different marking periods to remove the buds with significant differences. Ten flower buds were taken from each period and photographed with a Type microscope (SZX2–ILLT, Olympus Crop, Tokyo, Japan). The remaining buds were fixed separately using FAA fixative (50% alcohol:glacial acetic acid:formaldehyde = 18:1:1) and stored in a refrigerator at 4 °C to observe paraffin sections in developmental periods.

2.2.3. Paraffin Section

Flower buds placed in FAA fixative were removed, petals were peeled off in a fume hood, and the buds were numbered for different periods, subjected to routine paraffin sectioning, and stained with fenugreek solid green [19].

2.3. Experiment 2: Tartary Buckwheat Post-Flowering Grain Formation and Material Accumulation Research

The experiment was conducted in 2019 at the test site of Wufeng Town, Jintang County, Chengdu City, Sichuan Province (104°30′50.79″, 30°21′21.97″, 406 m above sea level). The experimental site was sandy loam soil with a pH of 6.59. In the 0~20 cm soil layer, the nutrient status was as follows: organic matter, 2.22%; available potassium, 116.0 mg·kg⁻¹; available phosphorous, 23.7 mg·kg⁻¹; alkaline dissolved nitrogen (ADN), 128.0 mg·kg⁻¹; total nitrogen (TN), 1.29 g·kg⁻¹; total phosphorus (TP), 0.87 g·kg⁻¹; and total potassium (K), 14.80 g·kg⁻¹. Base fertilizer (nitrogen fertilizer, 45 kg·ha⁻¹; phosphorus fertilizer, 120 kg·ha⁻¹; potash fertilizer, 40 kg·ha⁻¹) was applied before sowing, and nitrogen fertilizer, 45 kg·ha⁻¹, was applied at the seedling stage. The experiment followed a randomized block design with three blocks. Each plot had an area of 24 m² (3 m × 8 m) and used hole sowing with a hole spacing and row spacing of 0.3 m. In each hole, 8–10 seeds were sown, and the number of seedlings was reduced to 3 plants at the 5-leaf stage. Regular fertilizer and water management were employed to control weeds, diseases, and insect pests.

2.3.1. Dry Matter Transport and Accumulation

Tartary buckwheat plants were observed from the bud stage, with the first bud time recorded as 0 d. After budding, 10 plants were randomly selected at 10 d, 20 d, 30 d, 40 d, and 50 d. These plants were chosen for their similar growth and morphology, making them representative of the population. The plants were divided into roots, stems, leaves, flowers, and grains in separate kraft paper bags. The flowers and grains were further divided according to branches and main stems. The roots were washed with water and placed in bags. All materials were subjected to a temperature of 105 °C for 15 min to kill any living organisms and then dried at 75 °C until a constant weight was achieved. The dry weight of each organ was measured using one over ten thousand of electronic balance (FA2204, LICHEN, Ningbo, China).

Dry matter percentage of organs (%) = (dry weight of each organ/dry weight of whole plant) × 100%.

2.3.2. Distribution Rate of Dry Matter Growth

The growth distribution rate ($f_i$) reflects the growth, accumulation, and distribution dynamics of various organs in different periods [20]. That is, the ratio of the weight gain of each organ (the weight difference between two observations) to the weight gain of the whole plant (the aboveground part) during the growth process of Tartary buckwheat was the proportion of net assimilates distributed to each organ during this period.

$$f_i^j=\frac{W_i^{j+1}-W_i^j}{W^{j+1}-W^j}(W^{j+1}>W^j)$$

where $f_i^j$ denotes the growth distribution rate of each organ at different periods; $W^j$ is the total dry matter weight of the whole plant (above-ground part) observed at a certain period; $j$ is the developmental period; $W_i^j$ denotes the dry matter weight of an organ at a certain period; and $i$ denotes the stems, leaves, flowers, and grains. $f_i^j$ may be positive or negative since there is a material transfer between organs, and the absolute value may be greater than one.

2.3.3. Determination of Grain Morphology and Grain Weight

During the bloom period, 300 grains were picked from each of the following developmental periods on the main stem and branches: 20 d, 25 d, 30 d, 35 d, and 40 d. Samples were collected to measure the length and width of the grains, as well as the fresh and dry weights of the grains.

Grain length and width: ten grains were randomly selected and determined using a seed analyzer (MARVIN, Zealquest Scientific Technology Crop, Shanghai, China), and five replications were performed.

Grain fresh and dry weight: 50 grains were randomly selected for analysis. The fresh weight of the grains was measured using one over ten thousand of electronic balance (FA2204, LICHEN, Ningbo, China) in five replications. Subsequently, the grains were dried until they reached a constant weight, and the dry weight was then measured. Finally, the dry and fresh weights were converted to 1000-grain weights.

Grain moisture content (%) = (grain fresh weight − grain dry weight)/grain fresh weight × 100%.

2.3.4. Grain-Filling Characteristics

Logistic equation $Y=K/(1+e^{A+Bt})$ was used to fit the pattern of change in 1000-grain weight with days after flowering (t) [21], where K is the value of 1000-grain weight potential and A and B are the parameters of the equation. The derivation of the equation can be introduced:

Onset of grain-filling peak period: $t_1=[A-\ln (2+1.732)]/(-B)$

Grain-filling end time: $t_2=[A+\ln (2+1.732)]/(-B)$

Final grain-filling period (Assuming that Y reaches 99% K time): $t_3=-(4.89512+A)/B$

Grain-filling gradual increase period: $T_1=t_1$

Grain-filling rapid increase period: $T_2=t_2-t_1$

Grain-filling increasing period duration: $T_3=t_3-t_2$

Grain-filling duration: $T=t_3$

Average grain-filling rates: $R=K/t_3$

Maximum grain-filling rate arrival time: $T_{max}=-A/B$

Maximum grain-filling rate: $R_{max}=-BK/4$

Accumulation of grain-filling gradual increase period: $W_1=K/(1+e^{A+B{t}_1})$

Accumulation of grain-filling rapid increase period: $W_2=K/[1/(1+e^{A+B{t}_2})-1/(1+e^{A+B{t}_1})$

Accumulation of grain-filling increasing period: $W_3=K/[1/(1+e^{A+B{t}_3})-1/(1+e^{A+B{t}_2})$

The rate of grain filling during gradual increase period: $R_1=W_1/T_1$

The grain-filling rate of rapid increase period: $R_2=W_2/T_2$

Rate of grain-filling gradual increase period: $R_3=W_3/T_3$

2.3.5. Determination of Sucrose, Starch, and Soluble Sugar

Tartary buckwheat seeds of 20 d, 25 d, 30 d, 35 d, and 40 d were dried at 75 °C until they reached a constant weight. The dried grains were then hulled and ground into a powder. A 0.1 g powder sample was used to analyze the concentration of starch, sucrose, and soluble sugars. The sucrose concentration was determined using the resorcinol method, while the soluble sugar and starch concentrations were determined using the anthrone colorimetric method [22,23]. The experiment was performed with three repetitions.

2.3.6. Determination of Sucrose Synthase and Soluble Starch Synthase

A sucrose synthase activity (SS) assay kit and a soluble starch synthase activity (SSS) assay kit from Solarbio (Beijing Solarbio Science & Technology Co., Ltd., Beijing, China) were adopted.

2.4. Statistical Analysis

The raw data were digitized using Excel 2022 (Microsoft Corp, Montgomery, AL, USA). LSD analysis and ANOVA were conducted using SPSS 24.0 (IBM Corp, Chicago, AL, USA). Duncan’s multiple extreme variance test was used to assess significant sample differences (p ≤ 0.05). Graphs were created using GraphPad Prism 9.4 (GraphPad Software Corp, San Diego, CA, USA). Graph merging was performed using Adobe Illustrator 2022 (Adobe Crop, San Jose, CA, USA). Correlation analysis among indicators was conducted using CHIPLOT online software (https://www.chiplot.online (accessed on 25 October 2023)).

3. Results

3.1. Formation and Dynamic Changes of Tartary Buckwheat Flowers and Grains

3.1.1. Observation on the Dynamic Changes of Inflorescence Development, Bud Formation, Bud Development, and Grain Formation in Tartary Buckwheat

The inflorescence of Tartary buckwheat can be divided into three stages: elementary inflorescence formation period (Figure 2C(a)), inflorescence bud period (Figure 2C(b)), and inflorescence bud initiation period (Figure 2C(c)). Taking Qianku No.3 as an example, in the elementary inflorescence bud formation stage the flower stalks elongate, the number of flower buds increases, and the flower bud structure becomes apparent. Subsequently, some flower buds undergo pollination and fertilization, developing into young grains. Additionally, this study observed that the main stem inflorescence tends to bud, bloom, and form grains preferentially. As the inflorescence continued to develop, new flower buds would differentiate at the base of the flower stalk or grain stalk of the already formed flower buds or grains. Typically, a primary inflorescence could produce around 3–5 tiny flower buds, further developing to 10–20 buds upon maturity. However, it is worth noting that some inflorescences may have a more significant number of buds.

The corresponding period of bud sections (Figure 2B) revealed that Tartary buckwheat underwent fertilization within 4 d, even before the buds fully opened (Figure 2B(c)). Subsequently, the ovules started to decompose, providing nutrients and energy for the fertilized ovum, and a cavity was observed in the developing ovules (Figure 2B(d)). Concurrently, the shape of the ovary transformed from an initial oval shape (Figure 2B(a)) to a cone shape (Figure 2B(e)). By day 10, grain formation was evident (Figure 2B(f)), marking the initiation of the filling stage.

According to Figure 2D, the development of Tartary buckwheat buds and the formation of grains are closely interconnected. From 0 to 4 d, the buds continue to develop and increase in size while the petals start to unfold, reaching a semi-open state. By 6 d, the petals are fully unfolded, indicating blooming. Between 6 and 10 d, there is a phenomenon of petal convergence. At 10 d, the young grain becomes visible.

3.1.2. Dynamics of Total and Wilted Flower Numbers

The total number of flowers on Tartary buckwheat plants exhibited a ‘slow–fast–slow’ changing trend over time, observed in both cultivars (Figure 3A). When the number of flowers steadily increased, QK3 had 1014 flowers while XQ2 had 1539 flowers. The total number of flowers on the branches of both cultivars was 108–750 higher than that on the main stem. Moreover, there was a significant difference in the total number of flowers between the main stem and the branches of XQ2, with the branches having 750 flowers, which is 85.6% higher than QK3’s 108 flowers.

The number of wilted flowers on Tartary buckwheat plants exhibited a ‘slow–fast’ trend over time, observed in both cultivars (Figure 3B). At 60 d, the number of wilted flowers on the branches of QK3 increased faster than the main stem. Similarly, the number of wilted flowers on the branches of XQ2 surpassed those on the main stem at 48 d, 12 d earlier than QK3. At 54 d, XQ2 had 35–40 more wilted flowers than QK3. XQ2 had a higher number of wilted flowers compared to QK3.

3.1.3. Dynamics of Effective and Wilted Grains

The number of wilted grains on Tartary buckwheat plants exhibited a ‘slow–fast’ changing trend over time, observed in both cultivars (Figure 4A). Wilted grains appeared on the main stem 6–9 d earlier than on the branches. In comparison, the appearance of wilted grains on QK3 was delayed by 3 d compared to XQ2. The branches of XQ2 entered the rapid growth period 3 d earlier than the main stem. Additionally, the appearance time of wilted grains on QK3 (68th day) was 14 d later than that of XQ2 (54th day). On the 57th day, the number of wilted grains on XQ2 was 18–35 higher than on QK3.

The number of effective grains on Tartary buckwheat plants exhibited a ‘slow–fast’ changing trend over time, observed in both cultivars (Figure 4B). The main stem of QK3 showed an effective grain appearance time 12 d earlier than its branches. Additionally, the effective grain appearance time of QK3 was later than that of XQ2, with a difference of 9 d. The effective grain rapid growth of QK3’s main stem and branches occurred 9 d and 6 d later than XQ2. The two cultivars had a significant disparity in the number of effective grains. Specifically, on the 54th day, XQ2 had 41–55 more effective grains than QK3.

3.1.4. Spatial Distribution of Tartary Buckwheat Seed Setting

According to Figure 5, the main stem seed-setting rate of the two cultivars was significantly higher than that of the branches, with a difference of 8.47–10.49%. However, there is no significant difference in grain formation rate. The grain formation rate and seed-setting rate of the main stem, branches, and whole plant of QK3 were significantly higher than those of XQ2, by 3.44%, 4.58%, and 4.62%, respectively. Upon analyzing the grain formation rate and seed-setting rate, it was observed that the abortion rate of branched grains in both Tartary buckwheat cultivars was significantly higher than that of the main stem, with a difference of 11.70–14.98%. The abortion rates of the main stem, branches, and whole plant of QK3 were significantly lower than those of XQ2, by 2.33%, 5.61%, and 6.62%, respectively. However, the yield per plant of XQ2 was significantly higher than that of QK3, by 27.07%.

3.2. Tartary Buckwheat Grain Development and Grain-Filling Characteristics

3.2.1. The Process of Grain Development

During the development of Tartary buckwheat grains, various characteristics such as volume, length, width, color, and quality undergo continuous changes (Figure 6). For instance, for Qianku No.3, the color of Tartary buckwheat grains deepened as they developed, while the base of the grains expanded and the abdominal groove gradually filled. By the time they reached maturity, the ventral groove was no longer visible (Figure 6A(f)).

The length of the grain showed a gradual decrease as grain filling progressed (Figure 6B), and there were significant differences among different varieties (p ≤ 0.05). The difference between MS and Br also decreased as the grain-filling time extended. Throughout the same period, the grains on the branches of Tartary buckwheat were longer than those on the main stem, by 0.16–11.61%. Among different cultivars, QK3 exhibited a significantly greater length than XQ2, by 22.93–31.50%.

As the grain filling progressed, the width of the grain initially increased and then decreased (Figure 6C). Significant variations in grain width were observed among different cultivars and parts (p ≤ 0.05). Throughout the same period, except for on the 40th day, the grain width on Tartary buckwheat branches was 0.34–6.90% greater than the grain width on the main stem. Among the different cultivars, XQ2 exhibited a significantly wider grain width than QK3 and was also 6.81–12.56% longer.

As the grain filling progressed, the grain moisture content gradually decreased (Figure 6D). The differences in moisture content between the different parts of each cultivar also decreased. Throughout the same period (except for on the 40th day), the grains on the branches had a higher moisture content than those on the main stem, by 4.86% to 14.92%. However, on the 40th day, the moisture content of the main stem was 0.30% to 8.06% higher than that of the branches. Furthermore, among the different cultivars, the moisture content of XQ2 was significantly higher than that of QK3 on the 25th and 30th days, by 11.16% to 15.47%.

3.2.2. Grain-Filling Process and Characteristics

The logistic growth equation $\mathrm{Y}=\mathrm{K}/\left(1+e^{A+Bt}\right)$ was used to fit to the Tartary buckwheat grain-filling process, and the correlation coefficient was above 0.990, suggesting that this model accurately represents the Tartary buckwheat grain-filling process (

Table 1. Grain-filling curves.

Part	Simulation Equation	WS/g	WA/g	R2
QK3–MS	Y = 28.133/(1 + e5.900–0.231t)	28.133	27.314	0.999
QK3–Br	Y = 29.714/(1 + e5.727–0.215t)	29.714	28.592	0.996
QK3–WP	Y = 28.859/(1 + e5.814–0.223t)	28.859	27.953	0.998
XQ2–MS	Y = 26.947/(1 + e5.694–0.211t)	26.947	25.184	0.990
XQ2–Br	Y = 27.794/(1 + e5.185–0.188t)	27.794	25.138	0.992
XQ2–WP	Y = 27.323/(1 + e5.432–0.199t)	27.323	25.159	0.991

Note: XQ2 stands for Xiqiao No.2; QK3 stands for Qianku No.3; MS stands for main stem; Br stands for branch; WP stands for whole plant (g). Among them, W_S stands for theoretical maximum 1000-grain weight; W_A stands for actual maximum 1000-grain weight (g); R² stands for coefficient of determination.

). The theoretical 1000-grain weight and actual 1000-grain weight of QK3 were higher than those of XQ2, by 5.62% and 11.10%, respectively. Additionally, the theoretical 1000-grain weight of the branches was greater than that of the main stem, ranging from 3.14% to 5.62% higher than that of QK3. However, the actual 1000-grain weight of both cultivars was lower than the theoretical 1000-grain weight, indicating a possible potential for increasing the 1000-grain weight.

Differences in T_max and R_max were observed among grains in different parts of various cultivars. Specifically, the T_max of QK3 was 3.50–5.42% lower than that of XQ2 (

Table 2. Characteristic parameters of grain filling.

Part	Grain-Filling Characteristics Parameter
	Tmax	Rmax	T	R	T1	T2	T3	R1	R2	R3	W1	W2	W3
	(d)	(g/d)	(d)	(g/d)	(d)	(d)	(d)	(g/d)	(g/d)	(g/d)	(g)	(g)	(g)
QK3–MS	25.54	1.62	45.43	0.62	19.84	11.40	14.19	0.30	1.42	0.40	5.95	16.24	5.66
QK3–Br	26.64	1.60	48.01	0.62	20.51	12.25	15.25	0.31	1.40	0.39	6.28	17.16	5.98
QK3–WP	26.07	1.61	46.68	0.62	20.17	11.81	14.70	0.30	1.41	0.40	6.10	16.66	5.81
XQ2–MS	26.99	1.42	48.76	0.63	20.74	12.48	15.54	0.27	1.25	0.35	5.69	15.56	5.43
XQ2–Br	27.58	1.31	52.02	0.53	20.57	14.01	17.44	0.29	1.15	0.32	5.87	16.05	5.60
XQ2–WP	27.30	1.36	50.39	0.54	20.68	13.24	16.47	0.28	1.19	0.33	5.77	15.77	5.50

Note: XQ2 stands for Xiqiao No.2; QK3 stands for Qianku No.3; MS stands for main stem; Br stands for branch; WP stands for whole plant; T_max stands for maximum grain-filling rate arrival time; R_max stands for maximum grain-filling rate; T stands for grain-filling duration; R stands for average grain-filling rates; T₁ stands for grain-filling gradual increase period; T₂ stands for grain-filling rapid increase period; T₃ stands for grain-filling increasing period duration; R₁ stands for the rate of grain filling during gradual increase period; R₂ stands for the grain-filling rate of rapid increase period; R₃ stands for slow increase grouting rate of grain filling; W₁ stands for accumulation of grain-filling gradual increase period; W₂ stands for accumulation of grain-filling rapid increase period; W₃ stands for accumulation of grain-filling increasing period.

), while its R_max was 14.30–22.26% higher. Additionally, the T_max of the main stem was 2.15–4.11% lower than that of the branches, but the R_max was 1.73–8.81% higher compared to the branched grains.

There were variations between cultivars of T and R. QK3 had a lower T than XQ2, by 6.83–7.71%. However, the R of QK3 was higher than that of XQ2, with a difference of 10.05–15.84% (Table 2). There are also differences in T₁ and T₂ between different cultivars. QK3 has lower T₁ and T₂ than XQ2, by 0.31–4.36% and 8.66–12.66%, respectively. Among different plant parts, the main stem T₂ is 6.93–10.9% smaller than the branches. R₁, R₂, and R₃ all indicated that QK3 had higher values than XQ2, with differences of 7.24–9.16%, 14.30–22.26%, and 14.30–22.26%, respectively. Additionally, the accumulation amount during the corresponding period showed that QK3 has a higher value than XQ2, with a difference of 4.40–6.91%.

3.3. Accumulation of Substances and Their Physiological Changes after Flowering in Tartary Buckwheat

3.3.1. Dry Matter Accumulation and Distribution

There were noticeable variations in dry matter accumulation in different organs of Tartary buckwheat at various stages after flowering (

Table 3. Tartary buckwheat plant dry matter accumulation in different organs at different periods.

	Cultivar	Root		Stem		Leaf		Main Stem				Branch
								Grain		Flower		Grain		Flower
		Weight (g)	Ratio (%)	Weight (g)	Ratio (%)	Weight (g)	Ratio (%)	Weight (g)	Ratio (%)	Weight (g)	Ratio (%)	Weight (g)	Ratio (%)	Weight (g)	Ratio (%)
10 d	QK3	0.14 c	7.6	0.60 b	36.1	0.84 a	51.6	/	/	0.06 d	2.5	/	/	0.04 d	2.4
	XQ2	0.18 c	8.2	0.94 b	36.0	1.31 a	50.2	/	/	0.06 c	3.4	/	/	0.06 c	2.3
20 d	QK3	0.24 b	7.1	1.47 a	40.7	1.45 a	41.0	0.22 b	5.9	0.06 b	1.8	/	/	0.13 b	3.4
	XQ2	0.26 b	4.8	2.22 a	40.3	2.48 a	45.1	0.27 b	5.0	0.07 b	1.4	/	/	0.19 b	3.3
30 d	QK3	0.31 c	4.3	2.57 a	32.4	2.20 a	29.4	1.28 b	17.0	0.10 c	1.5	0.96 b	12.6	0.20 c	2.9
	XQ2	0.31 d	3.6	3.16 a	36.9	2.53 b	28.9	1.16 c	13.4	0.08 d	1.0	0.86 c	13.0	0.26 d	3.2
40 d	QK3	0.42 c	4.6	3.22 a	33.4	2.06 b	21.4	1.68 b	18.3	0.09 c	1.0	1.96 b	19.8	0.14 c	1.5
	XQ2	0.43 c	4.0	3.15 a	29.6	2.25 b	19.5	1.85 b	16.3	0.06 c	0.5	3.07 a	28.7	0.16 c	1.4
50 d	QK3	0.41 cd	3.9	3.53 a	32.6	0.89 c	8.1	2.03 b	18.5	0.07 d	0.6	3.84 a	35.5	0.10 d	0.9
	XQ2	0.36 e	2.5	4.17 b	28.6	1.59 d	10.8	2.37 c	16.1	0.07 e	0.5	5.41 a	40.3	0.21 e	1.4

Note: XQ2 stands for Xiqiao No.2; QK3 stands for Qianku No.3; 10 d, 20 d, 30 d, 40 d, and 50 d are days after flowering. Means indicated by different lowercase letters within a column refer to statistically significant differences at p ≤ 0.05.

). The weight of plant roots and stems gradually increased, while their proportions gradually decreased. The weight of leaves initially increased and then decreased, with a downward trend in proportion. The dry weight of flowers on the main stem decreased. Meanwhile, the proportion of branch flowers was greater than that of the main stem, and the dry weight of flowers in both parts did not exceed 6%. The dry weight of the main stem and branched grains showed a consistent upward trend. The changing trend in weight proportions of the roots, stems, main stem flowers, and branched grains in QK3 was smaller than in XQ2, by 35.09%, 52.70%, 34.48%, and 16.12%, respectively. Conversely, the changing trend in proportions of leaves, main stem grains, and branched flowers in QK3 was more significant than that of XQ2, by 10.41%, 14.91%, and 66.67%, respectively.

There were differences in the biomass growth distribution rates of different organs in different cultivars after flowering (

Table 4. Tartary buckwheat monocot biomass growth distribution rate of each organ over different time periods (%).

Period	Cultivar	Stem	Leaf	Main Stem		Branch
				Grain	Flower	Grain	Flower
10–20 d	QK3	49.52 a	34.07 b	11.19 a	0.72 a	/	5.02 a
	XQ2	45.12 a	40.86 a	8.96 a	0.80 a	/	4.25 a
20–30 d	QK3	28.87 a	20.20 a	28.49 a	1.08 a	29.53 a	1.38 a
	XQ2	31.08 a	8.13 b	31.00 a	0.07 b	31.12 a	2.23 a
30–40 d	QK3	36.03 a	−10.05 a	21.95 b	−1.28 a	50.78 a	−2.14 a
	XQ2	8.16 b	−9.21 a	28.69 a	−0.30 a	88.19 b	−3.54 b
40–50 d	QK3	23.95 a	−83.59 a	28.14 a	−6.85 a	144.58 a	−10.97 b
	XQ2	18.84 b	−19.21 b	10.61 b	−0.67 a	63.42 b	3.20 a

Note: XQ2 stands for Xiqiao No.2; QK3 stands for Qianku No.3; 10 d, 20 d, 30 d, 40 d, and 50 d are days after flowering. Means indicated by different lowercase letters in the table refer to statistically significant differences at p ≤ 0.05.

). The growth distribution rate of QK3 stems generally decreased, although the overall magnitude was small compared to XQ2. However, XQ2 exhibited a more significant rebound between 40 and 50 d. The growth distribution rates of QK3 leaves also showed a downward trend, with QK3 and XQ2 leaves displaying negative growth distribution rates after 30 d. The growth distribution rate of QK3 leaves was 95.87% higher than that of XQ2. Similarly, the growth distribution rates of the main stem and branched flowers of QK3 decreased overall, with QK3 experiencing a more significant decline than XQ2. However, the branched flowers of XQ2 rebounded after 40 d. The growth distribution rate of the main stem grains of QK3 exhibited an “up–down–up” trend, with minimal change after 20 d. Conversely, XQ2 displayed an “up–down” trend, with a significant drop in the growth distribution rate after 40 d. On the other hand, the growth distribution rate of QK3 branched grains showed an increasing trend. In contrast, XQ2 exhibited an “up–down” trend, but the overall change range was much smaller than that of QK3, with a change range of only 28.07%.

3.3.2. Grain Sucrose–Starch Conversion

As grain filling progresses, the soluble sugar concentration in the grains gradually increases. Different cultivars had variations in the soluble sugar concentration (Figure 7A). During the same period, the main stem grains consistently exhibited significantly higher soluble sugar concentration than those in the branches, ranging from 1.83% to 4.52%. As the grain filling progresses, the sucrose concentration in the grains exhibits a gradual decrease (Figure 7B). During the same period, the sucrose concentration of the grains on the main stem was consistently higher than that of the grains on the branches (except on the 40th day), with a difference of 3.06–8.37%. Furthermore, the sucrose concentration of QK3 was consistently higher than that of XQ2, by 0.29% to 25.99%. The starch concentration in grains gradually increased as the grain-filling process advanced. There was no significant difference in starch concentration among cultivars (Figure 7C). During the same period, the starch concentration in main stem grains was significantly higher than in branch grains, by 1.20% to 3.92%.

3.3.3. Sucrose–Starch Metabolism and Enzyme Activities

During the grain—filling process, the starch synthase (SS) activity in the grains gradually decreased (Figure 8A). The SS activity in the superior parts was higher than in the inferior grains, by 6.36% to 16.25%. Similarly, the SS activity in the superior and inferior QK3 grains was higher than in XQ2, by 7.13% to 12.15% and 3.67% to 21.01%, respectively.

The starch synthase sucrose synthase (SSS) activity also gradually decreased as the grain filling progressed (Figure 8B). The SSS activity in the superior parts of the grains was significantly higher than in the inferior parts, with a difference of 13.33% to 16.61%. Moreover, the SSS activities in the superior and inferior grains of QK3 were higher than those of XQ2, by 5.58% to 14.87% and 3.61% to 18.60%, respectively.

3.3.4. Pearson Correlation Analysis

Correlation analysis examined the relationships between various indicators at different flowering periods. At 30 d after flowering (Figure 9A), there were extremely significant positive correlations (p ≤ 0.01) between starch concentration, soluble sugar concentration, SS, sucrose concentration, and SSS. Additionally, a significant positive correlation (p ≤ 0.05) was observed between fresh weight and starch concentration, SSS, and SS. However, the correlations between other indicators were not found to be significant (p > 0.05). Similarly, at 35 d after flowering (Figure 9B), a highly significant positive correlation (p ≤ 0.01) was found between dry weight, fresh weight, SSS, starch concentration, and soluble sugar concentration. Moreover, starch concentration was significantly correlated with soluble sugar concentration, sucrose concentration, and SSS, while sucrose concentration and soluble sugar concentration were significantly correlated with SSS and SS (p ≤ 0.05). Again, the correlations between other indicators were not significant (p > 0.05). Finally, at 40 d after flowering (Figure 9C), a significant positive correlation (p ≤ 0.01) was observed between dry weight, fresh weight, sucrose concentration, starch concentration, soluble sugar concentration, SSS, and SS. Furthermore, starch concentration was significantly positively correlated with soluble sugar concentration and SS (p ≤ 0.05). However, the correlations between other indicators were not found to be significant (p > 0.05).

Combined with the results of correlation analysis, at 30 d the formation of fresh weight is primarily influenced by SS, SSS, and starch concentration. During this period, the grain has higher sucrose concentration and lower starch concentration (Figure 7B,C). The sucrose synthesis is mainly associated with SS and SSS, indicating that the grain is in the rapid grain−filling period. Additionally, starch synthesis is primarily related to SSS. By 35 d, there is a significant correlation between dry weight and fresh weight. During this time, starch formation in fresh weight is mainly influenced by SSS, sucrose, and soluble sugar. The sucrose concentration in the grain begins to decrease while the starch concentration starts to increase (Figure 7B,C), suggesting the conversion of sucrose into starch at this stage. By 40 d, there is an extremely significant correlation between dry weight and fresh weight. At this point, the sucrose concentration further decreases and the starch concentration continues to increase (Figure 7B,C). SS, SSS, sucrose, and soluble sugar primarily influence the starch formation in fresh weight.

4. Discussion

4.1. Tartary Buckwheat Flower and Grain Formation and Dynamics

Tartary buckwheat, belonging to the infinite inflorescence, has a flowering period that accounts for approximately two-thirds of its growth period. Interestingly, the same plant may exhibit flowers, inflorescences, and grains with varying degrees of development [24]. This study demonstrates that flower buds continue to form on the Tartary buckwheat plant even when mature grains are present (Figure 2A). Additionally, there is a notable discrepancy in the spatial distribution of flowers on the main stem and branches, aligning with the findings of Cawoy et al. [25]. Tartary buckwheat’s unique flowering and seed-setting characteristics lead to increased competition among seed-setting organs and a decline in source organs, decreasing the number of new flower buds and grains. This reduction may be a significant factor contributing to the decreased seed-setting rate.

It is reported that many wilted flowers and grains accumulate in Tartary buckwheat in the later stage, resulting in a reduction in the overall seed-setting rate of the plant [26]. Our research discovered that the total number of flowers on the branches of Tartary buckwheat is higher than that on the main stem. Furthermore, the accumulation rate of the total number of flowers and grains increases after entering the whole flowering period, and the number of wilted flowers, wilted grains, and effective grains also increases during the middle of the growth period. A significant amount of accumulation occurs during the later period. This rapid growth during the middle period may be attributed to a shift in focus from vegetative growth to reproductive growth [27]. The subsequent slow growth is mainly caused by intensified competition between flowers and grains. During this period, the leaves of Tartary buckwheat may experience problems such as ageing and abscission, leading to a reduction in total photosynthetic capacity. The grains consume a significant number of photosynthetic products, resulting in a decline in the supply of these products [28]. The formation of new inflorescences and flower buds follows a pattern of fast growth followed by slow growth. In the middle and late stages of growth, the accumulation of wilted flowers and grains in Tartary buckwheat accelerates. Analysis suggests this acceleration is primarily due to competition among seed-setting organs, leading to insufficient sources.

4.2. Tartary Buckwheat Grain Development and Matter Accumulation after Flowering

The accumulation and distribution of dry matter primarily influences the formation of crop yield. Enhancing dry matter accumulation in plants is crucial to achieving high yields [29]. This study found that, after flowering, the growth distribution rate of organ dry matter among different cultivars shifted from stems, leaves, and flowers to grains. Throughout the grain development process, the color of the grains became progressively darker, and the base of the grains continued to expand. At maturity, the moisture content significantly decreased (Figure 5A). During the same period, the grain length, width, and moisture content of the branches were higher than those of the main stems, and there were noticeable variations between the two cultivars. Our results also showed that cultivars with high dry matter accumulation distribute more grain material and promote seed setting, indicating that the grain dry matter distribution rate may be a crucial factor for the differences in seed-setting rate and 1000-grain weight [30].

The grain-filling period is a critical stage for crop grain filling and yield formation [31]. This study discovered that the rate of grain filling and the continuous grain-filling time could be significant factors contributing to the variations in the seed-setting rate of Tartary buckwheat. The grain-filling characteristics of both Tartary buckwheat cultivars follow a logistic curve, exhibiting a pattern of ‘slow–fast–slow’ grain-filling trend. Additionally, the time taken to reach the maximum grain-filling rate (T_max) for QK3 was shorter than that of XQ2, and the maximum grain-filling rate (R_max) was higher than that of XQ2. Despite the shorter continuous grain-filling time in QK3, it exhibits a higher grain-filling rate, increasing 1000-grain weight. This phenomenon may be attributed to the size of the leaf area, where a greater ‘source’ provided ample energy support for grain filling [32]. Correlation analysis revealed that the higher grain-filling rate in QK3 compared to XQ2 may be associated with the activity of SS and SSS. The higher activity of SS and SSS allows for earlier completion of grain filling and a significant amount of photosynthetic products and also provides more carbohydrate to other developing grains. Consequently, this leads to an increase in both 1000-grain weight and yield.

The starch synthesis rate and accumulation play a crucial role in determining the rate and degree of grain filling [33]. The starch synthesis relies on sucrose as its raw material, which is synthesized through photosynthesis and transported to the grain endosperm [34]. As a direct carbon source in grain filling, soluble sugar contributes to transposing photosynthetic products, enhances dry matter accumulation, and ultimately promotes grain development [35]. This study reveals a gradual increase in Tartary buckwheat starch and soluble sugar concentration during grain filling, with higher levels observed in the main stem grains than in those on the branches. Moreover, the sucrose concentration in the grains decreases as grain filling progresses, with higher levels found in the main stem grains than in the branch grains. Notably, QK3 exhibits higher sucrose concentration than XQ2 at all stages. The correlation analysis indicates that during the early stage of grain maturity, which is the rapid grain-filling stage, the sucrose concentration is high, and the synthesis of starch is not dependent on sucrose decomposition but is mainly associated with SSS. In the middle and late stages of grain maturity, the sucrose concentration gradually decreases while the starch concentration gradually increases. During this period, starch synthesis relies on sucrose decomposition and SSS. Overall, since the sucrose concentration in the main stem grains is higher than that in those on the branches, the conversion of starch into sucrose is relatively high in the later stage, leading to a higher seed-setting rate in the main stem compared to the branches.

Previous studies have demonstrated the involvement of SS in the synthesis of amylopectin [36]. SSS, on the other hand, is the initial enzyme responsible for sucrose degradation in grains and plays a crucial role in controlling starch synthesis [37]. In our study, it was observed that the activities of SS and SSS were higher in QK3 compared to XQ2 during the same period. Additionally, the enzyme activity was higher in the superior grains than in the inferior grains. As the grains matured, a gradual decline in the activities of SS and SSS was observed. This finding aligns with the previous discovery that QK3 has a higher grain-filling rate than XQ2. These results suggest that high SS and SSS activity during the early stages of Tartary buckwheat grain filling can contribute to an enhanced grain-filling rate and a reduced time for grain filling. Consequently, more carbohydrate can be allocated to the young and tender grains in the initial stages of development, increasing both the 1000-grain weight and the yield.

5. Conclusions

Two genotypes of Tartary buckwheat demonstrated that bud formation, flowering, seed setting, grain filling, and grain maturation co-occur in Tartary buckwheat plants. Further research revealed that the grain-filling process in Tartary buckwheat follows the crop’s ‘S’ growth curve, aligning with the logistic curve. The overall growth distribution rate of stems, leaves, and flowers showed a downward trend, while both the main stem grains and branch grains displayed an upward trend. The transfer of dry matter from roots, stems, and leaves to the grain is significantly higher in the main stem compared to the branches. Higher levels of SS, SSS, soluble sugar, and sucrose concentration in the grain of QK3 contribute to an increased grain-filling rate and shortened grain-filling time. This allows for reduced consumption of photosynthesis products and a greater allocation of nutrients to the developing grains, increasing the 1000-grain weight.