Timing Matters: The Impact of Sowing Dates on Photosynthetic Traits and Biomass Accumulation in Hybrid Winter Wheat

Abstract

At present, the use of hybrids is becoming more and more widespread, and the study of its traditional cultivation methods also needs to pay attention to the combination of good seed and good methods to achieve maximum economic benefits. “Good seed” is selected from varieties with high production potential and strong resistance to stress. To investigate the effects of sowing dates on biomass accumulation and photosynthetic characteristics of hybrid winter wheat, this study was conducted over two growing seasons (2021–2023) using a hybrid variety, Jingmai 17, and a conventional variety, Jimai 22. Three sowing dates were tested: October 13 (D1), October 23 (D2), and November 2 (D3). Results indicated that Jingmai 17 had a larger leaf area per culm (LAC) post-anthesis, and a lesser variation in SPAD values throughout the middle and late irrigation phases, leading to enhanced photosynthetic performance and higher biomass accumulation at maturity. It outperformed Jimai 22 by approximately 10% in yield. The optimal sowing date (D2) allowed both varieties to maintain a favorable LAC, which supported higher SPAD values, net photosynthetic rates (Pn), and stomatal conductance (Gs) during late grain filling. Correlation analysis revealed a positive relationship between post-anthesis biomass accumulation and yield, suggesting that increased accumulation at this stage underpins yield formation. Under D2 conditions, Jingmai 17 and Jimai 22 achieved yields of 9603.70 kg·ha⁻¹, 9058.68 kg·ha⁻¹ and 8785.44 kg·ha⁻¹, 8294.89 kg·ha⁻¹, respectively, which are 3.4%, 4.6% and 3.4%, 12.4% higher than under D1 conditions, respectively, and 10.7%, 12.4% and 12.8%, 14.5% higher than under D3 conditions, respectively. D1 had higher thousand-grain weight (TGW) but lower number of grains per unit area (GN); D3 had lower TGW, while the GN was not significantly higher, and D2 was effective in improving seed yield. Overall, Jingmai 17 demonstrated significant photosynthetic and yield advantages, maximizing yield potential through optimal interactions among spike number (SN), grain number per spike, and TGW at the October 23 sowing date.

1. Introduction

In recent years, the utilization of hybrid wheat has rapidly developed, being recognized as a crucial approach for significantly enhancing yield and overall production capacity [1,2]. The use of hybrid wheat exhibits remarkable characteristics such as high yield potential, stability, and broad adaptability [3]. To fully exploit the yield potential of hybrid wheat, appropriate cultivation techniques are essential; among them, the sowing date is one of the most direct and easily implemented measures for increasing winter wheat yield, significantly affecting growth and final yield [4]. Appropriate sowing dates can achieve higher economic yields without incurring additional costs [5]. The negative impacts of unfavorable environmental conditions on grain yield can be minimized by adjusting the sowing date [6,7].

Grain yield (GY) is determined by the spike number per unit area, grain number per spike, and the thousand-grain weight (TGW) [8]. The formation of yield in hybrid wheat results from the complementary and cumulative advantages of these components, while also being influenced by various internal and external factors [9]. Appropriate sowing effectively balances the relationships among effective spike number (SN), grain number per spike and TGW, leading to higher yields [10]. Research has shown that wheat yield tends to decrease gradually with delayed sowing dates, likely due to reductions in both SN and TGW [11,12]. Other studies have indicated that, within a certain range, delaying the sowing date can lead to a gradual decrease in SN, while the grain number per spike and TGW may increase, resulting in a trend of yield that first increases and then decreases. This suggests that appropriate delay sowing can enhance wheat yield [13,14].

Most studies suggest that hybrid wheat also exhibits significant advantages in light utilization, with pronounced single-leaf photosynthetic efficiency. The photosynthetic characteristics of the flag leaf significantly influence grain weight and yield formation [15,16,17], and this photosynthetic advantage is closely linked to high-yield cultivation practices [18,19]. The sowing date has a significant impact on chlorophyll content, photosynthetic rate (Pn), and stomatal conductance (Gs) in the flag leaf of winter wheat [20,21]. Delaying sowing can enhance SPAD values and leaf area index, thereby increasing the Pn and accumulation of photosynthetic products, which helps stabilize GY [22]. Biomass accumulation, which is the buildup of photosynthetic assimilates, forms the foundation for wheat yield formation [23]. Even slight increases in Pn can translate into significant biomass increases, leading to substantial yield enhancement [24]. Appropriate delays in the sowing date can enhance biomass accumulation at maturity and result in higher harvest indices (HI), thereby promoting yield increases [25]. However, some studies indicate that delayed sowing has little or no significant effect on biomass accumulation and HI [26]. GY formation primarily relies on the translocation of stored assimilates to grains before anthesis and the accumulation of photosynthetic products after anthesis [27], with increased post-anthesis assimilate accumulation particularly beneficial for enhancing HI and GY [28].

2. Methods and Materials

2.1. Experimental Site Overview

The experiment was conducted at the Jingkou Experimental Station of Qingdao Academy of Agricultural Sciences (36°30′ N, 120°39′ E) between 2021 and 2023. Wheat was bottom-treated with 1500 kg·ha⁻¹ of organic fertilizer and 750 kg·ha⁻¹ of compound fertilizer (N:P₂O₅:K₂O = 17:17:17) before sowing, and 150 kg·ha⁻¹ of urea was applied retrospectively at the pulling-out stage. The nutrient contents of the 0–20 cm soil, the amount of precipitation (mm), and the daily mean air temperature (°C) of the experimental sites in both years before sowing are shown in Table 1 and Figure 1.

Table 1. The 0–20 cm soil layer nutrient contents of the experimental field.

Year	Organic Matter (g·kg−1)	Soil pH	Total Nitrogen (g·kg−1)	Alkaline-Hydrolysable Nitrogen (mg·kg−1)	Available Phosphorus (mg·kg−1)	Available Potassium (mg·kg−1)
2021–2022	14.9 ± 0.91	6.7 ± 0.11	1.2 ± 0.08	94.6 ± 0.08	64.7 ± 1.12	111.6 ± 9.83
2022–2023	14.6 ± 0.63	6.6 ± 0.19	1.1 ± 0.03	110.4 ± 0.02	68.7 ± 1.40	129.6 ± 9.37

Table 1. The 0–20 cm soil layer nutrient contents of the experimental field.

Year	Organic Matter (g·kg−1)	Soil pH	Total Nitrogen (g·kg−1)	Alkaline-Hydrolysable Nitrogen (mg·kg−1)	Available Phosphorus (mg·kg−1)	Available Potassium (mg·kg−1)
2021–2022	14.9 ± 0.91	6.7 ± 0.11	1.2 ± 0.08	94.6 ± 0.08	64.7 ± 1.12	111.6 ± 9.83
2022–2023	14.6 ± 0.63	6.6 ± 0.19	1.1 ± 0.03	110.4 ± 0.02	68.7 ± 1.40	129.6 ± 9.37

Figure 1. Precipitation and daily mean air temperature during the winter wheat growing season.

Figure 1. Precipitation and daily mean air temperature during the winter wheat growing season.

2.2. Materials and Experimental Design

The test materials were the hybrid winter wheat variety Jingmai 17 (BS267×07Y Hua 91-5, Hybrid Wheat Research Institute of the Beijing Academy of Agriculture and Forestry Sciences, Beijing, China) and the conventional winter wheat variety Jimai 22 (Crop Research Institute of Shandong Academy of Agricultural Sciences, Jinan, China). Jingmai 17 is a winter variety, with wide and long leaves, moderate tillering ability, compact plant type, high resistance to collapse, and desirable maturity traits. The spikes are fusiform with long awns and red grains, and the grains are semi-hard and well-filled. Jimai 22 is a semi-winter variety, with high spike rate, compact plant type, high resistance to collapse, and desirable maturity traits. The spikes are fusiform, long-awned, with white grains, and are well-seeded and semi-horny.

The experiment was designed a split-plot design, with three sowing dates (October 13 (D1), October 23 (D2), and November 2 (D3)) as the main plot, and two varieties as the subplots, for a total of six treatments, with three replicates set up for each treatment. The area of the subplot measured 6 m² (2 m × 3 m), with equal row spacing of 20 cm. Both varieties were sown to achieve a target plant density of 300 seeds·m⁻² and were harvested uniformly on 13 June 2022 and 7 June 2023.

2.3. Measurement Traits and Methods

2.3.1. Flag Leaf SPAD

The flag leaf SPAD values were measured on 0, 7, 14, 21, and 28 days after anthesis (DAA) by randomly selecting 9 single stems from each plot using a SPAD-502 Minolta chlorophyll meter (Spectrum Technologies, Plainfield, IL, USA) and measuring the upper, middle and lower parts of each leaf and taking the average value; these measurements were taken between 10:00 and 11:00 a.m. on sunny days.

2.3.2. Leaf Area per Culm (LAC)

On 0, 7, 14, 21, and 28 DAA, in each treatment plot, 30 wheat plants were taken consecutively, and the green leaf area was measured with an LI-3100C Table type leaf area meter (LI-COR, Lincoln, NE, USA), and the single stem leaf area was calculated [29].

2.3.3. Photosynthetic Characteristics (Pn) of the Flag Leaves

On 0, 7, 14, 21, and 28 DAA, the net photosynthetic rate (Pn), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) of the flag leaves were measured between 9:30 a.m. and 12:00 a.m. on sunny days using a LI-6400 Portable plant photosynthesis and respiration instrument (LI-COR, USA). Three flag leaves were selected in each plot for determination with three replications, totaling 9 flag leaves per treatment.

2.3.4. Biomass Accumulation and Remobilization

At anthesis and maturity, 30 representative wheat plants were selected in each plot, and their above-ground parts were taken for sub-sampling treatment; they were divided into stems, leaves, and spikes at anthesis, and into stem leaves, glumes, and grains at maturity. Each green organ of the plant was oven-dried at 105 °C for 30 min, and at 75 °C until constant weight, and the biomass weight of each organ was recorded. The relevant calculations were as follows [30]:

$$\mathit{BR}=\mathit{BManthesis}-\mathit{BMmaturity} \mathit{without} \mathit{grain}$$

(1)

$$\mathit{BMPost}=\mathit{BMmaturity}-\mathit{BManthesis}$$

(2)

$$\mathit{HI}=\mathit{GY}/\mathit{DMtotal}$$

(3)

where BR is biomass remobilization, BManthesis and BMmaturity are biomass at anthesis and maturity, BMPost is post-anthesis biomass, DMtotal is total biomass weight of the plant.

2.3.5. Yield and Its Components

Before maturity, wheat was surveyed for spike number (SN) (investigating the number of spikes in a meter of double rows for conversion) and the grain number per spike (convert the number of grains in a spike by taking 50 spikes). At maturity, manual harvesting was carried out; an area of 2.4 m² was selected for harvesting in each plot. After harvesting, threshing, removal of impurities, weighing, and calculation of yield (at 13% moisture content), the thousand-grain weight (TGW) was also measured (at 13% moisture content).

2.4. Data Processing and Statistical Analysis

Data were processed using Microsoft Excel 2021, and analysis of variance (ANOVA) was conducted using SPSS 26.0 statistical software (SPSS Inc., Chicago, IL, USA). Differences among treatments were determined using the Least Significant Difference test (LSD; p = 0.05). Graphs and correlation analyses were performed using Origin 2021 software (Origin Lab Corporation, Northampton, MA, USA).

3. Results

3.1. Impact of Sowing Date on SPAD Values of the Flag Leaf

The SPAD values of the flag leaves for both varieties exhibited a consistent pattern across the two years. With the advancement of the grain-filling process, the SPAD values increased initially and then decreased from 0 to 28 DAA, peaking at 7 DAA. As the sowing date was delayed, the flag leaf SPAD values of Jingmai 17 and Jimai 22 tended to increase, reaching their highest values under D3 treatment (Figure 2). This indicates that late sowing enhances SPAD values, reduces the rate of decline, and delays senescence after anthesis. Under the same sowing date treatment, the SPAD values of Jingmai 17 were lower than those of Jimai 22 from 0 to 21 DAA, but significantly higher at 28 DAA. This suggests that Jingmai 17 decreased more slowly in SPAD value during the late filling stage, indicating a longer duration of effective photosynthesis.

3.2. Impact of Sowing Date on Leaf Area per Culm (LAC) of the Flag Leaf

The LAC for both varieties decreased from 0 to 28 DAA, showing consistent trends across the two years (Figure 3). As the sowing date was delayed, the LAC of Jingmai 17 and Jimai 22 gradually declined, with significantly higher values under D1 treatment. Early sowing facilitates the establishment of a larger LAC, thereby providing a better foundation for effective photosynthesis. Under the same sowing date treatment, Jingmai 17 consistently exhibited a higher LAC than Jimai 22 across both years, with Jingmai 17’s decline in leaf area post-anthesis occurring at a slower rate in contrast to Jimai 22. This indicates that Jingmai 17 is better at keeping leaves green during the post-anthesis period.

Figure 3. Leaf area per culm (LAC) after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 3. Leaf area per culm (LAC) after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.3. Impact of Sowing Date on Photosynthetic Characteristics of the Flag Leaf

3.3.1. Impact on Net Photosynthetic Rate (Pn)

During 0 to 28 DAA, the Pn for both Jingmai 17 and Jimai 22 initially increased and then decreased, peaking at 7 DAA, with consistent patterns across both years (Figure 4). As the sowing date was delayed, the Pn of both varieties showed an increasing and then decreasing trend, with the highest rates under D2 treatment, followed by D1, and the lowest under D3. In the 2021–2022 season, the Pn of Jingmai 17 was slightly greater than Jimai 22’s, while during the 2022–2023 season, it was lower between 21 and 28 DAA than Jimai 22’s, but higher between 0 and 14 DAA. Despite slight variations, Jingmai 17 consistently showed a higher overall Pn, indicating stronger photosynthetic performance and greater accumulation of photosynthetic products.

Figure 4. Net photosynthetic rate (Pn) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 4. Net photosynthetic rate (Pn) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.3.2. Impact on Stomatal Conductance (Gs)

Both Jingmai 17 and Jimai 22 revealed a downward trend in Gs from 0 to 28 DAA, with a faster decline from 0 to 21 DAA and a slower decline from 21 to 28 DAA (Figure 5). Additionally, Gs increased with delayed sowing, reaching the highest values under D3 treatment, suggesting that late sowing enhances Gs, creating favorable conditions for photosynthesis. Under the same sowing date treatment, Jingmai 17 consistently exhibited lower Gs than Jimai 22, indicating that Jingmai 17 does not have a Gs advantage over Jimai 22.

Figure 5. Stomatal conductance (Gs) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 5. Stomatal conductance (Gs) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.3.3. Impact on Intercellular CO₂ Concentration (Ci)

Both Jingmai 17 and Jimai 22 showed an increasing trend in Ci from 0 to 28 DAA, with consistent patterns across the two years (Figure 6). As the sowing date was delayed, Ci gradually increased, reaching the highest values under D3 treatment. This indicates that early sowing enhances the flag leaf’s ability to utilize intercellular CO₂, promoting photosynthesis. Under the same sowing date treatment, Jingmai 17 had slightly lower Ci than Jimai 22, suggesting that Jingmai 17 is more effective at utilizing intercellular CO₂, resulting in better photosynthetic performance.

Figure 6. Intercellular CO₂ concentration (Ci) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 6. Intercellular CO₂ concentration (Ci) of flag leaves after anthesis for different treatments in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.4. Effects of Sowing Date on Biomass Accumulation in Winter Wheat

3.4.1. Impact of Sowing Date on Biomass Accumulation at Anthesis and Maturity

As shown in Figure 7, the biomass accumulation of Jingmai 17 and Jimai 22 at anthesis decreased progressively with delayed sowing, reaching a maximum under D1 treatment. Conversely, the biomass accumulation at maturity initially increased and then decreased with delayed sowing, peaking under D2 treatment, with consistent patterns observed over two years. In the 2021–2022 season, Jingmai 17 demonstrated notable variations in biomass accumulation at both anthesis and maturity across treatments, with maximum values of 13,361.42 kg·ha⁻¹ and 19,240.08 kg·ha⁻¹, respectively. For Jimai 22, significant differences were noted in biomass accumulation at anthesis, peaking at 12,024.17 kg·ha⁻¹, while maturity accumulation showed no discernible variations between D1 and D2 but was much greater than D3, with a maximum of 17,801.54 kg·ha⁻¹. In the 2022–2023 season, both varieties exhibited no discernible variations in biomass accumulation at anthesis between D1 and D2, which were significantly higher than D3, reaching maximum values of 11,921.53 kg·ha⁻¹ and 10,901.37 kg·ha⁻¹, respectively. Significant differences were also observed in biomass accumulation at maturity, with peaks of 17,986.12 kg·ha⁻¹ for Jingmai 17 and 16,809.51 kg·ha⁻¹ for Jimai 22. Across both years, Jingmai 17 consistently outperformed Jimai 22 in both anthesis and maturity biomass accumulation, averaging 10.7% and 9.8% higher at anthesis, and 7.1% and 9.1% higher at maturity, respectively.

Figure 7. Biomass at anthesis and maturity for different treatments in 2021–2022 (a,c) and 2022–2023 (b,d) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 7. Biomass at anthesis and maturity for different treatments in 2021–2022 (a,c) and 2022–2023 (b,d) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.4.2. Impact of Sowing Date on Biomass Remobilization and Post-Anthesis Biomass Accumulation

Figure 8 indicates that with delayed sowing, biomass remobilization for both varieties decreased, peaking under D1 treatment. Post-anthesis biomass accumulation initially increased and then decreased, reaching a maximum under D2 treatment, with similar trends observed over two years. In the 2021–2022 season, Jingmai 17 showed significant differences in biomass remobilization across treatments, with a maximum of 3803.67 kg·ha⁻¹, which was 23.5% higher than the lowest treatment. For Jimai 22, no discernible variation was found between D2 and D3 for biomass remobilization, which was significantly lower than D1, where remobilization reached 2780.65 kg·ha⁻¹, a 20.0% increase over the lowest treatment. Post-anthesis biomass accumulation for both varieties showed no discernible variations between D1 and D3 but was significantly lower than D2, where accumulation reached 6287.83 kg·ha⁻¹ for Jingmai 17 and 6225.05 kg·ha⁻¹ for Jimai 22, an increase of 17.3% and 15.5% over the lowest treatment, respectively. In the 2022–2023 season, significant differences were observed in biomass remobilization between treatments, with maximum values of 3057.87 kg·ha⁻¹ for Jingmai 17 and 2641.87 kg·ha⁻¹ for Jimai 22, both showing a 35.7% increase over the lowest treatment. Post-anthesis biomass accumulation of both varieties was not significantly different under D1 and D3 treatments, but significantly lower than under D2 treatment, which accumulated 6328.67 kg·ha⁻¹ and 6066.28 kg·ha⁻¹, respectively, and increased by 15.3% and 20.6%, respectively, compared with the lowest treatment.

Figure 8. Biomass remobilization and post-anthesis biomass accumulation for different treatments in 2021–2022 (a,c) and 2022–2023 (b,d) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 8. Biomass remobilization and post-anthesis biomass accumulation for different treatments in 2021–2022 (a,c) and 2022–2023 (b,d) growing seasons. Different small letters indicate significant differences at 0.05 level.

Under the same sowing date, Jingmai 17 consistently exhibited significantly higher biomass remobilization than Jimai 22 across both years, with increases of 35.3% and 18.7%. In the 2021–2022 season, post-anthesis biomass accumulation for Jingmai 17 was slightly lower than Jimai 22 by 0.3%, but in the 2022–2023 season, Jingmai 17’s post-anthesis biomass accumulation significantly exceeded that of Jimai 22 by 7.7%.

3.4.3. Impact of Sowing Date on Harvest Index (HI)

As illustrated in Figure 9, the HI for both varieties first increased before decreasing with delayed sowing over two years. In the 2021–2022 season, Jingmai 17 reached the highest HI under D2 treatment, with no significant differences between D1 and the other treatments. Jimai 22 also peaked under D2, with significant differences observed among treatments. In the 2022–2023 season, the HI of Jingmai 17 was highest under D2, and D1 and D3 did not significantly differ from one another. For Jimai 22, the highest HI was, again, under D2, with no significant differences among treatments. Across both years and under the same sowing date treatment, Jingmai 17’s HI was substantially greater than Jimai 22’s, averaging 3.2% and 1.7% higher, respectively.

Figure 9. Harvest index (HI) of winter wheat in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

Figure 9. Harvest index (HI) of winter wheat in 2021–2022 (a) and 2022–2023 (b) growing seasons. Different small letters indicate significant differences at 0.05 level.

3.5. Impact of Sowing Date on Yield and Yield Components of Winter Wheat

According to

Table 2. Grain yield (GY) and its components for different treatments.

Year	Treatment	Spike Number (×104 ha−1)	Grain Number per Spike	Number of Grains per Unit Area (×103 m−2)	Thousand-Grain Weight (g)	Grain Yield (kg·ha−1)
2021–2022	JM17-D1	588.96 a	40.96 b	24,125.62 a	45.37 a	9289.48 b
	JM17-D2	580.28 a	43.31 a	25,129.44 a	44.78 a	9603.70 a
	JM17-D3	555.28 b	44.10 a	24,487.47 a	41.63 b	8673.27 c
	Mean	574.84 B	42.79 A	24,580.84 A	43.93 A	9188.82 A
	JM22-D1	734.97 a	31.18 c	22,914.41 b	43.63 a	8497.30 b
	JM22-D2	734.14 a	33.52 b	24,604.69 a	41.86 b	8785.44 a
	JM22-D3	648.35 b	35.06 a	22,729.19 b	40.26 c	7788.94 c
	Mean	705.82 A	33.25 B	23,416.10 B	41.92 B	8357.23 B
2022–2023	JM17-D1	581.16 a	41.91 b	24,359.67 b	41.89 a	8663.88 b
	JM17-D2	591.65 a	45.11 a	26,689.66 a	39.83 b	9058.68 a
	JM17-D3	528.23 b	46.55 a	24,581.38 b	38.62 b	8056.02 c
	Mean	567.01 B	44.52 A	25,210.24 A	40.11 A	8592.86 A
	JM22-D1	705.44 a	34.03 b	23,984.81 b	38.49 a	7832.43 b
	JM22-D2	700.49 a	37.01 a	25,925.80 a	37.51 ab	8294.89 a
	JM22-D3	619.71 b	37.95 a	23,511.15 c	36.21 b	7246.51 c
	Mean	675.21 A	36.33 B	24,473.92 B	37.40 B	7791.27 B
Y		**	**	**	**	**
V		**	**	**	**	**
D		**	**	**	**	**
Y × V		*	**	ns	ns	ns
Y × D		ns	ns	*	ns	ns
V × D		**	ns	ns	ns	ns
Y × V × D		ns	ns	ns	ns	ns

Different lowercase and uppercase letters after the same column of numbers indicate significant difference among treatments and between cultivars at p < 0.05. JM17: Jingmai 17. JM22: Jimai 22. D1: October 13; D2: October 23; D3: November 2. Y means year. V means variety. D means sowing date. * represents significant differences at the 0.05 level, ** represents extremely significant differences at the 0.01 level, and ns represents insignificant differences at the 0.05 level.

, the yields of both varieties increased and then decreased with delayed sowing, peaking under D2 treatment, with significant differences observed between sowing dates, consistent across both years. In the two years, Jingmai 17 achieved maximum yields of 9603.70 kg·ha⁻¹ and 9058.68 kg·ha⁻¹, which were 10.7% and 12.4% higher than under D3 treatment. Jimai 22 reached maximum yields of 8785.44 kg·ha⁻¹ and 8294.89 kg·ha⁻¹, representing increases of 12.8% and 14.5% over D3 treatment.

The SN gradually decreased with delayed sowing; there were no significant differences between D1 and D2 treatments for both varieties, but both were significantly higher than D3. The grain number per spike of Jingmai 17 in both years was not significantly different under D2 and D3, but was noticeably greater than that of D1 treatment. For Jimai 22, significant differences were observed across treatments in 2021–2022, while in 2022–2023, there were no significant differences between D2 and D3, but they were significantly higher than D1.

The TGW decreased with delayed sowing; in 2021–2022, Jingmai 17’s was not notable variations between D1 and D2, but it was significantly higher than that of D3, while Jimai 22’s TGW was significantly different among treatments. In 2022–2023, Jingmai 17 had the highest TGW under D1, with no significant differences between D2 and D3, while Jimai 22’s TGW in D2 was not considerably different from D1 or D3, but D1 was substantially greater than D3. In 2021–2022, the number of grains per unit area (GN) for Jingmai 17 was not significantly different among treatments, while Jimai 22 had the highest GN in D2, with no significant differences between D1 and D3. In 2022–2023, Jingmai 17 had the highest GN in D2, with no significant differences between D1 and D3, while Jimai 22 showed significant differences across treatments, and was significantly the highest under D2 treatment.

Overall, Jingmai 17 outperformed Jimai 22 in yield, TGW, and GN, indicating a clear yield advantage, with excess advantages of 10% and 10.3% over the two years. However, Jingmai 17 had a lower SN, which limited its potential for further yield increases, providing insight for future improvements in yield.

Furthermore, the variance analysis indicated that the effects of year, variety, and sowing date on yield and its three components were highly significant (p < 0.01); the interaction effect of year and variety on SN reached significance (p < 0.05), while the interaction between variety and sowing date was highly significant. The interaction of year and variety on the grain number per spike was also highly significant, and the interaction of year and sowing date on the GN was significant, with no other interaction effects observed.

3.6. Correlation Analysis

From the correlation analysis (Figure 10 and Figure 11), in the 2021–2022 season, GY was strongly related with GN, TGW, and LAC at anthesis and 28 DAA, Pn at anthesis and 28 DAA, biomass remobilization before anthesis, and HI; it was also positively correlated with SPAD values of the flag leaf at 28 DAA and post-anthesis biomass accumulation. TGW was strongly related with LAC at anthesis and 28 DAA, biomass remobilization before anthesis, and HI; it was also positively correlated with Pn at 28 DAA. GN showed significant and positive correlations with SPAD values of the flag leaf at 28 DAA, Pn at anthesis and 28 DAA, and HI, as well as significant correlations with LAC and post-anthesis biomass accumulation.

In the 2022–2023 season, GY was strongly related with GN, TGW, and LAC at anthesis and 28 DAA, Pn at anthesis, biomass remobilization before anthesis, post-anthesis biomass accumulation, and HI, with a strong and favorable association with SPAD values of the flag leaf at 28 DAA. TGW was strongly related with LAC at anthesis and 28 DAA, biomass remobilization before anthesis, and HI, with a significant positive correlation with SPAD values at 28 DAA. GN was strongly related with Pn at anthesis and 28 DAA, post-anthesis biomass accumulation, and HI.

These findings suggest that in this experiment, GY was primarily influenced by the GN, TGW, and SPAD values of the flag leaf at 28 DAA, LAC and Pn at anthesis, biomass remobilization before anthesis, post-anthesis biomass accumulation, and HI.

Figure 10. Correlation analysis among yield and indicators in 2021–2022. GY: Grain yield; GN: Grain number per m²; TGW: Thousand-grain weight; SPAD1: SPAD at anthesis; SPAD2: SPAD 28 d after anthesis; LAC1: Leaf area per culm at anthesis; LAC2: Leaf area per culm 28 d after anthesis; Pn1: Net photosynthetic rate at anthesis; Pn2: Net photosynthetic rate 28 d after anthesis; BR: Biomass remobilization; BMPost: Post-anthesis biomass; HI: Harvest index. Blue indicates a negative correlation, red indicates a positive correlation, and darker color indicates a stronger correlation. The same as Figure 11.

Figure 10. Correlation analysis among yield and indicators in 2021–2022. GY: Grain yield; GN: Grain number per m²; TGW: Thousand-grain weight; SPAD1: SPAD at anthesis; SPAD2: SPAD 28 d after anthesis; LAC1: Leaf area per culm at anthesis; LAC2: Leaf area per culm 28 d after anthesis; Pn1: Net photosynthetic rate at anthesis; Pn2: Net photosynthetic rate 28 d after anthesis; BR: Biomass remobilization; BMPost: Post-anthesis biomass; HI: Harvest index. Blue indicates a negative correlation, red indicates a positive correlation, and darker color indicates a stronger correlation. The same as Figure 11.

Figure 11. Correlation analysis among yield and indicators in 2022–2023.

Figure 11. Correlation analysis among yield and indicators in 2022–2023.

4. Discussion

4.1. Effects of Sowing Date on Yield and Yield Components of Winter Wheat

Numerous studies have revealed that increasing the number of grains per unit area (GN) (the product of the number of spikes (SN) per unit area and the grain number per spike) is essential to yield improvement. However, to further enhance yield, it is essential to optimize the synergistic interactions among yield components [31,32]. In hybrid wheat, yield advantage is roughly equal to the total of its component parts’ heterosis. Thus, to completely take advantage of the yield potential of hybrid wheat, maximizing the heterosis of its components is crucial [9,33]. Previous research has found that hybrid wheat has the most significant contribution of thousand-grain weight (TGW) to its yield compared to conventional wheat [34,35].

This study demonstrates that under the same sowing date conditions, the GN of Jingmai 17 was higher than that of Jimai 22, with significantly higher TGW, leading to significantly higher yield for Jingmai 17. Sowing date influences grain weight by affecting the thermal conditions during the grain-filling period, serving as a critical cultivation measure for achieving high yields. Delayed sowing initially increased yield before causing a decline, with a decrease in SN and TGW but an increase in the grain number per spike, consistent with findings by Li et al. [36]. However, Qiao et al. indicated that for “Heng za 102”, TGW and grain number per spike increased with delayed sowing, although the SN significantly decreased [37]. The differences from our results may be due to differences in varieties. Both varieties achieved the highest yield under D2 treatment, where although TGW was lower than D1, the higher GN compensated for this, leading to higher yield. Compared to D3, the higher GN, or a lack of significant difference, combined with significantly higher TGW contributed to the higher yield in D2.

Moreover, correlation analysis indicated that grain yield (GY) was positively correlated with the GN and TGW. Delayed sowing increased yield primarily through an enhanced GN. Thus, both varieties were more favorable, under the October 23 (D2) treatment, for coordinating the relationships between TGW, SN, and number of grains per spike, ultimately increasing GY.

4.2. Effects of Sowing Date on Photosynthetic Characteristics of the Flag Leaf

Photosynthesis plays a significant regulatory role in plant growth and yield. In the absence of other limiting factors, improving photosynthesis can enhance crop yield, and is a critical factor for achieving high-quality production [24,38]. Increasing the green leaf area per plant expands the photosynthetic area for the population, and maintaining a high level of leaf area post-tillering is beneficial for effective photosynthesis, which is key for achieving high GY in wheat [29]. A higher green leaf area helps maintain elevated SPAD values in the flag leaf.

This study indicates that delaying sowing increases SPAD values of the flag leaf, but reduces the leaf area per culm (LAC), consistent with previous findings [39]. This suggests that while early sowing promotes large LAC, it does not significantly enhance SPAD values or photosynthesis. Additionally, with the delay in the sowing date, the net photosynthetic rate (Pn) of the flag leaf in wheat first increases and then decreases, which is consistent with Liu et al. [40]. The stomatal conductance (Gs) and intercellular CO₂ concentration (Ci) show an increasing trend, consistent with the studies by Chen and Zhan et al. [41,42]. This indicates that appropriate late sowing (23 October) is beneficial for enhancing the leaf’s Pn and Gs, while also improving the flag leaf’s utilization of intercellular CO₂, thus promoting photosynthesis. Given the suitable population structure, flag leaf SPAD values, and LAC, appropriate late sowing treatment can delay the flag leaf post-anthesis senescence, which is conducive to maintaining better leaf greening in mid- and late-anthesis, and the leaf photosynthesis function period is longer and promotes photosynthesis, which in turn improves post-anthesis photosynthetic accumulation, and is conducive to the full filling of the seed grain.

Under the same sowing date treatment, Jingmai 17 exhibits a greater LAC during the late anthesis stage, with SPAD values of the flag leaf declining more slowly, resulting in prolonged leaf greenness. This helps maintain the greenness of individual leaves post-anthesis, allowing for more effective utilization of intercellular CO₂. Additionally, hybrids demonstrate clear advantages in light energy utilization, more conducive to increased biomass accumulation, providing a strong physiological basis for yield improvement.

4.3. Effects of Sowing Date on Biomass Accumulation and Remobilization in Winter Wheat

Biomass accumulation is the material basis for wheat yield formation. The accumulation and distribution of biomass not only represent the outcomes of photosynthesis but also directly relate to crop yield and overall growth conditions [43,44]. Particularly, post-anthesis biomass accumulation is critical for yield enhancement, showing a significant positive correlation with yield within certain limits [45,46].

Research indicates that late sowing can lead to a decrease in biomass accumulation at maturity [28], while other studies suggest that appropriate late sowing can improve biomass accumulation at maturity, thereby enhancing yield through an increase in harvest index (HI) [25]. In this experiment, with the delay of the sowing date, biomass accumulation at anthesis gradually decreased, while biomass accumulation at maturity and post-anthesis biomass accumulation initially increased and then decreased. This suggests that appropriate late sowing not only aids in post-anthesis biomass accumulation but also coordinates the relationship between biomass remobilization and post-anthesis biomass accumulation, leading to high GY [47,48].

Although the influence of sowing date on HI was minor, optimizing these factors can enhance both biomass remobilization and post-anthesis biomass accumulation, providing strong support for crop yield improvement.

5. Conclusions

Appropriate late sowing (23 October) is more beneficial for the hybrid variety Jingmai 17 in coordinating the relationships between the spike number, grain number per spike, and thousand-grain weight, maximizing the advantages of all three components for optimal yield. Under this sowing condition, both Jingmai 17 and Jimai 22 possess suitable leaf areas of per culm, which help maintain higher SPAD values in the flag leaf during the mid-late anthesis period, allowing for slower leaf senescence and better utilization of intercellular CO₂ for the accumulation of photosynthetic products. This leads to increased post-anthesis biomass accumulation and harvest index, while also achieving a higher number of grains per unit area and thousand-grain weight, effectively enhancing wheat yield.

Furthermore, the hybrid variety Jingmai 17 features a larger leaf area per culm, longer duration of green leaves post-anthesis, and clear photosynthetic advantages, resulting in more effective accumulation of photosynthetic products and higher post-anthesis biomass accumulation, alongside increased thousand-grain weight and grain number per spike. Thus, the hybrid demonstrates a significant yield advantage. Overall, hybrid wheat Jingmai 17 shows strong stability and potential for yield increase under appropriate late sowing conditions. These results verified that the spike number is an important factor limiting the increase of hybrid wheat yield, and how to increase the spike number, under the premise of ensuring that the thousand-grain weight and the grain number per spike are not reduced, has become the focus of improving hybrid wheat yield in the future, and at the same time, it provides valuable references for variety selection and optimal sowing date decisions in the Huang-Huai-Hai region, laying the theoretical groundwork for methods of high-yield farming.