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Article

Planting Density and Sowing Date Strongly Influence Canopy Characteristics and Seed Yield of Soybean in Southern Xinjiang

by
Naibo Xu
1,2,†,
Tingyong Mao
1,2,†,
Hengbin Zhang
3,
Xingjun Huang
1,2,
Yong Zhan
3,*,
Jiahao Liu
1,2,
Desheng Wang
1,2 and
Yunlong Zhai
1,2,*
1
College of Agriculture, Tarim University, Alar 843300, China
2
Key Laboratory of Genetic Improvement and Efficient Production for Specialty Crops in Arid Southern Xinjiang of Xinjiang Corps, Alar 843300, China
3
Crops Research Institute, Xinjiang Academy of Agricultural and Reclamation Sciences, Shihezi 832000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Agriculture 2024, 14(11), 1892; https://doi.org/10.3390/agriculture14111892
Submission received: 18 September 2024 / Revised: 20 October 2024 / Accepted: 23 October 2024 / Published: 25 October 2024
(This article belongs to the Special Issue Advances in the Cultivation and Production of Leguminous Plants)
Browse Figures
Versions Notes

Abstract

:
Southern Xinjiang is an important soybean production region in China. However, the short growing season and the cultivation of winter crops (such as wheat) in the region limit the expansion of soybean planting areas. An increased planting density can compensate for the loss in yield due to delayed sowing. To identify the quantitative relationship between increased density and delayed days, a two-year field experiment was conducted at the Tarim University Agronomy Experiment Station. Two sowing dates (April 7 (S1) and May 7 (S2)) and three planting densities of 206,800 plants·ha−1 (D1), 308,600 plants·ha−1 (D2), and 510,200 plants·ha−1 (D3) were used to compare various plant growth parameters and canopy characteristics. Late sowing and a high planting density significantly increased the plant height (S2 was 37.3% higher than S1, and D3 was 17.6% and 8.8% higher than D1 and D2), main stem internode, petiole length, and the mean tilt angle of the leaves (S2 was 22.5% higher than S1, and D3 was 11.7% higher than D2) but reduced the stem diameter (D3 was 28.6% and 12.5% lower than D1 and D2), branch number (S2 was 26.7% lower than S1, and D2 was 75% lower than D1), canopy light transmittance (S2 was 49.2% lower than S1, and D3 was 36.7% and 20.8% lower than D1 and D2), photosynthetic rate, and dry matter. The highest yield was achieved at S1D1, but the lowest yield was found for S2D1. Overall, the results suggest that earlier sowing and a lower planting density contribute to achieving an optimum canopy structure and higher yield. Our conclusions provide a reference for soybean production in southern Xinjiang.

1. Introduction

Soybean (Glycine max L.) is an important source of protein in the food industry, and its by-products provide high-quality feed for the livestock industry [1,2]. Due to its branching characteristics, the crop shows morphological adaptations to modifications in canopy structure in response to sowing date and plant density [3]. These morphological adaptations include yield, canopy development, light interception, and canopy microenvironment [4]. Xinjiang is one of the major agriculture growing provinces in China, with abundant thermal resources and vast land. Although southern Xinjiang has an arid climate and abundant sunshine, which are environmental factors known to encourage high soybean yields, there is still potential for significant improvement [5]. Therefore, crop management techniques such as high plant densities and adjusted sowing dates are often practiced to promote light and heat use efficiency [6].
Sowing date is an important determinant of yield and protein quality in soybean farming systems [7]. The timely planting of soybean is essential for building a reasonable canopy structure and vegetative growth for the optimum use of solar radiation [8]. Early-planted crops may experience frost damage at the seedling stage, and late planting can lead to crop fertility extension [9]. Both the plant density and sowing date strongly influence the soybean leaf area index, biomass accumulation, and canopy photosynthetic capacity [10,11]. Optimizing the soybean canopy structure is crucial for improving the photosynthetic active radiation (PAR) use efficiency [12]. PAR use efficiency is strongly related to canopy photosynthetic capacity and biomass accumulation, which, in turn, are associated with yield potential. Since canopy structure and plant density are strongly associated, adjusting the soybean plant density can not only lead to enhanced canopy PAR efficiency but also higher soybean branch numbers [13].
Compared to cotton and maize, the bean of the soybean plant is distributed throughout the main stem and branches [14]. Different varieties produce different yields, and this is particularly important when the soybean crop is grown more densely and at a late sowing date [15]. Adjusting the plant density and sowing date often causes changes in soybean’s branching number, stem node length, leaf area index, and photosynthetic rate [16]. Thus, plant density and sowing date can be important determinants of soybean crop phenology, growth, and development. However, limited information is available on their combined effect on morphology, photosynthesis, and yield distribution in the main stem or branches of soybean crops in southern Xinjiang. In this study, we explore the role of plant density and sowing date on (1) soybean morphology, photosynthesis, and yield distribution and (2) elucidate the quantitative relationship between plant density and sowing date. We believe that late planting and high planting densities will increase the height of the plant, main stem intercede, petiole length, and the mean tilt angle of the leaves but reduce the stem diameter, branch number, canopy light transmittance, photosynthetic rate, and dry matter. These data will provide crop management guidelines to increase soybean production and promote agricultural sustainability in southern Xinjiang.

2. Materials and Methods

2.1. Experimental Site

Experiments were carried out in 2021 and 2022 at the Agronomy Experimental Station of Tarim University in Alar City, Xinjiang (80°30′ N, 40°22′ E, altitude: 1100 m). The site has a warm continental arid desert climate. From April to October, there is low precipitation, and the average daily sunshine duration is 9.5 h. The highest temperature during vegetation in the test area in 2021 was 39.9 °C, and the total precipitation during the entire growth stage was 49.8 mm. The highest temperature in 2022 was 40.6 °C, and the total precipitation was 45.9 mm. Meteorological data were obtained from the China Meteorological Data Center (Figure 1). Additionally, 70–80 kg·ha−1 N and 100–110 kg·ha−1 P2O5 base fertilizers were applied. Topsoil from a depth of 0–20 cm was taken to measure the basic fertility levels. The organic matter, alkali-hydrolyzable nitrogen, available phosphorous, and available potassium contents were 8.12 g·kg−1, 22.6 mg·kg−1, 16.4 mg·kg−1, and 126 mg·kg−1, respectively. Field management was based on local measures, and the final thinning of seedlings was conducted according to the test density after germination (at the time of sowing, more seeds are usually sown to ensure seedling emergence in order to meet the density required for the trial, and as the seedlings grow, excess seedlings are removed to ensure density).

2.2. Experimental Design

The experiments were carried out using a split-plot design with three replicates. The main plot was used to assess the effect of sowing date (S): April 7 (S1) and May 7 (S2). The secondary plot was used to assess the effect of density (D): 206,800 plants·ha−1 (D1; 22 cm × 22 cm), 308,600 plants·ha−1 (D2; 18 cm × 18 cm), and 510,200 plants·ha−1 (D3; row spacing × plant spacing 14 cm × 14 cm), with a total of 6 treatments repeated three times. The test plots were 8 m long and 2.3 m wide, with an area of 18.4 m2. The soybean variety used in the experiment was Heinong 61.

2.3. Measurement Items and Methods

2.3.1. Plant Measurement

Five plants with uniform growth during the full seed (R6) stage were selected, and the total length and petiole length of each main stem node were measured. Ten plants from the field during the full maturity (R8) growth stage were selected (the samples were taken from each replication), and the plant height, stem diameter, number of main stem nodes and branches, and first pod height were measured. The stages of development were determined according to the criteria outlined by Fehr et al. [17].

2.3.2. LAI, CLT, and MTA

During the R6 stage, the LAI (leaf area index) was calculated by measuring the total leaf area of 10 individual leaves with a leaf area meter (LI-cor3100C; LI-COR, Inc., Lincoln, NE, USA). Between 12:00 and 14:00, a ceptometer (ACCUPAR LP-80; Decagon, Quincy, CA, USA) was used to measure the CLT (canopy light transmission rate) at the bottom 5 cm of the soybean canopy. During a time of day without direct sunlight, from 19:00 to 21:00, the MTA (mean tilt angle of the leaves) was measured using a plant canopy analyzer (LI-cor2200C).

2.3.3. Daytime Variation in CME and Pn

During the R6 stage, sunny and low-wind conditions were selected to measure the air temperature, relative humidity, and CO2 at half the plant height using an agricultural meteorological monitor (ZJTNHY-8-G; Zhejiang Top Cloud-Agri Technology, Hangzhou, China) with a connected air temperature and humidity sensor (TPJ-20-LG) and CO2 concentration sensor (TPJ-26-G); data were automatically recorded every 1 h between 08:00 and 20:00. During the R6 stage, a portable photosynthesis system (LI-6400; LI-COR, Inc.) was used to measure the net photosynthetic rate (Pn) in the middle of the third leaf from the top of each soybean plant, with measurements taken once every 2 h between 08:00 and 20:00.

2.3.4. Dry Matter Accumulation and Yield

During the V4 to R8 stage, five plants were selected, and the upper part of the cotyledonary node underwent enzymatic deactivation in an oven at 105 °C for 30 min before being dried at 80 °C to a constant weight to obtain the dry weight. During harvesting, the middle 3 m2 of each plot was manually harvested, and the yield and 100-seed weight were measured.

2.4. Data Processing

Microsoft Excel 2016 software was used for data collation and tabulation. Origin 2021 mapping and Pearson correlation analysis were used to analyze the relationship between plant-type structure and the canopy microenvironment (CME). DPS (Data Processing System) 7.05 was used to conduct analysis of variance (ANOVA) and the least significant difference method. The significance level was set to α = 0.05.

3. Results

3.1. Effect of Sowing Date and Planting Density on the Reproductive Process of Spring Soybean

Delayed sowing extends the spring soybean fertility period, while planting density has a smaller effect on the fertility period, with differences in fertility processes between densities of less than 3 d (three days) (Table 1). In 2021 and 2022, delaying sowing shortened the time from VE (soybean seedling stage) to R2 (peak flowering period of soybean) and R4 (peak period of soybean podding) to R6 and lengthened the time from R2 to R4 and R6 to R8, but the overall effect was a lengthening of the fertility period (V4: soybean third compound leaf expansion).

3.2. Effect of Sowing Date and Planting Density on Spring Soybean Yield and Components

In both years, there was no significant difference between the yields of years and significant differences between the sowing periods (Table 2 and Table 3). Delaying the sowing period significantly increased the number of main stem pods, the number of main stem grains, and the main stem grain weight but decreased the numbers of branched pods and grains and the branching grain hundred-grain weight, which reduced the yield. There was a significant difference between densities: increasing the planting density increased the number of main stem pods, the number of main stem grains, and main stem grain weight but reduced the hundred-grain weight, affecting the yield. Decreasing the planting density increased the number of branched pods, the number of branched grains, and the main stem grain weight. However, it decreased the 100-grain weight, thus affecting the yield.

3.3. Effects of Sowing Date and Planting Density on Spring Soybean Plant Structure

In two years, there were significant differences between soybean plant height, stem thickness, and number of main stem nodes between the years, and the studies in 2021 and 2022 showed that delayed sowing increased the plant height and number of main stem nodes, increased the number of beginning pod nodes, and elevated the height of the beginning pods. Conversely, reducing the planting density reduced the plant height and increased the number of main stem nodes, stem thickness, number of branches, and height of the bottom pods (Table 4 and Table 5). This indicates that sowing date and density have significant regulatory effects on soybean plant size. There were significant differences in plant height, stem thickness, and number of main stem nodes among the different years; there were significant differences in plant height among the different sowing periods, and delaying the sowing period significantly increased the plant height. There were highly significant differences in the plant height among the different planting densities, and increasing the planting density increased plant height. Moreover, there were significant differences between the year and the sowing period, and the sowing period and the year were interrelated.

3.4. Effects of Sowing Date and Planting Density on Main Stem Internode Length

The soybean main stem internode lengths were averaged under different sowing dates for R6 in 2021 and 2022, which indicated that there was little difference in the average petiole differential length of the main stem between sowing dates. However, longitudinally and section by section (as shown in Figure 2), delaying sowing significantly increased the internode length of nodes 5–15 of the main stem in 2021. In nodes 1–9 of the main stem, the internode lengths between different densities under each sowing period in 2021 and 2022 were D3 > D2 > D1, and some D1 internode lengths were greater than D2 and D3 above node 9, indicating that reducing the planting density could reduce the average internode lengths in the middle and lower parts of the main stem. Taken together, these findings indicate that delaying the sowing date and increasing the planting density would increase the internode length in the middle and upper part of the main stem.

3.5. Effects of Sowing Date and Planting Density on Main Stem Petiole Length

The analysis of petiole length at each node of main stem in 2021 and 2022 revealed that the bottom-most petiole node gradually increased at R6 with delayed sowing date, indicating that delaying the sowing date leads to the shedding of the lower petiole of the main stem (Figure 3). The average main stem petiole lengths of S1 and S2 in 2021 were 19.2 cm and 28.9 cm, respectively, indicating that the petiole lengths gradually increased with the sowing delay, as evidenced by the significant increase of 5 to 10 nodes in petiole length. Different densities of petiole length in the middle and lower part of the main stem over the two years showed that D3 > D2 > D1, and the average petiole length of the main stem in D1, D2, and D3 in 2022 was 21.7 cm, 23.3 cm, and 24.4 cm, respectively. Increasing the planting density increased the petiole length, but the differences were relatively small, and the increased length mainly occurred in the middle and lower parts of the main stem.

3.6. Effects of Sowing Date and Planting Density on CLT

There was a significant difference between the light transmission rate of soybeans in different years (Figure 4). The sowing period affected the canopy light transmission rate of the soybeans, and there was a significant difference between the canopy light transmission rate of soybeans at different densities. Increasing the planting density reduced the CLT rate. The canopy transmittance of S1 was the largest in both years, indicating that increasing the planting density and delaying the sowing period reduced the canopy transmittance. Reducing the planting density was shown to increase the canopy transmittance at all sowing dates in both years. For all treatments, light transmission was lower in 2021 than in 2022.

3.7. Effects of Sowing Date and Planting Density on MTA

There was a significant difference between the soybean MTA in different years. Delaying the sowing period increased the soybean MTA (Figure 5), and there was a significant difference in canopy transmittance between different the densities: increasing the planting density increased the MTA. In both years, the overall effect was that a delayed sowing period increased the soybean MTA, as the angle between the leaf blade and the horizontal increased. Different planting densities showed D3 > D2 > D1 at all sowing dates in both years, indicating that increasing the density significantly increased the angle between the leaf blade and the horizontal, prompting the leaf blade angle to favor the vertical.

3.8. Effects of Sowing Date and Planting Density on LAI

There was no significant difference in soybean LAI between the two years. Different sowing periods affected the LAI (Figure 6): delaying the sowing period was found to reduce soybean LAI, but the difference under the different sowing periods was not significant. Plant density also affected soybean LAI, whereby increasing the planting density increased the LAI. The LAI was largest in the D3 density treatment during all sowing periods, which indicated that increasing the planting density was the key to improving soybean LAI.

3.9. Effects of Sowing Date and Planting Density on Canopy Daytime Temperature

In both years, the peak period of maximum daytime temperature was inconsistent during different sowing periods (Figure 7). After averaging the daytime temperatures of the soybean canopy at each time period, it was found that the daytime average daily temperatures between the different sowing periods in 2021 and 2022 showed the pattern of S1 > S2, and the average canopy temperatures from 8:00 to 20:00 for S2 in the two years were 29.8 °C and 28.8 °C, respectively. This indicates that the temperature within the canopy increased at first and then decreased during the delayed sowing period. However, the temperature difference in the S2 sowing period was the largest over the two years. The canopy temperatures observed for the different planting densities in both years showed that reducing the planting density reduced the canopy temperature, with average temperatures of 29.0 °C, 29.9 °C, and 30.4 °C in 2021 and 28.8 °C, 28.9 °C, and 28.8 °C in 2022 for D1, D2, and D3, respectively. In terms of the treatments during daytime hours, in the morning canopy warming process, the temperatures for the low-density planting group were greater than those for the high-density planting group. However, the canopy temperatures for the time period of 12:00–20:00 were higher for the high-density planting group, and this was observed for both years, indicating that adjusting the soybean sowing date and planting density can regulate the soybean canopy temperature.

3.10. Effects of Sowing Date and Planting Density on Daytime Canopy Relative Humidity

The canopy relative humidity during the soybean bulging period was inversely proportional to the canopy temperature, and the canopy humidity decreased with the increase in canopy temperature during the day (Figure 8). The canopy humidity between different densities showed that D3 > D2 > D1; thus, the higher the density, the higher the relative humidity, and this pattern was consistent in both years. The average canopy humidity of D1, D2, and D3 was 50.6%, 49.8%, and 49.3% in 2021 and 48.9%, 51.6%, and 54.4% in 2022, respectively, indicating that the humidity in 2022 was higher than that in 2021 and that reducing the planting density significantly reduced the mid-canopy air humidity. However, the pattern of changes in the air humidity in the middle of the canopy was not consistent in the two years between planting periods, and the pattern was not obvious.

3.11. Effects of Sowing Date and Planting Density on Daytime Canopy CO2 Concentrations

The inter-day averaging of CO2 concentrations in the middle of the canopy during the bulging stage of spring soybean showed that S1 was the largest during the sowing period in both 2021 and 2022, suggesting that early sowing could enhance the internal CO2 concentration of the canopy. Different densities showed the pattern D3 > D2 > D1 during most of the day under each sowing period, indicating that increasing the planting density significantly increased the intra-canopy CO2 concentration (Figure 9). The CO2 concentration in 2021 was significantly higher than that in 2022 for all treatments during each sowing period.

3.12. Effects of Sowing Date and Planting Density on Daytime Canopy Pn Variation

The averaging of Pn at each time point during the day showed that D1, D2, and D3 were 11.72, 9.90, and 9.88 μmol m−2s−1 in 2021 and 13.88, 11.67, and 10.30 μmol m−2s−1 in 2022, respectively. This is a pattern of D1 > D2 > D3, suggesting that lowering the planting density increased Pn (Figure 10). In 2021, the inter-day Pn values in the different sowing periods showed an “M” curve, and the photosynthetic lunch break phenomenon was obvious, but the time of the lunch break was not consistent for the different sowing periods. There was no obvious pattern in the day-to-day variation in the Pn values between sowing periods, but the maximum values in both years occurred in S1 at 17.50 and 23.35 μmol m−2s−1, respectively.

3.13. Effects of Sowing Date and Planting Density on Dry Matter Accumulation

In 2021, the dry matter accumulation decreased gradually as the sowing date was delayed and increased gradually as the planting density increased. In the same year, the dry matter accumulation did not differ with respect to the sowing date under D1 (Figure 11). The dry matter accumulation was significantly lower under S1 than under S2. Overall, however, the sowing date had a negligible effect on dry matter accumulation in the high-density treatments. In both years, the values decreased in the order S2 > S1, indicating that delaying the sowing date significantly reduced the dry matter accumulation of individual plants under low-density planting. The trends in dry matter accumulation under various densities were not consistent across sowing dates: the differences were small at high densities, while those at low densities decreased significantly with the delay in sowing date.

4. Discussion

In this 2-year field study, we investigated the effects of plant density and sowing date on the yield distribution, growth, and physiology of soybean. The technique of increasing the plant density is practiced to increase the number of main stems per unit area, but it reduces the branch number [14]. This pattern is necessary when the sowing date is late. Planting crops late but with a high density has the potential to increase soybean yield under intensive field management [18]. Late-planted soybean with a high plant density and early-planted soybean with a low plant density have similar yields that are higher than those of other combinations. In the present study, the soybean yield was significantly greater in D3 compared with D2 and D1. Recent research has suggested that late-planted soybean yield can be increased by increasing the plant density.
The increased yield in the S2D3 crop was primarily attributed to the higher number of pods and grains in the main stem, caused by the increased density. The sowing date is also an important determinant for soybean production. In this study, the early-planted crop produced significantly more lint than the late-sown crop, which could be due to the fact that the S1 crop took advantage of more heat and sunlight over a longer growing season. Late-planted soybeans have fewer branch pods and branch grains and lower branch grain weight but more main stem pods. This can be explained by the different biomass distribution, resulting in different seed yield distribution. The high yield of the S1D3 treatment is closely related to the high number of branches, as more branches imply that more nodes are available to produce pods [19,20]. For the S1 sowing date, the D1 density had the highest yields, and the opposite pattern was observed for the S2 sowing date because an increase in the number of branches after early sowing leads to an increase in yield and vice versa. Late sowing also increases the number of main stem nodes, providing more pod formation points; therefore, yields under a high planting density lie mainly on the main stem [21]. The S1D1 combination had the highest yield in 2021 and 2022. The analysis of the main stem node number and first pod positions showed that the late-planted soybeans had more main stem pods per plant than the early-planted crop. This could be another reason for the different seed yield distribution observed in our study.
Early sowing and a lower planting density significantly decreased the plant height and first pod height, increased the stem diameter and branch number, and reduced the main stem internode length (Table 4). The differences in plant height can be explained by various factors, particularly plant density [22]. In this study, the number of main stem nodes increased with a delay in the sowing date, while the density was related to an increase in the main stem internode length. Plant height was positively correlated with the number of nodes and internode length on the main stem. The number of main stem nodes was mainly affected by the sowing date, as delaying the sowing date in the spring in southern Xinjiang resulted in increased ambient temperature during the node formation stage. Other studies have also found that an increase in temperature increases the number of main stem nodes [23,24]. Differences in internode length are caused by light and temperature differences in the node formation stage. In our experiment, CLT was negatively correlated with internode length and with a decrease in CLT, leading to an increase in internode length [25]. Increases in plant height and internode length increase canopy height, intensifying competition for light in a plant community. For leaves to reach the top of the canopy and obtain more solar radiation, plants must increase their petiole lengths. As a result, the petiole length increases with plant height. This is consistent with our results showing that petiole length is positively correlated with plant height and internode length but inversely correlated with CLT. Furthermore, the increases in petioles and main stem internode length mean that more photosynthetic products are transported to vegetative organs, thereby creating a suboptimal plant structure [26,27]. The first pod height depends on the effects of the canopy environment on the rate of flowering and pod formation in the lower part of the canopy [28]. In this study, the first pod height was negatively correlated with canopy temperature and canopy CO2 concentration; a delayed sowing date and increased planting density increased canopy temperature and CO2, resulting in the shedding of flowers and pods at the bottom of the canopy, thereby increasing the first pod height.
LAI depends on the number of leaves and the leaf area in the community. Reducing light transmittance at the bottom of the canopy reduces leaf light intensity, thereby shortening leaf life and reducing the specific leaf weight and LAI [29,30,31]. In this study, the low LAI for sowing date S4 could be explained by excessive vegetative growth, which reduced CLT. This resulted in the deterioration of the light environment at the bottom of the canopy, causing the shedding of leaves and petioles [19]. In this study, the CLT was negatively correlated with plant height, petiole length, and main stem internode length in soybean plants, respectively. Adjusting the sowing date and planting density can promote increases in plant height and petioles, thereby regulating CLT.
Genetic characteristics, cultivation management, and environment are the main determinants of dry matter accumulation and yield. The environment of crops can be divided into the macro-ecological environment and the microenvironment of the plant community. Adjusting the sowing date can improve the adaptability of crops to the external environment, and the planting density can regulate the relationship between individual plants and the community [32,33]. Thus, studying the interactions between sowing date and planting density can provide insights into the structure of soybean plants and a basis for improving the canopy microenvironment (CME) and structure [32]. Light, temperature, humidity, and CO2 concentration in the canopy are the main factors in studies of the CME [34]. In this study, the temperature increased between 08:00 and 12:00 under various densities, with the fastest increase under D3. Between 12:00 and 20:00, the canopy temperature was highest in the D3 treatments. This indicates that a high planting density forms a more closed canopy environment, causing the temperature to rise slowly in the morning and hot air to become trapped in the afternoon [35]. The net photosynthetic rate (Pn) is a key determinant of crop growth and is directly related to the CME, where leaves are located. In this study, the correlation coefficients between Pn and canopy temperature, humidity, and CO2 concentration were all negative. This indicates that high temperature and high humidity in the canopy reduce Pn. Furthermore, Pn, LAI, and photosynthetic potential are decisive factors of crop dry matter [36]. The decrease in dry matter accumulation per plant after a delay in the sowing date and the increase in the planting density in this study may be related to leaf shedding and reduced Pn caused by high temperature, high humidity, and low CLT (Figure 4 and Figure 12) [37].

5. Conclusions

The results of this study indicate that delaying the sowing date promotes vegetative growth in spring soybean; increases plant height, the number of main stem nodes, and the length of internodes and petioles of the main stem; reduces CLT; increases first pod height; and causes leaf shedding at the bottom of the plant. Furthermore, reducing the planting density reduces plant height, shortens the internode and petiole lengths, regulates plant development, increases CLT, decreases the MTA of leaves, reduces canopy temperature and humidity, and increases the net photosynthetic rate (Pn) of leaves. S1 produced the highest yields among the various sowing dates. In terms of density, the yield of D3 was high under S1 and S2, while S1D1 treatment led to the highest yield over the 2 years. Therefore, delaying the sowing date and reducing the planting density in southern Xinjiang can optimize the soybean canopy structure, improving the CME. Furthermore, it can also increase the spring soybean yield per unit area and regional productivity, thus helping to alleviate the soybean supply shortage.

Author Contributions

Conceptualization, Y.Z. (Yunlong Zhai) and D.W.; methodology, X.H.; software, N.X.; validation, Y.Z.(Yunlong Zhai), Y.Z. (Yong Zhan) and D.W.; formal analysis, T.M.; investigation, H.Z.; resources, J.L.; data curation, H.Z.; writing—original draft preparation, N.X.; writing—review and editing, T.M.; visualization, X.H.; supervision, D.W.; project administration, Y.Z. (Yunlong Zhai); funding acquisition, Y.Z. (Yunlong Zhai). All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the earmarked fund for Xinjiang Agriculture Research System (XJARS-04-08), a project of the key industrial support program of Xinjiang production and Construction Corps, South Xinjiang (2022DB015), Bingtuan Natural Science Foundation (2024DB024), and the President’s Fund of Tarim University (TDZKBS202408).

Data Availability Statement

The data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 01892 g001
Figure 1. Temperature and precipitation during the growth stage in the test area in 2021 (A) and 2022 (B).
Figure 1. Temperature and precipitation during the growth stage in the test area in 2021 (A) and 2022 (B).
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Figure 2. Effects of sowing date and density on internode length of the main stem in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 2. Effects of sowing date and density on internode length of the main stem in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 3. Effects of sowing date and planting density on petiole length of the main stem ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 3. Effects of sowing date and planting density on petiole length of the main stem ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 4. Effects of sowing date and planting density on CLT ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
Figure 4. Effects of sowing date and planting density on CLT ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
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Figure 5. Effects of sowing date and planting density on MTA ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
Figure 5. Effects of sowing date and planting density on MTA ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
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Figure 6. Effects of sowing date and planting density on LAI ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
Figure 6. Effects of sowing date and planting density on LAI ((A,B) show results in 2021, 2022, respectively. Different letters indicate significant difference at p = 0.05 level).
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Figure 7. Effects of sowing date and planting density on the canopy temperature in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 7. Effects of sowing date and planting density on the canopy temperature in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 8. Effects of sowing date and planting density on the canopy relative humidity in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 8. Effects of sowing date and planting density on the canopy relative humidity in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 9. Effects of sowing date and planting density on canopy CO2 concentration in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 9. Effects of sowing date and planting density on canopy CO2 concentration in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 10. Effects of sowing date and planting density on diurnal changes in the net photosynthetic rate (Pn) in spring soybean leaves ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 10. Effects of sowing date and planting density on diurnal changes in the net photosynthetic rate (Pn) in spring soybean leaves ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 11. Effects of sowing date and planting density on dry matter accumulation in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
Figure 11. Effects of sowing date and planting density on dry matter accumulation in spring soybean ((A,B) show results for S1, S2, respectively, in 2021; (C,D) show results for S1, S2, respectively, in 2022).
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Figure 12. Growth of single spring soybean plants at the full seed (R6) stage.
Figure 12. Growth of single spring soybean plants at the full seed (R6) stage.
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Table 1. Effect of sowing date and planting density on the reproductive process of spring soybean.
Table 1. Effect of sowing date and planting density on the reproductive process of spring soybean.
YearSowing DateDensitySeedtimeVEV4R2R4R6R8Growth Period/d
2021S1D104-0704-1505-1405-2606-0607-0407-1692
D204-0704-1505-1405-2606-0607-0407-1692
D304-0704-1505-1405-2506-0607-0407-1591
S2D105-0705-1306-0706-1307-1107-3108-1998
D205-0705-1306-0706-1307-1107-3108-1998
D305-0705-1306-0706-1307-0907-3108-1998
2022S1D104-0704-1505-0705-1406-0306-2407-1490
D204-0704-1505-0705-1406-0306-2407-1490
D304-0704-1505-0705-1406-0306-2407-1490
S2D105-0705-1406-0806-1707-1307-2808-22100
D205-0705-1406-0806-1707-1307-2808-22100
D305-0705-1406-0806-1707-1307-2808-22100
VE: soybean seedling stage; V4: soybean third compound leaf expansion; R2: peak flowering period of soybean; R4; peak period of soybean podding; R6: peak period of soybean grain filling; R8: soybean maturity.
Table 2. Effect of sowing date and planting density on spring soybean yield and components.
Table 2. Effect of sowing date and planting density on spring soybean yield and components.
Number of Main Stem Pods (pcs/m2)Number of Branched Pods (pcs/m2)Number of Grains in Main Stem (pcs/m2)Number of Branched Grains (pcs/m2)Main Stem Grain Weight (g/m2)Branching Grain Weight (g/m2)Hundred-Grain Weight (g)Yield (Kg/ha)
Y11056.0 ± 37.2 a321.9 ± 16.4 a2525.0 ± 148.2 a731.3 ± 8.1 a479.3 ± 10.2 a185.3 ± 23.5 a20.3 ± 0.3 b4216.3 ± 140.4 a
Y2832.6 ± 37.8 b245.6 ± 54.0 a2169.0 ± 147.7 a626.6 ± 231.3 a521.1 ± 25.3 a147.6 ± 54.7 a22.2 ± 0.4 a4476.7 ± 124.5 a
S1827.5 ± 44.3 b318.6 ± 33.8 a2082.5 ± 162.9 b764.4 ± 88.8 a427.8 ± 13.1 b174.9 ± 19.8 a22.1 ± 0.3 a4531.8 ± 101.5 a
S21061.1 ± 27.4 a249.0 ± 41.4 a2611.5 ± 24.5 a593.5 ± 173.6 a572.6 ± 22.1 a158.1 ± 28.0 a20.4 ± 0.4 b4161.2 ± 37.3 b
D1671.4 ± 20.9 c651.6 ± 75.7 a1647.0 ± 113.4 c1564.4 ± 230.7 a386.9 ± 36.4 c368.1 ± 55.0 a23.3 ± 0.5 a4584.9 ± 156.3 a
D2883.9 ± 38.5 b156.4 ± 81.2 b2269.5 ± 113.0 b375.5 ± 205.5 b497.5 ± 12.2 b114.4 ± 26.1 b20.9 ± 0.2 b3992.2 ± 135.4 b
D31277.6 ± 83.8 a43.4 ± 7.6 b3124.5 ± 144.6 a96.9 ± 33.2 b616.2 ± 22.6 a16.8 ± 6.6 c19.6 ± 0.5 c4462.3 ± 107.1 a
Y* **
S** ** ** ***
D****************
Y × S********** **
Y × D** ** **
S × D * ** ***
Y × S × D ******* *
Note: Different letters indicate significant difference at p = 0.05 level. * p < 0.05; ** p < 0.01.
Table 3. Effect of sowing date and planting density on spring soybean yield and components.
Table 3. Effect of sowing date and planting density on spring soybean yield and components.
YearSowing DateDensityNumber of Main Stem Pods (pcs/m2)Number of Branched Pods (pcs/m2)Number of Grains in Main Stem (pcs/m2)Number of Branched Grains (pcs/m2)Main Stem Grain Weight (g/m2)Branching Grain Weight (g/m2)Hundred-Grain Weight (g)Yield (Kg/ha)
YIS1D1692.8 ± 76.5 cd924.4 ± 60.0 a1844.7 ± 533.5 bc2481.6 ± 124.1 a385.3 ± 4.1 cd600.3 ± 41.9 a24.2 ± 0.4 ab5437.0 ± 298.0 a
D2987.5 ± 24.7 b225.3 ± 101.8 bcd2397.8 ± 9.3 b506.1 ± 240.7 b506.8 ± 5.6 bc106.7 ± 58.4 b21.1 ± 0.6 de4163.2 ± 242.8 cde
D31464.3 ± 76.5 a173.5 ± 30.6 bcd3642.8 ± 81.6 a387.8 ± 132.7 b616.3 ± 31.9 b67.2 ± 26.3 b18.2 ± 0.2 f4630.3 ± 100.2 bcd
S2D1655.6 ± 26.9 cde475.6 ± 173.7 b1379.4 ± 105.5 cd777.6 ± 335.0 b314.9 ± 8.1 de173.7 ± 61.3 b21.9 ± 0.1 cde3377.3 ± 291.4 f
D21015.3 ± 3.1 b132.7 ± 3.1 cd2370.0 ± 61.7 b234.5 ± 37.0 b443.8 ± 8.0 c163.8 ± 122.5 b18.5 ± 0.4 f4158.7 ± 385.5 cde
D31520.4 ± 224.5 a0.0 ± 0.0 d3515.3 ± 403.1 a0.0 ± 0.0 b608.9 ± 27.8 b0.0 ± 0.0 b18.0 ± 0.7 f4242.2 ± 181.7 cd
Y2S1D1441.2 ± 59.7 e392.9 ± 109.4 bc1013.3 ± 115.1 d820.3 ± 106.1 b226.7 ± 10.3 e183.2 ± 22.1 b24.9 ± 1.5 a4883.5 ± 269.9 abc
D2648.1 ± 163.3 de195.4 ± 170.0 bcd1861.9 ± 393.2 bc390.9 ± 340.0 b442.0 ± 36.4 c91.8 ± 79.9 b22.9 ± 0.1 bc4199.0 ± 74.1 cde
D3731.3 ± 106.2 cd0.0 ± 0.0 d1734.7 ± 417.6 bcd0.0 ± 0.0 b390.0 ± 35.8 cd0.0 ± 0.0 b21.5 ± 0.43877.8 ± 326.8 def
S2D1896.1 ± 140.8 bc813.4 ± 434.9 a2350.6 ± 401.7 b2178.3 ± 1293.5 a620.5 ± 158.8 b515.3 ± 302.8 a22.2 ± 0.5 cd4641.9 ± 446.3 bcd
D2884.7 ± 64.2 bcd72.0 ± 124.7 cd2448.2 ± 89.1 b370.3 ± 263.7 b597.6 ± 39.2 b95.6 ± 71.6 b21.3 ± 0.5 de3447.9 ± 275.4 ef
D31394.5 ± 106.2 a0.0 ± 0.0 d3605.4 ± 147.3 a0.0 ± 0.0 b849.8 ± 43.7 a0.0 ± 0.0 b20.6 ± 1.3 e5098.9 ± 262.4 ab
Note: Different letters indicate significant difference at p = 0.05 level.
Table 4. Effects of sowing date and planting density on plant structure in spring soybean.
Table 4. Effects of sowing date and planting density on plant structure in spring soybean.
Plant Height (cm)Stem thickness (cm)Number of Nodes of Main StemBranching NumberBottom Pod Height (cm)Alpha-Nodal Position (Botany)
Y176.6 ± 3.3 a0.9 ± 0.1 a14.5 ± 0.3 a1.3 ± 0.2 a10.4 ± 1.5 a4.2 ± 0.4 a
Y260.3 ± 2.0 b0.7 ± 0.0 b12.7 ± 0.5 b1.3 ± 0.4 a9.1 ± 1.2 a3.4 ± 0.4 a
S157.7 ± 2.7 b0.8 ± 0.1 a12.0 ± 0.4 b1.5 ± 0.5 a6.5 ± 0.4 b3.2 ± 0.2 b
S279.2 ± 2.7 a0.8 ± 0.0 a15.2 ± 0.3 a1.1 ± 0.2 a13.0 ± 2.2 a4.3 ± 0.4 a
D163.1 ± 2.6 c0.9 ± 0.1 a14.0 ± 0.5 a3.2 ± 0.5 a7.8 ± 1.2 b3.8 ± 0.5 a
D268.2 ± 1.9 b0.8 ± 0.0 b13.9 ± 0.4 a0.8 ± 0.5 b9.2 ± 0.4 b3.9 ± 0.3 a
D374.2 ± 3.0 a0.7 ± 0.0 b12.9 ± 0.3 b0.0 ± 0.0 c12.3 ± 2.2 a3.6 ± 0.3 a
Y******
S** ** ****
D**********
Y × S***
Y × D
S × D** **
Y × S × D*** **
Note: Different letters indicate significant difference at p = 0.05 level. * p < 0.05; ** p < 0.01.
Table 5. Effect of sowing date and density on plant structure of spring soybean plants.
Table 5. Effect of sowing date and density on plant structure of spring soybean plants.
YearSowing DateDensityPlant Height (cm)Stem Thickness (cm)Number of Nodes of Main StemBranching NumberBottom Pod Height (cm)Alpha-Nodal Position (Botany)
YIS1D146.8 ± 4.9 e1.3 ± 0.4 a12.6 ± 1.7 def3.8 ± 0.8 a6.1 ± 0.9 cd3.0 ± 0.4 d
D265.1 ± 7.0 bc0.8 ± 0.1 bc13.6 ± 0.5 cde0.8 ± 0.0 c4.9 ± 2.9 d3.8 ± 0.4 bcd
D368.6 ± 5.0 bc0.7 ± 0.1 cde11.8 ± 0.8 f0.0 ± 0.0 c8.5 ± 2.6 bcd3.6 ± 0.8 bcd
S2D196.2 ± 7.0 a1.0 ± 0.1 b16.6 ± 1.7 a2.4 ± 0.5 b10.5 ± 1.3 b5.6 ± 1.1 a
D288.6 ± 9.8 a0.9 ± 0.1 bc16.6 ± 1.3 a0.8 ± 0.8 c14.7 ± 3.3 a4.6 ± 0.5 ab
D394.6 ± 12.0 a0.6 ± 0.1 cde15.8 ± 0.8 ab0.0 ± 0.0 c18.3 ± 5.7 a4.4 ± 0.5 abc
Y2S1D149.7 ± 6.5 e0.6 ± 0.0 cde12.2 ± 0.8 ef3.4 ± 1.5 ab5.5 ± 1.5 d3.2 ± 0.7 cd
D254.7 ± 4.6 de0.6 ± 0.0 de11.6 ± 0.9 fg0.8 ± 1.1 c7.1 ± 1.2 bcd2.8 ± 0.8 d
D361.5 ± 4.5 cd0.6 ± 0.0 e10.2 ± 1.3 g0.0 ± 0.0 c7.1 ± 0.4 bcd2.8 ± 0.5 d
S2D159.8 ± 3.3 cd0.8 ± 0.1 bc14.6 ± 0.9 bc3.0 ± 1.2 ab8.9 ± 1.8 bcd3.4 ± 1.3 bcd
D264.2 ± 3.9 bcd0.8 ± 0.1 bcd14.0 ± 0.7 cd0.6 ± 0.5 c10.1 ± 1.4 bc4.4 ± 1.1 abc
D372.0 ± 4.3 b0.8 ± 0.1 bcd13.8 ± 0.4 cde0.0 ± 0.0 c15.4 ± 5.0 a3.6 ± 0.5 bcd
Note: Values represent mean, and different letters indicate significant differences at p = 0.05.
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Xu, N.; Mao, T.; Zhang, H.; Huang, X.; Zhan, Y.; Liu, J.; Wang, D.; Zhai, Y. Planting Density and Sowing Date Strongly Influence Canopy Characteristics and Seed Yield of Soybean in Southern Xinjiang. Agriculture 2024, 14, 1892. https://doi.org/10.3390/agriculture14111892

AMA Style

Xu N, Mao T, Zhang H, Huang X, Zhan Y, Liu J, Wang D, Zhai Y. Planting Density and Sowing Date Strongly Influence Canopy Characteristics and Seed Yield of Soybean in Southern Xinjiang. Agriculture. 2024; 14(11):1892. https://doi.org/10.3390/agriculture14111892

Chicago/Turabian Style

Xu, Naibo, Tingyong Mao, Hengbin Zhang, Xingjun Huang, Yong Zhan, Jiahao Liu, Desheng Wang, and Yunlong Zhai. 2024. "Planting Density and Sowing Date Strongly Influence Canopy Characteristics and Seed Yield of Soybean in Southern Xinjiang" Agriculture 14, no. 11: 1892. https://doi.org/10.3390/agriculture14111892

APA Style

Xu, N., Mao, T., Zhang, H., Huang, X., Zhan, Y., Liu, J., Wang, D., & Zhai, Y. (2024). Planting Density and Sowing Date Strongly Influence Canopy Characteristics and Seed Yield of Soybean in Southern Xinjiang. Agriculture, 14(11), 1892. https://doi.org/10.3390/agriculture14111892

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