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Article

Effects of Planting Density—Potassium Interaction on the Coordination among the Lignin Synthesis, Stem Lodging Resistance, and Grain Yield in Oil Flax

by
Bing Wu
1,2,
Zhengjun Cui
3,
Lijuan Ma
1,2,
Xiqiang Li
1,2,
Haidi Wang
2,4,
Yifan Wang
2,4,
Bin Yan
2,4,
Hongwei Dong
5 and
Yuhong Gao
2,4,*
1
College of Life Science and Technology, Gansu Agricultural University, Lanzhou 730070, China
2
State Key Laboratory of Aridland Crop Science, Lanzhou 730070, China
3
College of Agronomy, Tarim University, Alar 843300, China
4
College of Agronomy, Gansu Agricultural University, Lanzhou 730070, China
5
College of Agronomy, Northwest A&F University, Yangling 712100, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(10), 2556; https://doi.org/10.3390/agronomy13102556
Submission received: 14 August 2023 / Revised: 27 September 2023 / Accepted: 29 September 2023 / Published: 4 October 2023
(This article belongs to the Topic Plant Ecophysiology)
Browse Figures
Versions Notes

Abstract

:
To clarify the effects of planting density (PD) and potassium (K) application on the lignin synthesis of oil flax stalks and their relationship with lodging resistance, a two-year field experiment was conducted in the 2020 and 2021 growing seasons. The planting densities were 600 grains·m−2 (D1), 750 grains·m−2 (D2), and 900 grains·m−2 (D3); the potassium application levels were 0 kg K·hm−2 (K0), 60 kg K·hm−2 (K1,) and 90 kg K·hm−2 (K2). Then, the effects of PD and K on the stalk agronomic traits, lignin synthesis, lodging resistance, and grain yield (GY) of oil flax were studied. The results show that D3 increased the oil flax plant height and stem fresh weight, and K1 raised the height of the center of gravity. The enzyme activities of phenylalanine aminolyase (PAL), cinnamyl alcohol dehydrogenase (CAD), and peroxidase (POD) increased with an increase in planting density, whereby K1 promoted and K2 inhibited the lignin metabolic enzymes, respectively. The lignin and cellulose were significantly positively correlated with PAL. The combination of K with the D2 treatment increased the lignin and cellulose content in stems of oil flax at the kernel and maturity stages, and the effect of K1 on lignin metabolism and lodging resistance was better than that of K2. The increased GY was significantly correlated with the number of effective oil flax capsules per plant and the lodging resistance index, which were directly affected by K application. In conclusion, under the same ecological conditions as the experiment, the field management strategy of medium PD (750 grains·m−2) and low K (60 kg K·hm−2) was the best for improving the lodging resistance and GY formation of oil flax.

1. Introduction

Flax (Linum usitatissimum L.), known as oil flax, is one of the major oil crops in the world; it is widely cultivated in temperate areas and has important economic and agronomic value [1]. Flax is also one of the most important oil crops in the north of China [2]. Flaxseed is also a great source of industrial vegetable oil and essential fatty acids for the human diet, with many benefits for human health [3]. Flaxseed is rich in oil (41%), protein (20%), and dietary fiber (28%); it contains a high proportion of essential fatty acids, including 75% polyunsaturated fatty acids, and 57% alpha-linolenic acid [4]. With its widespread application as a functional food and other uses, its demand and additional value are increasing [5]. In order to meet the growing demand, this oil crop has received a lot of attention [3]. However, the poor yield of oil flax is one of the main factors restricting the development of the oil flax industry [3].
Increasing PD to increase the number of branches (capsules) per unit area is the main cultivation strategy used to further enhance the GY of oil flax [6]. However, a dense planting population leads to increased shading and community competition, as well as promotes stem elongation and reduces stem diameter, resulting in thinner stems and increased lodging risk [7]. In addition, oil flax lodges easily due to its delicate stems and large canopy in the late growth stage (kernel and maturity), resulting in a significant decrease in GY, especially under suboptimal cultivation systems [6,8]. Therefore, lodging is a prominent problem restricting oil flax production and industrial development [8]. Consequently, studying the mechanism of resistance to the lodging of oil flax with different PD has a crucial theoretical and productive significance.
The role of K in lodging resistance has long been recognized, with K nutrition being related to stem structure and strength, and susceptibility to root and stem rots [9]. Potassium can thicken collenchyma cells, increase the lignin content of collenchyma cells, improve the physical and chemical properties of stems, and thus improve the lodging resistance of stems [9]. Lignin is closely associated with the lodging resistance of crop stems [10]. Research has shown that K application can promote lignin metabolism and synthesis in plants [11].
Lignin is a major structural carbohydrate component that has irreplaceable biological functions for the normal growth and development, structural composition, mechanical support, and water transport of crop transport tissues [12]. Its content is often a major index used to measure the strength of crop lodging resistance [9], and the content of lignin and its related enzymatic activities are strongly related to the lodging resistance of the stalks [6]. The lignin synthesis process is catalyzed by complex enzyme systems such as PAL and tyrosine aminolyase (TAL) [13]. PAL, 4-coumaric acid COA ligase (4CL) and CAD are great regulatory enzymes in the lignin synthesis pathway [14]. Montero et al. [15] showed that the lignin content was positively correlated with the peel penetration strength, the fracture strength of the internode, and the PAL and 4CL activities, but was negatively correlated with the lodging rate. The PAL, 4CL, and CAD activities in wheat (Triticum aestivum) and soybean (Glycine max) could be improved by K application, which promoted the synthesis of lignin [11,16]. Liu et al. [8] reported that K significantly improved the activity of CAD of oil flax stems at the kernel stage.
In conclusion, it has been found that increasing the PD promotes the increase of GY of oil flax, and a higher PD increases the risk of lodging, while K plays an active role in lignin synthesis and the ability to resist lodging. Some studies suggest that, in the absence of lodging, the GY of oil flax increased with the increase in PD [6]. The lignin content of oil flax varieties with strong lodging resistance is relatively high, and the reasonable application of K fertilizer has a promoting effect on lignin metabolism [17]. Compared with no K application, the lignin content and lodging resistance index of oil flax increased by 12.18–68.78 and 19.80–86.37% with potassium application, respectively [18]. Previous studies have elucidated the effects of individual factors such as variety [6], PD [6,19], potassium fertilizer [8], and the combination of potassium and silicon fertilizers [17,18] on the lignin metabolism, carbohydrate synthesis, and lodging resistance of oil flax stems. In summary, the effects of PD, K, and the interaction of PD × K on the agronomic traits (plant height, height of the center of gravity, and fresh weight), lignin metabolism (lignin, cellulose content, and enzyme activities in the stems), and GY of oil flax, as well as their relationships with lodging resistance (breaking-resistant strength, stalk bending strength, lodging resistance index), need to be further studied.

2. Materials and Methods

2.1. Experimental Site

The experiment was conducted at the Dingxi Academy of Agricultural Science Experimental Station in Anding District, Gansu Province, China (34°26′ N, 103°52′ E, altitude 2050 m) in the 2020 and 2021 growing seasons. The air temperature and precipitation during the annual growing season of these sites are shown in Figure 1. The previous crop in 2020 and 2021 was spring wheat and oil flax, respectively. The annual precipitation shows an uneven characteristic, with nearly 60% occurring from June to September. In the study years (2020–2021), the average annual temperature and precipitation at the site were 8.07 °C and 425.5 mm, respectively. Soil samples (0–30 cm) were collected randomly at five points from the experimental location before sowing in 2020, and their physical and chemical properties were analyzed. The physical and chemical properties of the top 0–30 cm of soil are shown in Table 1.

2.2. Experimental Site

A two-factor randomized complete block design with three replications was used for this study. One factor was planting density (D1, 600; D2, 750, and D3, 900 grains·m−2), and the other was potassium (K0, 0; K1, 60 and K2, 90 kg K·hm−2) fertilization. Potassium sulfate (K2SO4) was applied in the treatment, which was evenly spread on the soil surface before sowing. Each plot was 5.0 m × 2.0 m (10 rows with row spacing of 20 cm) in size. Before sowing each year, 80 kg of N·hm−2 and 75 kg of P·hm−2 were distributed uniformly over the soil surface using urea (N 46%) and calcium superphosphate, respectively. In addition, 40 kg of N·hm−2 was applied at the budding stage of oil flax using urea. The oil flax variety ‘Longya No. 11’ (cultivated by the Gansu Academy of Agricultural Sciences, registration Number: GPD Flax (2018620012), variety source: 115 select-1-1 × Longya No. 7) was sown on 9 April 2020 and 7 April 2021 and harvested on 23 August 2020 and 13 August 2021, with a growth stage of 136 and 128 d, respectively.

2.3. Methodology

The plant height and height of the center of gravity were determined at the kernel and maturity stages by measuring 10 plants per plot and averaging these values for each plot. The plants were cut close to the surface from the stalk base with scissors, and the horizontal balance point of each plant was taken. The center of gravity height was measured from the stem base to the balance point of a single plant using a ruler and the fresh weight of a single plant was measured using an electronic balance at the kernel and maturity stages.
Assays of PAL and TAL were performed using the method of Kováčik et al. [20] with some modifications, and the improved method was described in detail in our previous literature study [6]. The activity of CAD and POD was determined using Morrison’s method [21]. CAD catalyzes cinnamyl alcohol and NADP to produce cinnamaldehyde and NADPH, and the NADPH formation rate at 340 nm can be measured to reflect the CAD activity. Peroxidase (POD) catalyzes the oxidation of H2O2 of a specific substrate with characteristic light absorption at 470 nm [8].
A CMT2502 electronic universal testing machine (SANS, Shenzhen, China, torque 2 cm, maximum test force 50 N, and speed range 1–500 mm min−1) was used to test the breaking-resistant strength of oil flax stems at the kernel and maturity stages. The plant lodging resistance tester (LS-1/LS-1S) was used to test the stalk bending strength of oil flax stems at the kernel and maturity stages.
The lignin content was determined using the Klason method [17]. Ten samples were collected from each plot, and the stalks were oven-dried at 105 °C for 30 min and then dried to a constant weight at 80 °C. The dried stalks were then crushed with a variable high-speed rotary crusher and passed through a 60-mesh screen for the determination of lignin content. The cellulose content was determined using the method of Updegraff et al. [22] with some modifications. The fresh samples were dried at a height of 5 cm above or below the center of gravity of the plants, crushed and weighed at 0.2 g, and placed in a boiling water bath for 4 h after each centrifuge tube was filled with 1.5 mL of a mixture of acetic acid: water: nitric acid (8:2:1) [22]. The tubes were cooled at room temperature and centrifuged at 10,000 RPM for 10 min. We removed the supernatant, the pellet was washed with distilled water four times and finally was washed with 90% ethanol. We repeated the centrifugation procedure after each wash. The tubes were dried at 80 °C and then the percentage of cellulose was measured on a dry-matter basis [22].
The lodging resistance index was calculated for all treatments according to the following formula [23]:
Lodging   resistance   index = B r e a k i n g − r e s i s t a n t   s t r e n g t h T h e   h e i g h t   o f   c e n t e r   o f   g r a v i t y × s t e m   f r e s h   w e i g h t

2.4. Statistical Analysis

A two-way (considering planting density and K application) analysis of variance (ANOVA) was performed to determine the interaction between the variables that met these assumptions of normality and homogeneity (SPSS statistical package v.24.0, SPSS Inst., Chicago, IL, USA). Treatment means were compared using an LSD test and a multiple-range test at the probability levels of 5% and 1%, respectively. The structural equation model (SEM) was used to evaluate the multivariate effects of the planting density and K on enzymatic activity, lodging resistance, and grain yield. Before SEM analysis, autoregressive correlation structures in IBM SPSS AMOS 24.0 (SPSS Inst., Chicago, IL, USA) were used to identify potential correlations. Graphs were generated using Origin 2021 (Systat Software Inc., CA, USA) and IBM SPSS Amos 24.0 (SPSS Inst., Chicago, IL, USA).

3. Results

3.1. GY and GY Components of Oil Flax

The GY of oil flax was significantly affected by PD, K, Y, and the interaction of PD × K, Y × PD, and Y × PD × K (p < 0.05), and was insignificantly affected by the interaction effects of Y × K (Table 2). When increasing the PD from 600 to 900 grains·m−2, the GY increased by 10.13% and 3.90% and 21.65% and 1.77% in 2020 and 2021, respectively. However, the GY of oil flax had no significant difference between the 600 and 900 grains·m−2 levels. Potassium significantly increased the GY of oil flax. K1 and K2, respectively, produced 18.47% and 18.47% (2020) and 13.17% and 14.03% (2021) greater GY than K0. Averaged across the two years, potassium application at 60 kg·hm−2 combined with 750 grains·m−2 achieved the highest GY, which increased by 3.38–33.62% compared to the other treatments, indicating that this was the best treatment combination for yield formation among the treatment combinations.
PD, K, and the interaction of PD × K and Y × K significantly affected the grain number (GN) per capsule. Compared with D1, D2 increased GN by 8.19% in 2020. Compared with K0, K1 increased GN and TKW by 16.20% and 8.40% in 2020, respectively. Potassium application increased GN by 6.49% and 8.61%, respectively, compared with K0 in 2021. Y and the interaction of PD × K, Y × K, and Y × PD × K had a significant effect on the number of effective capsules (EC) per plant. D1 increased EC by 17.09% and 14.17%, compared with D2 and D3 in 2021, respectively. PD, K, and their interaction had insignificantly effects on TKW in 2021, while high PD increased the TKW under low PD. Averaged across the two years, the highest EC and GN were observed under D2K2 and D2K1, increasing by 14.55% and 30.34%, respectively, compared to D1K0.

3.2. Agronomic Trait

3.2.1. Plant Height

The PH of oil flax was significantly affected by Y in 2020 and 2021, while K significantly affected PH in 2020, and the interaction of PD with K had a significant effect on PH (p < 0.05) in 2020 (Figure 2). Planting density and the amount of potassium applied affected the plant height of oil flax (Figure 2A–D). Compared with D1, D3 increased plant height at the kernel and maturity stages by 2.37% and 0.36% in 2020, and 2.81% and 1.18% in 2021, respectively. Compared with K0, K1 increased plant height at the kernel and maturity stages by 5.32% and 1.59% in 2020, and 0.90% and 1.71% in 2021, respectively. Meanwhile, K2 decreased plant height by 2.16 and 0.94% at the kernel stage, but increased it by 4.18% and 1.18% at the maturity stage, respectively, in 2020 and 2021 in comparison with K0.
High density increased the plant height at the maturity stage under the K0 application level and kernel stage under the K1 application level, respectively, but decreased the plant height at the kernel stage under the K2 application level. At the kernel stage, potassium application decreased the plant height under a low planting density level, but increased the plant height under a medium planting density level, while K1 significantly increased the plant height under a high planting density level (Figure 2A,B). At the maturity stage, potassium application decreased the plant height under a medium planting density level, K2 increased the plant height under a low planting density level, and K1 decreased the plant height under a high-density level (Figure 2C,D).
The highest plant height was observed under the D3K1 treatment at the kernel stage in both years, being 8.92–19.13% (2020) and 5.97–12.30% (2021) higher than that of the other treatments, while the lowest plant height was observed under the D1K0 treatment at the maturity stage in both years, being 5.19–10.68% (2020) and 0.52–4.46% (2021) lower than that of the other treatments.

3.2.2. The Height of the Center of Gravity

The center of gravity (HCG) of oil flax was significantly affected by Y in 2020 and 2021, while K and PD significantly affected HCG in 2020 (Figure 3). The HCG declined at the maturity stage, compared with the kernel stage. Compared with D1 and D3, D2 decreased the HCG at the kernel and maturity stages in 2020 and at the maturity stage in 2021 (Figure 3A,B,D), and improved the HCG at the kernel stage in 2021 (Figure 3C). K2 increased the HCG at the kernel stage in 2020 and 2021, but only increased it at the maturity stage in 2020. Compared with K1, K2 reduced the HCG under the D2 and D3 PD levels at the kernel and maturity stages in 2020, while it increased the HCG under the D3 PD level at the maturity stage in 2021. The influence of K on the HCG at the kernel and maturity stages in 2021 varied—K2 reduced the HCG under the D2 and D3 PD levels at the kernel stage, while it increased the HCG under the D3 PD level at the maturity stage compared with K1.

3.2.3. Fresh Weight

The FW of oil flax was significantly affected by Y, PD, K, and the interactions of Y × PD, Y × K, PD × K, and Y × PD × K (Figure 4). At the kernel stage, K2 increased the FW of oil flax leaves under the D1 PD level compared with K0, and the difference was significant in 2020. The effect of K2 on the FW of oil flax stems showed an opposite trend under the D2 PD level in 2020 and 2021—compared with K0 and K1, K2, respectively, significantly increased FW by 35.60% and 104.28% in 2020, but significantly decreased FW by 33.71% and 41.07% in 2021 (Figure 4). The FW of K1 under the D3 PD level was significantly increased by 37.90% and 48.26% compared with K2 and K3 in 2021, respectively. D2K2 and D3K1 had the highest FW at the kernel stage in 2020 and 2021, being 4.47–104.28% and 11.95–103.03% higher than that under other treatments, respectively.
At the maturity stage, K2 significantly increased the FW of oil flax leaves under the D1 and D3 PD levels in 2020 and under the D1 and D2 PD levels in 2021 compared with K0. K had no significant effect on FW under the D2 PD level in 2020, while K2 reduced FW under the D3 PD level in 2021. Compared with D1, D3 increased the FW of the oil flax stem under the same K application level, but D3 decreased FW under the K2 treatment in 2021. D1K2 had the highest FW at the kernel stage in 2020 and 2021, being 7.26–177.24% and 8.93–131.46% higher than that of other treatments, respectively.

3.3. Enzymatic Activity

The enzyme activity of TAL gradually increased from the budding to the kernel stage (except for individual treatments, such as the D2K2 and D3K1 treatments in 2020 and the D3K0 and D3K1 treatments in 2021). At anthesis, K0 and K2 treatments decreased and increased the enzyme activity of TAL under different PD levels in 2020, respectively (Figure 5A). The K1 and K2 treatments increased the TAL enzyme activity under D2 and D1 PD levels in 2021, respectively (Figure 5B). D2K2 and D2K1 had the highest TAL enzyme activity at anthesis in 2020 and 2021, being 8.41–64.59% and 21.54–117.93% higher than that of other treatments, respectively. At the kernel stage, an insignificant difference was discovered for TAL enzyme activity between the D2K1 and D3K1 treatments in 2020, which was significantly higher than that of other treatments by 22.19–43.56% and 23.41–45.00%, respectively. The D2K1 and D1K2 treatments significantly increased the TAL enzyme activity at the kernel stage in 2021, and an insignificant difference was observed between the two treatments.
The PAL enzyme activity first decreased and then increased from the budding to the kernel stage. An insignificant difference was discovered in the PAL enzyme activity between the D1K0 and D2K0 treatments in 2020, which significantly decreased the PAL enzyme activity from the budding to the kernel stage (Figure 5C). The D2K2 treatment significantly increased the PAL enzyme activity from the budding to the kernel stage, being significantly higher than other treatments by 6.44–110.00% (budding stage), 27.25–146.23% (anthesis), and 34.71–216.59% (kernel stage), respectively. PD and K application significantly affected PAL enzyme activity in 2021 (Figure 5D). The D1K0 and D3K0 treatments significantly decreased the PAL enzyme activity at the budding stage in 2021 compared with other treatments; K3 and K2 increased the PAL enzyme activity level under the D1 PD level and the D2 and D3 PD levels at anthesis, respectively; the D2K1 treatment significantly increased the PAL enzyme activity at the kernel stage, being significantly higher than that of other treatments by 13.55–44.13%.
The CAD enzyme activity of different PD and K application treatments displayed no significant differences from the budding to the kernel stage in 2020 (Figure 5E). The CAD enzyme activity at different growth stages in 2021 varied according to the treatments (Figure 5F). The K1 and K2 treatments increased the CAD enzyme activity under the D2 and D3 PD levels at the budding stage, respectively. K1 increased and decreased the CAD enzyme activity under the D1 and D3 levels at anthesis, respectively. The K1 and K2 treatments significantly increased the CAD enzyme activity under the medium (D2) and high (D3) PD levels at the kernel stage, respectively. The D2K1 treatment had the highest PAL enzyme activity at the kernel stage, being significantly higher than other treatments by 13.33–195.65%.
PD and K application significantly affected POD enzyme activity at the budding and kernel stages in 2020 and 2021. K2 increased the POD enzyme activity at the budding stage under the D1 and D3 levels in 2020, and K1 significantly increased the POD enzyme activity at the budding stage under the D2 PD level, being higher than that of K0 and K1 by 63.50% and 16.32% (2020), respectively (Figure 5G). The effects of K2 treatment on POD enzyme activity at the budding stage under D1 and D3 PD levels in 2021 showed an opposite trend—that is, K2 increased the POD enzyme activity under the D1 PD level, but decreased it under the D3 PD level (Figure 5H). The D1K2 treatment produced the highest POD enzyme activity at the budding stage in 2021, being 23.20–217.74% higher than that of other treatments. K0 significantly decreased the POD enzyme activity at the kernel stage, while K2 and K1 significantly improved the POD enzyme activity under the D2 and D3 levels in 2020, respectively. K application decreased the POD enzyme activity under the D1 and D3 PD levels, but improved it under the D2 PD level at the kernel stage in 2021.

3.4. Lignin and Cellulose Content

The contents of lignin and cellulose were significantly affected by Y at the budding, anthesis, kernel, and maturity stages; PD at the maturity stage; and K at the budding stage (Table 3). The contents of lignin and cellulose were significantly affected by the interaction of Y × K at the budding stage and by the interaction of PD × K at the maturity stage (p < 0.05). The interaction of Y × PD × K had an insignificant effect on the lignin content at the budding and maturity stages, but a significant effect on the cellulose content at the kernel and maturity stages.
The lignin content showed a single-peaked curve with an increase in potassium application under the same PD level at the budding stage and anthesis in 2020 (Figure 6A). K application reduced the lignin content under the D3 PD level at the budding stage and anthesis in 2021 (Figure 6B). There were differences in the effects of K application on the lignin content under different PD levels at the kernel and maturity stages in 2020 and 2021. The lignin content decreased first and then increased with the increase in K application under the D3 PD level, but displayed the opposite trend under the D1 PD level at the kernel stage in 2020; in 2021, this was consistent with that at the kernel stage. At the kernel and maturity stages, the lignin content increased with the increase in potassium consumption in 2020—initially increased and subsequently decreased in 2021 under the D2 PD level. D2K2 and D2K1 had the highest lignin content at the reproductive growth stages (kernel and maturity) in 2020 and 2021, respectively; compared with other treatments, lignin content was 7.00–24.09% and 7.63–38.89% (2020) higher in D2K2, and 4.78–45.75% and 14.90–39.26% (2021) higher in D2K1.
K1 decreased the cellulose content under the D2 PD level at the budding and flowering stages (Figure 6C,D), and the cellulose content with the increase in K application displayed a single-peaked curve under the D3 level at the budding stage and anthesis in 2020 and 2021. There were differences in the effects of K application on the cellulose content under different PD levels at the kernel and maturity stages in 2020 and 2021. At the kernel stage, the cellulose content with the increase in K application displayed a single-peaked curve under the D1 PD level in 2020, but decreased with the increase in the amount of K in 2021. K2 significantly increased the cellulose content under the D2 PD level in 2020, and K application increased the cellulose content under the D3 level in 2021 at the kernel stage. At the maturity stage, K1 significantly increased the cellulose content under the D1 PD level in 2020, but decreased the cellulose content in 2021. The K had insignificant effects on the cellulose under the D2 PD level in 2020 and 2021. The cellulose content improved with the increase in K under the D3 PD level in 2020 and 2021. D2K2 and D2K1 produced the highest cellulose content at the kernel and maturity stages in 2020, respectively, being 18.84–34.94% and 7.31–27.89% higher than that of other treatments, respectively.
In conclusion, the lignin content at the kernel stage and the cellulose content at the maturity stage were greatly affected by PD and K application, and the application of K combined with a medium PD level (D2) increased the lignin and cellulose content in the stems of oil flax at the kernel and maturity stages, which was conducive to improving the resistance of stems to lodging.

3.5. Lodging Resistance

3.5.1. Snapping Resistance

The SR was significantly affected by Y, K, and Y × PD × K (p < 0.05) in 2020 and 2021. The SR was significantly affected by Y × K (p < 0.05) in 2020, and by PD and PD × K (p < 0.05) in 2021 (Figure 7). K2 significantly increased the snapping resistance in 2020 (Figure 7A), but decreased the snapping resistance in 2021 under the D2 PD level (Figure 7B). Compared with D1 and D3, D2 significantly increased the snapping resistance by 33.85 and 47.96% in 2020, and significantly decreased the snapping resistance by 30.12% and 25.40% in 2021 under the K2 treatment, respectively. D2K2 and D2K1 produced the highest snapping resistance at the kernel stage, being 30.43–70.41% (2020) and 1.51–45.26% (2021) higher than that of other treatments.
At the maturity stage, snapping resistance showed a single-peaked curve with the increase in K application under the D1 and D3 PD levels, and K1 significantly decreased the snapping resistance under the D2 PD level in 2020. The snapping resistance increased with the increase in K application under the D1 PD level—the SR for K2 was significantly higher than K0 and K1 by 87.33% and 25.80%, respectively, but decreased under the D3 PD level in 2021. The effect of K application on snapping resistance showed an opposite trend under the D1 and D3 PD levels in 2021. K2 significantly increased the snapping resistance under the D1 PD level, but significantly decreased the snapping resistance under the D3 PD level. D2 significantly increased the snapping resistance in 2020, but D3 significantly decreased the snapping resistance in 2021 under the K2 treatment. D2K2 and D2K1 produced the highest snapping resistance at the maturity stage, being 9.19–98.91% (2020) and 0.39–88.60% (2021) higher than that of other treatments, respectively. Overall, results indicated that K2 with D2 PD increased the snapping resistance of oil flax stems, which is conducive to increasing the resistance of oil flax to lodging.

3.5.2. Stalk Bending Strength

The stalk bending strength (SBS) was significantly affected by Y, K, and Y × PD × K (p < 0.05) in 2020 and 2021. The SBS was significantly affected by Y × K (p < 0.05) in 2020, and by PD and PD × K (p < 0.05) in 2021. K significantly increased the stalk bending strength of oil flax under the D1 PD level at the kernel stage in 2020, with K1 and K2 increased by 48.95% and 67.19% compared to K0, respectively (Figure 8A). K2 significantly decreased the stalk bending strength under the D3PD level at the kernel and maturity stages in 2020 and 2021 (Figure 8A–D). The stalk bending strength increased with the increase of PD under the K0 treatment, but decreased with the increase in PD under the K2 treatment at the kernel stage in 2020. Compared with the D1 and D2 PD, D3 significantly decreased the stalk bending strength under the K2 treatments at the kernel and maturity stages in 2020 and 2021, and the decrease was by 50.00% and 51.28% (kernel) and 42.91% and 46.06% (maturity) in 2020, as well as 32.81% and 35.55% (kernel) and 6.74% and 13.99% (maturity) in 2021, respectively. The K1 and K2 treatments improved the stalk bending strength under the D2 PD level at maturity in 2020 and 2021, reaching 7.03 N in 2020 and 4.54 N in 2021.

3.5.3. Lodging Resistance Index

At the same density level, the lodging resistance index first increased and then decreased with the increase in potassium application amount. The LRI was the highest in both the kernel and the maturity stage under the K1 application level (except for the D2 treatment at the kernel stage in 2021) (Figure 9). The LRI decreased with the increase in PD at the maturity stage in 2020 under the K1 and K2 application levels (Figure 9B). D2 and D3 reduced the LRI at the kernel stage in 2021 under the K1 application level (Figure 9C). At the same potassium application level, D2 and D3 increased the LRI at the kernel stage in 2020 compared with D1, but there was an insignificant difference between D2 and D3 treatments (Figure 9A). PD had an insignificant effect difference on LRI at the maturity stage in 2021 under the K0 and K1 application levels (Figure 9D).

3.6. Correlation Analysis between Lignin Content and Lodging Resistance

At the kernel stage, the FW of stems was significantly negatively correlated with the lodging resistant index (LRI) in 2020 and 2021. Snapping resistance was significantly positively correlated with lignin, cellulose, PAL, CAD and POD in 2020 at the kernel stage, while it was only significantly positively correlated with cellulose, TAL, and PAL at the maturity stage (Figure 10A,B). Lignin and cellulose contents were significantly positively correlated with PAL and POD in 2020, and the lignin content was significantly positively correlated with PAL in 2021. At the maturity stage, the FW of the stem was negatively correlated with cellulose in 2020 and 2021, but there was no significant difference in 2020 (Figure 10C,D). Regardless of the growth stage, LRI was positively correlated with GY in 2020 and 2021, and there was a significant difference at the maturity stage (Figure 10).

3.7. The Effects of PD and Potassium on GY Based on SEM

The SEM was used to evaluate the direct and indirect relationships between farmland management (PD and potassium) and GY. In 2020 and 2021, potassium had a significant direct effect on GN and EC, and PD had a significant direct effect on EC, but both potassium and PD had no significant direct effects on TKW (Figure 11A,B). Potassium had a positive effect on GN and EC, but PD had a negative effect on TKW. Potassium had stronger effects on GN, EC, and TKW than PD.; the path coefficients (λ) were 0.325, 0.475, and 0.010 in 2020 and 0.378, 0.230, and 0.275 in 2021, respectively. The GN had the greatest effect on GY (λ was 0.269 and 0.319 in 2020 and 2021, respectively), and TKW had a weaker effect on GY (λ was 0.157 and 0.112 in 2020 and 2021, respectively), but the effect of both GN and EC on GY reached the level of significance (p < 0.05).
In 2020 and 2021, potassium had a significant direct influence on enzymatic activity and lodging resistance, and PD had a significant direct influence on agronomic traits and lodging resistance (Figure 11C,D). Potassium had a positive influence on enzymatic activity and lodging resistance, but PD had a negative influence on agronomic traits and lodging resistance. Overall, potassium had a stronger effect on enzyme activity, agronomic traits, and lodging resistance than planting density; the path coefficients (λ) were 0.884, −0.131, and 0.293 in 2020, and 0.598, −0.024, and 0.257 in 2021, respectively. The enzymatic activity and lodging resistance had a stronger influence on GY (λ was 0.973 and 0.450 in 2020, and 0.256 and 0.583 in 2021, respectively), and agronomic traits had a weaker influence on GY (λ was 0.248 and 0.229 in 2020 and 2021, respectively), but the effects of both GN and EC on GY reached the significant level (p < 0.05).

4. Discussion

Increasing PD to improve the target yield per unit area is the main cultivation practice used to further increase crop yields [7]. However, increasing PD brings about negative competition for solar radiation and the availability of other resources among individual plants and increases the risk of lodging [7,24]. The morphological traits (PH, HCG, stalk diameter, wall thickness, etc.) of crop stems were influenced by PD [7]. Our findings are in agreement with the previous findings of Sher et al. [25], who found that with the increase in planting density, the PH and the HCG also increased (Figure 2). When the PD was increased from 650 grains·m−2 to 900 grains·m−2, the plant height increased by 2.37% and 2.81% at the kernel stage in 2020. Previous research demonstrated that the proper application of K increased the PH and HCG of rice, improved the physical and chemical characteristics of the stems, and enhanced the lodging resistance [26]. In the present study, K2 increased the PH under a low planting density level, while K application decreased the PH under a medium planting density level. In summary, increasing PD brings about a high risk of lodging, while the increased application of K has reduced this risk to some extent. The results of this study suggested that high PD (900 grains m−2) increased the PH of oil flax at maturity, while the low-potassium treatment (60 kg hm−2) decreased the plant height.
PAL, TAL, CAD, and POD are key enzymes in the lignin synthesis process of plants [8]. The increase in enzyme activity in metabolic activities warrants the increase in lignin content and plays a key role in the resistance response of plants [27]. Our results showed that the enzyme activities of PAL, CAD, and POD increased with the increase in planting density, while TAL decreased with the increase in PD at the kernel stage in 2021. This may be related to the expression of enzyme activities, because Luo et al. [7] reported that the decreased range for the expression of PAL and CAD in wheat stalks at a high planting density suffering from shading stress was greater than that under low and medium planting density conditions. This study indicated that the low-potassium treatment increased the enzyme activities of PAL, TAL, and POD at the kernel stage in 2020, but the high-potassium treatment inhibited the enzyme activities of PAL, TAL, and POD in 2021 (Figure 5). These results are in agreement with those of Xu et al. [17], who demonstrated that low K significantly increased the enzyme activities of PAL, 4CL, and CAD, and although high potassium levels increased the activities of the three enzymes to varying degrees, the improvement effect was not significant. These results also indicate that appropriate potassium supply can improve the activity of lignin metabolic enzymes, but insufficient or excessive potassium supply can reduce the activity of lignin-related metabolic enzymes, affecting the synthesis of lignin.
Lignin is a phenolic polymer that is the main structural component of secondary cell walls in vascular plants [6,28]. Its main function is to increase the hardness of the plant cell wall, increase the thickness of the stem wall, and strengthen its resistance to pressure during the lignification process [17]. Genetic factors are the main factors influencing crop lodging; crop lodging is also influenced by environmental factors, especially the regulation of PD [29]. It has been shown that the lignin content of buckwheat (Fagopyrum esculentum M.) increased with increasing PD [30]. Gao et al. [6] found that PD is one of the main factors influencing the plant lignin content of oil flax. In the present study, lignin content showed a single-peak curve variation with an increase in PD at the kernel stage (Figure 6). There was a significant positive correlation between the cellulose content of cell wall components and the bending resistance [31], and the stem cellulose content can determine the strength of the stem and increase the stiffness of the stem [32]. Hu et al. [33] found that the lignin and cellulose content, GY, and lodging resistance in rapeseed (Brassica campestris L.) stems increased with the increase in PD. The cellulose content among different planting densities varied significantly—D2 had a higher cellulose content in the stem, resulting in greater bending strength of the oil flax stems. These results agree with Zhao et al. [34], who reported that the cellulose content in stems of highland barley (Hordeum vulgare var. Coeleste L.) with strong lodging resistance increases and then decreases with increasing planting density. The lignin content of the stem can be regulated by potassium fertilizer operation [17]. The lignin content of wheat stem was significantly increased after proper potassium application, while the lignin content decreased after insufficient or excessive potassium application [35]. The present study showed that K application at the kernel stage significantly increased the lignin content of oil flax stems. Similar results were shown by Xu et al. [17]. At a PD of 750 grains·m−2, the K2 treatments decreased the lignin content by 15.27% and 18.02%, compared with the K1 treatment, respectively, at the kernel and maturity stages in 2021. In conclusion, the influence of low potassium levels on lignin content was better than that of high potassium levels. These results indicate that a suitable amount of potassium fertilizer should be used to promote the accumulation of lignin in stems.
Stem lodging is not only affected by the varietal genotypes and agricultural measures, but is also related to growing conditions [24,36], such as precipitation and wind speed. The breaking-resistant strength and bending strength of stems have a strong correlation with lodging [37,38]—the greater the breaking-resistant strength and bending strength of the stem, the stronger the lodging resistance of the stem [17]. This study indicated that D2 significantly increased the snapping resistance of oil flax stem, being 19.48% and 16.68% (2020) and 28.81% and 15.97% (2021) higher than the D1 and D3 treatments, respectively, at maturity (Figure 7). The morphological structure and physiological characteristics of stems are closely related to lodging resistance, and the lodging resistance of crops can be significantly improved by increasing the bending strength of stems [39]. The results of this study showed that D3 significantly decreased the stalk bending strength under the K2 treatment at the kernel and maturity stages in 2020 and 2021 (Figure 8). Huang et al. [40] reported that potassium fertilizer can promote the synthesis and transportation of carbohydrates, reduce the accumulation of non-proteins in the stem, develop the mechanical organization, increase the strength of the stem, and improve the lodging resistance of the plant. The current study shows that K2 significantly decreases the stalk snapping resistance under the D3 PD level (Figure 7). At a PD of 750 grains m−2, K2 and K1 increased the stem snapping resistance and stalk bending strength in 2020 and 2021, respectively; this may be related to the accumulation of potassium in the soil, as the two-year experiment was repeated on the same plot, resulting in higher potassium content in the soil under the K2 treatment in 2021, which instead inhibited the improvement of snapping resistance and stalk bending strength.
Planting density, the potassium application amount, and the interaction effects of PD with K significantly influenced the GY of oil flax, shown as D2 > D3 > D1 and K2 ≈ K1 > K0 (Table 2). At a medium planting density of 750 grains·m−2, K2 significantly improved the GY of oil flax in 2020, but K1 significantly increased the GY in 2021. These results indicate that appropriate intensive planting significantly improved the GY of oil flax. However, when the density was higher, the individual competition for water, fertilizer, and light energy intensified, the photosynthetic rate of middle and lower leaves decreased, and the gas exchange within the population was blocked [41]. On the one hand, the supply capacity of flax plants decreased; on the other hand, it affects the development of flowers and increases the number of abortive grains, resulting in a decline in GY [42]. Appropriate K application increased the GY of oil flax, which may be related to the increased photosynthesis and cellular respiration of oil flax, provided more ATP for the formation of protein and seeds, and enhanced the transfer of carbohydrates to seeds and the capacity for grain [43]. In the present study, the K application significantly improved the GY of oil flax. Previous studies have shown that the GY of oil flax was not continuously increased with the amount of potassium fertilizer, but a high potassium application rate decreased the GY [44]. In this study, the highest GY of oil flax was observed in D2K2 and D2K1 treatments, being 1733 kg·hm−2 in 2020 and 1799 kg·hm−2 in 2021. These results are in agreement with the results of the previous study by Liu et al. [8].
Throughput analysis using the SEM showed that the number of effective capsules per plant directly influenced the GY of oil flax in the present study, while lodging resistance was directly affected by enzymatic activity and agronomic traits (Figure 11). PD had a negative effect on the 1000-kernel weight and agronomic traits, and had a positive effect on oil flax GY. The stem lignin content, snapping resistance, and stalk bending strength significantly influenced lodging resistance and also directly affected the GY.

5. Conclusions

The decreased plant height and the height of the center of gravity in oil flax, the increased fresh weight, the enzyme activities of TAL, PAL, POD, and COD, the lignin and cellulose content, the snapping resistance, and the stalk bending strength play crucial roles in increasing the lodging resistance index of oil flax stems in response to the stress caused by high PD. The TAL, PAL, POD, and COD activities, lignin content, and bending resistance of oil flax stems were increased by low potassium application (K1), while the lodging resistance index and the GY were improved by reducing the negative effects of an increase in density. The increased GY was significantly correlated with the number of effective oil flax capsules per plant and the lodging resistance index, which were directly affected by K application. In conclusion, under the same ecological conditions as the experiment, the field management strategy of medium PD (750 grains·m−2) and low K (60 kg K·hm−2) was the best for promoting the lodging resistance and GY of oil flax.

Author Contributions

B.W.: Writing the original draft, conceptualization, methodology, and funding acquisition. Z.C.: Writing the original draft and making figures. L.M., X.L., H.W. and H.D.: Data collection and analysis. Y.W. and B.Y.: writing—review and editing the manuscript. Y.G.: Conceptualization, methodology, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Talents of Gansu Agricultural University (GAU-KYQD-2018-21), the National Natural Science Foundation of China (31760363), the State Key Laboratory of Aridland Crop Science, Gansu Agricultural University (GSCS-2020-Z6), the China Agriculture Research System of MOF and MARA (CARS-14-1-16), the Gansu Education Science and Technology Innovation Industry Support program (2021CYZC-38), and the Fuxi Outstanding Talent Cultivation Plan of Gansu Agriculture University (Gaufx-02J05).

Data Availability Statement

All data included in this study are available upon request by contact with the first author.

Acknowledgments

Authors are thankful to the editors and anonymous reviewers for providing valuable comments and suggestions for the improvement of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Average monthly precipitation and temperature in 2020 and 2021. P, Precipitation; T, Temperature.
Figure 1. Average monthly precipitation and temperature in 2020 and 2021. P, Precipitation; T, Temperature.
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Figure 2. The plant height under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020; (C,D), kernel and maturity stage, respectively, in 2021; PD, planting density; K, potassium; Y, year; **, significant at p ≤ 0.01; ***, significant at p ≤ 0.001; ns, not significant.
Figure 2. The plant height under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020; (C,D), kernel and maturity stage, respectively, in 2021; PD, planting density; K, potassium; Y, year; **, significant at p ≤ 0.01; ***, significant at p ≤ 0.001; ns, not significant.
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Figure 3. The height of the center of gravity under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020; (C,D), kernel and maturity stage, respectively, in 2021; PD, planting density; K, potassium; Y, year; *, significant at p ≤ 0.05; ***, significant at p ≤ 0.001; ns, not significant.
Figure 3. The height of the center of gravity under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020; (C,D), kernel and maturity stage, respectively, in 2021; PD, planting density; K, potassium; Y, year; *, significant at p ≤ 0.05; ***, significant at p ≤ 0.001; ns, not significant.
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Figure 4. The fresh weight under different PD and K levels at the kernel and maturity stage in 2020 and 2021. Note: A and B, 2020 and 2021. The small letters next to the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year; **, significant at p ≤ 0.01; and ***, significant at p ≤ 0.001.
Figure 4. The fresh weight under different PD and K levels at the kernel and maturity stage in 2020 and 2021. Note: A and B, 2020 and 2021. The small letters next to the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year; **, significant at p ≤ 0.01; and ***, significant at p ≤ 0.001.
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Figure 5. The enzymatic activity under different PD and K levels at different growth stages in 2020 and 2021. Note: (A,C,E,G), enzyme activities of TAL, PAL, CAD, and POD, respectively, in 2020; (B,D,F,H), enzyme activities of TAL, PAL, CAD, and POD, respectively, in 2021. **, significant at p ≤ 0.01.
Figure 5. The enzymatic activity under different PD and K levels at different growth stages in 2020 and 2021. Note: (A,C,E,G), enzyme activities of TAL, PAL, CAD, and POD, respectively, in 2020; (B,D,F,H), enzyme activities of TAL, PAL, CAD, and POD, respectively, in 2021. **, significant at p ≤ 0.01.
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Figure 6. The lignin and cellulose content under different PD and K levels at different growth stages in 2020 and 2021. Note: (A,B), the lignin content in 2020 and 2021, respectively; (C,D), the cellulose content in 2020 and 2021, respectively.
Figure 6. The lignin and cellulose content under different PD and K levels at different growth stages in 2020 and 2021. Note: (A,B), the lignin content in 2020 and 2021, respectively; (C,D), the cellulose content in 2020 and 2021, respectively.
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Figure 7. The snapping resistance under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), the snapping resistance in 2020 and 2021, respectively; The small letters above the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year.
Figure 7. The snapping resistance under different PD and K levels at the kernel and maturity stages in 2020 and 2021. Note: (A,B), the snapping resistance in 2020 and 2021, respectively; The small letters above the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year.
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Figure 8. The stalk bending strength under different PD and K levels at the kernel and maturity stage in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020, (C,D), kernel and maturity stage, respectively, in 2021. The small letters above the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year; *, significant at p ≤ 0.05; **, significant at the p ≤ 0.01; ***, significant at p ≤ 0.001; ns, not significant.
Figure 8. The stalk bending strength under different PD and K levels at the kernel and maturity stage in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020, (C,D), kernel and maturity stage, respectively, in 2021. The small letters above the error bars indicate significant differences between treatments at the p < 0.05 level; PD, planting density; K, potassium; Y, year; *, significant at p ≤ 0.05; **, significant at the p ≤ 0.01; ***, significant at p ≤ 0.001; ns, not significant.
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Figure 9. The lodging resistance index under different PD and K levels in 2020 and 2021. Note: (A,B), kernel and maturity stage in 2020, (C,D), kernel and maturity stage in 2021. Different small letters over the error bars indicate significant differences between treatments at the p < 0.05 level.
Figure 9. The lodging resistance index under different PD and K levels in 2020 and 2021. Note: (A,B), kernel and maturity stage in 2020, (C,D), kernel and maturity stage in 2021. Different small letters over the error bars indicate significant differences between treatments at the p < 0.05 level.
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Figure 10. The correlation analysis of lignin content and lodging resistance at the kernel and maturity stage in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020, (C,D), kernel and maturity stage, respectively, in 2021. PH, plant height; HCG, height of the center of gravity; FW, fresh weight of stem; SR, snapping resistance; SBS, stalk bending strength; TAL, tyrosine ammonia-lyase activity; PAL, phenylalanine ammonia-lyase activity; CAD, cinnamyl alcohol dehydrogenase activity; POD, peroxidase activity; LRI, lodging resistant index; GY, grain yield.
Figure 10. The correlation analysis of lignin content and lodging resistance at the kernel and maturity stage in 2020 and 2021. Note: (A,B), kernel and maturity stage, respectively, in 2020, (C,D), kernel and maturity stage, respectively, in 2021. PH, plant height; HCG, height of the center of gravity; FW, fresh weight of stem; SR, snapping resistance; SBS, stalk bending strength; TAL, tyrosine ammonia-lyase activity; PAL, phenylalanine ammonia-lyase activity; CAD, cinnamyl alcohol dehydrogenase activity; POD, peroxidase activity; LRI, lodging resistant index; GY, grain yield.
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Figure 11. Structure equation models fitted to the linear relationships among planting density, potassium application rate, GN, EC, TKW, enzymatic activity, agronomic traits, lodging resistance, and GY in the 2020 and 2021 growing seasons. Note: (A,B), structure equation models of GY and yield components in 2020 and 2021, respectively; (C,D), structure equation models of GY and structure equation models in 2020 and 2021, respectively. PD, planting density; K, potassium application amount; GN, grain number per capsule; EC, effective capsule per plant; TKW, thousand kernel weight; GY, grain yield; TAL, tyrosine aminolyase; PAL, phenylalanine aminolyase; CAD, cinnamyl alcohol dehydrogenase; POD, peroxidase; PH, plant height; HCG, height of the center of gravity; FW, fresh weight of stems. The green and red arrows indicate positive and negative relationships, respectively. The thickness of the arrows is proportional to the strength of the causal effect. The numbers next to the arrows are standardized path coefficients or covariates. R2 values indicate the proportion of variance explained by each variable.
Figure 11. Structure equation models fitted to the linear relationships among planting density, potassium application rate, GN, EC, TKW, enzymatic activity, agronomic traits, lodging resistance, and GY in the 2020 and 2021 growing seasons. Note: (A,B), structure equation models of GY and yield components in 2020 and 2021, respectively; (C,D), structure equation models of GY and structure equation models in 2020 and 2021, respectively. PD, planting density; K, potassium application amount; GN, grain number per capsule; EC, effective capsule per plant; TKW, thousand kernel weight; GY, grain yield; TAL, tyrosine aminolyase; PAL, phenylalanine aminolyase; CAD, cinnamyl alcohol dehydrogenase; POD, peroxidase; PH, plant height; HCG, height of the center of gravity; FW, fresh weight of stems. The green and red arrows indicate positive and negative relationships, respectively. The thickness of the arrows is proportional to the strength of the causal effect. The numbers next to the arrows are standardized path coefficients or covariates. R2 values indicate the proportion of variance explained by each variable.
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Table 1. Soil porosity and chemical characteristics of the soil of the experimental site.
Table 1. Soil porosity and chemical characteristics of the soil of the experimental site.
YearTotal NTotal PSoil Organic MatterAvailable NAvailable PAvailable KpH
(g·kg−1)(g·kg−1)(g·kg−1)(mg·kg−1)(mg·kg−1)(mg·kg−1)
202010.070.810.6948.8527.40108.248.14
Table 2. Effects of PD and K application, and their interaction on yield and yield components in 2020 and 2021.
Table 2. Effects of PD and K application, and their interaction on yield and yield components in 2020 and 2021.
Treatment20202021
GYECGNTKWGYECGNTKW
kg·ha−1No. Plant−1Grain No·Capsule−1gkg·ha−1No. Plant−1Grain No·Capsule−1g
D1K01277d41.4bc6.43e7.35c1175d13.6ab6.39c6.17a
K11495bc45.1ab8.22abc7.85bc1246cd11.7ab8.23ab6.20a
K21465c43.1bc7.69bcd7.48c1474bc15.8a8.60ab6.88a
D2K01332d35.4c7.87bcd7.53c1382bc11.3ab7.70b6.45a
K11600b45.3ab8.67a9.00a1799a13.3ab8.04ab6.47a
K21733a52.7a7.64cd8.30b1555b10.3b8.19ab6.45a
D3K01338d44.8ab7.35d7.62c1292cd13.9ab8.58a6.68a
K11583bc45.5ab8.29ab7.53c1312bcd11.1ab7.84ab6.48a
K21480bc44.8ab7.30d7.55c1360bc11.1ab7.82ab6.70a
Source of variance
PD** ns ** *** *** * ns ns
K*** * *** ** ** ns * ns
PD × K* * * * ** ** *** ns
GY
kg·ha−1		EC
No·Plant−1		GN
Grain No·capsule−1		TKW
g
Y*** *** ns ***
Y × PD** ns ns ***
Y × Kns ** ** **
Y × PD × K*** * ns *
Note: GY, grain yield; EC, effective capsule; GN, grain number; TKW, thousand kernel weight; Different lowercase letters in the same column represent least significant difference (LSD) at p < 0.05. PD, planting density; K, potassium; Y, year; * Significant at p ≤ 0.05; ** Significant at p ≤ 0.01; *** Significant at p ≤ 0.001; ns, not significant.
Table 3. Effects of PD and K application, and their interaction on lignin and cellulose content at different growth stages in 2020 and 2021.
Table 3. Effects of PD and K application, and their interaction on lignin and cellulose content at different growth stages in 2020 and 2021.
ItemLigninCellulose
BuddingAnthesisKernelMaturityBuddingAnthesisKernelMaturity
Y*** *** *** ns ** 0.014 *** ***
PD*** ns ** ** ns ns ns ***
K*** ns * ns * ns ns ns
Y × PDns ns ns ns ns *** ** ns
Y × K** ns ns * * ns ns ns
PD × K** ns ns ** ns *** ns *
Y × PD × Kns ns ns ns ns ns ** *
Note: Y, year; PD, planting density; K, potassium; *, significant at p ≤ 0.05; **, significant at p ≤ 0.01; *** significant at p ≤ 0.001; ns, not significant.
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MDPI and ACS Style

Wu, B.; Cui, Z.; Ma, L.; Li, X.; Wang, H.; Wang, Y.; Yan, B.; Dong, H.; Gao, Y. Effects of Planting Density—Potassium Interaction on the Coordination among the Lignin Synthesis, Stem Lodging Resistance, and Grain Yield in Oil Flax. Agronomy 2023, 13, 2556. https://doi.org/10.3390/agronomy13102556

AMA Style

Wu B, Cui Z, Ma L, Li X, Wang H, Wang Y, Yan B, Dong H, Gao Y. Effects of Planting Density—Potassium Interaction on the Coordination among the Lignin Synthesis, Stem Lodging Resistance, and Grain Yield in Oil Flax. Agronomy. 2023; 13(10):2556. https://doi.org/10.3390/agronomy13102556

Chicago/Turabian Style

Wu, Bing, Zhengjun Cui, Lijuan Ma, Xiqiang Li, Haidi Wang, Yifan Wang, Bin Yan, Hongwei Dong, and Yuhong Gao. 2023. "Effects of Planting Density—Potassium Interaction on the Coordination among the Lignin Synthesis, Stem Lodging Resistance, and Grain Yield in Oil Flax" Agronomy 13, no. 10: 2556. https://doi.org/10.3390/agronomy13102556

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