1. Introduction
Rye (
Secale cereale), an annual gramineous herbaceous plant, is particularly resistant to environmental stress, which manifests in cold resistance, drought resistance, lodging resistance, and disease resistance [
1,
2]. Rye is mainly distributed in Germany, Poland, and France and China’s Heilongjiang, Inner Mongolia, Qinghai, and Tibet regions. Rye can be used for both grain and forage, and its grain nutrients are abundant, often being made into rye beer, rye bread, and rye tea [
3]. The plant grows easily, with wide leaves and a high grass yield, which can be used as a livestock concentrate feed. Winter rye exhibits high nutritional value and planting it may effectively alleviate the production crisis caused by the shortage of feed in winter due to the current rapid development of the animal husbandry industry. In order to safely overwinter, wintering rye must be sown around September each year, which effectively improves the loss of topsoil and increases the coverage of ground vegetation in winter pastoral areas and has good applications for scientific research [
4].
Population structure is an important factor for ensuring crop production. Reasonable planting density can alleviate the stresses between individuals and the group and promote the coordinated development of spike number per unit area, grain number per spike, and thousand-grain weight [
5]. Keles, et al. [
6,
7,
8,
9] showed that with an increase in planting density, rye yield first increases and then decreases, while the total tiller number, plant height, and stem leaf ratio increase gradually with an increase in planting density.
Goncharenko et al. [
10,
11] found that with an increase in wheat planting density, the ability for light transmission and ventilation between groups gradually weakens, stems become thinner, and lodging resistance decreases. Therefore, it was concluded that high-density planting was the direct cause of wheat lodging. Reducing the planting density of wheat enhances lodging resistance but reduces the number of effective panicles and grain yield. Many studies have found that the planting density of wheat is negatively correlated with the lodging resistance index; stems are gradually refined, and the length of the basal internodes increases [
12]. The overall length of the stems reduces, and thus, the mechanical strength of the stems decreases [
13]. Studies have shown that the planting density of winter wheat is closely related to leaf senescence and grain weight. The higher the planting density of winter wheat after flowering, the lower the content of chlorophyll and SOD activity, while the content of soluble protein and MDA increases [
14]. Photosynthates are the main source of dry matter in wheat grains after anthesis. After wheat flowering, photosynthetic products, which are the main source of dry matter in the grain, can effectively delay leaf senescence, enhance the physiological function of the leaves, and promote grain weight [
15]. Yu et al. [
16] stated that reducing the planting density of rye is an important way to optimize its population structure. Further, low-density treatment effectively prolongs the functional period of flag leaves, alleviates the senescence speed of flag leaves, increases the accumulation of dry matter in the late growth stage, and plays a positive role in the formation of yield. Research has indicated that the senescence of rice leaves is related to a decrease in leaf chlorophyll content and an increase in free proline content, indicating that the senescence process of leaves is related to proline content [
17]. Wintering rye 001 was bred by the Laboratory of Wheat Research, Northeast Agricultural University and has been shown to overwinter safely in the Heilongjiang Province, which is located at high latitudes with cold weather. Due to a short frost-free period, it is often planted only once a year by those with insufficient experience in the cultivation of overwintering crops, which often results in a weaker cultivation of winter rye. Determining the optimum planting density and planting schedule of winter rye requires an in-depth study [
18]. To establish the best planting plan for overwintering rye in high-latitude areas, and to explore the physiological basis of yield formation under different planting densities, the physiological characteristics, stem morphological characters, aging characteristics, and feeding quality of winter rye under different planting densities were studied [
5]. This study provides a practical reference for establishing the production mode of winter rye feeding and for resolving the quality problems of rye cultivation in early spring cold forages by establishing the optimal planting density of early spring rye and winter forage in cold regions. Further, we clarify the cultivation basis for optimum rye yields.
4. Discussion
Planting density is an important factor affecting the lodging resistance of rye stems. In the later stages of wheat growth and development, with an increase in planting density, the lodging resistance of the stem decreases [
23]. The lodging resistance of wheat is related to the basal internode thickness and the wall thickness of the wheat stem. The thicker the stem, the thicker the stem wall, and the stronger the lodging resistance [
24]. The lodging resistance of stems is closely related to plant height and stem base traits [
25]. Rebetzke et al. [
26] found that plant height was the most important factor affecting the lodging resistance of wheat stems. The higher the plant, the greater the probability of lodging. Related studies in wheat have found that within a certain range, the higher the seeding density, the higher the amount of storage substances that are transported from the stem to the ear, and the higher the probability of stem lodging [
27]. Increasing sowing density increases population density resulting in a closed population, poor stem quality, and a population that is prone to lodging [
28]. The length of the first and second internodes, center of gravity height, and height of the basal internodes of the wheat stem have been shown to be positively correlated with lodging rate [
29]. Stem lodging occurs mostly in the first and second basal internodes; therefore, the shorter the internodes, the better the lodging resistance [
30,
31]. Zhirov et al. [
32] found that the internode thickness and stem diameter between the first and second internodes of the stem are important indexes that affect the lodging resistance of the stem. Miranda et al. [
33,
34] found that with an increase in planting density, the plant height of wheat and the length of the first and second internodes of the stem base increased, thereby increasing the proportion of the first and second internodes of the base to the total length of the upper internodes. As density increases, the center of gravity increases, and the stem diameter decreases, which reduces the mechanical strength of the stem and the lodging resistance index [
34]. The mechanical strength and lodging resistance index of stems are also crucial indicators for determining the lodging resistance of plants [
35], and the lodging resistance index of stems can be used to directly reflect the lodging resistance of overall plants. The ANOVA and correlation analyses between stem characteristics and lodging resistance index showed that stem strength, stem wall thickness, and stem diameter were closely related to lodging resistance, and further, contributed greatly to lodging resistance, which is consistent with previous research results [
35]. The longer the length of the second section of the stem base, the weaker the lodging resistance of the stem, indicated by a significant negative correlation found in our study. However, the correlation was higher between the length of the first section of the stem base and the lodging resistance, which is consistent with the findings of Zhang. The results of this study showed that plant height, barycenter height, and internode length of the first and second internodes at the stem base of rye increased with an increase in density. Stem wall thickness, diameter, strength, and lodging resistance index decreased gradually with an increase in planting density, which is similar to the results of Miranda. Therefore, at lower planting densities, rye stem lodging resistance is higher, and the stem is stronger, which is conducive to yield formation.
Several studies have shown that, in wheat, flag leaves exhibit their highest photosynthetic efficiency at the late growth stage compared to other leaves, which promotes the formation of grain yield, and thus, significantly improves overall yield [
36]. A decline in the physiological and biochemical functions of flag leaves in the late growth stage of rye are the main indictors of senescence. Grain filling and leaf senescence in post-anthesis wheat have been characterized extensively [
36,
37]. In the plant internal defense system, the primary role of antioxidant phosphatase is to effectively remove reactive oxygen species so that the generation and removal of reactive oxygen species achieves a dynamic balance [
38,
39]. In this study, at the late growth stages of the rye, the SOD and POD activities in the flag leaves first increased after anthesis, and then decreased after reaching a maximum value. The SOD and POD activities decreased mostly from the late flowering stage; SOD activity reached its peak on the 14th day after anthesis. The activities of SOD and POD in leaves gradually decrease with an increase in planting density [
40]. The results of this study showed that as time progressed, the leaves gradually senesced, and there was a negative correlation between SOD activity and POD activity and planting density, indicating that increasing planting density would significantly reduce SOD activity and POD activity and aggravate the leaf senescence process. MDA is the product of membrane peroxidation and can be used as an indicator of membrane lipidization [
41]. The results showed that MDA content gradually accumulated with the senescence of the flag leaves and increased after the 21st day of flowering. We found a negative correlation between SOD activity and MDA content in the wheat. The decrease in SOD and POD activity in leaves accelerated the senescence of wheat, which directly resulted in a decrease in the function of scavenging reactive oxygen species and the accumulation of MDA content. With an increase in planting density, leaf senescence was aggravated, and MDA content showed an upward trend, a finding which is similar to the results of Zhang et al. [
42]. Our results showed that low planting density had a positive effect on optimizing the population structure of rye and could effectively extend the functional period of flag leaves and slow down their senescence, which was conducive to the increase in dry matter accumulation during the late growth stage and an observed improvement in yield.
High planting density has been shown to promote the accumulation of NDF and ADF and reduce the content of crude protein in plants. In this study, the content of crude protein showed a gradual downward trend with planting density; however, neutral and acid detergent fiber content showed an upward trend with the increase in rye planting density, which is consistent with previous research results [
43,
44]. The crude protein content of rye under the D2 planting density was higher than the other planting densities. Further, the neutral detergent fiber and acid detergent fiber contents under the D2 planting density were lower than other higher planting densities, indicating that, to a certain extent, a higher planting density could improve yield but reduce feed quality.
Adjusting planting density is an important method for farmers to improve the yield and economic coefficients of their crops [
43]. Many studies have shown that suitable planting density has a positive effect on crop photosynthesis and intra-population gas exchange, which can improve crop yield [
44,
45]. Planting density affects the grain yield of wheat, as well as other yield components such as spike number, grain number per spike, and thousand-grain weight. You [
46] showed that rye exhibits a higher grain number per spike and thousand-grain weight under a low planting density, resulting in high yields. Ma [
47] found a significant effect of wheat spike number on yield. In this study, when rye was planted at a density of 225 × 10
4 plants hm
−2, the spike number was significantly lower than in other density treatments. In addition, the thousand-grain weight and grain number per spike fully compensated for the disadvantage of a low spike number, resulting in a much higher grain yield in the high-density treatment. When the density of rye planting was 325 × 10
4 plants hm
−2, the spike number of the rye was higher than the other density treatments; in this treatment, grain yield reached its maximum, and the yield of the rye was much higher than the other density treatments, which is consistent with previous research [
48]. We concluded that the planting density had a comprehensive effect on the grain yield of winter rye, and a too high or too low planting density had adverse effects on the formation of grain yield.