1. Introduction
In China, hybrid maize comprises more than 95% of maize cultivation [
1], and the annual demand for hybrid seeds is approximately 1.1 billion kg [
2]. The production of high-quality hybrid maize seeds is one of the foundations of successful agriculture and can comprehensively enhance the development level of modern agriculture. For a long time, China has many seed companies (5808 companies in 2018, data from China Seed Congress 2019, Beijing, China) and crop varieties. It is worth noting that the seed production levels of different seed enterprises are inconsistent. This may be due to the low entry threshold of China’s seed enterprises in the past. In contrast, most maize growers in the USA depend on a relatively small number of relatively large seed companies that maintain advanced standards of seed quality. More importantly, the seed quality in China has remained unclear in recent years. Thus, it is important to investigate the seed quality of the main crops, i.e., maize, rice, and wheat, in China.
Recently, many studies about seed vigor, as an important index, reflecting seed quality have been reported [
3,
4,
5]. In particular, the quality of maize seeds has attracted wide attention. For example, regarding maize seed production and storage, the ideal harvest time for the seed production of XY335 and ZD958 with the highest vigor across eight environments has been identified [
6]. Seeds of the maize inbred line Zheng 58 from the middle and bottom section of the ear have lower abscisic acid (ABA)/gibberellin acid (GA) ratios in the embryos and higher seed vigor, stress resistance, and carbon and nitrogen metabolism [
7]. Twenty-three candidate genes for association with maize seed vigor traits coinciding with 13 quantitative trait loci (QTLs) with functions in the glycolytic pathway and in protein metabolism under artificial aging conditions, were mapped using recombinant inbred line (RIL) maize populations (Yu82 × Shen137 and Yu537A × Shen137) [
8]. Six known genes and five novel candidate genes related to germination in aged seeds of the maize inbred lines P39 and EP44 were identified by simple sequence repeats (SSRs) [
9]. The raffinose family oligosaccharides play important roles in modulating maize seed vigor and seed longevity in storage [
10]. Regarding the genetics of maize seed vigor-related traits, Jiang et al. (2011) detected 10 QTLs controlling coleorhiza length, which were closely related to seed vigor [
11], and Li et al. [
12] detected 43 QTLs related to seed low-temperature germination ability in two cold-tolerant inbred lines (220 and P9-10), two susceptible lines (Y1518 and PH4CV), and three connected F2:3 populations.
It is very important to analyze the maize seed quality by collecting and testing the seed samples of cultivated varieties in the main maize production areas in China. However, there are few reports on the market research analysis of maize seed quality; for example, Zhang et al. [
13] investigated the maize seed quality in the Shandong province of China through a questionnaire survey in 2014. However, the relationships between indices of maize seed’s physical and chemical characteristics and seed vigor need to be further elucidated [
14], and high-vigor seed traits also need to be continuously examined.
In this study, we investigated the seed quality of commercial maize hybrid samples which were collected from the main maize-producing areas of China from 2013 to 2018 and performed a comparative analysis of seed quality of these maize samples in different years. At the same time, the prediction model of seed quality correlation was established. This work will provide a basis to better understand the changes of maize quality in China over the past six years and offer an important reference for improving the maize seed quality.
4. Discussion
Maize is a high-yielding crop used for food, feed, energy, and industrial materials. It is vital for global food security. Moreover, China is an important maize-producing country globally. Understanding the changes of maize seed quality in the main production areas of China in the past six years will be important for further improving the quality of China’s maize seeds, breeding high-vigor maize, and promoting the rapid development of modern agriculture.
From 2013 to 2018, we collected 1196 maize hybrid cultivar samples in the market from the NHNSS regions of China. We acquired the basic information of these samples, including the variety name, collecting location, packaging type, seed type, and package label information. Meanwhile, the seed quality tests were performed mainly according to the International Rules for Seed Testing made by the ISTA [
16] and Rules for Agricultural Seed Testing [
17]. In particular, the maize seed quality indices were performed for comparative analysis in different years, and the relationships between seed quality-related indices and FSE were investigated.
Some results attracted our attention. First, the sample packaging rate was only 84.8% in 2013, and it has been on the rise in recent years; for example, it was 97.4% in 2018. This is closely related to the application and implementation of the Seed Law of the People’s Republic of China [
26] and the promotion of people’s seed quality consciousness. With the single seed sowing technology popularized and applied to maize, high-quality seeds are an important prerequisite for applying this technology to obtain high yield [
35]. The sample coating rate, namely the percentage of samples with coated seed obviously increased from 2013 to 2018. Seed coating has been used widely in the production if many crops production. This may be due to its specialized functions to improve handling, protection, and, to a lesser extent, germination enhancement and plant establishment [
36,
37,
38,
39]. Packaging according to seed number has changed the unit of measurement of maize packaging, which mainly depends on the variety characteristics such as the number of plants in the unit area. The SRPASN climbed to 58.6% in 2018 from 24.5% in 2013. Wang et al. [
14] concluded that the seed quality of samples packaged according to seed number was better than that of samples packaged according to seed weight, and multiple types of seed packaging number were related to the characteristics of maize varieties and the requirements of mechanization precision planting. Therefore, the increase in sample coating rate and SRPASN are important aspects that signify the improvement in maize seed quality in China.
Second, the mean standard germination percentage (SGP) increased from 91.8% in 2013 to 95.1% in 2018. However, the index slightly declined in 2017 compared with 2016. The main reason for this may be attributed to the excessive production capacity of maize in China in 2015 and/or 2016, which led to the samples with old seeds making up a high proportion of the total in 2017. Furthermore, the overproduction of maize seeds led to a large amount of seeds being in stock. Seeds stored for a long time will obviously age, which leads to decreased seed quality-related indices, such as seed vigor. According to the Seed Law of the People’s Republic of China, seeds stored for a long time can be sold on the market as long as the standard germination percentage is not lower than the national standard germination percentage (85%) [
31]. It is noteworthy that seed vigor declines continuously during seed storage, and the degree of seed vigor decline is commonly associated with different maize varieties and seed high-vigor maintenance techniques. According to the quality standards for grain crop seeds in China (GB4404.1-2008) [
27] and the Seed Law of the People’s Republic of China [
26], maize samples seeds in different years of production also can be sold on the market as long as the standard germination percentage of these seeds is not less than 85%. For example, in 2017, many sample packaging bags were not marked with the production date; instead, the seed testing date or seed processing date was indicated. Therefore, the quality of seeds stored for a long time has already declined. How to effectively improve seed stand storage has become a great concern at the present stage of maize seed destocking in China.
Third, the sample qualification rate, namely the percentage of samples which met the prescribed quality standards continued to increase and even reached 98.4% in 2018. This finding reflects the increase in maize seed quality. However, compared with developed countries, there is still great room for the improvement and control of seed quality. We think there are two aspects of the problem that need to be solved: on the one hand, at present, there are only four indicators for evaluating seed quality in China, including varietal purity, seed purity, germination percentage, and seed moisture content, and there is no form of explanation or requirement for the indices of seed vigor; on the other hand, compared with seed companies in developed countries (e.g., DuPont Pioneer, which had an identifying germination percentage of ≥95% for maize seed production six years ago), the current national standard germination percentage of China is only 85%. Thus, it is necessary to raise the current standards of seed quality in China while adding the indices of seed vigor to meet the future challenge of maize seed quality. Based on the analysis of this research, we suggest increasing the GP of current national standards from 85% to 92%. Finally, there are more types of purity, clarity, standard germination percentage, moisture, seed packing number, and seed packing weight of the packaging label. This is because different varieties have different characteristics. More importantly, China has many seed companies and maize varieties. This may be due to the low entry threshold of China’s seed enterprises in the past (8700 companies in 2010, data from China Seed Congress 2019, Beijing, China), and this leads to seed production levels becoming uneven. Therefore, in the future, China will continuously raise the entry of threshold of seed enterprises, constantly improve the level of seed production and processing, and guarantee the production of high-quality seeds.
In addition, the 14 seed quality indices data of 100 maize samples per year were used for comparative analysis. Five principal typical components from 14 seed quality indices’ data each year were extracted by the principal component analysis. The top three components were ‘resembles the seed germination and field seedling emergence factor’, ‘resembles the seed physical characteristics factor’, and ‘resembles the seed chemical characteristics factor’, respectively. Among them, the first component that explained a 25.0% in 2013, 31.3% in 2014, 27.2% in 2015, 24.5% in 2016, 28.8% in 2017, and 31.0% in 2018 of the data variation received more attention (
Table 2 and
Table S4). In particular, the results of cluster analysis showed that the indices of the first principal component (resembles the seed germination and field seedling emergence factor) can be classified into the same cluster annually (
Figure S2, Table S5).
It is of great significance to understand the relationship between different seed quality indices and FSE for seed quality control. In this research, based on the samples collected in the market from 2013 to 2018 and 30 samples of seeds from different hybrid varieties intercrossed by hand in 2018, we obtained the following main conclusions: (1) there were significantly correlations (
p < 0.01) for standard germination energy (SGE), standard germination percentage (SGP), cold test germination percentage (CTGP) and accelerated aging test germination percentage (AATGP) with mean field seedling emergence (FSE). Among them, the cold test germination percentage and accelerated aging test germination percentage were the two most widely parameters of seed vigor [
5]; In this study, the cold test was better than the accelerated aging test in predicting the field emergence rate (
Table 5); (2) the variance analysis showed that these five indices in 2018 all reached significant differences (
p < 0.01) compared with 2013 (
Table 5). Moreover, SGE, SGP, CTGP, AATGP, and FSE had a very significant positive correlation (
Table 4). Furthermore, we took SGE, SGP, CTGP, and AATGP as independent variables, FSE as a dependent variable, and carried out the multiple regression analysis each year (2013–2018) and for the total six years. According to the model parameters to evaluate the model fitting, we established the regression equations of each year, and the equation of six years, which was the appropriate regression model and could better predict the field seedling emergence (
Figure 4 and
Figure S3, Tables S9–S11); (3) there were significant correlations (
p < 0.05 or
p < 0.01) between seed bulk density, flat compression yield stress, and FSE. Zhang et al. [
40] analyzed the correlation of the seed density, yield, and quality of maize seeds and concluded that to increase the seed bulk density of maize, the total starch content of maize seeds should be increased first. Related studies on seed hardness of crops have been reported [
41,
42]. Li et al. [
43] isolated chromosome 5ra-specific genes responsible for seed hardness. Hardness is closely related to the mechanical properties of maize seeds during seed production and processing. The study demonstrated that flat compression yield stress, as one of the seed hardness indices, deserves to be further investigated; (4) starch metabolism plays an important role in seed germination and the vigor of crops [
44]. Here, we found that total starch content was positively correlated with FSE (significantly correlated in some years,
p < 0.05 or
p < 0.01). Therefore, total starch content and related indices such as the activity of amylase activity can be used as a typical characteristic of high-vigor maize seeds; however, further studies are needed to confirm this.
5. Conclusions
In this study, a comparative analysis of seed quality indicators of 1196 hybrid maize seed samples from the main maize-producing areas in China from 2013 to 2018 was carried out. The results show that the maize seed quality in China has changed obviously in the past six years. This was mainly reflected by the following aspects: In 2013 and 2014, the percentage of samples with coated seed were 62.8% and 86.8%, respectively, and compared with them, the percentage increased obviously in 2015–2018, all exceeding 97%. The sample rate of packaging according to seed number was from 24.5% in 2013 to 58.6% in 2018, and the percentage of samples which met the prescribed quality standards continuously was 89.2% in 2013 to 98.4% in 2018. Based on the principal component analysis, the first principal component each year “resembles the seed germination and field seedling emergence factor”; taken together, mainly including standard germination energy (SGE), standard germination percentage (SGP), cold test germination percentage (CTGP), accelerated aging test germination percentage (AATGP), and mean field seedling emergence (FSE), and by cluster analysis, they could be divided into the same group. More importantly, the mean SGE, SGP, CTGP, AATGP, and FSE in other years were higher than in 2013 and reached significant differences (p < 0.05 or p < 0.01), except the mean SGP (2017). Furthermore, SGE, SGP, CTGP, and AATGP were used to establish the field emergence rate prediction models of each year (2013–2018) and SGE, SGP, and CTGP were selected to establish the field seedling emergence prediction models for the total six years. In addition, a series of important results were also obtained, which need to be further studied; the seed bulk density, total starch content, and flat compression yield stress were closely correlated with seed vigor. This study provided a theoretical basis and data support to better understand the changes of maize quality in China over the past six years and offered an important reference to further improve the maize seed quality in the future.