We selected wheat cultivars data on four quality-related parameters from the 2006–2018 China Wheat Quality Report published by the Ministry of Agriculture of the People’s Republic of China, and box chart method was performed to eliminate outliers. In total, a set of 1,055 winter-wheat quality data for 211 cultivars was collected. According to the classification guide of wheat quality regions in China (He et al., 2002), winter-wheat growing areas were divided into six regions (Figure 1), including North strong-gluten wheat region (1), North Huanghuai strong- and medium-strong-gluten wheat region (2), South Huanghuai medium-gluten wheat region (3), the medium-weak-gluten wheat region in the middle and lower Reaches of the Yangtze River (4), Sichuan Basin medium-weak-gluten wheat region (5), and Yungui Plateau wheat region (6).

SRs of different grades of winter-wheat were analyzed according to the national standard GB/T 17320-2013 (Table 2). In terms of HI, the SRs of the medium-gluten wheat reached 91%, but that of weak-gluten wheat was only 9%. The SRs of CPC for medium-, medium- strong-, strong-, and weak-gluten wheat were 87, 79, 54, and 13%, respectively. In terms of the SV, the SRs of medium-, medium- strong-, weak-, and strong-gluten wheat were 54, 28, 46, and only 11%, respectively. For WG, 90% of the medium-gluten, 79% of the medium-strong-gluten, 61% of the strong-gluten and only 10% of the weak-gluten wheat samples met the corresponding standards. The SRs of the medium-, medium- strong-, strong-, and weak-gluten wheat for WA were 77, 60, 41, and 23%, respectively. When it came to the ST, the SRs of the medium-, medium- strong-, weak-, and strong-gluten wheat were 69, 33, 31, and 24%, respectively. The SRs of MAXR for medium-, medium- strong-, and strong-gluten wheat were 93, 82, and 67%, respectively. Finally, the SRs of SA for the medium-, medium- strong-, and strong-gluten wheat were 90, 68, and 27%. The results showed that the SRs of medium-gluten and medium-strong-gluten winter-wheat were relatively higher compared with those of strong-gluten and weak-gluten wheat in China. Analysis of quality traits showed that for weak-gluten wheat, the SRs of HI, WG, CPC, and WA were relatively low, whereas for strong-gluten wheat, the SRs of SV, ST, and SA were relatively low.

Based on the variation analysis of quality traits (Table 3), the CVs were listed as follows: ST > MAXR > SA > DT > GI > TRA > SV > FN > AC > WG> HI>CPC >WC> FER > WA > NS > TW. The CV of ST was the highest among all quality traits, reaching 79.3%. The CVs of MAXR, SA, DT, GI, TRA, and SV were 49.9, 48.4, 43.2, 28.4, 23.3, and 22.2%, respectively. The CVs of FN, AC, and WG were 16.5, 15.4, and 12.2%, respectively. The CVs of HI, CPC, WC, FER, WA, and NS were relatively low at 9.5, 8.9, 8.2, 6.4, 6.1, and 4.6%, respectively. The CV of TW was the smallest at only 2.9%.

The correlations of winter-wheat quality-related parameters are summarized in Figure 2. On the whole, wheat quality indicators were closely correlated, especially WG and CPC, SA and SV, SA and GI, MAXR, and GI. In terms of grain quality, only FN was significantly correlated with HI, test weight (TW), water content (WC), and CPC. This indicated that FN could be the most important indicator for evaluating grain quality, followed by HI, TW, and WC. In terms of flour quality, WG was correlated with the other quality traits, which involved flour extraction rate (FER), SV, AC, and WG, and GI was correlated with the other five quality traits (FER, SV, AC, WG, and GI). This suggested that WG and GI were the most important indicators of flour quality. Moreover, with regard to dough quality, DT, ST, and tractility (TRA) were significantly correlated with other dough quality, indicating that DT and ST could be the core trait of dough quality.

On the whole, WA was significantly correlated with 13 other traits of winter-wheat quality. WG and DT were significantly correlated with 12 other traits of winter-wheat quality. HI, CPC, FN, ST, and TRA were significantly correlated with 11 other traits. These results indicated that WA, WG, DT, HI, CPC, FN, ST, and TRA directly affected the overall quality of winter-wheat.

The correlation analysis of wheat samples meeting the standard of medium-gluten and noodle scores is presented in Figure 3. There was a close correlation between indicators of wheat samples achieving medium-gluten SRs (GB/T 17320-2013), especially the relationship between WG and CPC, ST and DT, ST and GI, SA and GI, SA and ST, MAXR and GI, MAXR and DT, MAXR and ST, and MAXR and SA. Noodles score (NS) has a significant correlation with HI, TW, WC, AC, GI, DT, ST, SA, and MAXR.

According to the analysis of variance, the effects of growing locations, cultivars, and years varied across different traits (Table 4). Although interaction effects existed, winter-wheat quality parameters were mainly affected by single factor effect (i.e., locations, cultivars, or years). Specifically, HI, TW and CPC, FER, and SV were significantly affected by location and cultivars, but were not influenced by years, because these index interactions between locations and years significantly affected HI and FER. WC, FN, GI, WA, and TRA were significantly affected by locations, years, and cultivars. SA and MAXR were only significantly affected by cultivars. AC was only significantly affected by growing locations. For ST and DT, no significant influences were detected between locations, cultivars, years, and their interactions.

In recent years, China has made remarkable achievements in increasing wheat yield and total production. At the same time, with the increased income of residents, their demand for wheat quality is also upgrading. In general, the quality of medium- and medium-strong-gluten wheat in China is relatively high and can basically meet the market quality requirements. However, the strong-gluten wheat used for making bread and the weak-gluten wheat for cake are far from meeting the market demand, making it less competitive compared with high-quality agricultural products from other countries (Deliberali et al., 2010). As a result, a significant quantity of high-quality strong- and weak-gluten wheat was imported each year.

Currently, many researches focus on identifying how the wheat quality could meet the strong-gluten or weak-gluten standard more quickly and accurately, so as to obtain optimal traits for wheat breeding. Compared with other testing techniques, rheometry is considered as a better technique to predict final product quality (Dobraszczyk and Morgenstern, 2003; Hrušková et al., 2006). Based on China National Standard (GB/T 17320-2013), the SRs of Chinese wheat quality indicators were different among strong-, medium-, medium-, and weak-gluten. SRs of HI, WG, MAXR, and WA were all greater than 90% (Table 2) in medium-gluten wheat, indicating that the quality of medium-gluten wheat was relatively high in China. These results suggested that HI, WG, MAXR, and WA were no longer limiting factors in quality improvement of medium-gluten wheat in China. However, the SRs of weak-gluten wheat were relatively low in China, especially for HI (9%), CPC (13%), WG (10%), and WA (23%) (Table 2). However, the rheological index is time-consuming and laborious, which is not conducive to quick grasp. Our results indicate that HI, CPC, WG, and WA had become major factors affecting the development of weak-gluten wheat in China. Therefore, it could be applied to determine whether the wheat meets the standard of weak-gluten wheat by determining the quality of HI, CPC, WG, and WA. The latest research also suggests that WA, CPC, and WG should be the main factors in the quality selection of weak-gluten wheat (Hu et al., 2020). For weak-gluten wheat in China, WG and CPC are not insufficient, but even high. Hu et al. (2020) think field management is difficult to take into account the quality, leading to the high CPC and WG. Therefore, the extreme value in the national standard should be relaxed appropriately.

The overall compliance rate of strong-gluten wheat was higher than that of weak-gluten wheat, but strong-gluten wheat quality in China still needs to be improved. This study found that the improvement of strong-gluten wheat was limited by SV, ST, and SA (Table 2). Regarding ST and SA, the low SRs in association with extremely high CV (Tables 2, 3) implied that the large variability of ST (Zhao et al., 2019) and SA should be a key reason responsible for the low SRs of strong-gluten wheat in China. Consequently, special attention should be paid to select strong-gluten wheat cultivars with long dough ST and strong tensile properties. Furthermore, providing a suitable growth environment is essential to ensure the quality stability.

Multiple quality parameters for winter-wheat that are closely related to each other made it difficult to evaluate wheat quality. Based on the correlation analysis of 13 years’ data of six major wheat regions in China, CPC and WG are significantly correlated (Figure 2), and this result is consistent with the that of Rozbicki et al. (2015). Previous study showed that gliadin and glutenin are the main components of wet gluten (Shewry et al., 2000), and gluten comprises some 75% protein on a dry weight basis, with most of the remainder being starch and lipids (Shewry et al., 2002). This may be the main reason for the significant correlation between CPC and WG. In addition, we found that GI, MAXR, and SA were all significantly correlated. On the one hand, the high GI value indicated that the quality of glutenin and gliadin was high, and high-quality glutenin provides the tensile resistance of the dough, whereas high-quality gliadin provides the adhesion required for dough fluidity and ductility. However, there was no significant correlation between WG and MAXR. This result indicates that the composition and structure of protein may have a more important effect on quality than protein quantity, which is similar to the result of Xue et al. (2016b). On the other hand, both SA and MAXR are extensibility indexes of dough. SA indicates dough strength: higher value of SA reflects greater strength of dough. There is a significant positive correlation between SA and MAXR.

The correlation between the quality indicators of medium-gluten wheat and NS showed that NS has no significant correlation with quantitative parameters of proteins (CPC, SV, and WG), but significant correlation (P < 0.001) with protein composition index (GI) (Figure 3). Previous research found that the ratio of gliadin to glutenin and their subunit composition, especially the composition and proportion of high-molecular-weight glutenin subunits, had a significant effect on the baking quality of wheat (Xue et al., 2016a). This may be one of the reasons that the correlation between noodle quality and protein content indexes of medium-gluten wheat was not significant, but highly correlated with protein composition index.

Genotype and environmental conditions are internal and external factors affecting the quality of winter-wheat. This study selected three influencing factors including growing locations, years, and cultivars. Growing locations represented husbandry practices and the spatial characteristics of soil and climatic conditions. Wheat cultivars represented genetic characteristics of the winter-wheat. Growing years mainly reflects the interannual climate conditions. Our results indicated that SV, ST, and SA were the main factors limiting the development of strong-gluten wheat (Table 2). The CV of ST (79.3%) was the largest (Table 3); the high CV of ST may be due to the fact that there was no effect of locations, cultivars, and years on ST, which was consistent with the study of Baric et al. (2004). However, cultivars had significant effects on SV and SA (P < 0.01), and especially SV had a high genetic stability (Moonen et al., 1982; Yong et al., 2004). Therefore, there is high potential for future development of strong-gluten wheat in China through breeding new varieties with excellent genes.

On the contrary, the quality of weak-gluten wheat was mainly restricted by HI, CPC, WG, and WA (Table 2). The results in Table 4 showed that HI, CPC, WG, and WA were significantly affected by locations and cultivars (P < 0.01). However, Rozbicki et al. (2015) found that CPC and WG were more significantly affected by environment than genotypes (varieties). Therefore, the core influencing factors of CPC and WG in different countries are not consistent. The interaction among wheat locations, varieties, and years had little effect on wheat quality, which was consistent with the results of Peterson et al. (1998). In addition, an important portion of the observed variability in quality was determined by the environment, and the nitrogen supply was the principal factor determining protein content and composition (Xue et al., 2019). The total protein content increases with higher supplies of nitrogen, and as the grain protein increases, the gliadin and glutenin contents and their ratio also increased (Triboi et al., 2000). As a consequence, breeding of wheat cultivars with low protein content and WG should be placed at the first priority in improving weak-gluten winter-wheat quality in China. It is suggested to strictly control the topdressing of wheat at the later stage and to ensure the irrigation amount before and after grain filling stage (Xue et al., 2016a).