The total cultivars or accessions investigated from 2017 to 2020 in Harbin and Hailun locations in this study is shown in Table 1.
Correlations among R1, R2, R6, R7 and R8 within the same year
At first, R1 and R2 were highly correlated in all years and locations at p <0.001(Fig 1, Fig 2A). In Hailun (HL) location, the correlation coefficients (r) were 0.9795***, 0.9867***, 0.9759*** and 0.9869*** for the years of 2017, 2018, 2019 and 2020, respectively (Fig 1). The correlations between R1 and R6 or R7 or R8 were also statistically significant at p<0.001, although not as higher as that between R1 and R2. In 2018, the r values were 0.5815*** between R1 and R6; 0.5630*** between R1 and R7; and 0.5581*** between R1 and R8 (Fig 2 B-D). Similarly, in 2019, the r values were 0.7141*** between R1 and R6; 0.6914*** between R1 and R7; and 0.6649*** between R1 and R8 (Fig 2 F–H). A similar trend was observed in 2020 (Fig 2 J-L). The r values in 2018 between R2 and R6 (Fig 1) or R7 or R8 were 0.6679***, 0.6582*** and 0.6440***, respectively. In 2019, the r values between R2 and R6 (Fig 1) or R7 and R8 were 0.7820***, 0.7587*** and 0.7308***, respectively. Similar correlations were observed in 2020 in Hailun (Fig 1). At the late maturity stage, R6 was highly correlated with R7 or R8 with the r values of 0.8493*** and 0.8150*** in 2018; 0.8875*** and 0.8320*** in 2019; and 0.9883*** and 0.9679*** (Fig 1) in 2020. A very high correlation was observed between R7 and R8 in all years, with the r values of 0.9264*** in 2018, 0.9288*** in 2019 and 0.9840*** in 2020. Also in Harbin, the r values between R1 and R2 were 0.8843*** in 2017; the r values between R1 and R6 or R7 or R8 were 0.7974***, 0.6885*** and 0.5088***, respectively. Furthermore, the r values between R6 and R7 or R8 were 0.8644*** and 0.8141***, respectively.
The correlations of the same R1, R2, R6, R7, R8 between different years in Hailun
The correlations of DAE to the same maturity stages between 2017 and 2018 or 2019 and 2020 were shown in Hailun (Fig 1). At the early reproductive stage, the r values were 0.9377*** for R1 between 2018 and 2019 and 0.9672***for R1 between 2018 and 2020. The r value of 0.9500*** was for R2 between 2018 and 2019 and 0.9683*** for R2 between 2018 and 2020 (Fig 1). Meanwhile, at the late reproductive stage, the r values of 0.6560*** and 0.5801*** were for R6 between 2018 and 2019 (Fig 1) and 0.5801*** for R6 between 2018 and 2020. Similarly, the r value of 0.6365*** was for R7 between 2018 and 2019 and 0.5902*** for R7 between 2018 and 2020. Furthermore, the r value of 0.5932*** was R8 between 2018 and 2019 and 0.5729*** for R8 between 2018 and 2020 (Fig 1). Apparently, R2 was a consistently stable trait among different years.
The correlations of R1, R2, R6, R7, R8 between two latitudinal locations
The phenotypic data of flowering time and maturity between Hailun and Harbin in 2017 were analyzed. The r values of 0.8204***, 0.9327***, 0.7697***, 0.6130*** and 0.6205*** were respectively for R1, R2, R6, R7 and R8 between Hailun and Harbin in 2017 (Fig 1). Apparently, the R2 was the most stable trait between the two locations.
Correlation among agronomic traits and maturity traits
Plant height
The traits of plant height (PH) were highly correlated between different years. The r values of 0.6981*** were for PH between 2017 and 2018, 0.5994*** between 2018 and 2019 and 0.5875*** between 2018 and 2020. Also, PH was significantly correlated to R1, R2, R6 and R8. The r values between PH and R1, R2, R6, R7, or R8 were 0.5019***, 0.5109***, 0.5017***, 0.4787*** and 0.4956***, respectively (Fig 1; Fig 3A-D) in 2018 at Hailun. Similarly, r values between PH and R1, R2, R6, R7, or R8 were 0.3876***, 0.4066***, 0.3626***, 0.3370*** and 0.3083***, respectively, in 2019 at Hailun (Fig 1; Fig 3E-H). A similar correlation trend was observed in 2020 at Hailun.
Node numbers
The trait of node number was correlated with different years. The r value of 0.5825*** was for node number between 2017 and 2019, 0.4630*** between 2017 and 2020 and 0.4418*** between 2019 and 2020. The r between node number and time for different reproductive stages (R1, R2, R6, R7 and R8) was statistically significantly correlated with a range from 0.2333*** to 0.5882*** (Fig 1). The node number showed a higher correlation with plant height at r = 0.8573*** in 2017, 0.6618*** in 2019 and 0.8121*** in 2020 (Fig 1).
Branch number (BN)
Based on the phenotypic data of BN in 2017, 2018, 2019 and 2020, the r value between branch numbers among different years was statistically significant, with a range from 0.4512*** (2019 vs. 2017) to 0.5727*** (2019 vs. 2020). Also, the trait of branch number was statistically significantly correlated with the R1, R2, R6 and R8. In 2018, the r values between BN and R1, R2, R6, R7, or R8 were 0.4001***, 0.3958***, 0.3875***, 0.3254*** and 0.3159***, respectively (Fig 1). In 2019, the corresponding r values were 0.5862***, 0.5891***, 0.3931***, 0.4023*** and 0.3991***, respectively (Fig 1). Furthermore, a similar correlation trend was observed in 2020 at Hailun (Fig 1). The trait of BN was significantly correlated with plant height, with r = 0.3704*** in 2018, 0.2899*** in 2019 and 0.5244*** in 2020. Also, BN was significantly correlated with the node number, with the r values of 0.3204***in 2019 and 0.4560*** in 2020.
Height of the 1st effective node
The trait of height of the first effective node was statistically significant between different years, with the r values of 0.2745*** between 2018 and 2019, 0.2959*** between 2018 and 2020 and 0.2109*** between 2019 and 2020. Also, the trait of height of the first effective node was significantly correlated to R1, R2, R6 and R8 (Fig 1). At Hailun in 2018, the r values between the height of the 1
st effective node and R1, R2, R6, R7, or R8 were 0.2327***, 0.2831***, 0.4083***, 0.4022*** and 0.3909***, respectively (Fig 1). Correspondingly, the r values were 0.2518***, 0.2610***, 0.2246***, 0.1847*** and 0.1672***, respectively, in 2019 at Hailun. Furthermore, a similar correlation trend was observed in 2020 at Hailun, with a range of r values from 0.3194*** to 0.3664*** (Fig 1). In 2020, the first effective node was significantly correlated with plant height at the value of 0.5196 ***, with the node number at r = 0.41169*** and with branch number at r = 0.2384 *** (Fig 1).
Pod numbers per plant
This yield-related trait, pod number per plant, in 2017 was positively correlated with R1, R2, R6, R7, or R8 with the r values of 0.3913***, 0.4526***, 0.5075***, 0.5395*** and 0.5179***, respectively. Also, this trait was significantly related to plant height at r = 0.4422***, nod number at r = 0.5615*** and branch number at r = 0.4740*** (Fig 1). Surprisingly, this trait was not or marginally correlated with flowering time and maturity traits in 2018 and 2019, with a range of r values from -0.0970 (R8 in 2018, p = 0.0297) to 0.0436 (R1 in 2019, p = 0.3702***) (Fig 1). In 2018, the pod number per plant was not significantly correlated with plant height (r = 0.0029) but significantly correlated with branch number (r = 0.2905***) and significantly negatively correlated with the height of the first effective node (r = -0.2355***). However, in 2019, the pod number per plant was not significantly correlated with either plant height (r = 0.0410), or branch number (r = 0.0167), or height of the first effective node (r = 0.0497), or node number (r = -0.0069) (Fig 1).
The overall correlation among all traits
From the heatmap of correlation coefficients matrix, all maturity related traits were highly correlated and classified into a group, especially for the traits of R1 and R2 (Fig 1). The three architecture traits,
e.g. branch number, plant height and node, were not only correlated each other, but also significantly related with maturity traits. Furthermore, the height of 1st effective nodes and pod number per plant were least or inconsistently correlated to above maturity and architecture traits (Fig 1).
The domestication of soybean occurred in China about 5000 years ago and there are a large number of germplasms, including landrace and modern cultivars, in China
(Qiu et al., 2013). The characterization of the effects of different planting dates on flowering time and maturity in soybeans and other plant species triggered the discovery of photoperiodism (
Garner and Allard, 1920). These reproductive-related traits are crucial for soybean production, breeding, soybean functional studies on gene regulatory networks controlling maturity and other related agronomic traits,
e.g., plant height and node numbers
(Goyal et al., 2015). As progress is made on the various omics and accomplishments of the T2T genomes of many soybean cultivars
(Zhang et al., 2023), accurate phenotypic data will greatly facilitate the cloning of new or minor genes as well as functional study. Currently, the main soybean production area is located in the northern China with higher latitude. Accurate phenotyping of flowering time, maturity and important agronomic traits will enable us to judge the ecological or latitudinal adaptation and yield potential of a given cultivar or accession
(Ige et al., 2021).
In this study, Fehr’s classification of reproductive stage was basically used for the phenotyping of traits of flowering time and maturity
(Fehr et al., 1971). Consistent correlation results among different reproductive stages, different years and different locations indicated that the phenotyping of these traits is accurate and repeatable, reflecting the ecological or latitudinal adaptation of these accessions or cultivars. Especially, the R2 was the most stable trait based on the correlation analysis. Higher correlations were observed between R1 and R2, or between R6 and R8, but moderate correlations were revealed between R1 or R2 and R6 or R7 or R8. This result is in coincidence with many reports that, although many maturity genes can control flowering and maturity, some of them may mainly function in the late reproductive stages,
e.g. Gmfulb might function most on the maturity time and reproductive length other than flowering time (R1)
(Kumar et al., 2015; Escamilla et al., 2024).
As to the three architecture traits, branch number, plant height and node, were classified into a group, which showing a significant correlation each other, as well as a higher correlation with maturity traits (Fig 1). This relationship between these traits disclosed in this study is consistent with previous reports
(Jain et al., 2018; Sayama et al., 2010). Variations in genes underlying photoperiod sensitivity and growth habit can have pleiotropic effects on other agronomic characteristics other than the major effects on flowering time and maturity
(Cober et al., 2000).
The trait of the height of the first effective node approximately corresponds to the height of bottom pods
(Cober et al., 2010). In this study, the correlation of this trait were significantly correlated with maturity traits, however, some inconsistences were shown with the same trait between different years or the correlation between this trait with maturity traits in some years. This phenomenon might be ascribed to the influence of the environmental changes in temperature, planting density, flooding and agricultural practices,
e.g., intertilling. At the molecular level, the interaction between
E1,
DT1 and other genes,
e.g.,
GmHY2a, can determine the height of bottom pods (
Cober and Tanner, 1995;
Zhang et al., 2022). In the past, harvesting losses could be greatly resulted from low bottom pods
(Curtis et al., 2000). The “high-bottom pods” might not be so demanding in that resent technical advances have been made in the harvesting machine to harvest soybean cultivars having low-bottom pods. Similarly, the yield-related trait, the pod number per plant, showed some consistent correlation with maturity, plant height and node number in some years, but not all the years. Yield-related traits are quantitatively inherited and easily subject to the influence of environmental changes,
e.g. drought, flooding and agricultural practices such as intertilling, irrigation and the application of herbicides and fertilizer.