Excessive plant height is an important factor that can lead to lodging, which is closely related to soybean yield. Gibberellins are widely used as plant growth regulators in agricultural production. Gibberellic acid (GA₃), one of the most effective active gibberellins, has been used to regulate plant height and increase yields. The mechanism through which GA₃ regulates internode elongation has been extensively investigated. In 2019 and 2020, we applied GA₃ to the stems, leaves, and roots of two soybean cultivars, Heinong 48 (a high-stalk cultivar) and Henong 60 (a dwarf cultivar), and GA₃ was also applied to plants whose apical meristem was removed or to girded plants to compare the internode length and stem GA₃ content of soybean plants under different treatments. These results suggested that the application of GA₃ to the stems, leaves, and roots of soybean increased the internode length and GA₃ content in the stems. Application of GA₃ decreased the proportion of the pith in the soybean stems and primary xylem while increasing the proportion of secondary xylem. The apical meristem is an important site of GA₃ synthesis in soybean stems and is involved in the regulation of stem elongation. GA₃ was shown to be transported acropetally through the xylem and laterally between the xylem and phloem in soybean stems. We conclude that the GA₃ level in stems is an important factor affecting internode elongation.

Internode length and diameter in soybean (Glycine max (L.) Merr.) are closely associated with lodging. The pattern of internode elongation and increase in internode diameter and factors involved were studied in two soybean cultivars, HN48 (tall-stem cultivar) and HN60 (dwarf cultivar), in the growing seasons of 2017 and 2018. Four treatments included natural lighting, shading of the apical meristem, covering of all internodes with aluminium foil, and whole-plant shading with plastic shading nets. When the number of internodes (N) on the main stem was >3, internode N began to elongate. Internode N – 1 exhibited the most rapid elongation, and internode N – 2 elongated slowly. Internode N – 3 stopped elongating, but the increase in internode diameter did not cease as internode elongation stopped. Shading the soybean apical meristem, the stem, and the whole plant all led to internode elongation. Different shading conditions did not alter the pattern of internode elongation. Soybean stem and apical meristem were both light-sensitive tissues. With an increase in shading, internode length increased, whereas internode diameter decreased. Contents of gibberellic acid (GA3) and salicylic acid in the stem also increased, but abscisic acid content decreased. Shading reduced the size of starch grains but increased the number of osmiophilic granules in the chloroplast. Elevated GA3 level was the main cause of the changes in internode length and diameter induced by shading. These results suggest that reduction in GA3 synthesis and enhancement in carbohydrates formation could a strategy for soybean plants to avoid lodging.

Increasing planting density is among the most effective ways to increase soybean yield, but high planting density increases lodging risks in soybean plants. To examine the effects of shading on soybean plant lodging, the tall cultivar Heinong 48 (HN48) and dwarf cultivar Henong 60 (HN60), two soybean cultivars with large plant height differences (approximately 40 cm), were selected, and their internode lengths were measured under different planting densities in 2019 to determine the key internode influencing lodging. In 2020, we treated soybean plants with different shading treatments to examine the morphology, anatomical structure and stem fiber composition of the fifth internode, changes in the GA₃ (gibberellic acid) content, and relative expression of GA₃-related enzyme genes in the stem under different shading conditions. The results showed that as the planting density increased, the lengths of the internodes changed significantly, starting from the fifth internode. Under different shading conditions, the length of the fifth internode increased for both cultivars, but the diameter and the diameter-to-length ratio decreased; the area of the xylem decreased, and the pith cells stretched more longitudinally and were more orderly; the lignin, cellulose, and hemicellulose contents decreased; the GA₃ content in the stem increased, and the expression levels of GmGA3ox6, GmGA20ox1-D, and GmGA2ox4 enzyme genes were all upregulated; and the change in PSN treatment was more significant than other treatments. Therefore, the fifth internode was the key internode that affected lodging in soybean plants, shading could increase the GA₃ content significantly in soybean stems, and the increase in GA₃ content was positively correlated with the shaded area. This study provides a new direction for future research on lodging resistance in densely planted or intercropped soybean plants.

The soybean (Glycine max L. Merr.) is a crop with a high demand for nitrogen (N). The root nodules that form in soybeans can fix atmospheric N effectively, yet the goal of achieving high yields cannot be met by relying solely on nodule-fixed N. Nonetheless, the application of N fertilizer may inhibit nodule formation and biological N fixation (BNF), but the underpinning mechanisms are still unclear. In this study, we grafted the roots of non-nodulated soybeans onto nodulated soybeans to generate plants with dual root system. The experiment included three treatments conducted under sand culture conditions with ${\mathrm{NO}}_3^{-}$ and ${\mathrm{NH}}_4^{+}$ as N sources. Treatment I: The non-nodulated roots on one side received 50 mg·L^{−1 15} ${\mathrm{NO}}_3^{-}$ or ¹⁵NH₄⁺, and the nodulated roots on the other side were not treated. Treatment II: The non-nodulated roots received 50 mg·L^{−1 15} ${\mathrm{NO}}_3^{-}$ or ¹⁵ ${\mathrm{NH}}_4^{+}$ , and the nodulated roots received 50 mg·L^{−1 14} ${\mathrm{NO}}_3^{-}$ or ¹⁴ ${\mathrm{NH}}_4^{+}$ . Treatment III: Both non-nodulated and nodulated roots received 50 mg·L^{−1 15} ${\mathrm{NO}}_3^{-}$ or ¹⁵ ${\mathrm{NH}}_4^{+}$ . The results showed the following: (1) Up to 81.5%–87.1% of the N absorbed by the soybean roots and fixed by the root nodules was allocated to shoot growth, leaving 12.9%–18.5% for root and nodule growth. Soybeans preferentially used fertilizer N in the presence of a ${\mathrm{NO}}_3^{-}$ or ${\mathrm{NH}}_4^{+}$ supply. After the absorbed fertilizer N and nodule-fixed N was transported to the shoots, a portion of it was redistributed to the roots and nodules. The N required for root growth was primarily derived from the ${\mathrm{NO}}_3^{-}$ or ${\mathrm{NH}}_4^{+}$ assimilated by the roots and the N fixed by the nodules, with a small portion translocated from the shoots. The N required for nodule growth was primarily contributed by nodule-fixed N with a small portion translocated from the shoots, whereas the ${\mathrm{NO}}_3^{-}$ or ${\mathrm{NH}}_4^{+}$ that was assimilated by the roots was not directly supplied to the nodules. (2) Based on observations of the shoots and one side of the roots and nodules in the dual root system as an N translocation system, we proposed a method for calculating the N translocation from soybean shoots to roots and nodules during the R₁–R₅ stages based on the difference in the ¹⁵N abundance. Our calculations showed that when adding N at a concentration of 50 mg·L⁻¹, the N translocated from the shoots during the R₁–R₅ stages accounts for 29.6%–52.3% of the N accumulation in nodulated roots (Root_n) and 9.4%–16.6% of the N accumulation in Nodule_n of soybeans. Through the study of this experiment, the absorption, distribution and redistribution characteristics of fertilizer N and root nodule N fixation in soybean can be clarified, providing a theoretical reference for analyzing the mechanisms of the interaction between fertilizer N and nodule-fixed N.