Abstract
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.
Highlights
Light plays an important role in regulating plant growth and development
Cheng et al [9] showed that when soybeans were intercropped, decreasing their planting density could increase the activity of lignin-related enzymes and the accumulation of carbohydrates, thereby enhancing the lodging resistance of soybean stems
The results showed that the relative expression levels of GmGA3ox6, GmGA20ox1-D, and GmGA2ox4 in shading of the apical meristem (SAM) and CAF under long-term shading treatment were significantly higher than that in short-term shading treatment
Summary
Light plays an important role in regulating plant growth and development. Under high planting densities, the crop canopy structure is negatively affected, while the competition for light resources within the plant population intensifies, leading to a high tendency toward lodging. In [4], shading promoted rapid elongation of stems and petioles and reduced branches while significantly decreasing the photosynthetic rate, harvest index, and stem mechanical strength. The mechanical strength of plant stems depends on the contents of cell wall components, such as cellulose and lignin [5]. Liu et al [6] showed that soybean–corn intercropping significantly reduces the cellulose accumulation and stem tension of soybean stems. Hussain et al [8] argued that shading reduces the amount of lignin deposited in the cells of vascular bundles and sclerenchyma, rendering the stems slender and more fragile. Cheng et al [9] showed that when soybeans were intercropped, decreasing their planting density could increase the activity of lignin-related enzymes and the accumulation of carbohydrates, thereby enhancing the lodging resistance of soybean stems
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