Lignin Content Alone Does Not Determine Lodging Resistance in Forage Sorghum: Roles of Stem Anatomy and Divergent Accessions

Forage sorghum (Sorghum bicolor (L.) Moench) has excellent stress resistance and feeding quality. Lodging is one of the most critical factors influencing its yield and overall quality. However, there remains no definitive conclusion regarding the key indicators affecting the lodging of forage sorghum and their correlations. In this study, 149 forage sorghum lines, including 26 maintainer lines and 123 restorer lines, from the United States, China, and Japan were used to analyze the correlation between agronomic and mechanical properties at maturity. In addition, we selected forage sorghum with high, medium and low lignin content and observed the tissue sections, which further revealed the essence of lodging of forage sorghum from the microscopic point of view. Correlation analysis showed that lodging index (LI) was positively associated with plant height (PH), stem bending moment (SBM), lignin content (LC), stem center of gravity height (SGH), and fresh weight from the breaking point to the top of the panicle (FWP); lodging index was significantly negatively associated with breaking resistance (BR) and exhibited no significant correlation with stem diameter (SD) and lodging resistance of individual plant (LR). LI showed no significant association with BR and SD in the maintainer line subgroup, whereas LI was significantly negatively correlated only with BR and showed significant positive correlations with the other seven traits in the restorer line subgroup. Histological observation of the longitudinal section of the stem stained with Safranin O/Fast Green showed that the lodging resistance of sorghum could not be simply defined by the lignin content, and the structure of stem tissue played a more decisive role in determining the lodging resistance.