Optimising Potassium Levels Improved the Lodging Resistance Index and Soybean Yield in Maize-Soybean Intercropping by Enhanced Stem Diameter and Lignin Synthesis Enzyme Activity

Abstract

Lodging is a major factor limiting soybean yield in maize–soybean intercropping system (IS). Potassium fertilisation significantly enhances the lodging resistance index by promoting dry matter accumulation in soybean. However, the physiological mechanisms through which potassium affects the lodging resistance index remain unclear, particularly under different planting systems. In this study, we analysed the relationships between photosynthetic characteristics, root system, stem physiology, stem morphological characteristics, dry matter and lodging resistance index of soybean based on field experiments. The soybean cultivar Jinong 40 was used in both maize–soybean intercropping (maize: soybean as 6:6) and monoculture soybean systems (MS) in a two-year field experiment (2022–2023), with five potassium fertilisation levels (0 kg ha⁻¹, 30 kg ha⁻¹, 60 kg ha⁻¹, 90 kg ha⁻¹ and 120 kg ha⁻¹). Potassium application significantly improved chlorophyll fluorescence parameters, dry matter accumulation, stem lignin synthesis enzyme activity (phenylalanine ammonia-lyase, tyrosine ammonia-lyase and cinnamyl alcohol dehydrogenase), lodging resistance index and grain yield, regardless of the planting system. However, no significant differences in lodging resistance index or grain yield were observed between the potassium rates of 90 kg ha⁻¹ and 120 kg ha⁻¹. Compared to 0 kg ha⁻¹, increased potassium rates increased stem diameter by 17.8% and 15.5%, while the ratio of stem length to stem diameter ratio (L/D) decreased by 27.2% and 26.8% in maize–soybean intercropping and monoculture soybean systems, respectively. Across the high potassium inputs (90 kg ha⁻¹ and 120 kg ha⁻¹), phenylalanine ammonia-lyase (2.6%) and cinnamyl alcohol dehydrogenase (3.9%) were higher in the maize–soybean intercropping system compared to the monoculture soybean system. For the two planting patterns, the lodging resistance index was found to be more dependent on stem enzyme activity (93.5% for IS and 75.3% for MS) and L/D ratio (−81.0% for IS and −83.8% for MS), rather than stem length or root characteristics. We conclude that potassium application optimises stem diameter, enhances stem lignin synthesis and reduces the L/D ratio, therefore improving the soybean lodging resistance index and yield, especially in the maize–soybean intercropping system.