Endophytic Symbiosis Enhances the Growth, Yield, and Antioxidant Defense in Soybean (Glycine Max (L.) Merrill) Exposed to Various Light Intensities

Soybean is a crucial crop in the modern world. Light plays a significant role in the growth and development of crops, as well as the prevailing environmental conditions during the growth period. This study aimed to investigate how fungal symbiosis affects the growth, biochemical properties, and grain yield (GY) of soybeans under various light intensities. The experiment was designed as split-plot based on randomized complete block design with three replications in the research farm of Sari Agricultural Sciences and Natural Resources University (SANRU) in 2022. The main factor consisted of five light intensities (20, 40, 50, 70, and 100% of natural light), and the sub-factor involved four levels of fungal inoculation (control, inoculation with Bjerkandera adusta (ST1) or Chaetomium globosum (SE2) fungi, and co-inoculation of ST1 + SE2). According to the results, as the light intensity decreased from 70 to 20%, the grain yield GY also decreased. The highest GY was observed at the 70% light intensity level with the symbiosis of two fungi, resulting in a significant increase of 50.4% compared to the control level. It seems that fungal symbiosis partially compensated for the light-imposed reduction. Furthermore, when the average intensity for light declined from 100 to 70%, the harvest index (HI) initially increased by 0.7% and then decreased. By comparison, the activity of catalase (CAT) and glutathione peroxidase (GPX) enzymes increased with higher shade intensity. However, fungal symbiosis at different light levels decreased the activity of these enzymes. In conclusion, the use of symbiotic fungi, particularly the combination of the two fungi, is recommended to increase soybean yield, especially in areas with lower light intensity.