Alleviation mechanism of intercropping with wheat for faba bean autotoxicity under benzoic acid stress

Continuous cropping of faba bean causes serious growth obstacle, and benzoic acid is one of the main autotoxins in faba bean continuous cropping. Faba bean-wheat intercropping is a general planting pattern in Southwest China. To investigate the effect of benzoic acid on faba bean seedling growth and Fusarium wilt occurrence, and to explore the effect and alleviation mechanism of intercropping with wheat on faba bean autotoxicity, a hydroponic experiment of faba bean-wheat intercropping with benzoic acid application was conducted in the greenhouse of Yunnan Agricultural University. Four benzoic acid rates were set, which were C₀ (0 mg·L^-1), C₁ (50 mg·L^-1), C₂ (100 mg·L^-1) and C₃ (200 mg·L^-1), with two planting patterns, faba bean monocropping and faba bean and wheat intercropping. The growth, enzymes activities and occurrence of faba bean Fusarium wilt were investigated. The results showed that in comparison with C₀ treatment, seedling growth was inhibited with increasing benzoic acid concentration. Benzoic acid increased the incidence and disease index of monocropped faba bean Fusarium wilt, significantly increased root and leaf MDA contents, and decreased the activities of antioxidant enzyme (POD and CAT) and pathogenesis-related proteins (β-1, 3-glacanase and chitinase) with increasing application rates. Comparison with monocropped faba bean, intercropped faba bean increased shoot dry weight by 17.0%-47.1%, reduced disease incidence and disease index by 11.1%-25.0% and 20.0%-42.1%, respectively, under benzoic acid application. Furthermore, the peroxidase (POD) activity increased by 12.9%-16.9% in root and by 9.3%-24.9% in leaf; and the catalase (CAT) activity increased by 10.3%-54.0% in root and by 6.6%-20.5% in leaf under intercropped condition. Compared with monocropping of faba bean, intercropping with wheat increased faba bean β-1, 3-glacanase and chitinase activities in root by 4.7%-13.1% and 6.7%-15.8%, while decreased malondialdehyde (MDA) contents in both root and leaf significantly by 19.5%-25.4% and 20.5%-29.9%, respectively, with benzoic acid application. Under C₂ (100 mg·L^-1) treatment of benzoic acid stress, faba bean-wheat intercropping system showed the best effect of improving pathogenesis-related proteins and antioxidative enzymes activity. Intercropping of faba bean-wheat significantly increased physiological resistance to Fusarium oxysporum f. fabae and significantly decreased the incidence of Fusarium wilt. Therefore, it alleviated the damage of Fusarium wilt caused by benzoic acid, and promoted growth of faba bean. Thus, intercropping with wheat was an effective method to alleviate faba bean autotoxicity caused by benzoic acid. The results also provided an experimental evidence for developing an ecologicaleconomic and effective approach to control soil-borne diseases caused by continuous crop cultivation.