Autotoxicity refers to the process by which plants or their residues release toxic chemicals into the environment during decomposition, thereby inhibiting the germination and growth of the same plant, and is a common cause of plant continuous cropping obstacles (Huang XX et al,2010). Currently, it has been proven that there are many autotoxic substances in plant root, stem and leaf extracts (J. C. Alías et al ,2006 ; Guo, K et al༌2016). This experiment showed that all concentrations tested of the extracts of faba bean stems, leaves, and roots significantly inhibited the growth of faba bean seedlings compared with the control, and the pronounced inhibition of root growth was particularly significant, which is similar to the results of Singh, N.B.et al (2008) on a study of tomato extracts. Plant cells accumulate free radicals owing to reduced antioxidant capacity during adverse conditions, leading to the oxidative damage of cellular macromolecules and membranes (Yin, Y.Q. et al, 2012). Furthermore, autotoxic metabolites produced by the stressed plants accelerate free radical-induced membrane peroxidation and breakdown, thereby providing nutrients to the pathogens and enhancing their ability to invade plant roots. In fact, the activity of the antioxidant enzymes POD and CAT are reliable indicators of disease resistance in plants (Ren LX. et al, 2008). Wang et al (2019) showed that exogenous syringic acid and phthalic acid significantly reduced the activities of POD and CAT in strawberry roots and increased the content of MDA. This is identical to the results obtained in this experiment. Compared with the control, medium and low concentrations of faba bean stem and leaves and root extracts significantly inhibited the activities of the antioxidant enzymes POD and CAT of the faba bean root system, while significantly enhancing the accumulation of MDA in faba bean roots. This could be because the extract of faba bean stems and leaves and roots contains a substantial amount of phenolic acids (Guo, K et al༌2016). They destroy the functional pathways of antioxidant enzymes and cause enormous damage to the defense system of faba bean roots, which in turn clears obstacles for the pathogens to invade faba bean roots. The accumulation of pathogen is the root cause of soil-borne diseases, and these microorganisms are difficult to remove from the soil. Experiments have proven that the three biological forms of F. oxysporum can survive for more than 11 years without changing their morphology (Sun, Y et al, 2017). Long-term continuous crops have formed a stable and suitable environment with increased temperature and humidity, sufficient nutrients and host conditions that are more conducive to the propagation and growth of pathogen, resulting in the aggravation of disease (Yang et al, 2001; Li et al, 2014a). In this experiment, the extracts of faba bean stems, leaves, and roots at low concentrations inhibited the spore germination and mycelial growth of FOF. However, with the increase in concentration, the inhibitory effect gradually disappeared, and at high concentration, the extracts significantly promoted germination and growth of the fungus. In actual agricultural production, owing to years of continuous cropping, the faba bean extracts had accumulated to large amounts in the soil, and the concentration of extract in the soil is very high. Therefore, in actual production, the faba bean extract has an enormous stimulatory effect on the germination and growth of FOF. Based on these results, we hypothesize that the autotoxicity of faba bean may promote the growth of pathogen by destroying the defense system of faba bean root system and enhancing the invasion of pathogen to the root system of faba bean, finally resulting in strong inhibition of the growth of faba beans.
Intercropping is a green and efficient planting model, particularly in terms of increasing yield and controlling diseases. Now this advantage has been verified in many intercropping systems, such as corn and soybean intercropping that effectively controls corn crown rot and rice/watermelon intercropping that effectively controls watermelon wilt (Gao, X et al,, 2014; Ren LX et al, 2008). Similarly, we found that in field experiments, intercropping faba bean and wheat significantly inhibited the incidence of faba bean wilt in the faba bean branching and flowering stages, and during the branching, flowering and podding stages of faba bean, the disease index of faba bean wilt was significantly inhibited. Most research on the mechanism of intercropping disease control focuses on allelopathic substances secreted into the soil through the root system, but in actual production, these compounds can also enter the soil through the leaching and evaporation of plant roots, stems and leaves. These allelopathic substances are easily overlooked (Hao, Z. P et al, 2007). For the research on the mechanism of control by faba bean and wheat intercropping, we added different concentrations of wheat stem and leaf and root extracts in the faba bean hydroponic experiment. Compared with the faba bean extracts, we found that the wheat extracts significantly promoted the growth of faba bean seedlings at all treatment concentrations; similar conclusions have been obtained in the moringa and wheat intercropping system (Khan et al, 2017). We also simultaneously found that, compared with the treatment of faba bean extracts, wheat extracts significantly enhanced the activities of faba bean root POD and CAT and effectively reduced the accumulation of faba bean root MDA. The ability of the faba bean root system to resist the invasion of pathogen had improved. A series of results show that in actual production, wheat extracts can effectively alleviate the autotoxicity of faba beans. We hypothesize that this may be one of the important mechanisms of wheat and faba bean intercropping for disease control. This result is consistent with previous studies on rice/watermelon, rice/water chestnut, and corn/sunflower intercropping systems (Ren et al, 2008; Chen et al, 2012; Qin et al, 2013). On the basis of the significant improvement of the faba bean root defense system by wheat extracts, compared with the faba bean extracts, the wheat extracts could significantly inhibit the mycelial growth and germination of spores, thereby fundamentally reducing the possibility of pathogen infection of faba beans. This is consistent with the results of Ren et al (2010) on the wheat/watermelon intercropping system. This shows that in the wheat/faba bean intercropping system, the extracts of wheat can effectively relieve the stimulatory effects of the faba bean extracts on the occurrence of faba bean wilt, thereby further reducing the occurrence of faba bean wilt. Unexpectedly, compared with the control, the wheat extracts were effective at a low concentration, but they enhanced the inhibition of the growth of faba beans at high concentration. However, in actual production, unlike the large accumulation of faba bean extract, wheat has no continuous cropping history. The concentration of extract in the field is very low and is easily degraded by microorganisms in the soil. Therefore, in the actual field intercropping mode, the concentration of wheat extracts will not be very high. This experiment was a hydroponic one, and our aim was to explore the allelopathy of wheat extracts of different concentrations. This does not examine the decomposition of allelochemicals by soil rhizosphere microorganisms. However, it also reminds us that in actual agricultural production, we should pay attention to controlling the ratio of faba bean and wheat and avoiding an excessive planting density of wheat that leads to an excessive concentration of rhizosphere wheat extract that could inhibit the growth of faba bean. After verification, we believe that the 2:6 ratio of faba bean:wheat used in this experiment is the best ratio to inhibit FOF, which is of substantial significance for the guidance of field production and disease control.