Allelopathic effects and components analysis of root exudates of faba bean cultivars with different degrees of resistance to Fusarium oxysporum

Fusarium wilt is one of the most common and destructive soil-borne fungal diseases of faba bean because of continuous monocropping. It has been responsible for severely reduced yield, quality and production of faba bean, especially in Yunnan Province. Numerous studies on soil-borne diseases have mainly focused on changes in soil nutrients, enzyme and microbe. Little has remained known about the relationship between root exudates and soil-borne diseases. In fact, there has been less report confirming the occurrence of fusarium wilt in different cultivars of faba bean affected by different faba bean root exudates. The objectives of this study were to compare the differences in root exudates contents and components of different cultivars of faba bean. The study also aimed to lay the basis for further studies on resistance mechanism of faba bean fusarium wilt, which could in turn lay the basis for ecological control of faba bean fusarium wilt. Cultivars of different resistances to fusarium wilt were used in the experiment - '89-147' (high resistant cultivar), '8363' (medium resistant cultivar) and 'YD324' (susceptible cultivar). Root exudates of the three cultivars of faba bean were collected in nutrient solution cultures. The effects of faba bean root exudates on spore germination and mycelia growth of Fusarium oxysporum f. fabae were determined using the culture medium method. The contents and components of organic acids were identified by HPLC while free amino acids were identified by amino acid analyzer. The content of soluble sugar was tested by anthrone colorimetry. The results suggested that compared with the control, root exudates of resistant cultivars inhibited spore germination and mycelia growth of F. oxysporum. Exudates of medium resistant cultivar promoted mycelia growth with the addition of 5 mL of root exudates, it otherwise had no significant effect on mycelia growth. Exudates of susceptible cultivar promoted spore germination and mycelia growth with the addition of 1 mL of root exudates. Total content of free amino acids and soluble sugar increased while organic acids decreased with increasing resistance of the three faba bean cultivars to fusarium wilt. Fifteen types of amino acids were detected in root exudates of medium resistant and susceptible faba bean cultivars while fourteen types of amino acids were detected in root exudates of high resistant faba bean cultivars. Histidine was found only in root exudates of medium resistant faba bean cultivar while praline was only in root exudates of susceptible faba bean cultivar. Arginine was not found in root exudates of the three faba bean cultivars. The occurrence and development of faba bean fusarium wilt were enhanced when the contents of aspartic, glutamate, phenylalanine, tyrosine and leucine in root exudates were high. Faba bean fusarium wilt was inhibited when the contents of methionine, lysine and serine in root exudates were high. Tararic was detected only in root exudates of high resistant cultivar. The richness of organic acid species in root exudations enhanced faba bean resistance to fusarium wilt. Root exudates of different resistance faba bean cultivars had different effects on F. oxysporum. These differences were related to the contents and components of soluble sugars, free amino acids and organic acids in root exudates of different faba bean cultivars.