Effects Of Allelochemicals From Watermelon And Rice Root Exudates On Fusarium Wilt Of Watermelon

Root exudates play a key role in communications between plants and microbes in the rhizosphere. Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (FON), drastically reduces watermelon yields in continuous cultivation systems, but it can be significantly alleviated using watermelon/aerobic rice intercropping system as shown by the research previously carried out in this laboratory. It is worthwhile to evaluate the interaction between the root exudates from the two crops and the pathogen and thus to clarify the mechanism of disease suppressiveness in the intercropping system. Here we firstly evaluated the allelopathic effects of root exudates of watermelon and rice plants on FON and the allelochemicals that present in these two root exudates to clarify the mechanism of disease suppressiveness in the intercropping system. The results obtained are listed as follows:1. Root exudates of watermelon and rice plants were collected by nutrient solution culture. The effects of root exudates from watermelon and rice plants on FON were determined by culture medium method. Compared with the control, addition of 1.0 ml and 5.0 ml of watermelon root exudates increased the numbers of germinating spores by 46.9% and 59.2%, respectively, while the spore formation was enhanced by 10.8% to 84.6%, respectively, when watermelon root exdates were added in the range of 0.1-2.0 mL. In contrast, addition of 1.0 ml and 5.0 mL of rice root exudates decreased spore germination by 6.1% and 14.3%, respectively, while addition of rice root exdates from 0.1 to 2.0 mL decreased spore formation by 4.6% to 37.5%, respectively.2. The phenolic acids of root exudates from watermelon and rice plants were identified by HPLC. Exogenous phenolic acids were also used to investigate the effect of some phenolic acids, which existed in watermelon and rice root exudates, on FON. Salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified both in watermelon and rice root exudates by HPLC analysis, but ferulic acid or p-coumaric acid was detected exclusively in watermelon or rice root exudates, respectively. Moreover, the content of p-coumaric acid was the highest among phenolic acids in rice root exudates, accounting for 37.9% of the total amount of phenolic acids, which was 1.4-fold as high as that in watermelon root exudates. Furthermore, exogenous ferulic acid increased spore germination by 28.6% to 114.3% and spore formation by 17.7% to 54.8% whereas exogenous p-coumaric acid reduced spore germination by 9.1% to 70.5% and spore formation by 24.1% to 100.0%, as compared with the controls.3. Two watermelon cultivars and rice were used to study the contents and components of the soluble sugars and free amino acids in the root exudates and their effects on the growth of FON. The contents and compositions of soluble sugars and free amino acids in root exudates of the two watermelon cultivars with different resistance to FON were both much more than the rice plants, while those parameters increased with increased degree of susceptibility in these two watermelon varieties. Compared with susceptible watermelon cultivar (Zaojia 84-24), the root exudates of resistant cultivar (Jingxing 1) were lack of the Tyr, and those of the rice cultivar were lack of the Asp, Ser, Glu, Val, Met, Ile, Tyr, Phe and Arg. According to their effects on spore germination, sporulation and mycelial growth of FON, Ala, Glu, Asp, Arg, Gly and Phe were the promoting amino acids whereas Met and Ser were the inhibitory amino acid.4. To establish ecological relationships of decomposing products of plant tissues and pathogen during the interactions of plant-microbe, extracts of decaying watermelon (two cultivars with different resistance) and rice tissues were added into media to investigate the allelopathic impact of their decaying tissues on the growth of FON in a laboratory study. Extracts from the decomposed root of susceptible watermelon cultivar (Zaojia 84-24) significantly enhanced the spore germination of FON, while those from the decomposed stems and leaves remarkably promoted the sporulation of FON at their higher concentrations. The inhibiting allelopathic effects of different decomposing products from resistant watermelon cultivar (Jingxing 1) on spore germination of FON were found, and there were significantly promoting effects of the decomposing products from roots on sporulation of FON, followed by that from leaves. But, the decomposing products from roots and stems of rice tissues remarkably suppressed the growth of FON.