| Intercropping mode is a traditional agricultural management in China, with the advantages such as enhancing photosynthesis areas, increasing land use efficiency and avoiding nutrient competition by roots. In addition, intercropping can also control plant disease. Watermelon is susceptive to wilt disease, which usually results in reducing its yield and quality. Previous pot experiment showed that intercropping with aerobic rice could suppress wilt disease occurrence in watermelon, which is related to that rice root exudates suppressed F. oxysporum. Intercropping with aerobic rice or arbuscular mycorrhizal fungi (AMF) colonization alleviated watermelon wilt disease, which is likely attributed to rice root exudates or AMF depressing watermelon wilt pathogen. Coumaric acid is the specific substance in rice root exudates, with the ability to inhibit spore germination and spore procreation of F. oxysporum. However, further research is needed to investigate whether rice root exudates can transfer to watermelon rhizosphere soil, whether rice root exudates can induce systemic resistance against wilt disease in watermelon and whether AMF can affect the transfer of rice root exudates to watermelon rhizosphere soil. Ferulic acid is the specific substance in watermelon root exudates, with the ability to improve spore germination and spore procreation of F. oxysporum. Salicylic acid is exudated by both rice and watermelon roots. It has no effect on spore germination and spore procreation of F. oxysporum. Salicylic acid is an essential hormone in plant immunity. A pot experiment with14C labeling method was performed to investigate the transfer of rice root exudates to watermelon rhizosphere. A pot experiment with separation between rice and watermelon roots was conducted to detect the role of rice root exudates in alleviating watermelon wilt disease. Exogenous coumaric acid, ferulic acid and salicylic acid were applied to watermelon to investigate the effect of these phenolic acids on watermelon disease resistance. Chitinase gene and PAL gene expression as well as enzymatic acitvities were determined to show watermelon disease resistance.The methods and results were listed as below:1.A rhizobox experiment,with aerobic rice under14C02,was conducted to investigate the effect of AMF colonization on carbon(C)transfer from rice to watermelon and phosphorus(P)uptake by both watermelon and rice.The rhizobox was separated into labelling side(L side)and sampling side(S side)by inserting nylon mesh in the middle of the box.The L side was planted with aerobic rice,and the S side was aerobic rice (monocropping)or watermelon(intercropping).When14CO2was added to rice canopy at the L side,14C activities of rice roots and rhizosphere soils in the L side were increased by intercropping with watermelon or AMF colonization.The14C was detected in roots and rhizosphere soils of rice and watermelon in the S side,but no differences were found among different treatments.14C activities in leaves were improved by AMF inoculation in the S side,regardless of rice or watermelon.Mycorrhizal colonization stimulated P absorption and translocation to rice in intercropping system.These findings suggest that AMF colonization could increase C transfer from rice to watermelon while intercropping with watermelon could promote AMF colonization and P uptake by rice.2. Pot experiment with different separation methods between aerobic rice and watermelon was performed to investigate the effect of separation methods on watermelon wilt disease and its mechanism.The results showed that the wilt disease occurrence was delayed6-7days and was alleviated in the treatment with no separation between aerobic rice and watermelon roots,compared with film separation treatment.The wilt disease occurrence was delayed3-4days in the treatment with nylon mesh separation between aerobic rice and watermelon roots.Compared to film separation,the F. oxysporum number, bacterial number and fungal number decreased while the actinomycetes number increased in watermelon rhizosphere soil in no separation treatment.The similar results were obtained in nylon mesh separation treatment.3.Pot experiment was carried out to investigate the effect of exogenous coumaric acid supply to watermelon rhizosphere on chitinase and phenylalanine ammonia lyase (PAL) genes expression and related enzymatic activities.The results showed that chitinase and PAL genes in watermelon leaf were down regulated.The β-1,3-glucanase activityin leaf was decreased by75.8%with F. oxysproum inoculation.Chitinase gene in root and PAL gene in leaf were up-regulated with exogenous supply of coumaric acid to watermelon.The activities of chitinase and β-1,3-glucanase in roots were increased,and more cinnamic acid was excreted from watermelon root with supplying exogenous coumaric acid. Under supplying both F. oxysporum and coumaric acid, chitinase gene and PAL gene expression in leaf increased with increasing the supply. Compared with control, the activity of β-1,3-glucanase in leaf was increased by53.7%when100μmol L-1coumaric acid was applied to watermelon. Therefore, exogenous coumaric acid improved watermelon disease resistance and100μmol L-1is the best dose for exogenous supply.4. Pot experiment was carried out to investigate the effect of exogenous ferulic acid supply on watermelon resistance. The results showed that chitinase gene in leaf and PAL gene in root were down-regulated with ferulic acid supply. Compared with control, chitinase activity in leaf was decreased by65.2%with100μmol L-1ferulic acid supply. The β-1,3-glucanase activity in watermelon leaf was reduced by75.8%with F. oxysporum inoculation, and decreased by32%-37%under supplying both F. oxysporum and30-300μmol L-1ferulic acid, compared with the control. Therefore, supplying exogenous ferulic acid decreased chitinase activity in leaves and roots and β-1,3-glucanase activity in leaves of watermelon, and thus decreased disease resistance throuth down regulating chitinase gene and PAL gene, and induced wilt disease occurrence in watermelon.5. Exogenous supply of salicylic acid to watermelon rhizosphere was performed to study the effect of hormone secreted by rice and watermelon roots on watermelon disease resistance. The results showed that watermelon root growth was improved by300βmol L-1salicylic acid supply for9days. In addition, the PAL gene expression in watermelon leaf and root was up regulated, as well as the chitinase gene in root with300μmol L-1salicylic acid supply for9days. Compared with control, the chitinase activity in leaf was promoted by2.25and2.45times with30μmol L-1and100μmol L-1salicylic acid application, respectively. The chitinase activity in root was increased by52.2%with300μmol L-1exogenous salicylic acid. Compared with control, the β-1,3-glucanase activity in leaf was enhanced by56.7%and50.7%with adding exogenous30μmol L-1and100μumol L-1salicylic acid, respectively. However, the β-1,3-glucanase activity decreased by76.6%with300μmol L-1salicylic acid. Commanic acid concentration in watermelon root exudates was improved by salicylic acid, and with the highest at100μmol L-1. The commanic acid concentration was increased by9.63times with100μmol L-1salicylic acid supply, compared with the control. Therefore,100μmol L-1salicylic acid aupply was better in up-regulating chitinase gene and PAL gene expression, in improving chitinase and β-1,3-glucanase activities and in secreting more cinnamic acid to improve watermelon wilt disease resistance.The present study demonstrated the mechanism of suppression watermelon wilt disease by intercropped with aerobic rice. Rice root exudates transferred to watermelon rhizosphere to inhibit F. oxysporum growth. On the other hand, rice root exudates induced systemic acquired resistance of watermelon to decrease wilt disease incidence. |
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