The Mechanism Of Enhancement The Growth Condition And Suppression Of Fusarium Wilt In Watermelon By Wheat As Companion Crop

Watermelon (Citruiius lanatus (Thunb.) Matsum.&Nakai) is one of the most popular greenhouse plants. With the rapid development of facility agriculture in our country, watermelon continuous cropping is inevitable. However, continuous cropping can lead to serious problems including poor plant growth, declined production and disease resistance. The growth of watermelon is often threatened by Fusarium in the continuous cropping soil, which causes wilting. Fusarium wilt of watermelon is caused by the pathogen Fusarium oxysporum f. sp. niveum (FON) and has become a constraint of the production of watermelon. Our studies showed that, as a companion crop, wheat could enhance watermelon growth performance and suppress Fusarium wilt of watermelon. However, the mechanism is still unclear.Based on the screened wheat varieties with growth-enhancing and Fusarium wilt-suppressing characters, this study analyzed the influences of wheat on (1) the expression of expansin gene and of morphological chacracteristics of watermelon root with qRT-PCR and agar culture methods;(2) and watermelon rhizosphere microorganism community with PCR-DGGE, qPCR and Miseq sequencing methods;(3) and the expressions of disease resistance genes, secondary metabolism substance content, the pathogenesis-related proteins activities in watermelon with qRT-PCR, bioinformatics methods. The purpose of this study is to reveal the mechanism of how wheat could enhance watermelon growth performance and suppress Fusarium wilt of watermelon as a companion crop from the view of allelopathy, soil microorgasm and molecular biology. Overall, this study could provide theoretical foundation and technical support for the application of the wheat as a companion crop of watermelon and the reduction of the soil sickness in watermelon production. It can also provide ideas and technical basis for correlation studies.Main results of this paper were as flowing:1. The lower concentration of wheat root exudates stimulated the growth of watermelon seedlings, the higher concentration of wheat root exudates had inhibitory effects.60%wheat cultivar root exudates of5mL plant-1stimulated the growth of watermelon seedlings in this study, Dj25wheat root exudates significantly stimulated the growth of watermelon seedlings. D123wheat root exudates significantly inhibited the mycelia growth of FON,5mL plant-1had the strongest inhibitory effects. 2. Compared with monoculture, companion cropping with wheat significantly increased root length, surface area, volume, number of roots, root mean diameter, root dry weight in watermelon seedlings. Companion cropping with wheat significantly increased ratio of0.5-1.0mm,1.0-1.5mm,1.5-2.0mm root diameter in watermelon seedlings compared with monoculture. Two cultivar wheat root exudates significantly increased root length, surface area, volume, number of roots, root dry weight, increased ratio of0-0.5mm and decreased ratio of0.5-1.0mm,1.0-1.5mm,1.5-2.0mm,2.0-2.5mm root diameter in watermelon seedlings. After treatment with the inhibited solution of wheat root exudates, two wheat root exudates had no effect on root length, surface area, volume, number of roots, root mean diameter, root dry weight in watermelon seedlings compared with control, and no significantly effect on ratio of0-0.5mm,0.5-1.0mm,1.0-1.5mm,1.5-2.0mm,2.5-3.0mm,3.0-3.5mm,3.5-4.0mm root diameter in watermelon seedlings, reduced the ratio of2.0-2.5mm roor diameter in watermelon seedlings.3. At8,9,10,11days accompanied with wheat under continuous cropping soil, no significant difference were detected to the expression of expansin gene in watermelon roots between the treatments. Two cultivar wheat root exudates significantly upregulated the expression of expansin gene in watermelon roots and radical.4. Compared with the monoculture, the rhizosphere soil saccharase and polyphenol oxidase activities increased in companion cropping, bacterium, total number increased in companion cropping.D123companion cropping increased fungus and actinomyce number in watermelon rhizosphere soil. The content of microbial N, and P in watermelon rhizosphere soil accompanied with wheat were significantly higher than in monoculture, however, C/N in the rhizosphere soil increased in monocropping treatment, D125companion cropping significantly increased content of microbial C. Watermelon rhizosphere fungi was positively correlated with urease, polyphenol oxidase activities. Watermelon rhizosphere bacterium was positively correlated with polyhenol oxidase and catalase. Microbial population and actinomyce were positively correlated with urease, polyhenol oxidase and saccharase.5. Companion cropping with wheat affected watermelon rhizosphere microorganism community structure. Companion cropping with wheat increased abundance of Alphaproteobacteria, Gemmatimonadetes, Anaerolineae and Aeromicrobium. Companion cropping with D125wheat enhanced abundance of Pseudomonas in watermelon rhizosphere soil. Companion cropping with D123wheat increased abundance of Sphingomonas in watermelon rhizosphere soil. Companion cropping with wheat reduced the FON population in watermelon rhizosphere soil. As to watermelon rhizosphere bacterial and fungus community, companion cropping with D125wheat treatment and companion cropping with D123wheat treatment had a lot in common.6. Companion cropping with wheat increased the PPO, PAL, chitinase, B-1,3-glucanase activities flavonoid, total soluble phenolics and lignin contents, decreased MDA contents in watermelon roots.7. Inoculation of FON induced genes involved in JA biosynthesis and shikimate-phenylpropanoid-lignin pathways in the watermelon roots in both companion cropping and monoculture systems. However, the timing and levels of the induced expressions differ between the two cropping system. In the companion cropping system, PL, PAL, CCR, AOS, SK, DAHPS,4CL, AOC, CCOMT genes were strongly induced during the early stages of FON infection and the expression levels were higher than in the monoculture system. The timing and levels of the induced expressions slightly differ between companion with D123wheat and companion with D125.Overall, two cultivar wheat root exudates significantly stimulated the expression of expansin gene and the growth in watermelon seedlings roots. After treatment with the inhibited solution of wheat root exudates, two wheat root exudates had no effect on the growth of root in watermelon seedlings. It is suggested that wheat exudates is one of the reasons for promoting the growth of watermelon in the companion cropping system. Companion cropping with wheat increased bacterium, fungus, actinomyce population and the ratio of bacterium and actinomycetes, and reduced the ratio of fungus and FON population in watermelon rhizosphere soil, and affected watermelon rhizosphere microorganism community structure. Companion cropping with wheat increased the PPO, PAL, chitinase,β-1,3-glucanase activities in watermelon roots and he flavonoid, total soluble phenolics and lignin contents, and decreased MDA contents in watermelon roots compared with monoculture. In the companion cropping system, the expression of genes in JA biosynthesis and shikimate-phenylpropanoid-lignin pathways were strongly induced during the early stages of FON infection and the expression levels were higher than in the monoculture system.