No abstract

No abstract

BackgroundAutotoxicity of cucumber root exudates or decaying residues may be the cause of the soil sickness of cucumber. However, how autotoxins affect soil microbial communities is not yet fully understood.Methodology/Principal FindingsThe aims of this study were to study the effects of an artificially applied autotoxin of cucumber, p-coumaric acid, on cucumber seedling growth, rhizosphere soil microbial communities, and Fusarium oxysporum f.sp. cucumerinum Owen (a soil-borne pathogen of cucumber) growth. Abundance, structure and composition of rhizosphere bacterial and fungal communities were analyzed with real-time PCR, PCR-denaturing gradient gel electrophoresis (DGGE) and clone library methods. Soil dehydrogenase activity and microbial biomass C (MBC) were determined to indicate the activity and size of the soil microflora. Results showed that p-coumaric acid (0.1–1.0 µmol/g soil) decreased cucumber leaf area, and increased soil dehydrogenase activity, MBC and rhizosphere bacterial and fungal community abundances. p-Coumaric acid also changed the structure and composition of rhizosphere bacterial and fungal communities, with increases in the relative abundances of bacterial taxa Firmicutes, Betaproteobacteria, Gammaproteobacteria and fungal taxa Sordariomycete, Zygomycota, and decreases in the relative abundances of bacterial taxa Bacteroidetes, Deltaproteobacteria, Planctomycetes, Verrucomicrobia and fungal taxon Pezizomycete. In addition, p-coumaric acid increased Fusarium oxysporum population densities in soil.Conclusions/SignificanceThese results indicate that p-coumaric acid may play a role in the autotoxicity of cucumber via influencing soil microbial communities.

The diversity of soil microbial communities as affected by continuous cucumber cropping and alternative rotations under protected cultivation were evaluated using community level physiological profiles (CLPP) and random amplified polymorphic DNA (RAPD) analysis. The soils were selected from six cucumber cropping systems, which cover two cropping practices (rotation and continuous cropping) and a wide spectrum for cucumber cropping history under protected cultivation. Shannon-Weaver index and multivariate analysis were performed to characterize variations in soil microbial communities. Both CLPP and RAPD techniques demonstrated that cropping systems and plastic-greenhouse cultivation could considerably affect soil microbial functional diversity and DNA sequence diversity. The open-field soil had the highest Shannon-Weaver index (3.27 for CLPP and 1.50 for RAPD), whereas the lowest value occurred in the 7-year continuous protected cultivation soil (3.27 for CLPP and 1.50 for RAPD). The results demonstrated that continuous plastic-greenhouse cultivation and management can cause the reduction in the species diversity of the biota. Higher Shannon-Weaver index and coefficients of DNA sequence similarity were found in soils under rotation than those under continuous cropping. Cluster analysis also indicated that microbial community profiles of continuous cultivation soils were different from profiles of rotation soils. The reduction in diversity of microbial communities found in continuous cultivation soils as compared with rotation soils might be due to the differences in the quantity, quality and distribution of soil organic matter.

The continuous cropping of cucumber in the same potting soils may result in a reduction of yield and quality of the crop, a phenomenon described as soil sickness. The changes of soil microbial communities as affected by continuous cropping and the link between these changes and soil sickness of cucumber are still not clear. In the present study, cucumber was cropped in pots under greenhouse conditions for nine successive cropping cycles. Structures and sizes of rhizosphere fungal and Fusarium (Ascomycota, Fungi) communities, both ubiquitous and ecologically important in soils, were analysed with PCR-denaturing gradient gel electrophoresis and quantitative reverse transcription PCR, respectively. Cucumber showed retarded growth in the seventh cropping cycle. The RNA- and DNA-based fungal community structures derived from the same sample differed from each other, and the active soil fungal communities were more sensitive to continuous cropping. The RNA-based fungal and Fusarium community sizes were larger in the seventh cropping cycle than in the other cropping cycles. Overall, the findings of this study indicate that the population sizes rather than the diversity of fungi and Fusarium communities are linked to the soil sickness associated with cucumber cultivation.

Intercropping plays a vital role in greenhouse production, and affects soil physicochemical properties and soil microbial communities structure, but influences of intercropping on the relationship of microorganisms are reported in continuous cropping soil rarely. Here, we investigated the effects of seven intercropping systems [alfalfa (Medicago sativa L.)/cucumber, trifolium (Trifolium repens L.)/cucumber, wheat (Triticum aestivum L.)/cucumber, rye (Secale cereale L.)/cucumber, chrysanthemum (Chrysanthemum coronrium L.)/cucumber, rape (Brassica campestris L.)/cucumber, mustard (Brassica juncea L.)/cucumber] on soil bacterial and fungal communities compared to the cucumber continuous cropping system in the greenhouse. The results showed that intercropping increased microbial OTU richness and fungal communities diversity, soil bacterial communities diversity was abundant in the trifolium-cucumber and mustard-cucumber systems. Nevertheless, there was no significant differences of microbial communities structure between intercropping and monoculture systems. Redundancy analysis indicated that soil microbial communities composition was indirectly influenced by soil properties. In addition, network analysis demonstrated that simple inter-relationships of fungal taxa were observed in the intercropping soil, and trifolium, wheat, and mustard intercropping systems had a complex connection between bacterial taxa. Taken together, trifolium and mustard as the intercrops significantly increased cucumber continuous cropping soil bacterial and fungal communities diversity. Moreover, intercropping strongly changed the relationships of microbial taxa, though did not shape notably soil microbial communities structure.

Phenolic compounds have been implicated as autotoxins of cucumber under mono‐cropping management systems. Inhibition of cucumber growth may result from direct uptake of phenolic compounds or an indirect effect resulting from changes in soil microflora. In the present study we monitored the dynamics of soil phenolics in a continuously mono‐cropped cucumber system and then assessed the effects of these compounds on soil microbial communities. Six phenolic compounds were identified in all soil samples in the continuously mono‐cropping system. Soil total phenolic content increased extensively in the first cropping, but maintained a relatively stable level in the following croppings. Amendments of phenolics at the concentration detected in the soil showed inhibitory effects on cucumber seedling growth and stimulatory effects on soil dehydrogenase activity, soil microbial biomass carbon content and soil bacteria and fungi community sizes. Amendments of phenolics caused shifts in soil microbial community structures and soil bacteria and fungi communities had different responses. Our results suggested that direct phytotoxic effects of phenolics on cucumber probably did not happen in continuous mono‐cropping systems, but they might indirectly influence cucumber performance by changing soil microbial communities.

No abstract