Interactions Between Brown Planthopper And Root-feeding Nematodes Affected By Cultivar And Fertilizer Management

The interactions between aboveground and belowground subsystems have become a focus of terrestrial ecology now. In the past two decades, it has been increasingly recognized that belowground and aboveground subsystems are tightly linked with each other through a series of plant-mediated mechanisms and feed-backs. Examining fundamental relationships between herbivores and plants is thought to be the first stage to understand aboveground-belowground interactions. It has been documented that aboveground and belowground herbivores can affect each other by inducing quantitative and qualitative changes in host plant. However, few studies have focused on investigating interactions between aboveground and belowground herbivores of rice (Oryza sativa), which does not match the role of rice in world food production. Rice is one of the most important food crops in China, which are attacked by multiple herbivores feeding on both roots and shoots during the whole growth period. Brown planthopper (Nilaparvata lugens Stal) is the most destructive aboveground pest to rice in the Yangtze River valley, southern and southwest area of China. It is a migrating sucking insect. Through sucking phloem sap of rice plants, brown planthopper causes serious plant growth retardation and yield loss. Rice is also frequently attacked by a number of belowground pests. In particular, the flooding condition in paddy fields stimulates the development of some root-feeding nematodes, for example, rice root nematode, Hirschmanniella spp., which is one of the most serious root pests of rice. To date, the plant-mediated interactions between brown planthopper and root-feeding nematodes are seldom considered in rice system. Besides, it has been documented that both rice cultivar and fertilizer management can greatly affect rice plant resistance to pests. However, few studies have investigated effects of rice cultivar and fertilizer on interactions between aboveground and belowground pests of rice. This thesis focuses on studying how brown planthopper and root-feeding nematodes affect each other via plant-mediated changes, and whether rice cultivar and fertilizer management affect these interactions. We also investigate effects of rhizosphere soil multitrophic organism community on interactions between brown planthopper and rice root nematodes. A series of greenhouse experiment were carried out to examine effects of brown planthopper and root-feeding nematodes on rice plant growth, labile resource allocation in plant-soil (rhizosphere soil) system and soil nematode community. Our results may contribute to a better understanding of interactions between aboveground and belowground herbivores in terrestrial ecosystem and will help the development of integrative rice pest management and environmental-friendly agriculture. The main results were as follows:1. Effects of brown planthopper and rice cultivar on soil labile resources and nematode communityThe extent of the negative effects of brown planthopper on rice plant growth decreased with the increased level of rice cultivar resistance to brown planthopper. For example, brown planthopper decreased shoot water content by more than 46% and 25% respectively for the susceptible and intermediate susceptible rice cultivars, but by less than 20% and 1% respectively for the intermediate resistant and resistant cultivars. Shoot height and root length of rice showed similar response tendency to brown planthopper across four rice cultivars. Rice cultivar resistance also mediated herbivore effects on labile resources and food web structure in rhizosphere soil, which is indicated by trophic structure of soil nematode community. Under susceptible cultivar, brown planthopper decreased labile resources in rhizosphere soil, such as concentrations of soluble sugars, amino acids, dissolved organic carbon and nitrogen, microbial biomass carbon and nitrogen (P<0.05), and reduced percentage of bacterial-feeding nematodes by 31% while increased percentage of root-feeding nematodes by 28%. However, under resistant rice cultivar, rhizosphere soil labile resources and the percentages of root-feeding and bacterial-feeding nematodes responded to brown planthopper in the opposite way (P<0.05). Therefore, rice cultivar resistance to brown planthopper significantly modified the effects of brown planthopper on plant growth and rhizosphere soil ecosystem. The results suggest that the agronomic use of resistant cultivars could not only resist aboveground pest effectively but also restrict the development of root-feeding nematodes and promote resource availability in rhizosphere soil in the presence of brown planthopper.2. Effects of aboveground herbivory intensity on rice growth and soil labile resourcesThe herbivory intensity of brown planthopper, which depend on herbivore density and infestation duration, significantly affected shoot and root biomass of rice, and labile resources in rhizosphere soil, such as microbial biomass carbon and nitrogen, dissolved organic carbon and nitrogen, and ammonium and nitrate concentrations. Compared to the control treatment, the high density of brown planthopper (12 inds./plant) significantly decreased shoot and root mass of rice, soil dissolved organic carbon and nitrogen, and microbial biomass carbon and nitrogen (P<0.05), while low (4 inds./plant) and moderate planthopper densities(8 inds./plant)did not affect or even showed beneficial effects on rice plant growth and labile resources in rhizosphere soil after a short infestation duration (9 days). However, the negative effects of moderate and high herbivore densities on plant growth and soil labile resources increased with the prolonged infestation duration(15 days), while the low herbivore density treatment exerted more beneficial effects (P<0.05). Our results showed that intermediate herbivory intensity of brown planthopper exerted positive effects on rice seedling performance and, further, induced increases in concentrations of labile resources in rhizosphere soil. Such a trend may further affect rhizosphere soil organism community.3. Effects of rice cultivar and root-feeding nematodes on soil labile resourcesThe effects of rice cultivar on amount of rice root nematodes did not depend on rice cultivar resistance to aboveground herbivore, brown planthopper. The amount of rice root nematodes substantially increased under all cultivars, reaching 2.7 to 3.2 times the introduced amount, which may attribute to the lack of free-living nematodes in soil. The amount of rice root nematodes did differ among rice cultivars (F=3.24, P=0.050), which showed a similar tendency with soluble sugar and amino acid concentrations in root across cultivars. Rice root nematodes exerted negative effects on plant growth, shoot and root soluble sugar and amino concentrations, and concentrations of labile resources (soluble sugars, amino acids, dissolved organic carbon and nitrogen, microbial biomass carbon and nitrogen) in rhizosphere soil in all cultivar treatments(P<0.001). Rice cultivar resistance to brown planthopper alleviated the extent of nematode-induced reductions in root growth, concentrations of shoot labile resources, soil soluble sugar and dissolve organic carbon, although it did not affect the amount of rice root nematodes. The results implied that integrated complex mechanisms are involved in rice resistance to aboveground and belowground herbivores.4. Rice cultivar and fertilizer modify impacts of brown planthopper on soil labile resources and nematode communityRice cultivar and fertilizer management significantly affected performance of brown planthopper (P<0.001). Both resistant rice cultivar and organic fertilizer increased rice plant resistance to brown planthopper and decreased planthopper-induced reductions in plant biomass, concentrations of labile resources(dissolved organic carbon and nitrogen, microbial biomass carbon and nitrogen) in rhizosphere soil and the percentages of microbial-feeding nematodes, while suppressed the development of root-feeding nematodes. Fertilizer management exerted more significant effects than rice cultivar on soil microbial biomass carbon and nitrogen(P<0.001), dissolved organic carbon and nitrogen(P<0.001), and percentages of root-feeding (PO.001) and bacterial-feeding nematodes (P=0.008). Compared to chemical fertilizer, organic fertilizer led to significant reductions in percentage of root-feeding nematodes (P<0.001) while showed beneficial effects on percentages of microbial-feeding nematodes (P<0.001). Although the interactions between rice cultivar and fertilizer did not significantly modify effects of brown planthopper on plant and soil properties, our results suggested that employing resistant rice cultivar and organic fertilizer could increase plant resistance to both shoot and root pests of rice and positively affect rhizosphere soil labile resources.5. Effects of soil multitrophic community structure on interactions between aboveground and belowground herbivores of riceBrown planthopper and rice root nematodes affected the performance of each other (P<0.05)via inducing changes in concentrations and allocations of labile resources within rice plant. The infestation of brown planthopper decreased soluble sugar concentration in shoots but increased it in roots, which increased the amount of rice root nematodes by 27%. However, rice root nematodes caused negative effects on both shoot and root soluble sugar and amino acid concentrations, which therefore decreased the amount of brown planthopper by 28%. The interactions between brown planthopper and rice root nematodes could be significantly altered by other trophic groups of nematodes in soil (free-living nematodes). The amount of rice root nematodes in mixed nematode community treatment only reached 21% of the amount in single rice root nematode treatment and the negative effects of rice root nematodes on brown planthopper decreased when soil free-living nematodes were present. Soil free-living nematodes also modified herbivore effects on rice plant and labile resources in rhizosphere soil. Our results implied the importance of soil multitrophic community structure in integrated pest management of rice.To summarize, the research is the first one to discuss interactions between aboveground and belowground herbivorous pests of rice. The results show that brown planthopper and rice root nematodes can affect each other via inducing quantitative and qualitative changes in rice plant and both herbivores can exert effects on labile resources and food web structure in rhizosphere soil. Rice cultivar, fertilizer management and rhizosphere soil multitrophic community structure greatly affect interactions between brown planthopper and rice root nematodes. The study may contribute to a better understanding of interactions between aboveground and belowground herbivores in terrestrial ecosystem and have important implications for integrated pest management of rice.