Plants need to defend themselves against their natural enemies without compromising their interactions with beneficial organisms. Chemical mechanisms underpin many of these interactions and changes in plant metabolism are critical to both robust defences against antagonists and effective signals to mutualists. Further, such plant responses can be systemic, so mediating interactions between spatially separated organisms above and below ground. This thesis aimed to characterise the changes in the ragwort (Senecio jacobaea L.) metabolome caused by two different belowground organisms, an antagonistic herbivorous nematode (Pratylenchus penetrans (Cobb, 1917) Filipjev & Schuurmans Stekhoven, 1941) and a mutualist arbuscular mycorrhizal fungus (AMF) (Glomus intraradices Smith & Schenck). Initially, vegetative and reproductive stage ragwort plants were sampled in the field and the chemical composition of leaf and flower tissues was assessed using a metabolomic approach. Techniques for the identification of key ragwort secondary metabolites were trialled and results demonstrated that plants of different ages differed in their allocation of within plant defences such as flavanoids, pyrrolizidine alkaloids (PAs) and chlorogenic acids. Subsequent experiments with nematodes and AMF focussed on the analysis of leaf tissues from vegetative stage plants. Feeding by the nematode species P. penetrans resulted in increased concentrations of metabolites associated with plant defence, including the main class of ragwort defence compounds PAs. In contrast, colonisation of root material by AMF caused increases in the concentrations of metabolites associated with the maintenance of the beneficial interaction between plant and fungi, such as a number of apocarotenoids known as blumenols. The findings of both experimental studies detected unexpected and previously unreported changes in plant metabolism, highlighting the importance of an untargeted approach when examining the chemical ecology of plant interactions.

Fallopia japonica, or Japanese knotweed is a rhizomatous perennial herb native to East Asia; most notably Japan, China and Korea. Upon discovery of this species and subsequent import to Europe in the 1840's, it was considered an esteemed ornamental plant – winning the medal for the ‘most interesting new plant of the year' in 1847. F. japonica soon became known as a menace rather than a champion, when it began to spread throughout its new environment, spreading to gardens and nurseries and regenerating from discarded plant fragments. The species ability to cause environmental damage has earnt it a place in the ‘top 100 world's worst most invasive alien species' list. Commercially available herbicides have proven have little effect on F. japonica, and to be successful require many repeat applications. The plant can grow up to 10 cm per day during the early budding and shoot stage and can easily dominate an environment when left unchecked. A key objective of this research was to determine the biochemical pathways of energy generation particularly during the rapid phase of growth with the longer term goal of identifying potential inhibitors of this process which may have commercial opportunities. Very little research is available regarding the biochemistry of growth of F. japonica, thus detailed protocols were required to be established and optimised prior to biochemical investigations. Mitochondrial isolations and following respiratory activity measurements were performed on F. japonica prepared from naturalised plants. Such mitochondrial samples were found to have a very low respiratory rates when compared to mitochondria isolated from other species such as Arum maculatum. This was confirmed following an analysis of the respiratory complexes via electrophoresis, which revealed that all complexes were of low abundance in comparison with other plant species. Transmission electron microscopy also revealed that the numbers and volumes of mitochondria in budding tissue were considerably fewer and larger than those observed in other rapidly expanding plant tissues - providing further confirmation of the respiratory measurements. In an attempt to overcome the small yield associated with mitochondrial isolations, research is also presented on the generation, optimisation and characterisation of suspension cultures from F. japonica explants. Suspension cultures were shown to have almost identical characteristics in terms of mitochondrial protein complement and respiratory capacity as observed in bud and shoot isolations. Preliminary mass spectroscopy data indicated a large proportion of ATP synthase subunits were present in the isolated mitochondrial fractions from leaf, bud, shoot and suspension cultures. Glycolytic analysis of fractions isolated from suspension cultures were also undertaken the outcome of which are discussed in terms of the energy generation pathways within F. japonica and the implications of how such pathways may be controlled.

Interactions between insect herbivores and host plants are fundamental, shaping both ecosystem functions and community structure. One aspect of insect-plant interactions that has received considerable attention recently is the indirect linkages between aboveground and belowground insect herbivores via a shared host plant. To date, the relationship between a maternal insect aboveground and her soil dwelling offspring has been largely overlooked. This study aimed to examine the interactions between the adult insect and soil dwelling larvae of the vine weevil (Otiorhynchus sulcatus) with reference to the preference– performance hypothesis, using an agronomically important host plant, red raspberry (Rubus ideaus). A meta–analysis of aboveground–belowground insect herbivore interactions highlighted that belowground Coleopteran herbivores positively impacted aboveground Homoptera and that general predictions from conceptual models in the literature regarding the direction of interactions between insects were correct, but not statistically significant. In addition it was found that aboveground insect herbivores negatively influenced the survival of belowground herbivores. The preference–performance hypothesis was not supported in the findings for the vine weevil on raspberry. Instead a conflict between larval development and adult egg laying behaviour was observed, with a significant reduction in larval mass recorded when maternal adults fed on the same host plant. Larval mass was decreased by 19% after prior conspecific root feeding, but maternal weevils did not distinguish between plants with and without larvae for oviposition. Significant differences between larval performance (abundances and mass) on the raspberry cultivars Glen Rosa and Glen Ample were not correlated with adult oviposition aboveground. Instead, in some instances, maternal egg laying was correlated with foliar nitrogen content, suggesting that this may be an influential factor in the oviposition behaviour of adult vine weevils. Significant differences seen in larval performance in the laboratory were not reflected in the field, with adult vine weevil populations on Glen Rosa and Glen Ample showing no significant differences in terms of abundance. The findings from this study suggest that mother–offspring relationships in an aboveground–belowground context warrant further consideration. In particular, the identification of potential conflict between mother and offspring highlights another factor influencing aboveground–belowground relationships that could consequently influence terrestrial ecosystems.

Plants come under attack by a variety of organisms, including insects and pathogenic microorganisms such as viruses. Plant viruses can interact indirectly with their vectors by inducing changes to plant chemistry which may alter its attractiveness as a host for herbivore vectors. Using red raspberry as a study system, this study aimed to investigate the host plant mediated interactions occurring between the large raspberry aphid, Amphorophora idaei, and two of the viruses that it transmits, Black raspberry necrosis virus (BRNV) and Raspberry leaf mottle virus (RLMV). In whole plant bioassays, BRNV and RLMV-infected plants were shown to be initially more attractive to A. idaei and aphids remained on the initially selected host plant for a period of approximately 30 minutes. In addition, A. idaei took three days longer to reach reproductive maturity compared with those feeding on non-infected plants, suggesting a virally-induced manipulation of aphid behaviour whereby a deceptive attraction of the vector to a host plant found to be nutritionally poor, presumably acts to promote virus transmission. Investigations of the underlying plant chemistry revealed that raspberry viruses may be capable of facilitating aphid feeding by reducing leaf phenolic concentration when aphids are feeding and that infection with BRNV and RLMV resulted in significantly elevated levels of carbon and free amino acids in the leaves. While increased concentrations of amino acids might be expected to promote aphid performance, the amino acid composition was dominated by glutamate (77% of total content of infected plants), a previously suggested indicator of reduced host-plant suitability for aphids. Volatile entrainments from virus-infected plants showed elevated levels of the green leaf volatile (Z)-3-hexenyl acetate. Bioassays subsequently revealed that this compound acted as an aphid attractant at a concentration of 50 ng ml-1 but that aphid behaviour was unaffected by lower concentrations. The combined utilisation of PCR diagnostics developed from newly sequenced viral genomes and the implementation of a non-invasive, targeted method of sampling plant headspace volatiles enabled this study to provide novel insights into the nature of host plant mediated interactions between aphids and the viral pathogens that they transmit.

An apoplastic pathway, the so-called bypass flow, is important for Na+ uptake in rice under saline conditions. The primary aim of this thesis was to identify the point of entry for bypass flow into rice roots subjected to salinity. Investigations using lateral rootless mutants (lrt1, lrt2), a crown rootless mutant (crl1), their wild types (Oochikara, Nipponbare and Taichung 65, respectively) and seedlings of rice cv. IR36 showed that the entry point, quantified using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), was not at the sites of lateral root emergence. However, PTS was identified in the vascular tissue of lateral roots using both epifluorescence microscopy and confocal laser scanning microscopy. Cryo-scanning electron microscopy and epifluorescence microscopy of sections stained with berberine-aniline blue and Fluorol Yellow 088 revealed that an exodermis was absent in the lateral roots, suggesting that the lack of the exodermis allowed PTS to pass through the cortical layers, enter the stele and be transported to the shoot via the transpiration stream. These findings suggest a role for the lateral roots of rice in bypass flow. The addition of polyethylene glycol (PEG) and silicon (Si) to the culture solution significantly reduced Na+ uptake to the shoot by reducing bypass flow through the lateral roots. PEG was found to be more effective than Si. It was also shown that changing the relative humidity in the air around the shoots had a significant effect on the magnitude of bypass flow and the flux of water across the roots: the greater the flux of water through the roots, the greater the Na+ uptake and bypass flow. Furthermore, results showed that recombinant inbred lines of rice with low Na+ transport possessed low magnitudes of bypass flow, whereas lines with high Na+ transport had a high degree of bypass flow, indicating that bypass flow could be used as a criterion for screening salt resistance in rice varieties.