Browsing by Subject "EPG"

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    Morphological and chemical plant properties mediate host plant selection of whiteflies (Hemiptera: Aleyrodidae)
    (2021) Stoll, Nina Sara; Zebitz, Claus P. W.
    Whiteflies are among the most important pests causing severe damage to numerous cultivated and ornamental plants worldwide. The present dissertation comprises four studies and contributes to the knowledge of the host plant selection process by whiteflies. In the first study, host preferences were determined in dual choice tests for Aleyrodes proletella (L.), Bemisia tabaci (Genn.), and Trialeurodes vaporariorum (Westw.) on several host plants. On the one hand, this study extends the knowledge on the food spectrum of these economically important pests; on the other hand, the results highlight the host adaptation of whiteflies. The second study elucidated potential sources of host plant resistance against A. proletella, B. tabaci, and T. vaporariorum by recording their probing and feeding behaviour on two host plants each using the electrical penetration graph (EPG) method. It is concluded that whiteflies decide upon host plant acceptance by evaluation of multiple plant factors located in epidermal and/or mesophyll tissues of leaves as well as in the phloem sap of plants. Moreover, epicuticular leaf waxes are a key factor in the host selection process of A. proletella. It is hypothesized that constituents of the leaf surface wax act as stimulants promoting leaf penetration and phloem accession. The findings of this study shed light on the whitefly-host adaptation. The goal of the third study was to identify the role of epicuticular leaf waxes of several Brassica cultivars in the host selection process of A. proletella. For this purpose, dual choice tests were carried out on both waxy and dewaxed plant leaves as well as on Parafilm® treated with different leaf wax extracts. Also, life-history traits were monitored on waxy and dewaxed leaves, and the feeding activity of A. proletella was recorded on Parafilm® with and without leaf wax extracts. Scanning electron microscopy (SEM) imaging was used to visualize epicuticular leaf waxes on the plant surface. Finally, it was proved that leaf surface waxes of host plants promote feeding and act as phagostimulants. Although the wax compounds mediating host plant selection remain unknown, these findings offer breeding potential for resistant crop cultivars. In the fourth study, the influence of free phloem amino acids on the host plant selection of T. vaporariorum was investigated. Via liquid chromatography-mass spectrometry (LC-MS), the amino acid profiles in the phloem sap of six vegetable crops varying in their host plant attractiveness were analysed. Subsequently, stepwise multiple regressions of the relative amino acid compositions and the pre-determined host plant preferences were performed. To verify the contribution of single amino acids on host choice, dual choice tests on sucrose media with and without added single amino acids were carried out. Single amino acids play an active role in phagostimulation, whereas some amino acids exert strong inhibitory effects. This indicates that the dominant presence of such amino acids might reduce phloem sap uptake, thus contributing to host plant resistance towards T. vaporariorum. Overall, this research compared the host selection process of three whitefly species to identify their underlying mechanisms. It is hypothesized that the observed host selection strategies are the result of evolutionary adaptations between whiteflies and their host plants. Depending on the occupied ecological niche, species-specific host plant ranges of varying complexity were formed. Accordingly, the host selection process of the more specialised species A. proletella is particularly efficient by consideration of characteristic leaf surface wax stimuli. In contrast, host selection of the extreme generalists B. tabaci and T. vaporariorum is regulated by simple gustatory stimuli in order to take advantage of the host diversity they are offered. The findings of this research provide the basis for new approaches to optimizing breeding programs for whitefly resistant crops.
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    Resistenz von Hopfen, Humulus lupulus L., gegen die Hopfenblattlaus, Phorodon humuli (Schrank)
    (2010) Kryvynets, Oleg; Zebitz, Claus P. W.
    Non-chemical methods controlling the damson-hop aphid, Phorodon humuli (Schrank) have recently gained importance, due to its great economical relevance and increasing problems with its chemical control. Breeding for the classic 'Genetically controlled resistance', where the plant exerts negative influence on the pest's behaviour during feeding, in this case appears as the only promising method because so far no other efficient biological control methods are available. In order to analyse the host-parasite relationship between hop and damson-hop aphid, and to provide a reliable, standardised screening method based on line-specific host selection behaviour, 1. the composition of the aphid's food, 2. hardness of hop tissue, 3. host selection behaviour and 4. parameters of aphid growth and development were investigated. In the apoplast (Intercellular washing fluid) of leaves and cones from all hop lines resveratrol was found, which also indicates an accumulation of its glycosidic form in the plant tissue. In contrast, no resveratrol could be found in samples of non-transgenic hop. An analysis for bitter acids detected alpha- and beta-acids in the cones only, not however in the intercellular space of the leaves. In a comparative study of plant tissues the contents of lupulones in leaf extracts from transgenic plants (except Tk160 and Tk424) was lower, the contents of humulones higher compared with those from non-transgenic plants. Generally leaf extracts showed higher contents of alpha- than of beta-acids. Resveratrol was not found in any of the leaf- or cone-extracts. Tissue hardness differed between transgenic and non-transgenic plants as well as between those from the glasshouse and from open land. In the field-grown plants it was definitely higher. Leaves of transgenic lines were harder than those of non-transgenic ones. This difference was pronounced particularly in young leaves. Significant differences were observed among parameters of aphid growth and development. The individual transgenic lines exert significantly different influence on P. humuli in respect of relative growth rate of individuals, rate of population increase and fecundity. There was no significant difference between the variants regarding the parameters rDS-value, life span and number of embryos. Host selection behaviour was different on transgenic versus non-transgenic plants. On leaves of transgenic plants a slowing down of food intake from the phloem and an extension of water intake from the xylem were observed. On the cones no similar effect ? general reduction or extension of intake from phloem or xylem in comparison to non-transgenic plants ? could be ascertained. Generally a strong decrease in food intake from the phloem of cones compared to that of the leaves was observed, caused by frequent phases of aphid inactivity on cones. Aphid behaviour on artificial diets was definitely influenced by different concentrations of bitter acids and resveratrol. A gradual increase in bitter acid concentration first prolonged or raised some EPG-parameters and then shortened or reduced them subsequently. This means that the aphids first reacted negatively, from a certain concentration of bitter acids on, however, positively to these substances. A raise in resveratrol concentrations led in the whole course either to a notable increase or decrease of values for the individual EPG-parameters. This suggests a xenobiotic resistance effected by this substance. Correlations between the experimental data showed the following dependencies: The humulones and lupulones found in intercellular washing fluid and in tissue extracts influence EPG-parameters that indicate factors located in the mesophyll. The duration of those phases of the penetration that take place in the intercellular space is reduced with increasing concentrations of humulones and lupulones. Food quality and quantity in plants from the glasshouse had no negative effects on the damson-hop aphid. On field-grown plants different effects of alpha- and beta-acids in the extracts on the aphids were ascertained. The contents of lupulones in cone- and leaf-extracts exerted a more inhibiting, the humulones a more promotional effect on the aphids during feeding. As with the overall contents of humulone and lupulone a different influence of individual bitter acids on aphid behaviour was recognisable in EPG-parameters. On cones the strong behaviour modification by the examined substances took effect on the mesophyll level and in the phloem. Positive correlations between the adult weight of the aphids as well as the amount of offspring per life span per insect and the bitter acid contents indicate an influence in favour of the insect. Negative correlations between the rm -value and the bitter acid contents, however, show an influence on the aphids in favour of the plant. Higher bitter acid concentrations reduce the development rate of aphid populations. Notable differences between transgenic and non-transgenic plants as well as varying effects of examined substances on the aphids, partly favourable for the plant, partly favourable for the pest, were found. These differences were partially clarified by comparing the effects of the substances when administered with artificial diets. The exact mode of action of resveratrol and its derivatives in transgenic hop lines on the damson-hop aphid requires further examination. A direct toxic effect on the aphids is very probable. As a conclusion from the clustering by examined parameters the transgenic hop line Tk424 can be designated as optimal.