Browsing by Subject "Epicuticular wax"

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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.
Untersuchungen zur Mikromorphologie und chemischen Zusammensetzung der Cuticularwachse bei den Gattungen Calicorema, Arthraerua und Leucosphaera (Amaranthaceae) in Namibia
(2008) Dinter, Ina; Haas, Klaus
Epidermal characters and especially cuticular waxes were examined in the genera Calicorema, Arthraerua and Leucosphaera (Amaranthaceae). They are chlorophyllous-stemmed shrubs of the Retama type. These drought resistant xerophytes maintain normal metabolic activity in more or less intact shoot systems even at longer lasting periods of severe dryness. Adaptations for drying delay are of crucial importance in Namibia, especially in the Namib desert which represents the dryest part of Southern Africa. Therefore, the investigations focused on the possibilities for improved water uptake, efficient water conduction, water storage, and the restriction of transpiration in the four species of Amaranthaceae. The most important adaptations for survival under drought stress serve to minimize transpiration. These are xeromorphic characters referring the interface between plants and their environment. The combined occurrence of epicuticular and intracuticular waxes, cuticular layers, and trichomes is a remarkable feature of the species examined. These properties act synergistically to optimize the surface functions. Cuticular transpiration is considerable reduced by the wax incrustation of cuticular layers. The dense arrangement of wax crystals in the epistomatal spaces minimizes stomatal transpiration without affecting gas exchange. The hydrophobic chemistry of the epicuticular waxes, their micro-roughness, and resulting unwettability cause the self-cleaning properties (so-called lotus-effect) of the surfaces to protect e.g. against pathogens. Reflectance of irradiation is a further effect of epicuticular wax crystals which prevents the plants from overheating. The mechanical protection mediated by cuticular layers (so-called thick cuticles), epicuticular wax crystals, and trichomes enables resistance to abrasion by wind-borne sand particles which turn out to increased transpiration, overheating, and infections. For the first time, epicuticular wax crystals are shown to occur in the Amaranthaceae. Intracuticular wax is also present. These wax parts are extracted separately from different shoot regions. The wax extracts contain the aliphatic substance classes of alkanes, esters, aldehydes, primary alcohols and free fatty acids. Partial solvent resistance of epicuticular wax crystals in Calicorema capitata and Arthraerua leubnitziae is indicative of polymeric aldehydes. They are insoluble at room temperature but readily dissolve in hot chloroform by cleavage of polymer bonds. A similar solubility was found in primary alcohols with the characteristic increase of amounts in hot solvent. The solvent resistance of epicuticular waxes could be explained by a co-crystallization of primary alcohols and polymeric aldehydes. The mechanical stability mediated by the chemical properties could be of considerable ecological importance in arid habitats: Increased protection against surface abrasion would reduce cuticular transpiration.