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According to the nutrient-translocation hypothesis, yellowing tree leaves are colonized by aphids at the end of the growing season owing to improved availability of nutrients in the phloem sap after chlorophyll degradation. We measured aphid densities on potted Betula pendula seedlings in a field site where a small proportion of foliage rapidly turned yellow before normal autumn coloration as a consequence of root anoxia. The number of adults and nymphs of the birch-feeding specialist aphids Euceraphis betulae, Betulaphis brevipilosa and Callipterinella tuberculata were counted from leaves on each of the 222 plants. Aphids were detected on 19 per cent of green leaves and on 41 per cent of yellow leaves. There was no indication of aphid avoidance of yellow leaves, and the number of winged (alate) viviparous E. betulae adults and their nymphs were significantly higher on yellow leaves than on green leaves, while the numbers of apterous B. brevipilosa and C. tuberculata did not differ between the leaf colour types. Our result suggests that only aphid species with alate generation during colour change can take advantage of yellowing leaves. This may explain the exceptional abundance of E. betulae compared with other aphid species on birches.

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Plant defence can be induced by exposing plants to the plant hormone jasmonic acid (JA) or its volatile ester, methyl jasmonate (MeJA). Carrageenans (Carr) - sulphated D-galactans extracted from red algae - can also induce plant defences. In this study, the effects of exogenous MeJA and Carr application (concentration 300 and 12.7 μmol, respectively) on volatile emissions from two widespread evergreen woody species, Pinus sylvestris (nine Turkish and one Finnish provenance) and Quercus ilex (Italian provenance) were investigated. We collected headspace samples from seedlings and analysed the quality and quantity of volatile compounds emitted by treated and control plants. In total, 19 monoterpenes, 10 sesquiterpenes, 10 green leaf volatiles (GLVs) and two aromatic compounds were emitted by P. sylvestris from all the provenances studied. Foliar MeJA application clearly affected the volatile profiles of trees from all the provenances. Effects of Carr were genotype specific. In Q. ilex, emissions of sesquiterpenes, GLVs and the homoterpene (E)-DMNT were all induced by MeJA application. However, emissions of most constitutively emitted monoterpenes were significantly reduced. Carr application also led to a significant reduction in monoterpene emissions, but without corresponding increases in other emissions. Our results indicate that exogenously applied MeJA and Carr can both significantly modify the volatile profiles of P. sylvestris and Q. ilex, but also that there are important provenance- and species-specific differences in the overall degree of elicitation and compositions of elicited compounds.

This study reports a comparative screening of four species of Origanum in Turkey, based on their essential oil composition, total phenolic content, antioxidant and antibiofilm activities. The major components of essential oils were p-cymene, linalool, and thymol. The total phenolic contents differed from 3.81 to 47.54 mg of GAE/g of extract. The concentrations of flavonoids varied from 12.74 to 58.39 mg of Ru/g of extract. Antioxidant activity was determined in vitro using DPPH reagent and expressed as concentration of each extract required to inhibit radical by 50% (IC 50 ) values that ranged from 16.03 to 48.94 μg/ml. Our results indicated that chloroform extracts of species O. majorana and O. onites, with a total content of polyphenols (47.54 mg of GAE/g and 45.17 mg of GAE/g, respectively) and an IC 50 of 16.03 μg/ml and 16.89 μg/ml, respectively were more antioxidant. Among the essential oil concentrations tested, maximum antibiofilm activity was found as 92.24% against M. luteus NRRL-B 1013 by O. majorana essential oil at 50 mg/ml. ARTICLE HISTORY