Emission of Volatile Organic Compounds After Herbivory from Trifolium pratense (L.) Under Laboratory and Field Conditions

Plants emit a wide range of volatile organic compounds in response to damage by herbivores, and many of the compounds have been shown to attract the natural enemies of insect herbivores or serve for inter- and intra-plant communication. Most studies have focused on volatile emission in the laboratory while little is known about emission patterns in the field. We studied the emission of volatiles by Trifolium pratense (red clover) under both laboratory and field conditions. The emission of 24 compounds was quantified in the laboratory, of which eight showed increased emission rates after herbivory by Spodoptera littoralis caterpillars, including (E)-β-ocimene, the most abundant compound, (Z)-β-ocimene, linalool, (E)-β-caryophyllene, (E,E)-α-farnesene, 4,8-dimethyl-1,3,7-nonatriene (DMNT), 1-octen-3-ol, and methyl salicylate (MeSA). While most of these compounds have been reported as herbivore-induced volatiles from a wide range of plant taxa, 1-octen-3-ol seems to be a characteristic volatile of legumes. In the field, T. pratense plants with varying herbivore damage growing in established grassland communities emitted only 13 detectable compounds, and the correlation between herbivore damage and volatile release was more variable than in the laboratory. For example, the emission of (E)-β-ocimene, (Z)-β-ocimene, and DMNT actually declined with damage, while decanal exhibited increased emission with increasing herbivory. Elevated light and temperature increased the emission of many compounds, but the differences in light and temperature conditions between the laboratory and the field could not account for the differences in emission profiles. Our results indicate that the release of volatiles from T. pratense plants in the field is likely to be influenced by additional biotic and abiotic factors not measured in this study. The elucidation of these factors may be important in understanding the physiological and ecological functions of volatiles in plants

Tri-trophic consequences of UV-B exposure: plants, herbivores and parasitoids.

In this paper we demonstrate a UV-B-mediated link between host plants, herbivores and their parasitoids, using a model system consisting of a host plant Brassica oleracea, a herbivore Plutella xylostella and its parasitoid Cotesia plutellae. Ultraviolet-B radiation (UV-B) is a potent elicitor of a variety of changes in the chemistry, morphology and physiology of plants and animals. Recent studies have demonstrated that common signals, such as jasmonic acid (JA), play important roles in the mechanisms by which plants respond to UV-B and to damage by herbivores. Plant responses elicited by UV-B radiation can affect the choices of ovipositing female insects and the fitness of their offspring. This leads to the prediction that, in plants, the changes induced as a consequence of UV damage will be similar to those elicited in response to insect damage, including knock-on effects upon the next trophic level, predators. In our trials female P. xylostella oviposited preferentially on host plants grown in depleted UV-B conditions, while their larvae preferred to feed on tissues from UV-depleted regimes over those from UV-supplemented ones. Larval feeding patterns on UV-supplemented tissues met the predictions of models which propose that induced defences in plants should disperse herbivory; feeding scars were significantly smaller and more numerous – though not significantly so – than those on host plant leaves grown in UV-depleted conditions. Most importantly, female parasitoids also showed a clear pattern of preference when given the choice between host plants and attendant larvae from the different UV regimes; however, in the case of the female parasitoids, the choice was in favour of potential hosts foraging on UV-supplemented tissues. This study demonstrates the potential for UV-B to elicit a variety of interactions between trophic levels, most likely mediated through effects upon host plant chemistry