SummaryBiogenic volatile organic compounds produced by plants are involved in plant growth, development, reproduction and defence. They also function as communication media within plant communities, between plants and between plants and insects. Because of the high chemical reactivity of many of these compounds, coupled with their large mass emission rates from vegetation into the atmosphere, they have significant effects on the chemical composition and physical characteristics of the atmosphere. Hence, biogenic volatile organic compounds mediate the relationship between the biosphere and the atmosphere. Alteration of this relationship by anthropogenically driven changes to the environment, including global climate change, may perturb these interactions and may lead to adverse and hard-to-predict consequences for the Earth system. Abbreviations: ACC, 1-aminocyclopropane-1-carboxylic acid; BVOC, biogenic volatile organic compound; FACE, free-air CO 2 enrichment; IPCC, Intergovernmental Panel on Climate Change; JA, jasmonic acid; MeJA, methyl jasmonate; MPAN, peroxymethacrylic nitric anhydride; MeSA, methyl salicylate; PAN, peroxyacetylnitrate; SA, salicylic acid; SOA, secondary organic aerosol. I. IntroductionThe Earth is a single and partially self-regulating system that consists of interlinked physical, chemical and biological components. The terrestrial biosphere is one subsystem of this and, by acting as a source of biogenic volatile organic compounds (BVOCs) to the atmosphere, provides a strong link between the Earth's surface, atmosphere and climate. Most of these BVOCs are synthesized by one of three major biochemical routes: the isoprenoid, the lipoxygenase or the shikimic acid pathways (Feussner & Wasternack, 2002; Dudareva et al., 2006;Matsui, 2006;Xiang et al., 2007; New Phytologist (2009) Qualley & Dudareva, 2008). A number of low-molecularweight (C < 5) BVOCs are also emitted by plants, for example methanol, ethylene, formaldehyde, ethanol, acetone and acetaldehyde (Kreuzwieser et al., 1999;Fall, 2003; Argueso et al., 2007). These pathways have been relatively well studied and the routes of formation are now well understood (Fig. 1). However, the biochemical regulation and function of most of these compounds are not clearly known BVOCs are released from above-and below-ground plant organs. In general, flowers and fruits release the widest variety of BVOCs, with emission rates peaking on maturation (Dixon & Hewett, 2000;Soares et al., 2007), but leaves have the greatest mass emission rates. The vegetative parts of woody plants are more likely to release diverse mixtures of terpenoids, including isoprene, monoterpenes, sesquiterpenes and some diterpenes (Owen et al., 2001;Keeling & Bohlmann, 2006), whereas grass species emit relatively large amounts of oxygenated BVOCs and some monoterpenes (Kirstine et al., 1998;Fukui & Doskey, 2000). When plants are damaged, the emissions of these compounds may be increased and other, so-called, green leaf volatiles (C 6 aldehydes and ketones) may also be prod...

Contents  Summary 677 Introduction 678 Light as an environmental variable 678 Long‐term effects of light on plant–herbivore or plant–pathogen interactions 679 Mechanisms of responses to the light environment: the whole‐plant perspective 684 Short‐term responses to the light environment: induced defences 686 Mechanisms for light‐dependent induced defences 688 Interpreting interactions between light and defence responses 690  Acknowledgements 693  References 693 Summary Plants frequently suffer attack from herbivores and microbial pathogens, and have evolved a complex array of defence mechanisms to resist defoliation and disease. These include both preformed defences, ranging from structural features to stores of toxic secondary metabolites, and inducible defences, which are activated only after an attack is detected. It is well known that plant defences against pests and pathogens are commonly affected by environmental conditions, but the mechanisms by which responses to the biotic and abiotic environments interact are only poorly understood. In this review, we consider the impact of light on plant defence, in terms of both plant life histories and rapid scale molecular responses to biotic attack. We bring together evidence that illustrates that light not only modulates defence responses via its influence on biochemistry and plant development but, in some cases, is essential for the development of resistance. We suggest that the interaction between the light environment and plant defence is multifaceted, and extends across different temporal and biological scales.

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Summary Priming of defence is a strategy employed by plants exposed to stress to enhance resistance against future stress episodes with minimal associated costs on growth. Here, we test the hypothesis that application of priming agents to seeds can result in plants with primed defences. We measured resistance to arthropod herbivores and disease in tomato (Solanum lycopersicum) plants grown from seed treated with jasmonic acid (JA) and/or β‐aminobutryric acid (BABA). Plants grown from JA‐treated seed showed increased resistance against herbivory by spider mites, caterpillars and aphids, and against the necrotrophic fungal pathogen, Botrytis cinerea. BABA seed treatment provided primed defence against powdery mildew disease caused by the biotrophic fungal pathogen, Oidium neolycopersici. Priming responses were long‐lasting, with significant increases in resistance sustained in plants grown from treated seed for at least 8 wk, and were associated with enhanced defence gene expression during pathogen attack. There was no significant antagonism between different forms of defence in plants grown from seeds treated with a combination of JA and BABA. Long‐term defence priming by seed treatments was not accompanied by reductions in growth, and may therefore be suitable for commercial exploitation.