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Volatile organic compounds emitted by plants represent the largest part of biogenic volatile organic compounds (BVOCs) released into our atmosphere. Plant volatiles are formed through many biochemical pathways, constitutively and after stress induction. In recent years, our understanding of the functions of these molecules has made constant and rapid progress. From being considered in the past as a mere waste of carbon, BVOCs have now emerged as an essential element of an invisible language that is perceived and exploited by the plants' enemies, the enemies of plant enemies, and neighbouring plants. In addition, BVOCs have important functions in protecting plants from abiotic stresses. Recent advances in our understanding of the role of BVOC in direct and indirect defences are driving further attention to these emissions. This special issue gathers some of the latest and most original research that further expands our knowledge of BVOC. BVOC emissions and functions in (1) unexplored terrestrial (including the soil) and marine environments, (2) in changing climate conditions, and (3) under anthropic pressures, or (4) in complex trophic communities are comprehensively reviewed. Stepping up from scientific awareness, the presented information shows that the manipulation and exploitation of BVOC is a realistic and promising strategy for agricultural applications and biotechnological exploitations.

Summary In order to test whether xylem‐transported carbohydrates are a potential source for isoprene biosynthesis, [U‐ 13 C]‐labelled α‐ d ‐glucose was fed via cut ends of stems into the xylem of Quercus robur seedlings and the incorporation of 13 C into isoprene emitted was studied. Emission of 13 C‐labelled isoprene was monitored in real time by proton‐transfer‐reaction mass spectrometry (PTR‐MS). A rapid incorporation of 13 C from xylem‐fed glucose into single (mass 70) and double (mass 71) 13 C‐labelled isoprene molecules was observed after a lag phase of approx. 5–10 min. This incorporation was temperature dependent and was highest (up to 13% 13 C of total carbon emitted as isoprene) at the temperature optimum of isoprene emission (40–42°C), when net assimilation was strongly reduced. Fast dark‐to‐light transitions led to a strong single or double 13 C‐labelling of isoprene from xylem‐fed [U‐13C]glucose. During a period of 10–15 min up to 86% of all isoprene molecules became single or double 13 C‐labelled, resulting in a 13 C‐portion of up to 27% of total carbon emitted as isoprene. The results provide evidence that xylem‐transported glucose or its degradation products can potentially be used as additional precursors for isoprene biosynthesis and that this carbon source becomes more important under conditions of limited photosynthesis.

Monoterpene emissions, monoterpene synthase activities, photosynthesis, fluorescence yield in the dark and drought stress indicators (stomatal conductance and mid‐day water potential) were concurrently measured under similar temperature and illumination in current‐year leaves of Quercus ilex L. of plants grown in open‐top chambers at ambient (350 ppm) and elevated (700 ppm) CO2. The study was undertaken to understand the effect of CO2 on monoterpene biosynthesis, and to predict and parameterize the biogenic emissions at growing CO2 concentrations. The results of the 1998 and 1999 studies show that at elevated CO2, and in the absence of persistent environmental stresses, photosynthesis was stimulated with respect to ambient CO2, but that the emission of the three most abundantly emitted monoterpenes (α‐pinene, sabinene and β‐pinene) was inhibited by approximately 68%. The enzyme activities of the monoterpene synthases catalysing the formation of the three monoterpenes were also inhibited at elevated CO2 and an excellent relationship was found between monoterpene emission and activity of the corresponding enzyme both at ambient and elevated CO2. Interestingly, however, limonene emission was enhanced in conditions of elevated CO2 as it was also the corresponding synthase. The ratio between enzyme activity and emission of the three main monoterpenes was high (above 20) at ambient CO2 but it was below 10 at elevated CO2 and, for limonene, on both treatments. Our results indicate that the overall emission of monoterpenes at elevated CO2 will be inhibited because of a concurrent, strong down‐regulation of monoterpene synthase activities. When the enzyme activity does not change, as for limonene, the high photosynthetic carbon availability at elevated CO2 conditions may even stimulate emission. The results of the 1997 study show that severe and persistent drought, as commonly occurs in the Mediterranean, may inhibit both photosynthesis and monoterpene (α‐pinene) emission, particularly at ambient CO2. Thus, emission is probably limited by photosynthetic carbon availability; the effect of elevated CO2per se is not apparent if drought, and perhaps other environmental stresses, are also present.

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