The heat wave of summer 2003 was the largest and the most persistent ever experienced in Central Europe and has fuelled concern about the effects of climate change on European ecosystems. Since forests constitute the most important European ecosystems, in this review article we assess current knowledge on the effects of heat and drought on key metabolic processes for growth and productivity of forest trees. In particular, the general consequences of heat and drought on (1) photosynthesis and respiration at the cellular and community level, and (2) on nutrient uptake, partitioning and competition for nutrients are summarized. The latter are a major sink for photosynthetic energy and, therefore, are indirectly but strongly connected to the performance of photosynthesis. In addition, the interaction of heat and drought with stress compensation mechanisms and emission of biogenic volatile organic compounds (BVOC) are discussed, since these processes are directly connected to carbon metabolism. Effects on the emission of BVOC are also included because they constitute an important feedback mechanism on ozone formation and, thus, on atmospheric pollution. As far as available, data collected during the 2003 heat wave are included and discussed.

There is an urgent need for new sustainable solutions to support agriculture in facing current environmental challenges. In particular, intensification of productivity and food security needs require sustainable exploitation of natural resources and metabolites. Here, we bring the attention to the agronomic potential of volatile organic compounds (VOCs) emitted from leaves, as a natural and eco-friendly solution to defend plants from stresses and to enhance crop production. To date, application of VOCs is often limited to fight herbivores. Here we argue that potential applications of VOCs are much wider, as they can also protect from pathogens and environmental stresses. VOCs prime plant’s defense mechanisms for an enhanced resistance/tolerance to the upcoming stress, quench reactive oxygen species (ROS), have potent antimicrobial as well as allelopathic effects, and might be important in regulating plant growth, development, and senescence through interactions with plant hormones. Current limits and drawbacks that may hamper the use of VOCs in open field are analyzed, and solutions for a better exploitation of VOCs in future sustainable agriculture are envisioned.

• The mechanism uncoupling isoprene emission and photosynthesis under drought was investigated in Populus alba saplings. • Isoprene emission, incorporation of ¹³C into the isoprene molecule, isoprene synthase (ISPS) activity, concentration and gene expression, and photosynthesis were measured as a function of the fraction of transpirable soil water (FTSW) and in plants recovering from drought. • Photosynthesis sharply declined below FTSW₃₀ (a FTSW of 30%) and its inhibition was not caused by metabolic factors. A decline in isoprene emission was only evident towards the FTSW endpoint. ¹³C incorporation into isoprene was lower when photosynthesis was constrained by drought. ISPS activity was inhibited by mild drought, while ISPS gene expression and concentration declined in concert with isoprene emission at the FTSW endpoint. Following rewatering, isoprene emission was higher than, and photosynthesis was similar to, prestress values. ISPS activity and concentration, and ¹³C incorporation into isoprene, also rapidly recovered to prestress values, while ISPS gene expression remained low in rewatered plants. • Our experiment revealed a larger contribution of alternative carbon sources to isoprene emission only when photosynthesis was dramatically constrained by drought. Isoprene emission was likely controlled at the posttranscriptional level under severe drought.

Proton transfer reaction-time of flight (PTR-TOF) mass spectrometry was used to improve detection of biogenic volatiles organic compounds (BVOCs) induced by leaf wounding and darkening. PTR-TOF measurements unambiguously captured the kinetic of the large emissions of green leaf volatiles (GLVs) and acetaldehyde after wounding and darkening. GLVs emission correlated with the extent of wounding, thus confirming to be an excellent indicator of mechanical damage. Transient emissions of methanol, C5 compounds and isoprene from plant species that do not emit isoprene constitutively were also detected after wounding. In the strong isoprene-emitter Populus alba, light-dependent isoprene emission was sustained and even enhanced for hours after photosynthesis inhibition due to leaf cutting. Thus isoprene emission can uncouple from photosynthesis and may occur even after cutting leaves or branches, e.g., by agricultural practices or because of abiotic and biotic stresses. This observation may have important implications for assessments of isoprene sources and budget in the atmosphere, and consequences for tropospheric chemistry.

Summary: Black poplar (Populus nigra L.) plants grown at 25 and 35°C were subjected to drought stress to assess the combined impact of two consequences of global climate change – rising temperature and drought – on isoprene biosynthesis and emission. At both temperatures, photosynthesis was inhibited by moderate drought, but isoprene emission only decreased when drought was prolonged. The mRNA transcript level, protein concentration and activity of isoprene synthase (ISPS) changed in concert with isoprene emission during drought stress. However, ISPS activity decreased before isoprene emission during drought development, indicating a tighter control of the emission at a transcriptional or post‐transcriptional level under moderate drought stress, and at both temperatures. A residual isoprene emission was measured when photosynthesis was totally inhibited after 35 days of drought. This photosynthesis‐independent emission of isoprene was probably dependent on a cytosolic carbon supply as all the properties of ISPS were drastically inhibited. Isoprene emission was associated with dark respiration during the entire drought stress period, and at both temperatures, indicating that the two processes are sustained by, but do not compete for, the same carbon source. Isoprene emission was directly related to phosphoenolpyruvate carboxylase activity in plants grown at 25°C and inversely related in plants grown at 35°C, suggesting a strong temperature control on the regulation of the pyruvate flowing from the cytosol to the plastidic isoprenoid biosynthetic pathway under drought stress and recovery. In re‐watered plants, the temperature control on isoprene emission was suppressed, despite complete recovery of photosynthesis and ISPS activity similar to levels in plants subjected to mild drought stress. Our results reveal the overriding effects of drought on isoprene emission, possibly affecting protein level or substrate supply. These effects may largely offset the predicted impact of rising temperatures on the emission of isoprene in terrestrial ecosystems.