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When plants are damaged by herbivorous insects, blends of volatile organic compounds (VOCs) are induced and released and can also be used by parasitoids to locate hosts. The aim was to determine whether VOCs induced by water stress affect the plant-herbivore-para- sitoid system represented by broad bean (Vicia faba; Fabales: Fabaceae) stink bug (Nezara viridula; Hetero- ptera: Pentatomidae) egg parasitoid (Trissolcus basalis; Hymenoptera: Platygastridae). The effects of water stress (expressed as the percentage fraction of transpirable soil water [FTSW] supplied) alone and in combination with N. viridula damage (feeding plus oviposition) were deter- mined according to: (1) the behavioural response of the egg parasitoid in a Y-tube olfactometer and (2) the plant VOCs collected and analysed by thermal desorption-gas chromatography-mass spectrometry. With pot water capacity as FTSW100, water stress was applied as mild (FTSW80), moderate (FTSW50) and severe (FTSW10). Bioassays with plants under abiotic stress alone showed that egg parasitoids are more attracted by FTSW10 plants than by well-watered plants. When plants were under abiotic and biotic stress interactions, the egg parasitoids are more attracted by FTSW10 and FTSW50 plants than by well-watered plants infested with N. viridula. Considering VOCs emissions, projection to latent structures discrimi- nant analysis (PLS-DA) separated treatments according to egg parasitoid responses. Water stress alone and in com- bination with biotic stress induced changes in VOC emis- sions of V. faba plants that attract egg parasitoids. These findings contribute to our understanding of how water stress affects the interactions between plants, insect pests and egg parasitoids.

The anatomy, biochemistry and physiology of Phragmites leaves have been investigated. Biochemical and physiological measurements indicate that Phragmites australis leaves have a C 3 mechanism of carbon fixation. However, structural and ultra-structural observations of young leaves are more reminiscent of a C 4 -like anatomy. In addition, chloroplasts apparently fully competent for photochemical and biochemical reactions were in both the mesophyll and in the bundle sheath cells of young leaves. The activity of the principal enzyme involved in carbon metabolism, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), was high under ambient conditions (59.8 and 64.2 μmol m −2 s −1 , respectively in mature and young leaves). In contrast, phosphoenolpyruvate carboxylase activity was low in both mature and young leaves (7.8 and 8.1 μmol m −2 s −1 , respectively) Despite the high Rubisco activity, the rate of photosynthesis of Phragmites leaves on a leaf area basis was low. We investigated if resistances to CO 2 entry in the leaves could limit photosynthesis. However, stomatal and mesophyll resistances to CO 2 diffusion in Phragmites leaves were comparable to those of terrestrial plants and did not restrict intercellular CO 2 concentration significantly.

The direct and indirect impact of ozone on Populus alba was studied by exposing leaves enclosed in specially designed cuvettes for 30 days to high ozone (150 ppb, 11 h per day), while leaves developing above the cuvettes were exposed to ambient ozone. Gas exchanges and histo-anatomical parameters were measured to specifically understand whether ozone indirectly affects the anatomy and physiology of leaves. Three leaf classes were investigated: (1) those expanding above the cuvettes (A leaves); (2) those already developed inside the cuvettes (B leaves) and (3) those developing inside the cuvettes, since the beginning of the ozone treatment (C leaves). The anatomy and morphology of the first leaf developing outside the cuvette (A1) were strongly affected by ozone, whereas photosynthesis was not perturbed. However, in leaves of ozone-treated plants developing after A1, a large reduction of starch accumulation was observed, which suggests a delayed biosynthesis, or a very rapid export of starch toward other sinks. Isoprene emission was higher and isoprene synthase messenger RNA was more expressed in ozone-treated A1 leaves than in control leaves with similar ontogeny. This indicates that isoprene synthesis is stimulated by ozone, and reveals that isoprene emission is controlled at a transcriptional level. Leaves already developed inside the cuvette (B leaves) rapidly sensed ozone stress, which inhibited photosynthesis, stomatal conductance and isoprene emission. The observation that new leaves were developing inside the cuvettes during the treatment (C leaves) suggests that resistance to ozone may be acquired by plants. Leaves C showed a more packed and thinner mesophyll than controls of similar development, which may help reduce ozone penetration inside cells. They also showed a lower photosynthesis in comparison to controls and to other leaf classes, probably because of ribulose 1,5-bisphosphate carboxylase/oxygenase activity limitation, as inferred from photosynthesis response to intercellular CO2. However, isoprene emission was slightly stimulated also in C leaves, confirming that a large fraction of carbon is invested into isoprene formation under ozone stress.

Summary • Isoprene reduces visible damage (necrosis) of leaves caused by exposure to ozone but the mechanism is not known. Here we show that in Phragmites leaves isoprene emission was stimulated after a 3-h exposure to high ozone levels. • The photosynthetic apparatus of leaves in which isoprene emission was inhibited by fosmidomycin became more susceptible to damage by ozone than in isoprene-emitting leaves. Three days after ozone fumigation, the necrotic leaf area was significantly higher in isoprene-inhibited leaves than in isoprene-emitting leaves. • Isoprene-inhibited leaves also accumulated high amounts of nitric oxide (NO), as detected by epifluorescence light microscopy. • Our results confirm that oxidative stresses activate biosynthesis and emission of chloroplastic isoprenoid, bringing further evidence in support of an antioxidant role for these compounds. It is suggested that, in nature, the simultaneous quenching of NO and reactive oxygen species by isoprene may be a very effective mechanism to control dangerous compounds formed under abiotic stress conditions, while simultaneously attenuating the induction of the hypersensitive response leading to cellular damage and death.

Leaf discs of the ozone tolerant tobacco ( Nicotiana tabacum L.) cv. Bel B and of the ozone sensitive cv. Bel W3, were exposed to an acute ozone fumigation (300 ppb) for 3 h. We measured ozone uptake by leaves and physiological characteristics before, during and after the treatment, in order to determine if the different O 3 sensitivity was correlated to the leaf uptake. In the tolerant cv. Bel B, O 3 uptake was high during the first 2 h of ozone exposure and then decreased. In the sensitive cv. Bel W3, the rate of O 3 uptake decreased constantly during ozone fumigation. The estimated cumulative uptake over the treatment time was higher (200 ± 30 μmol m –2 ) in Bel B than in Bel W3 (130 ± 12 μmol m –2 ). Thus, the ozone sensitivity was not correlated with ozone uptake. Stomatal conductance and photosynthesis were significantly inhibited during the fumigation in both cultivars. However, these reductions were strong and irreversible in the cv. Bel W3, while in the cv. Bel B both parameters recovered in the post-fumigation period. Thus, ozone tolerance may be related to a sustained capacity of recovery. There was no linear correlation between ozone uptake and photosynthesis reduction, but a threshold of ozone uptake was found after which photosynthesis was substantially impaired. This threshold may or may not be reached under the same external ozone level, indicating that the AOT40 may not be a sufficiently accurate index for the detection of ozone damage in plants.

Two cultivars of tobacco with contrasting sensitivity to ozone have similar photosynthesis at low light intensity, but at high light, photosynthesis of the sensitive cultivar Bel-W3 was lower than that of the resistant cultivar Havana. In contrast, the electron transport rate at photosynthesis saturated light intensity was higher in Bel-W3 than in Havana. This excess may require efficient mechanisms to dissipate energy and oxygen radicals. We found that the non-radiative dissipation was lower in Bel-W3 than in Havana and that the conversion between violaxanthin and anther-axanthin + zeaxanthin was not complete in the ozone sensitive cultivar. Violaxanthin may have been partly converted in ABA. The high content of ABA may have caused stomatal closure and explain why photosynthesis at high light was reduced in Bel-W3 in comparison to Havana. β-Carotene content was also lower in Bel-W3 than in Havana. A low β-carotene content coupled with an inefficient use of the xanthophyll mechanism may contribute to determine the ozone sensitivity of Bel-W3 leaves