Variation in the holm oak leaf proteome at different plant developmental stages, between provenances and in response to drought stress

Major proteins of the holm oak leaf proteome have been previously identified using a combination of 2-DE, MS analysis and BLAST similarity search (Jorge et al., Proteomics 2005, 5, 222-234). That study, conducted with field samples from mature trees, revealed the existence of a great variability in the 2-DE protein map, with qualitative as well as quantitative changes, both analytical and biological. A similar study has been carried out with 2-year-old seedlings to analyze and study: (i) changes in the 2-DE protein profile at different tree developmental stages; (ii) the 2-DE protein map variability between three different Spanish provenances; and (iii) variations in the 2-DE protein profile in response to drought stress. Although the protein profile of leaves from seedlings and mature trees was fairly similar, the biological variance found was lower in the former. In the present study, new proteins have been identified. At least four different protein spots differentiated Spanish provenances, two of them identified as an ATP synthase alpha chain, and a 2,3-bisphosphoglycerate-independent phosphoglycerate mutase. Fourteen different protein spots were qualitatively variable between well-watered and drought-stressed seedlings, with some of them corresponding to enzymes of carbohydrate and protein metabolism. Data presented indicated the mobilization of storage proteins and carbohydrates, as well as photosynthesis inhibition under drought conditions.     

Overaccumulation of glycine betaine enhances tolerance to drought and heat stress in wheat leaves in the protection of photosynthesis

We investigated the different responses of wheat (Triticum aestivum L.) plants to drought- (DS) and heat stress (HS), and analyzed the physiological mechanisms of glycine betaine (GB) involved in the improvement of wheat tolerance to the combination of these stresses. The transgenic wheat T6 line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into the wild-type (WT) Shi4185 line. The gene was cloned from the Garden Orache plant (Atriplex hortensis L.). Wheat seedlings were subjected to drought stress (30%, PEG-6000), heat stress (40°C), and their combination. Photosynthetic gas exchange, water status and lipid peroxidation of wheat leaves were examined under different stresses. When subjected to a combination of drought and heat, the inhibition of photosynthesis was significantly increased compared to that under DS or HS alone. The increased inhibition of photosynthesis by the combined stresses was not simply the additive stress effect of separate heat- and drought treatments; different responses in plant physiology to DS and HS were also found. HS decreased the chlorophyll (Chl) content, net photosynthetic rate (P _N), carboxylation efficiency (CE) and apparent quantum yield (AQY) more than DS but DS decreased the transpiration rate (E), stomata conductance (g _s) and intercellular CO₂ concentration (C _i) more than HS. GB over-accumulation led to increased photosynthesis not only under individual DS or HS but also under their combination. The enhancement of antioxidant activity and the improvement of water status may be the mechanisms underlying the improvement of photosynthesis by GB in wheat plants.     

Overaccumulation of glycine betaine enhances tolerance of the photosynthetic apparatus to drought and heat stress in wheat

To investigate the role of glycine betaine in photosynthesis under stress, a transgenic wheat (Triticum aestivum L.) line T6 overaccumulating glycine betaine and its wild type Shi4185 were used. Seedlings were exposed to conditions of drought (30%, PEG-6000), heat (40°C) and their combination. The results revealed ultrastructural damage to the chloroplast and thylakoid lamellae with the withered phenotype by both drought and heat stress, and the damage was exacerbated by the combination of drought and heat. The appearance of a K step in the typical O-J-I-P curve and the decrease of Hill activity indicated a reduction of oxygen evolving complex function caused by stress. The greater damage was found in wild type than T6. Overaccumulation of glycine betaine in T6 could protect lipids in the thylakoid membrane from damage and stabilize the index of unsaturated fatty acids under stress. A lower ratio of monogalactosyl diacylglycerol/digalactosyl diacylglycerol and higher phosphatidylglycerol content in the thylakoid membrane of T6 were also observed under stress. These effects can promote stability of the thylakoid membrane. Otherwise, glycine betaine overaccumulation decreased photoinhibition of PSII under stress. The results also suggest that xanthophyll cycle-dependent non-radiative energy dissipation may be involved in the GB-mediated effects on PSII function under stress conditions.     

Hexaconazole induces antioxidant protection and apigenin-7-glucoside accumulation in Matricaria chamomilla plants subjected to drought stress

In this experiment, the possibility of enhancing the water deficit stress tolerance of chamomile (Matricaria chamomilla L.) during two growth stages by the exogenous application of hexaconazole (HEX) was investigated. To improve water deficit tolerance, HEX was applied in three concentrations during two different stages (50 and 80 days after sowing). After HEX applications, the plants were subjected to water deficit stress. Although all HEX concentrations improved the water deficit stress tolerance in chamomile plants, the application of 15 mg L⁻¹ provided better protection when compared to the other concentration. The exogenous application of HEX provided significant protection against water deficit stress compared to non-HEX-treated plants, significantly affecting the morphological characteristics and aspects of productivity, the relative water, protein and proline contents; non-enzymatic and enzymatic antioxidants; and the flower's apigenin-7-glucoside content. These results suggest that the HEX-induced tolerance to water deficit stress in chamomile was related to the changes in growth variables, antioxidants and the apigenin-7-glucoside content.     

Overexpression of Arabidopsis ABF3 gene confers enhanced tolerance to drought and heat stress in creeping bentgrass

The Arabidopsis, abscisic acid responsive element-binding factor 3, ABF3 is known to play an important role in stress responses via regulating the expression of stress-responsive genes. In this study, we introduced pCAMBIA3301 vector harboring the ABF3 gene into creeping bentgrass (Agrostis stolonifera) through Agrobacterium-mediated transformation in order to develop a stress-tolerant variety of turfgrass. After transformation, putative transgenic plants were selected using the herbicide resistance assay. Genomic integration of the transgene was confirmed by genomic PCR and Southern blot analysis, and gene expression was validated by northern blot analysis. Under drought-stressed condition, the transgenic plants overexpressing ABF3 displayed significantly enhanced drought tolerance with higher water content and slower water loss rate than the control plants. Furthermore, the stomata of the ABF3 transgenic plants closed more than those of wild-type creeping bentgrass plants, under both non-stressed and ABA treatment conditions. In addition, the transgenic plants showed enhanced tolerance to heat stress. These results suggest that the overexpression of the ABF3 gene in creeping bentgrass might enhance survival in water-limiting and high temperature environments through increased stomatal closure and reduced water losses.     

Heat shock protein 70 regulates the abscisic acid-induced antioxidant response of maize to combined drought and heat stress

We investigated the interaction between heat shock protein 70 (HSP70) and abscisic acid (ABA)-induced antioxidant response of maize to the combination of drought and heat stress. First, the increased activities of enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT), induced by drought were less than those by heat or combined drought and heat stress, except some individual cases (e.g. CAT in leaves, GR in roots). Second, both HSP70 synthesis and H₂O₂ production increased prominently under drought, heat or their combination stress; the increase in leaves induced by drought and heat combination was the highest, followed by heat and by drought, while the increase in roots had not visible difference. Third, either in leaves or roots, pretreatment with ABA inhibitor, HSP70 inhibitor and H₂O₂ scavenger, significantly arrested the stress-induced increase of antioxidant enzyme activities, and ABA inhibitor and H₂O₂ scavenger obviously suppressed HSP70 synthesis, while HSP70 inhibitor slightly heightened H₂O₂ accumulation. Finally, 100 μM ABA significantly enhanced the activities of antioxidant enzymes, HSP70 expression and H₂O₂ production under stresses in comparison with ABA-deficient mutant vp5 maize plants without pretreatment. Thus, ABA-induced H₂O₂ production enhances the HSP70 synthesis and up-regulates the activities of antioxidant enzymes, resulting in the suppression of cellular reactive oxygen species (ROS) levels. Our results suggest that HSP70 may play a crucial role in ABA-induced antioxidant defense of maize to drought and heat combination.     

Changes in response to drought stress of triticale and maize genotypes differing in drought tolerance

Direct effects and after-effects of soil drought for 7 and 14 d were examined on seedling dry matter, leaf water potential (ψ), leaf injury index (LI), and chlorophyll (Chl) content of drought (D) resistant and sensitive triticale and maize genotypes. D caused higher decrease in number of developed leaves and dry matter of shoots and roots in the sensitive genotypes than in the resistant ones. Soil D caused lower decrease of ψ in the triticale than maize leaves. Influence of D on the Chl b content was considerably lower than on the Chl a content. In triticale the most harmful D impact was observed for physiologically younger leaves, in maize for the older ones. A period of 7-d-long recovery was too short for a complete removal of an adverse influence of D.     

Expression of ethylene response factor JERF1 in rice improves tolerance to drought

Ethylene response factor (ERF) proteins regulate a variety of stress responses in plant. JERF1, a tomato ERF protein, can be induced by abscisic acid (ABA). Overexpression of JERF1 enhanced the tolerance of transgenic tobacco to high salt concentration, osmotic stress, and low temperature by regulating the expression of stress-responsive genes by binding to DRE/CRT and GCC-box cis-elements. In this research, we further report that overexpression of JERF1 significantly enhanced drought tolerance of transgenic rice. The overexpression activated the expression of stress-responsive genes and increased the synthesis of the osmolyte proline by regulating the expression of OsP5CS, encoding the proline biosynthesis key enzyme deltal-pyrroline-5-carboxylate synthetase. JERF1 also activated the expression of two ABA biosynthesis key enzyme genes, OsABA2 and Os03g0810800, and increased the synthesis of ABA in rice. Analysis of cis-elements of JERF1-targeted genes pointed to the existence of DRE/CRT and/or GCC box in their promoters, indicating that JERF1 could activate the expression of related genes in rice by binding to these cis-elements. Unlike some other ERF proteins, constructive overexpression of JERF1 did not change the growth and development of transgenic rice, which makes JEFR1 a potentially useful source in breeding for greater tolerance to abiotic stress.     

Developmental instability as an index of adaptation to drought stress in a Mediterranean oak

An increase in temperature and water deficits caused by the ongoing climate change might lead to a decline growth rates and threaten the persistence of tree species in drought-prone areas within the Mediterranean Basin. Developmental instability (the error in development caused by stress) may provide an index of the adaptability of woody plants to withstand climatic stressors such as water shortage. This study evaluated the effects of drought stress on growth variables in three stands of a Mediterranean oak (Quercus faginea) exposed to differing climatic conditions (xeric, mesic and cooler) along an altitudinal gradient in northeastern Spain, in two climatically contrasting years (wet and dry years). Two indices of developmental instability, fluctuating and translational asymmetries, which reflect environmental stress, were measured in leaves and current-year shoots, respectively. We also measured branch biomass and fractal complexity of branches as indicators of the species’ performance. After a period of drought the individuals’ at the most xeric site presented lower developmental instability and less branch biomass than did the individuals from the mesic and cooler sites. We interpret that difference as an adaptive response to drought which reflects a trade-off between maintenance of homeostasis and growth when water is scarce. The study demonstrated that developmental instability constitutes a useful index to assess the degree of adaptation to stressful environmental conditions. The assessment of developmental instability in sites and years with contrasting climatic conditions provides a means of quantifying the capacity of plants to develop plastic adaptive responses to climatic stress.     

Tree growth,mortality, and above-ground biomass accumulation in a holm oak forest under a five-year experimental field drought

A holm oak forest was exposed to an experimental drought during 5 years to elucidate the growth responses of the dominant species Quercus ilex, Arbutus unedo and Phillyrea latifolia. Soil water availability was partially reduced, about 15% as predicted for this area for the next decades by GCM and ecophysiological models, by plastic strips intercepting rainfall and by ditch exclusion of water runoff. The stem diameter increment was highly correlated with annual rainfall in all species, and drought treatment strongly reduced the diameter increment of Q. ilex (41%) and specially of A. unedo (63%), the species showing higher growth rates. Stem mortality rates were highly correlated with previous stem density, but drought treatment increased mortality rates in all species. Q. ilex showed the highest mortality rates (9% and 18% in control and drought plots, respectively), and P. latifolia experienced the lowest mortality rates (1% and 3% in control and drought plots, respectively). Drought strongly reduced the increment of live aboveground biomass during these 5 years (83%). A. unedo and Q. ilex experienced a high reduction in biomass increment by drought, whereas P. latifolia biomass increment was insensitive to drought. The different sensitivity to drought of the dominant species of the holm oak forest may be very important determining their future development and distribution in a drier environment as expected in Mediterranean areas for the next decades. These drier conditions could thus have strong effects on structure (species composition) and functioning (carbon uptake and biomass accumulation) of these Mediterranean forests.     

Recent methods of drought stress tolerance in plants

In the climate change scenario the drought has been diagnosed as major stress affecting crop productivity. This review demonstrates some recent findings on the amelioration of drought stress. Nanoparticles, synthetic growth regulators viz. Trinexapac-ethyl, and Biochar addition helps to economize the water budget of plants, enhances the bioavailability of water and nutrients as well as overcomes drought induced osmotic and oxidative stresses. Besides ABA, SA and JA are also involved in inducing tolerance to drought stress through modulation of physiological and biochemical processes in plants. Plant growth promoting rhizobacteria (PGPR) offer new opportunities in agricultural biotechnology. These beneficial microorganisms colonize the rhizosphere/endo-rhizosphere of plants and impart drought tolerance by improving root architechture, enhancing water use efficiency, producing exopolysaccharides, phytohormones viz, ABA, SA and IAA and volatile compounds. Further PGPR also play positive role in combating osmotic and oxidative stresses induced by drought stress through enhancing the accumulation of osmolytes, antioxidants and upregulation or down regulation of stress responsive genes. In transgenic plants stress inducible genes enhanced abiotic stress tolerance by encoding key enzymes regulating biosynthesis of compatible solutes. The role of genes/cDNAs encoding proteins involved in regulating other genes/proteins, signal transduction process and strategies to improve drought stress tolerance have also been discussed.     

Identification of drought tolerance determinants by genetic analysis of root response to drought stress and abscisic Acid

Drought stress is a common adverse environmental condition that seriously affects crop productivity worldwide. Due to the complexity of drought as a stress signal, deciphering drought tolerance mechanisms has remained a major challenge to plant biologists. To develop new approaches to study plant drought tolerance, we searched for phenotypes conferred by drought stress and identified the inhibition of lateral root development by drought stress as an adaptive response to the stress. This drought response is partly mediated by the phytohormone abscisic acid. Genetic screens using Arabidopsis (Arabidopsis thaliana) were devised, and drought inhibition of lateral root growth (dig) mutants with altered responses to drought or abscisic acid in lateral root development were isolated. Characterization of these dig mutants revealed that they also exhibit altered drought stress tolerance, indicating that this root response to drought stress is intimately linked to drought adaptation of the entire plant and can be used as a trait to access the elusive drought tolerance machinery. Our study also revealed that multiple mechanisms coexist and together contribute to whole-plant drought tolerance.     

Stress induced injury and antioxidant enzymes in relation to drought tolerance in wheat genotypes

The role of plant antioxidant system in water stress tolerance was studied in three contrasting wheat genotypes. Water stress imposed at different stages after anthesis resulted in a general increase in lipid peroxidation (LPO) and decrease in membrane stability index (MSI), and contents of chlorophylls (Chl) and carotenoids (Car). Antioxidant enzymes like glutathione reductase and ascorbate peroxidase significantly increased under water stress. Genotype C 306, which had highest glutathione reductase and ascorbate peroxidase activity, also showed lowest LPO and highest MSI, and Chl and Car contents under water stress in comparison to susceptible genotype HD 2329, which showed lowest antioxidant enzyme activity as well as MSI, Chl and Car contents and highest LPO. HD 2285 which is tolerant to high temperature during grain filling period showed intermediate behaviour. Thus, the relative tolerance of a genotype to water stress as reflected by its comparatively lower LPO and higher MSI, Chl and Car contents is closely associated with its antioxidant enzyme system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Indirect effects of tending ants on holm oak volatiles and acorn quality

The indirect effect of ants on plants through their mutualism with honeydew-producing insects has been extensively investigated. Honeydew-producing insects that are tended by ants impose a cost on plant fitness and health by reducing seed production and/or plant growth. This cost is associated with sap intake and virus transmissions but may be overcompesated by tending ants if they deter or prey on hebivorous insects. The balance between cost and benefits depends on the tending ant species. In this study we report other indirect effects on plants of the mutualism between aphids and ants. We have found that two Lasius ant species, one native and the other invasive, may change the composition of volatile organic compounds (VOCs) of the holm oak (Quercus ilex) blend when they tend the aphid Lachnus roboris. The aphid regulation of its feeding and honeydew production according to the ant demands was proposed as a plausible mechanism that triggers changes in VOCs. Additionally, we now report here that aphid feeding, which is located most of the time on acorns cap or petiole, significantly increased the relative content of linolenic acid in acorns from holm oak colonized by the invasive ant. This acid is involved in the response of plants to insect herbivory as a precursor or jasmonic acid. No effect was found on acorn production, germination or seedlings quality. These results suggest that tending-ants may trigger the physiological response of holm oaks involved in plant resistance toward aphid herbivory and this response is ant species-dependent.Key words: tended aphid, invasive ants, linolenic acid, jasmonic acid, monoterpene emissionsTo achieve an indirect effect it is necessary to have a minimun of three species, two focal species that interact directly and an associate species whose presence promotes an indirect effect on one or both focal species. In general, indirect effects of a third species are defined by how and to what degree a pairwise species interaction is influenced by the presence and density of this third species.¹ There are several examples of interactions presenting indirect effects: apparent competition,¹ facilitation,² tri-trophic level interactions,³ cascading effects⁴ and exploitative competition. 5 But, indirect effects have been studied most extensively in the context of trophic cascades when top predators are removed⁶ or added⁷ and in the context of mutualisms.^8–10 Usually, indirect effects are investigated as changes in abundance of the focal species occur. However, indirect effects may result in biologically significant changes in a species that are not reflected only to its abundance.¹¹ There are many examples of changes in physiology, behavior, morphology and/or genotypic composition of the focal species.^11,12 These changes on density and/or morphological, physiological and behavioral traits of the focal species are not mutually exclusive, and all can act at the same time.¹³ The magnitude and direction of both direct and indirect effects should influence the relative resilience of communities to perturbation, which in turn will affect species coexistence and community evolution.¹⁴ In this regard, indirect effects had been postulated as one of the main forces structuring communities² and shaping the evolution of communities.¹⁴In terrestrial communities ants interact with plants both directly and indirectly. They can disperse or consume seeds, feed from specialized plant structures such as food bodies and extrafloral nectaries, act as or deter pollinitators, prey on herbivorous insects and/or develop mutualisms with honeydew-producing insects indirectly modifying plant fitness.^15–17 Additionally, through their nesting activities in soil, ants increase soil nutrient content available to plants, may change water infiltration and soil holding-capacity and modify biodiversity and abundance of soil organisms related to the decomposition process.^18,19 As a consequence of their activities, ants may thus change behavior, density, physiology or fitness of other species.^12,22,23 In the case of ants that tend honeydew-producing insects, evidence shows that their attention may change some traits of insect life history, 22 their abundance or physiology.¹⁸ For the plant, the net outcome of the mutualism between ants and honeydew-producing insects will depend on the balance between the costs for plant fitness via consumption of plant sap and transmission of plant pathogens and the benefit of ants deterring herbivorous insects.^18,23 As a consequence, plant seed production, pod production or even plant growth may decrease when the cost of honeydew-producing insects exceed the benefit provided by tending ants.^18,23Recently, we have described the changes that two tending ant species may exert indirectly on monoterpene emissions of holm oak (Quercus ilex) saplings through its mutualism with Lachnus roboris aphids.²⁴ One of these tending ant species was Lasius neglectus, an invasive ant species that displaces the local ant Lasius grandis. We found that aphids feeding on holm oak increased the emission of total volatile organic carbon (VOCs) by 31%. In particular, aphids feeding elicited the emission of a new monoterpene, Δ³-carene, and increased the emission of myrcene (mean ± SE; sapling alone: 0.105 ± 0.011 µg g⁻¹ h⁻¹; sapling plus not tended aphid: 0.443 ± 0.057 µg g⁻1 h⁻1) and γ-terpinene (sapling alone: 0.0013 ± 0.0001; sapling plus not tended aphid: 0.0122 ± 0.0022 µg g⁻1 h⁻1) (Mann-Whitney, sapling alone vs. sapling plus not tended aphids, U_4,4 = 0, p < 0.05 for both compounds). Changes of VOC emission in response to aphid infestation were noticed also in boreal trees.²⁴ When the aphids became tended by the invasive ant, L. neglectus, VOCs emissions increased only 19% because myrcene, the main compound of the blend, decreased significantly (25 When our data was recalculated on leaf area basis (nmol m⁻² s⁻¹), the general pattern was the same independently of the units, but the differences among treatments were not statistically significant (26 These slight differences in the statitiscal significance of the differences of VOC emissions depending on the reference unit may be due to differences in leaf morphology, i.e., changes of leaf area and mass. However, in our study, all holm oaks showed a similar leaf morphology among treatments (Kruskal-Wallis, leaf mass: H_3,20 = 2.16, p = 0.53; leaf area: H_3,20 = 2.64, p = 0.45) (24^,27 This lack of consistence of aphid effect on leaf area and mass limits the development of a clear pattern linking aphids feeding, leaf area or mass and VOC emissions. On the other hand, to achieve statistical significance of emitted VOCs among treatments, values should differ strongly given the high variability of VOC emission within treatments.²⁶ Under this scenario, we recommend giving the values of leaf morphology and to give VOC emissions on both unit bases to facilite comparisons among different studies.

Table 1

Means and standard error of the emission rates of the main compounds emitted by Quercus ilex saplings (n = 4 for T1 and T2 and n = 8 for T3) infested with untended aphids (T1) or infested with aphids tended by the native ant Lasius grandis (T2) or by the invasive ant Lasius neglectus (T3)

Variation in the genetic structure and reproductive biology of holm oak populations

Five polymorphic enzymatic loci were studied in 30 populations of Quercus ilex distributed from Crete to North Africa. A high within-population genetic diversity was found for each locus studied. Most of the alleles were common to all the populations, indicating that the holm oak corresponds to a single genetic entity. Observation of breeding system characteristics showed that the holm oak is monoecious, wind-pollinated, and likely to possess genetic autoincompatibility. Furthermore, studies of flowering phenology in a single population showed that the period of flowering varied notably among trees. An average of 29% of the trees showed inter-annual variation in flowering time. The male, female or vegetative investment also varied widely among trees from one year to another. This results in a diversification of pollen source received by a given tree each year, and also from one year to another.     

Common cranesGrus grus and habitat management in holm oak dehesas of Spain

Changes in agricultural policies have favored tree clearing and removal of shrubs to favor intensive farming in the holm oakQuercus ilex dehesas of Iberia. The effect of these changes for bird species wintering in this agricultural landscape has been rarely analyzed. Here the effects are studied of farming changes in traditional holm oak dehesas on population size, social structure and time budget of common cranesGrus grus in eight traditional wintering sites in Spain. The role of acorn availability, the principal winter food of the species, in relation to management is also assessed. The number of cranes in each wintering area was not related to degree of agricultural intensification, nor to the availability of acorns. However, crane flock size decreased, and the relative occurrence of isolated family groups increased in less intensively transformed wintering areas characterized by the predominance of fallow lands. The age composition of crane flocks varied with acorn availability since more juveniles were reported in areas with relatively lower acorn availability. Time budget was also related to acorn availability, since cranes spent relatively more time preening in those wintering areas with higher availability of acorns. Globally evidence suggests the need to maintain the fallow land (posíos) to sustain the wintering populations of cranes in Iberia, since this agro-grazing system maintains most of the juvenile cranes using the western migratory route.     

Effect of water stress on monoterpene emissions from young potted holm oak (Quercus ilex L.) trees

We investigated the effects of a short period of water stress on monoterpene emissions from Quercus ilex, a common oak species of the Mediterranean vegetation and a strong emitter of monoterpenes. The experiment was carried out on two young saplings with a branch enclosure system under semi-controlled conditions. Under unstressed conditions, small qualitative (cis--ocimene, trans--ocimene, -caryophyllene and 1,8-cineol) and large quantitative (as much as 40% for the main compounds emitted) differences were observed between the two apparently similar trees. Nevertheless these differences did not affect the short-and long-term responses to temperature and water stress. Daily courses of emissions and gas exchanges were similar before and after the stress. During the most severe stress, emissions were reduced by a factor of two orders of magnitude and the log-linear relationship between emissions and temperature no longer existed. Photosynthesis and transpiration rates decreased as soon as the soil started to dry, whereas monoterpene emissions slightly increased for few days and then dropped when the daily CO₂ balance approached zero. We concluded that under water stress monoterpene emissions were highly limited by monoterpene synthesis resulting from a lack of carbon substrate and/or ATP. After rewatering, both emissions and gas exchanges recovered immediately, but to a level lower than the pre-stress level. These results have many implications for monoterpene emission modelling in the Mediterranean area, since the dry period generally extends from May to August. If our results are confirmed by field experiments, water stress could lead to a large overestimation of the emissions under summer conditions, when the algorithms based on light and temperature would give high emission rates.