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First published online September 24, 2004; 10.1104/pp.104.050005 Plant Physiology 136:2937-2947 (2004) © 2004 American Society of Plant Biologists Airborne Ethylene May Alter Antioxidant Protection and Reduce Tolerance of Holm Oak to Heat and Drought Stress1Departament de Biologia Vegetal, Universitat de Barcelona, Facultat de Biologia, 08028 Barcelona, Spain (S.M.-B.); and Unitat d'Ecofisiologia CSIC-CEAB-CREAF, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain (J.P., D.A., J.L.)
Plant-emitted ethylene has received considerable attention as a stress hormone and is considered to play a major role at low concentrations in the tolerance of several species to biotic and abiotic stresses. However, airborne ethylene at high concentrations, such as those found in polluted areas (20–100 nL L–1) for several days, has received far less attention in studies of plant stress tolerance, though it has been shown to alter photosynthesis and reproductive stages (seed germination, flowering, and fruit ripening) in some species. To assess the potential effects of airborne ethylene on plant stress tolerance in polluted areas, the extent of oxidative stress, photo- and antioxidant protection, and visual leaf area damage were evaluated in ethylene-treated (approximately 100 nL L–1 in air) and control (without ethylene fumigation) holm oak (Quercus ilex) plants exposed to heat stress or to a combination of heat and drought stress. Control plants displayed tolerance to temperatures as high as 50°C, which might be attributed, at least in part, to enhanced xanthophyll de-epoxidation and 2-fold increases in  -tocopherol, and they suffered oxidative stress only when water deficit was superimposed on temperatures above 45°C. By contrast, ethylene-treated plants showed symptoms of oxidative stress at lower temperatures (35°C) than the controls in drought, as indicated by enhanced malondialdehyde levels, lower -tocopherol and ascorbate concentrations, and a shift of the redox state of ascorbate to its oxidized form. In addition, ethylene-treated plants showed higher visual leaf area damage and greater reductions in the maximum efficiency of the PSII photochemistry than controls in response to heat stress or to a combination of heat and drought stress. These results demonstrate for the first time that airborne ethylene at concentrations similar to those found in polluted areas may reduce plant stress tolerance by altering, among other possible mechanisms, antioxidant defenses.
Ethylene, the simplest unsaturated hydrocarbon, regulates many diverse metabolic and developmental processes in plants, ranging from seed germination to organ senescence, and it is considered to play a major role as a signal molecule at low concentrations in the tolerance of several species to environmental stresses (for review, see Abeles et al., 1992  ; Morgan and Drew, 1997; Bleecker and Kende, 2000). Ethylene is a volatile hormone that diffuses freely in air and from cell to cell across membranes, and it therefore easily reaches target cells of the same organ, organs other than those responsible for its biosynthesis, or even target cells of neighbor plants acting in plant-plant signaling (Pierik et al., 2004). Ethylene concentrations in free air in rural areas may well be below 2 nL L–1. However, in a greenhouse replete with plants, airborne ethylene may increase to 6 nL L–1 and reach 25 to 80 nL L–1 within dense canopies of ethylene-emitting plants (Heilman et al., 1971; Pierik et al., 2004). Irrespective of plant ethylene emission, airborne ethylene may reach 20 nL L–1 on annual averages and 1 µL L–1 at hourly maximum concentrations as a result of industrial activity in polluted areas (as a reference, ethylene concentrations may be as high as 3 µL L–1 close to polyethylene factories or 1,000 µL L–1 in vehicle exhaust; for review, see Cape, 2003).
Although high ethylene concentrations of industrial origin in air may be dispersed fairly rapidly by the wind, their source at ground level means that there may be local fumigation of the natural vegetation in surrounding areas at concentrations ranging from 20 to 100 nL L–1 (or even higher) for several days, and effects of ethylene of industrial origin on vegetation should not be disregarded. In fact, pollutant-specific effects of ethylene have been reported in the field downwind from major industrial sources, showing significant effects on epinasty (Tonneijck et al., 1999
Environmental stresses, such as heat and drought stress, may lead to an imbalance between antioxidant defenses and the amount of activated oxygen species (AOS) resulting in oxidative stress (Smirnoff, 1993
Holm oak (Quercus ilex) is one of the most typical dominant Mediterranean forest species and shows tolerance to heat stress (Larcher, 2000 In an attempt to study the effects of airborne ethylene at concentrations found in polluted areas on stress tolerance in holm oak, photosynthesis, the extent of oxidative stress, photo- and antioxidant protection, and visual leaf area damage were evaluated in ethylene-treated (approximately 100 nL L–1 in air) and control (without ethylene fumigation) plants exposed to heat stress, or to a combination of heat and drought stress. It is shown here that airborne ethylene may reduce tolerance of holm oak plants to heat and drought stress by altering, among other processes, antioxidant defenses.
Airborne Ethylene Reduces Photosynthesis and Increases Stress-Induced Damage The effects of airborne ethylene on leaf gas exchange and several indicators of leaf injury were investigated in holm oak plants exposed to heat stress or to a combination of heat and drought stress. Plants were progressively exposed to increasing temperatures up to 45°C under irrigated conditions, or to a combination of high temperatures and drought stress (up to 45°C and 6 d of water deficit). After these treatments, each group of plants (irrigated and water stressed) were subject to strong temperature fluctuations (50/25°C), i.e. exposure at 50°C for 1 d followed by recovery at 25°C for another day, to evaluate the ability of holm oak to prevent oxidative damage under extreme climatic conditions. Ethylene fumigation allowed us to compare the response of plants to stress under contrasting ethylene concentrations in the air. The airborne ethylene concentrations in the fumigated chamber ranged from around 100 to 125 nL L–1, whereas those in the control chamber ranged between 2 and 3 nL L–1 throughout the experiment (Fig. 1). In addition, measurements of ethylene emitted by leaves revealed that ethylene production by holm oak leaves did not increase under heat or drought stress, and if any increase occurred it remained below detectable amounts (<0.1 nL [g dry wt h]–1) throughout the experiment in ethylene-treated and control plants.
Leaf hydration was constant throughout the experiment (around 0.8 gwater [gdry wt]–1) in irrigated and water-stressed plants exposed to increasing temperatures (from 25°C–45°C) or temperature fluctuations (50/25°C), thereby indicating that neither airborne ethylene nor drought stress resulted in leaf water loss during the experiment (Fig. 2). The maintenance of leaf water content was associated with significant reductions in stomatal conductance in water-stressed plants, and ethylene-treated plants showed earlier stomatal closure under water deficit than controls as temperatures increased (Fig. 2). Net CO2 assimilation rates decreased as stomatal conductance was progressively reduced in drought. However, while stomatal conductance was constant, net CO2 assimilation rates decreased at 35°C in irrigated plants, thus indicating heat-induced non-stomatal limitations of photosynthesis. Airborne ethylene also caused non-stomatal limitations of photosynthesis, as indicated by the reductions in net CO2 assimilation accompanied by constant stomatal conductance rates between 35°C and 45°C in drought (Fig. 2).
Indicators of heat- and drought-induced damage were measured in ethylene-treated and control plants (Fig. 3). While chlorophyll levels remained unaltered throughout the experiment in both plant groups, the maximum efficiency of PSII photochemistry (Fv/Fm ratio), which is an indicator of photoinhibitory damage to PSII, was significantly reduced under heat and drought stress, especially in ethylene-treated plants. The Fv/Fm ratio decreased concomitantly with lipid peroxidation, as indicated by malondialdehyde concentrations, which increased in both plant groups. In control plants, malondialdehyde levels increased significantly under strongly fluctuating temperatures (50/25°C) in drought stress only (Fig. 3), which was associated with strong reductions in photosynthesis (Fig. 2). Ethylene-treated plants showed higher malondialdehyde levels than controls at 45°C under drought stress and also under strong temperature fluctuations (50/25°C) both in irrigated and water-stressed plants (Fig. 3). The higher malondialdehyde levels were associated with higher visual leaf area damage at the end of the experiment in ethylene-treated plants as compared to controls. Control plants showed some discolored and brown areas (approximately 10%) as a result of heat stress under irrigated conditions, and leaf area damage increased 4-fold when drought was superimposed on heat stress. Levels of injury were much higher in ethylene-treated plants both under irrigated and water-stress conditions, and maximum levels of leaf damage area (approximately 83%) were attained under a combination of heat and drought stress in ethylene-treated plants (Fig. 4).
Airborne Ethylene Alters Photo- and Antioxidant Protection under Stress
The effects of airborne ethylene on mechanisms of photo- and antioxidant protection were investigated in holm oak plants exposed to heat stress or to a combination of heat and drought stress. The concentrations of total carotenoids and those of the major carotenoids, lutein and
The extent of antioxidant protection differed in ethylene-treated and control plants (Figs. 6 and 7). In control plants, levels of -tocopherol increased 2-fold in response to heat stress and then decreased under strong temperature fluctuations (50/25°C). By contrast, -tocopherol levels did not increase under heat stress in ethylene-treated plants, and stayed at levels similar to (under strong temperature fluctuations) or even lower (at 35°C and 45°C) than those that appeared at 25°C (Fig. 6). Changes in -tocopherol levels under a combination of heat and drought stress did not differ from those observed under heat stress in either ethylene-treated or control plants. Ascorbate levels increased as well in response to heat stress, but the response differed from that of -tocopherol (Fig. 7). Maximum ascorbate levels were attained at 35°C and then decreased progressively as temperature increased. Ethylene-treated plants showed lower levels of ascorbate than controls under strong temperature fluctuations (50/25°C) and also above 35°C under drought stress. This was associated with higher oxidation of ascorbate in ethylene-treated plants as compared to controls, as indicated by the redox state of ascorbate. In irrigated plants, maximum oxidation of ascorbate was attained in ethylene-treated plants exposed to strong temperature fluctuations. In drought, the redox state of ascorbate shifted toward its oxidized form at 35°C in ethylene-treated plants, while this shift was not observed until 45°C in control plants.
Plant-emitted ethylene has been shown to be involved in plant responses to environmental stresses, conferring protection against heat and drought stress in several, although not all, species (Abeles et al., 1992 ; Weyers and Paterson, 2001; Larkindale and Knight, 2002). Ethylene mediates the response to stress, acting as a signal molecule at low concentrations, and, in cooperation with other signaling compounds (abscisic acid, salicylic acid, and calcium), it may reduce heat-stress-induced oxidative damage in some species (Larkindale and Knight, 2002). However, the effect of airborne ethylene at high concentrations such as those found in polluted areas (20–100 nL L–1) for several days on plant stress tolerance has been poorly studied so far, giving contrasting results (for review, see Cape, 2003). While increasing airborne ethylene concentrations to 15 to 60 nL L–1 for 11 d resulted in reduced wilting under drought stress in Helianthus annuus (Reid and Watson, 1985), it has been shown that the reaction of ozone with ethylene in the substomatal cavity may increase ozone-induced damage in plants (Mehlhorn et al., 1991). Furthermore, the effects of ethylene at concentrations found in polluted areas on plant thermotolerance have not been investigated so far. Other studies on exogenous applications of ethylene or ethylene-releasing compounds and their effects on plant responses to environmental stresses have been done, but the ethylene concentrations applied are not representative of those found in nature (for review, see Cape, 2003).
For many physiological effects, the dose-response curves for airborne ethylene have thresholds around 10 nL L–1 and half-maximal responses in the range of 100 to 500 nL L–1 (Gunderson and Taylor, 1988
Holm oak plants without ethylene treatment displayed tolerance to temperatures as high as 50°C, which might be attributed, at least in part, to enhanced xanthophyll de-epoxidation (of both lutein epoxide and violaxanthin) and transient increases in
Holm oak plants without ethylene treatment suffered oxidative stress, as indicated by a shift of the redox state of ascorbate toward its oxidized form and enhanced malondialdehyde levels, only when water deficit was superimposed on temperatures above 45°C. In addition, drought stress for 8 d increased visual leaf area damage 4-fold under heat stress as compared to irrigated plants. This is indicative of this species' high degree of tolerance for heat stress and of its sensitivity to drought when combined with heat stress, which is in agreement with previous studies (Tetriach, 1993
To our knowledge, ethylene emission by holm oak leaves has not been studied so far. In this study, no increase in ethylene emission in holm oak under heat stress or under a combination of heat and drought stress was detected, which indicates that hormones other than ethylene may be directly involved in the activation of mechanisms of photo- and antioxidant protection in this species under stress. The absence of stress-induced ethylene synthesis in this study is in agreement with previous studies on other Mediterranean plants, including other species of the genus Quercus, which do not show enhanced ethylene synthesis under various stress treatments (Finch-Savage et al., 1996
By contrast, holm oak plants exposed to airborne ethylene concentrations found in polluted areas (100 nL L–1) showed reduced stress tolerance and alterations in mechanisms of photo- and antioxidant protection, thus indicating a sensitivity of this species to airborne ethylene under stress. Ethylene-treated plants showed symptoms of oxidative stress at lower temperatures (35°C) than controls in drought, as indicated by enhanced malondialdehyde levels, lower
Since it is known that oxidative damage occurs during recovery from heat stress in some species (Gong et al., 1997;
The mechanisms by which airborne ethylene may reduce tolerance of holm oak plants to heat and drought stress may provoke uncontrolled production of AOS, as indicated by increased lipid peroxidation and reduced antioxidant defenses, under stress. This is partly in agreement with previous studies, which have shown that ethylene at high concentrations (>1 µL L–1) may reduce antioxidant defenses, increase lipid peroxidation and induce cell death in some species (Lee et al., 1998
In conclusion, we have shown that (1) holm oak plants activate several mechanisms of photo- and antioxidant protection to withstand heat and drought stress; (2) these mechanisms are able to protect the leaves from oxidative damage, when stress is not extremely severe; (3) ethylene reduces photosynthesis and alters antioxidant protection under heat and drought stress; and (4) ethylene at concentrations found in polluted areas may decrease the stress tolerance of native Mediterranean vegetation. The mechanisms by which airborne ethylene at concentrations found in polluted areas may reduce plant stress tolerance are still not fully understood, but our results support the contention that airborne ethylene may trigger the uncontrolled production of AOS, overwhelm antioxidant defenses, and induce cell death under stress. Further studies are needed to better understand the signal transduction pathway involved in the response of stressed holm oak plants to airborne ethylene, especially considering the effect of ethylene in conjunction with other air pollutants.
Experimental System, Ethylene Treatment, and Variables Measured Two-year-old holm oak plants (Quercus ilex), which were obtained from a nursery (Forestal Catalana S.A., Breda, Spain), were grown in a greenhouse in 2-L pots with peat and sand (2:1, v/v) containing slow-release fertilizer (2 g L–1, 15:8:11 N,P,K + 2Mg) under irrigated conditions. For each experimental set, 20 plants were transferred into a 1.59-m3 (1.75 m high x 1.25 m wide x 0.725 m deep) chamber (Vötsch-industrietechnik Bio Line model VB 1014; Balingen-Frommern, Germany), in which environmental conditions (photosynthetically active photon flux density, relative humidity, and air temperature) were programmed independently by an automated control mechanism. Light was supplied by 8 400-W halogen lamps (model HQI-T; Vötsch-industrietechnik), each supplying a photosynthetically active photon flux density of 500 to 600 µmol m–2 s–1 at a height of 0.9 m on a plane over the plant during a 12-h photoperiod; relative humidity in the chamber was kept at around 50% throughout the experiment. Plants were exposed to two watering regimes: one-half of the plants (10 individuals) were watered daily, once with tap water and once with Hoagland solution (irrigated plants), while the other half did not receive water at all for 8 d (water-stressed plants). During this period, each group of plants was exposed to temperature increases from 25°C to 45°C, followed by a final temperature fluctuation (50/25°C) treatment. For high temperature treatments, plants were progressively exposed to from 25°C to 35°C and then to 45°C and were kept at such temperatures for 2 d before measurements. After exposure to 45°C, plants were subjected to 50°C for 1 d, followed by a sharp decrease to 25°C, at which temperature plants were kept for one more day before measurements. Two different atmospheres were assayed as a result of ethylene fumigation and non-fumigation (control) in four experimental sets, two sets for each treatment, in plants of the same height (approximately 0.9 m) transferred to the environmental growth chamber between May 19, 2003, and July 22, 2003. Ethylene treatment was applied by regulating the entrance of 1% (v/v) ethylene (Abelló Linde S.A., Barcelona) with a manometer-coupled valve (model VXG41.15R; Landis & Gyr., Geneva) controlled by a Sho-rate rotameter (model 1350/D1A9D1B00000; Brooks Instruments B.V., Veenendaal, The Netherlands). The airflow was measured with a mass flow meter (model 5811 N; Brooks Instruments B.V.) attached to the chamber. The airborne ethylene concentrations in the fumigated chamber were kept at around 100 to 125 nL L–1, whereas those in the control chamber ranged between 2 and 3 nL L–1 throughout the experiment, as estimated by gas chromatography coupled to mass spectrometry (GC-MS; see below). Airflow in the chamber was 0.125 m3/min, and air in the chamber was completely renewed every 12.7 min. Ethylene emission, leaf water status, gas exchange, chlorophyll fluorescence, chlorophylls, lipid peroxidation, and the extent of photo- and antioxidative protection (levels of carotenoids, tocopherols, and ascorbate) were determined in fully developed young leaves of randomly chosen plants grown under irrigated and water-stress conditions in non-fumigated (control) and ethylene-treated atmospheres. For measurements of lipid peroxidation, photosynthetic pigments, and antioxidants, leaves were collected around 1.5 h after the start of the light period, frozen in liquid nitrogen, and stored at –20°C until analyses. Measurements of ethylene emission, leaf water status, gas exchange, and chlorophyll fluorescence were performed around 2 h after the start of the light period. Estimation of leaf area damage in plants was performed at the end of the experiment. Leaves showing necrotic lesions were discarded for all other measurements. Measurements at each temperature were made on four to six individuals belonging to duplicated experiments for each assayed atmosphere.
Ethylene emission by leaves and ethylene concentrations in fumigated and non-fumigated chambers were determined by GC-MS. A Ciras-2 gas exchange system (PP Systems, Hertfordshire, England) was used for sampling foliar emissions. Intact leaves were clamped in an automatic Parkinson leaf cuvette (Std Broad 2.5). Air coming out of the cuvette flowed through a T system to a glass tube (11.5 cm long and 0.4 cm i.d.) filled with adsorbents Carbotrap C (300 mg), Carbotrap B (200 mg), and Carbosieve S-III (125 mg) from Supelco (Bellefonte, PA) separated by plugs of quartz wool. The tubes' hydrophobic properties minimized sample displacement by water and produced no chemical transformation as checked with standards. Prior to use, they were conditioned for 3 min at 350°C with a stream of purified helium. The sampling time was 5 min and the flow was 500 mL min–1 with a calibrated sampler pump (Escort Elf Pump; Mine Safety Appliances, Pittsburgh). Blanks of 15 min free air sampling without clamping leaves were carried out immediately before and after each measurement. The glass tubes (with trapped volatiles) were stored at –28°C until analysis (no more than 24–48 h later). For sampling airborne ethylene in fumigated and non-fumigated chambers, part of the air exiting the chamber flowed through a T system to a glass tube filled with adsorbents Carbotrap C, Carbotrap B, and Carbosieve S-III (Supelco) as described above. Ethylene analyses were performed by using a GC-MS system (Hewlett Packard HP59822B; Palo Alto, CA). Samples of trapped ethylene were desorbed (Thermal Desorption Unit, model 890/891; Supelco) at 250°C for 2 min and injected into a 30 m x 0.25 mm x 0.25 mm film thickness capillary column (Supelco HP-5, Crosslinked 5% Me Silicone; Supelco). After sample injection, the initial temperature (46°C) was increased at 30°C min–1 up to 70°C and thereafter at 10°C min–1 up to 150°C, a temperature that was maintained for another 5 min. Helium flow was 1 mL min–1. The identification of ethylene was conducted by comparison with the mass spectra of an authentic standard from Abelló Linde S.A.
The absence of ethylene emission by holm oak leaves under heat and drought stress was confirmed by using a high-sensitivity Proton-Transfer-Reaction Mass Spectrometer (PTR-MS-FTD hs) system with a fast drift tube (Ionicon Analytik GesmbH, Innsbruck, Austria). Sampling was performed as described before, except that part of the air exiting the leaf cuvette flowed through a T system to the PTR inlet. The PTR-MS system was operated as described elsewhere (Lindinger et al., 1998
Leaves were weighed (fresh wt) and leaf area was immediately measured using a LI-COR LI-3100 area meter (LI-COR, Lincoln, NE). Then leaves were oven dried at 60°C to constant weight (dry wt). The hydration of leaves (H) and leaf mass per area ratio (LMA) were determined as (fresh wt – dry wt)/dry wt and dry wt/leaf area, respectively.
Calibrated Ciras-2 porometers (PP Systems) were used for determination of CO2 and water exchange. Intact leaves were clamped into a Parkinson leaf cuvette (Std Broad 2.5), and net CO2 assimilation and stomatal conductance rates were calculated from variations in gas exchange concentrations according to Von Caemmerer and Farquhar (1981)
Measurements of Fv/Fm were conducted in situ on attached leaves with a portable pulse-modulated fluorimeter PAM-2000 (Walz, Effeltrich, Germany). Leaves were dark adapted with leaf holder clips for 20 min prior to measurements. The Fv/Fm ratio was calculated as (Fm – Fo)/Fm, where Fm and Fo are the maximum and basal fluorescence yields, respectively, of dark-adapted leaves (Genty et al., 1989
The extent of lipid peroxidation in leaves was estimated by measuring the amount of malondialdehyde using the method described by Hodges et al. (1999)
The extraction and HPLC analyses of photosynthetic pigments and antioxidants (chlorophylls, carotenoids, tocopherols, and reduced and oxidized ascorbate) were carried out essentially as described by Munné-Bosch and Alegre (2003)
Chlorophylls and carotenoids (neoxanthin, lutein, lutein epoxide, violaxanthin, antheraxanthin, zeaxanthin, and
Ascorbate was isocratically separated on a Spherisorb ODS C8 column (Teknokroma, St. Cugat, Spain) using the mobile phase (24.25 mM Na-acetate/acetic acid, pH 4.8; 0.1 mM diethylenetriaminepentaacetic acid; 0.015% [w/v] m-phosphoric acid; 0.04% [w/v] octylamine; 15% [v/v] methanol) at a flow rate of 0.8 mL min–1. Detection was carried out at 255 nm (Diode array detector 1000S). Ascorbate was identified by its characteristic spectrum and by coelution with an authentic standard from Sigma (Steinheim, Germany).
The percentage of damaged (discolored or brown) leaves was measured at the end of the experiment by measuring the damaged area of all the plant leaves. Leaf area (separating damaged and healthy leaves) was measured using a LI-COR LI-3100 area meter.
Statistical differences between controls and ethylene-treated plants were analyzed following the Student's t test using SPSS (Chicago). Differences were considered significant at a probability level of P < 0.05. Data shown in the figures correspond to the overall treatment mean ± the range observed between the two chambers for either controls (non-fumigated) or ethylene-treated plants progressively exposed to increasing temperatures under irrigated and water-stress conditions.
We are very grateful to the Serveis Científico-Tècnics (Universitat de Barcelona) for technical assistance. Received July 16, 2004; returned for revision August 6, 2004; accepted August 11, 2004.
1 This work was supported by the Spanish Government (grant nos. REN2001–0003 and REN2003–04871).  Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.050005. * Corresponding author; e-mail smunne{at}ub.edu; fax 34–934112842.
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ABSTRACT
RESULTS
-carotene, did not change significantly in ethylene-treated plants as compared to controls, and remained unaltered throughout the experiment in response to heat and drought stress (
