Article Cite This: J. Agric. Food Chem. 2017, 65, 9579-9586
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Passive Adsorption of Volatile Monoterpene in Pest Control: Aided by Proximity and Disrupted by Ozone Adedayo O. Mofikoya,*,† Tae Ho Kim,† Ahmed M. Abd El-Raheem,†,‡ James D. Blande,† Minna Kivimaë npaä ,̈ † and Jarmo K. Holopainen† †
Department of Environmental and Biological Sciences, University of Eastern Finland, Post Office Box 1672, 70211 Kuopio, Finland Department of Economic Entomology and Agricultural Zoology, Faculty of Agriculture, Menoufia University, Shebin El Kom, Menoufia Post Office Box 32514, Egypt
‡
S Supporting Information *
ABSTRACT: Plant volatiles mediate a range of interactions across and within trophic levels, including plant−plant interactions. Volatiles emitted by a plant may trigger physiological responses in neighboring plants or adhere to their surfaces, which, in turn, may affect the responses of the neighboring plant to herbivory. These volatiles are subject to chemical reactions during transport in air currents, especially in a polluted atmosphere. We conducted a field experiment to test for the adsorption of dispenserreleased myrcene on the surfaces of cabbage plants and the effects of distance from the dispenser and elevated ozone levels (1.4× ambient) on the process. We also tested the effects of the same treatments on oviposition on cabbage plants by naturally occurring Plutella xylostella. Under low ambient ozone conditions of central Finland, there was evidence for the adsorption and re-release of myrcene by cabbage plants growing at a distance of 50 cm from myrcene dispensers. This effect was absent at elevated ozone levels. The number of eggs deposited by P. xylostella was generally lower in plots under elevated ozone compared to ambient control plots. Our results indicate that passive adsorption and re-release of a volatile monoterpene can occur in nature; however, this process is dependent upon the distance between emitter source and receiver plants as well as the concentration of atmospheric pollutants in the air. We conclude that, in the development of field-scale use of plant volatiles in modern pest control, the effects of distances and air pollution should be considered. KEYWORDS: Brassica oleracea, volatile organic compounds, ozone, Plutella xylostella, myrcene, plant−plant interactions, adsorption, pollution
1. INTRODUCTION Volatile organic compounds (VOCs) emitted by plants are involved in a number of ecological and atmospheric processes.1 The emission of these volatile compounds can be constitutive or induced by mechanical damage, herbivore feeding, oviposition, and abiotic environmental stresses.2−6 Plants use VOCs in both direct and indirect defense against herbivores. Direct defense involves the deterrence of feeding or oviposition, whereas indirect defense involves the attraction of natural enemies of herbivores.7,8 VOCs are also mediators of plant−plant interactions and within plant signaling.9−11 The mode of action of volatile-mediated plant−plant interactions can be either active or passive. Active interactions require the VOCs released by an emitter plant to trigger a physiological response in the receiver plant, whereas passive interactions involve volatiles released by emitter plants adsorbing to the surfaces of neighboring plants.1 Active and passive processes may structure interactions that result in significant ecological effects.12,13 The roles of VOCs in pollination, herbivore foraging, and plant−plant interactions is dependent upon transport in air currents.14−17 For example, induced resistance in tobacco mediated by volatiles from clipped sagebrush neighbors is restricted when there is no air contact between the two plants.9 Volatiles disperse from their point source by diffusion and turbulent motion in the air, where they occur as intermittent © 2017 American Chemical Society
odor strands of high-volatile concentrations interspersed by volatile-free pockets.18−20 The intermittency of a volatile signal is dependent upon the release rate, air turbulence, and distance between the volatile plume and the source.21 The presence of reactive compounds, such as ozone in the atmosphere, may also degrade or change the blend of volatile compounds and subsequently affect the ecological processes that they mediate.13,22 VOCs have varying atmospheric lifetimes ranging from as low as half a minute to several days.23 These lifetimes are dependent upon chemical properties, such as the number of CC double bonds in the compound as well as the concentration of oxidizing agents in the atmosphere.24,25,1 VOCs in the atmosphere can be either removed or transformed via wet and dry deposition, photolysis, or reacting with hydroxyl (OH) or nitrate (NO3) radicals and ozone (O3).26 Tropospheric ozone degrades terpenes and green leaf volatiles (GLVs), which are key compounds in plant−plant and plant−insect interactions.27−29 The concentration of ozone in the troposphere varies based on region and is projected to increase in the coming decades as a result of climate change.30 Increasing ozone levels reduced the effective distance of active Received: Revised: Accepted: Published: 9579
July 14, 2017 September 12, 2017 October 9, 2017 October 9, 2017 DOI: 10.1021/acs.jafc.7b03251 J. Agric. Food Chem. 2017, 65, 9579−9586
Article
Journal of Agricultural and Food Chemistry
Figure 1. Daily mean ozone volume mixing ratio (ppb) for daylight hours (8:00−22:00) for experimental periods (from June 30th to July 7th, 2016 and from July 8th to July 15th, 2016).
plant−plant communication in lima beans.31 Ozone affects the ability of neighboring plants to detect volatiles or degrades adsorbed volatile compounds on neighboring plant surfaces in passive interactions.12 The degradation of volatiles by ozone disrupts host location in Plutella xylostella larvae and oviposition by their adults.32,12,13 Hence, volatile-based plant−plant interactions are dependent upon the concentrations of ozone in the atmosphere in both natural and agricultural systems. Dispenser-released volatiles are commonly used to manipulate arthropod behavior in forest and agricultural systems.33,34 Controlled release of methyl salicylate from sticky cards and slow-releasing dispensers have been used for pest control strategies in grape and hop yards.35,36 A mixture of monoterpenes released from a dispenser reduced carrot psyllid damage in carrot crops,37 and dispenser-released myrcene along with other pheromone components was also used for the attraction of western pine beetle in a forest ecosystem.38 In a Belgian wheat field, slow release of a GLV, (Z)-3-hexenol, was successfully used to attract aphids, while employment of the sesquiterpene (E)-β-farnesene reduced aphid numbers by recruitment of aphid natural enemies to the wheat plots.39 Myrcene is emitted by a wide variety of plants, including brassicaceous species, and in relatively high quantities by Rhododendron tomentosum.40 Some semi-volatile compounds from R. tomentosum adsorb to surfaces of neighboring plants and make them less susceptible to herbivory.41,42 Myrcene is one of the most reactive monoterpenes in the atmosphere as a result of its acyclic nature and presence of three double bonds.23 Its reaction with ozone and OH radicals produce secondary compounds and secondary organic aerosol (SOA) particles.43 The atmospheric lifetime of myrcene can be significantly shortened in atmospheres with high ozone levels.44 Birch trees growing in close proximity with R. tomentosum in boreal and subarctic environments had higher emissions of myrcene compared to trees growing without R. tomentosum neighbors (private communication). This suggests that, given the right conditions, there is the possibility of uptake or adsorption and re-emission of VOCs by neighboring plants. In this study, we investigated the effects of dispenser-released myrcene on the volatile profile of cabbage plants. We also tested the effect of distance between plants and dispenser and elevated ozone concentration on the re-emission of myrcene. The effects of myrcene and ozone treatment on natural oviposition by P. xylostella was also tested. This study centered around four main hypotheses: (a) dispensed myrcene adsorbs
to the surface and is re-released by neighboring cabbage plants; (b) adsorption and re-emission is dependent upon the distance between the emission source and receiver; (c) ozone breaks down the process of myrcene adsorption; and (d) myrcene and ozone treatment affects P. xylostella oviposition on cabbage plants.
2. MATERIALS AND METHODS 2.1. Plant Material. Brassica oleracea convar. capitata var. alba Jetma RZ (white cabbage) seeds were sown in 1 L pots in a substrate mixture of peat, sand, and mull (3:1:1). Plants were watered daily and fertilized once a week with 100 mL of 0.1% solution of nitrogen, phosphorus, and potassium (19−4−20, Taimi Superex, Kekkilä Oyj, Eurajoki, Finland). They were grown in plant growth chambers (75 cm width, 128 cm length, and 130 cm height, Weiss Bio 1300, Weiss Umwelttechnik Gmbh, Reiskirchen-Lindenstruth, Germany) [day, 16 h (photosynthetically active radiation of 300 μmol−2 s−1), 23 °C, and 60% humidity; night, 8 h dark, 18 °C, and 80% humidity] for 5 weeks prior to the start of field experiments. 2.2. Experimental Plan. A field experiment was conducted in the Ruohoniemi open-air ozone exposure field of the University of Eastern Finland, Kuopio campus (62° 53′ 42′ N, 27° 37′ 30′ E) between June 30th and July 15th, 2016. The first field exposure was between June 30th and July 7th, 2016, and the experiment was repeated between July 8th and July 15th, 2016. The exposure field consisted of eight circular plots (Ø of 9 m); four had elevated ozone levels, and the other four had ambient ozone levels (control). Both ambient and elevated ozone plots were divided into plots with and without myrcene treatment. The plot treatments were ambient ozone (A), ambient ozone with myrcene treatment (AM), elevated ozone (O), and elevated ozone with myrcene treatment (OM) (n = 2 for each treatment in both exposures). Myrcene treatments within ambient and elevated plots were interchanged for the second field exposure. There was a mean distance of 28 m between the plots, and the minimum distance between two plots was 18 m, as described by Karnosky et al.45 Ozone was produced from pure oxygen by an ozone generator (Pacific G22, Pacific Ozone Technology, Inc., Benicia, CA, U.S.A.) and released via computer-controlled valves (Hoke 2315G4Y, Hoke, Inc., Spartanburg, SC, U.S.A.) into the plots through vertical perforated pipes. Each plot was surrounded by 40 of these pipes in 8 units each having 5 pipes. Ozone was released from 20 upwind side pipes at a time until the main wind direction changed. Then, the next 20 pipes best fitting the main wind direction were selected for ozone release. Elevated ozone plots had an average ozone level of 42 ppb during the experiment; in ambient plots, the ozone concentration averaged 29 ppb (Figure 1). The highest hourly ozone concentration measured during the course of the experiment in the elevated ozone plot was 116 ppb. Six 5-week-old white cabbage plants were placed at three distances (0.5, 1.5, and 3 m) to the east and west of the center of each plot (Figure 2a). Myrcene dispensers (Figure 2b) were placed at the 9580
DOI: 10.1021/acs.jafc.7b03251 J. Agric. Food Chem. 2017, 65, 9579−9586
Article
Journal of Agricultural and Food Chemistry
Figure 2. (a) Schematic representation of the experimental setup. The red dot in the middle of each plot represents a myrcene dispenser. Each pair of plots represents a different treatment: ambient ozone (A), ambient ozone and myrcene (AM), elevated ozone (O), and elevated ozone and myrcene (OM). (b) Diagrammatic representation of a myrcene dispenser (n = 2 for each treatment/exposure). had been pre-cleaned at 120 °C for a minimum of 1 h. We made a hole at the top of the bag through which filtered air was passed via Teflon tubing at a flow rate of ≈350 mL min−1 for 10 min to clean the system. A Tenax tube was attached to a second hole in the bag and connected to a suction pump that pulled the plant headspace air through the tube at approximately 200 mL min−1 for 20 min. The inlet and suction air was calibrated with a mini-Buck calibrator (model M-5, A.P. Buck, Inc., Orlando, FL, U.S.A.). Tenax tubes were stored at 4 °C before analyses. Volatile analyses were conducted by gas chromatography−mass spectrometry (GC−MS, MSD 5973, Hewlett-Packard GC type 6890, Wilmington, DE, U.S.A.). The compounds were desorbed at 250 °C for 20 min, cryofocused at −30 °C by an ATD 400 automatic thermal desorption system (PerkinElmer, Ltd., Waltham, MA, U.S.A.), and injected into a HP-5 capillary column (50 m length × 200.0 μm diameter × 33.0 μm film thickness), at 72.4 kPa pressure and 7 cm s−1 average velocity, with helium as the carrier gas. The oven was programmed with an initial temperature of 40 °C for 1 min. The temperature was raised to 210 °C at a rate of 5 °C min−1 and reached 250 °C at a rate of 20 °C·min−1. The compounds (terpenes and GLVs) were identified by comparing their mass spectra and retention times to those in the Wiley and National Institute of Standards and Technology (NIST) libraries and with pure standards. The emission rates were expressed in nanograms per gram per hour. The cabbage plants were oven-dried at 60 °C for 72 h and weighed after inspection for oviposition by P. xylostella. 2.5. Egg Counting. After volatile collection, eggs and neonate larvae on the leaves were counted under a microscope (Nikon SMZ800 zoom stereomicroscope, Nikon Corporation, Tokyo, Japan). The P. xylostella egg is oval-shaped with a shiny yellow color and a length of
