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An assessment of VOC and ozone fluxes in crop plantations from the leaf to the ecosystem level. Relationships with the plant physiology and implications for air quality

Final Report Summary - CITROVOC (An assessment of VOC and ozone fluxes in crop plantations from the leaf to the ecosystem level. Relationships with the plant physiology and implications for air qualit

Summary description of Project background and objectives

The overall objective of the proposed project was to provide critical information to assess the contribution of Citrus orchards to ozone (O3) cycling (removal or formation) in the atmosphere. Citrus is a major crop in California and Italy, where it yields economically valuable, high-quality productions in selected, well-suited, and intensively cultivated areas. Citrus plants, like all members of the Lamiaceae family, also produce large quantities of volatile isoprenoids and other reactive biogenic volatile organic compounds (VOC) that may influence the chemistry of the troposphere, especially in presence of anthropogenic pollutants. Both heavy VOC production, and the intensive cultivation in areas that are also densely populated, make Citrus plants likely important players in the relationships between the biosphere and atmosphere. The researcher was involved in studying whether VOC emissions by Citrus plants interact with other chemical species in the atmosphere and in the plants. The main species investigated were lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), the most widespread Citrus crops on the territory of California and Italy. In a three-year project, a three-level study was proposed based on greenhouse, laboratory and field measurements:

Level I. Measurements of physiology, ozone uptake and VOC emission for major citrus types.

Ozone is one of the most dangerous oxidants for plants and human health. Chronic and acute exposure to elevated ozone concentrations usually produce biochemical and physiological changes with detrimental effects on forests and crop yield, in turn decreasing the benefits (economical, social) that natural and cultivated plant ecosystems can offer. Ozone is formed through photochemical reactions, and its production dramatically increases when VOC react with oxides of nitrogen (NOx) emitted by anthropogenic (e. g. cars) or biogenic (e. g. soils) sources in the presence of sunlight. Ozone is considered as a greenhouse gas, being responsible of an increase of temperature of 0. 5 °C after the industrial revolution, and likely further contributing to global warming in the next decades. Evidences have been presented that plants act as a suitable and natural sink for ozone, and can usefully be employed to phytoremediate the atmosphere, provided they do not get damaged by this dangerous pollutant. Further research is now keyed at understanding how ozone removal by plants can be optimised. The uptake of ozone is attributed to stomatal and non-stomatal sinks. The stomatal absorption is often the major contributor to the total uptake of ozone and is considered to be the main responsible for plant injuries. After entering stomata, ozone reacts directly with cell surfaces. Here ozone can be completely detoxified but can also induce membrane denaturation and the formation of free radicals, with a possible cascade of negative effects on plant physiology and biochemistry.

The first objective of CITROVOC was be to study the mechanisms of uptake of ozone from Citrus species using plant enclosures.

The non-stomatal mechanism of ozone uptake is mainly represented by the adsorbtion of the pollutant by cuticles, and by reactions in the gas phase between ozone and VOC. VOC may indeed represent a relevant sink of ozone already in the boundary layer, but also in the leaf mesophyll where they also act as antioxidant, scavenging ROS (Reactive Oxygen Species) produced by ozone. Plants emit 1-1. 5 TgC year-1 VOC in the atmosphere. These emissions account for no more than 3 % of the total carbon exchange between biota and the atmosphere, but they play an indirect and often important role in the oxidative chemistry of the atmosphere, because they may be involved, in the presence of anthropogenic pollutants, namely NOx, in photochemical reactions leading to formation of ozone, photochemical smog, and particulate matter. This may be the case in all Citrus growing areas of California (especially in the southern San Joaquin Valley, the Los Angeles basin, and the Ventura-Santa Barbara area) and central and southern Italy.

In non-stressed conditions, isoprenoids (isoprene, monoterpenes, sesquiterpenes) are the main constituent of VOC. These compounds are typically more reactive than VOC emitted from anthropogenic sources, and can have higher ozone-forming potential. Citrus species, in particular, are able to produce isoprenoids (mainly monoterpenes and sesquiterpenes) from leaves, flowers and fruits, emitting the compounds in the atmosphere, or storing them in specific organs. Monoterpenes (e. g. linalool, a-and ß-pinene, limonene, and cis and trans ß-ocimene) are 10-carbon atoms very common in plants living in the regions with Mediterranean climate. Monoterpenes play a decisive and important roles in the atmospheric chemistry, and contribute more than isoprene (the most abundant isoprenoid species, and the monomer of monoterpenes) to form particles and secondary organic aerosols. Sesquiterpenes are 15-carbon VOC which have been reported to be emitted by vegetation of southern and central Europe. They are even more reactive with ozone than monoterpenes. These rapidly reacting compounds are strongly emitted by Citrus, and are likely to be the key drivers of ozone uptake.

Given the importance and the need to know the relationships between ozone and VOC, a second objective of CITROVOC was to use branch enclosures to estimate VOC emission of major Citrus species and varieties. These information are useful to determine whether isoprenoids contribute to ozone formation, or if they clean the air removing ozone through gas-phase reactions.

Level II: Measurements of VOC precursors and internal pools for major Citrus types.

Biochemical studies were not the main focus of this project. However, measurements of pools and precursors are important to understand the biochemical potential of emissions in terms of source strength and main biochemical limitations. At a cellular level two activities were proposed to perform on leaf material collected during key stages of phenology, in plants grown in ozone-free air or under chronic levels of ozone:
1. Detection of internal pools of isoprenoids in leaves. When considering that Citrus is storing high levels of monoterpenes and sesquiterpenes, this assay will allow to assess how the internal pools changes, at varying environmental and phenological conditions, contribute to the changes of emissions. 2. Determination of precursors of monoterpenes (GDP – Geranyl diphosphate) and sesquiterpenes (FDP – Farnesyl diphosphate) formation. Within this level of investigation, the applicant proposed to investigate whether ozone perturbs the origin of released isoprenoids, as assessed by 13C labeling. The hypothesis supporting this research, as elaborated during previous studies at CNR-IBAF, is that a higher amount of unlabelled isoprenoids can be released in plants exposed to ozone, due to the breakage of storage organs, at least until these reservoirs are depleted. However, as many isoprenoids are induced by stresses, it was also hypothesized that de-novo synthesis may successfully meet the larger release of isoprenoids, and may sustain emissions for longer than predicted on the basis of constitutive formation of these compounds.

Level III. VOC flux measurements from selected Citrus canopies.

While ozone removal at a plant scale is driven by plant physiology and mainly by diffusion through stomata and following detoxification in the mesophyll through reactions with antioxidants, at a canopy scale a more complex interaction between plant physiology (including VOC emission) and the transport of air masses containing ozone exists. To understand the mechanisms that drive ozone removal at canopy scale, simultaneous measurements of the physiological parameters (water and CO2 exchanges), ozone fluxes, and reactive compounds emitted by the plants (VOC) are required. Although it is now assumed that a metric based on ozone fluxes is more tenable than measurements of exposure, there is a limited number of studies aimed to measure the ozone fluxes. Modelling approaches which are mostly based on calculation of stomatal ozone fluxes are instead used to estimate fluxes. Few laboratories in the world are equipped with all the instruments necessary to measure simultaneously fluxes of CO2, water, ozone and VOC through micrometeorological techniques. The main objective of the field (level III) experiments of CITROVOC was to measure ozone deposition at canopy-level, in parallel with CO2 uptake and evapotranspiration, in Citrus plantations. In addition, VOC fluxes were measured in-situ above the crop canopy over multiple periods including key stages of Citrus phenology (e. g. flowering) and agricultural treatments (pruning and harvesting) to describe the complex dynamics of Citrus emissions. Environmental parameters, together with fluxes of water, CO2, H2O, O3, and VOC were measured with eddy covariance techniques.

Description of the work performed since the beginning of the project and the main results achieved so far

During his outgoing phase, the researcher had interaction with experts at Berkeley and Riverside University (Dr. John Karlik), and started planning experiments in the greenhouse facility of UC Berkeley using north American and Italian Citrus varieties, and at the same time identifying suitable locations for field experiments. In the first 1, 5 years of project duration, Level I, Level II and Level III objectives were accomplished.

Level I: At plant level, a plant enclosure for gas-exchange studies were designed to expose the whole plant under observation to known ozone concentrations, and to directly measure the ozone fluxes, CO2 and water exchange. Plants were measured at varying conditions of light and temperature during the different phenological stages. The experiments were carried out in a greenhouse on a set of plants grown in ozone-free air. Air temperature and light intensity were modulated to induce different levels of stomatal aperture. Ozone uptake of different Citrus species was ultimately quantified. VOC measurements were performed simultaneously to the measurements of ozone uptake. Large emissions were detected during flowering episodes. Measurements showed distinct emissions from different plant varieties during both day and night, and allowed to derive diurnal profiles and light dependencies.

Level II: On a monthly basis, leaf samples from Citrus trees in the field site were collected and stored. Laboratory measurements of pools of isoprenoids content in leaves revealed an inverse correlation with isoprenoid fluxes, suggesting that during summer and hot periods, thickening of wax coatings on leaf cuticles favour isoprenoids accumulation in leaves, thus limiting fluxes.

Level III: A suitable field site was identified in the California central valley, Exeter, thanks to the cooperation with experts at UC Davis cooperative extension (DR. John Karlik). During one-year long continuous measurements, routine meteorological data were measured from the top level of a tower installed in the middle of an orange orchard. Vegetation metrics used in VOC emission models (canopy area, leaf area index, leaf mass density, etc.) were collected simultaneously during the measurements. Additionally, concentrations of O3, CO2, H2O, VOC, and energy were measured continuously at high temporal resolution, and fluxes were calculated by the Eddy Covariance method above the crop canopy as the covariance of the concentration with the vertical component of the wind speed. An extensive set of VOC measurements were performed during key stages of crop phenology. Observations allowed to describe temporal and spatial variations of emission rates and chemical processes, coupling measurements with GC-MS and PTR-MS, thus providing all information about specific VOC emitted in a high time resolution for flux calculation.

Expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far)

The study was aimed to bring innovation to the sector through a very interdisciplinary approach. For the first time, reactive VOC emitted from Citrus plantations in real time, understanding the basis for their control by phenology, agricultural practices, and environmental parameters, and particularly focusing on the interactions between VOC emissions and ozone uptake by Citrus plants. At a plant level, the applicant designed and customised a new Teflon plant enclosure for gas exchange studies which allowed precise and on-line measurements of reactive molecules (e. g. ozone, NOx, VOC) at whole canopy level and in an integrated way with detection of other gas exchange parameters to assess plant functionality. The applicant`s idea to build a unique system for measurements of all these gaseous compounds, assembling the plant enclosures with multiple detectors (IRGAs, ozone detector, GC-MS and PTR-MS) was transformed into action. The laboratory studies had an important scientific value since they allowed a mechanistic comprehension of ozone uptake through stomatal and non-stomatal processes involving reactions with VOC is to be achieved.

The micrometeorological part of the work was also characterised by high innovation. Citrus large plantations in areas of California and Italy emit a high blend of reactive compounds and the potential of these emissions for interaction with the chemistry of troposphere was still not fully unrevealed. Limited data have been developed for some agricultural crops including Citrus, but the biological bases for ozone deposition and VOC emissions from Citrus species were not been characterised, and integrated field scale measurements of these exchanges using in-situ eddy covariance flux measurements were missing. The proposed study filled these gaps, specifically testing the idea that emissions of fast reacting isoprenoid species may currently be significantly underestimated by past approaches, due to ozone destruction within plant canopies. The data collected in the field will be used to parameterise and validate models on ozone fluxes for crop species which emit VOC.

Overall, laboratory and field experiments using advanced and novel instrumentation on ozone deposition and destruction, CO2 sequestration, VOC compound identities, emission rates and fluxes, and dependence of emissions on environmental and physiological parameters will reduce uncertainty in VOC emission inventories for Citrus.

Two more innovative outputs of this project were achieved. First, mechanistic and empirical description of VOC contribution to ozone regulation in the atmosphere (i. e. ozone formation or scavenging) will impact on environmental policies, and in particular on regulations concerning air quality control and agricultural policies in polluted areas. It may ultimately bring to modification of agricultural practices and crop selections to help with environmental control. Second, results of this study will provide basic data needed for extrapolation to larger scale emission estimates. In particular, the project data are providing the background for modification of existing algorithms and models to predict current and future interactions between biosphere and the atmosphere in areas with strong anthropogenic pressure and fragile environmental conditions.