Human activities result in a wide array of pollutants being released to the atmosphere. A number of these pollutants have direct effects on plants, including carbon dioxide (CO 2 ), which is the substrate for photosynthesis, and ozone (O 3 ), a damaging oxidant. How plants respond to changes in these atmospheric air pollutants, both directly and indirectly, feeds back on atmospheric composition and climate, global net primary productivity and ecosystem service provisioning. Here we discuss the past, current and future trends in emissions of CO 2 and O 3 and synthesise the current atmospheric CO 2 and O 3 budgets, describing the important role of vegetation in determining the atmospheric burden of those pollutants. While increased atmospheric CO 2 concentration over the past 150 years has been accompanied by greater CO 2 assimilation and storage in terrestrial ecosystems, there is evidence that rising temperatures and increased drought stress may limit the ability of future terrestrial ecosystems to buffer against atmospheric emissions. Long-term Free Air CO 2 or O 3 Enrichment (FACE) experiments provide critical experimentation about the effects of future CO 2 and O 3 on ecosystems, and highlight the important interactive effects of temperature, nutrients and water supply in determining ecosystem responses to air pollution. Long-term experimentation in both natural and cropping systems is needed to provide critical empirical data for modelling the effects of air pollutants on plant productivity in the decades to come.

IntroductionBy mid-century, global atmospheric carbon dioxide concentration ([CO2]) is predicted to reach 600 μmol mol−1 with global temperatures rising by 2 °C. Rising [CO2] and temperature will alter the growth and productivity of major food and forage crops across the globe. Although the impact is expected to be greatest in tropical regions, the impact of climate-change has been poorly studied in those regions.ObjectivesThis experiment aimed to understand the effects of elevated [CO2] (600 μmol mol−1) and warming (+ 2 °C), singly and in combination, on Panicum maximum Jacq. (Guinea grass) metabolite and transcript profiles.MethodsWe created a de novo assembly of the Panicum maximum transcriptome. Leaf samples were taken at two time points in the Guinea grass growing season to analyze transcriptional and metabolite profiles in plants grown at ambient and elevated [CO2] and temperature, and statistical analyses were used to integrate the data.ResultsElevated temperature altered the content of amino acids and secondary metabolites. The transcriptome of Guinea grass shows a clear time point separations, with the changes in the elevated temperature and [CO2] combination plots.ConclusionField transcriptomics and metabolomics revealed that elevated temperature and [CO2] result in alterations in transcript and metabolite profiles associated with environmental response, secondary metabolism and stomatal function. These metabolic responses are consistent with greater growth and leaf area production under elevated temperature and [CO2]. These results show that tropical C4 grasslands may have unpredicted responses to global climate change, and that warming during a cool growing season enhances growth and alleviates stress.Electronic supplementary materialThe online version of this article (10.1007/s11306-019-1511-8) contains supplementary material, which is available to authorized users.

In omics experiments, variable selection involves a large number of metabolites/ genes and a small number of samples (the n < p problem). The ultimate goal is often the identification of one, or a few features that are different among conditions- a biomarker. Complicating biomarker identification, the p variables often contain a correlation structure due to the biology of the experiment making identifying causal compounds from correlated compounds difficult. Additionally, there may be elements in the experimental design (blocks, batches) that introduce structure in the data. While this problem has been discussed in the literature and various strategies proposed, the over fitting problems concomitant with such approaches are rarely acknowledged. Instead of viewing a single omics experiment as a definitive test for a biomarker, an unrealistic analytical goal, we propose to view such studies as screening studies where the goal of the study is to reduce the number of features present in the second round of testing, and to limit the Type II error. Using this perspective, the performance of LASSO, ridge regression and Elastic Net was compared with the performance of an ANOVA via a simulation study and two real data comparisons. Interestingly, a dramatic increase in the number of features had no effect on Type I error for the ANOVA approach. ANOVA, even without multiple test correction, has a low false positive rates in the scenarios tested. The Elastic Net has an inflated Type I error (from 10 to 50%) for small numbers of features which increases with sample size. The Type II error rate for the ANOVA is comparable or lower than that for the Elastic Net leading us to conclude that an ANOVA is an effective analytical tool for the initial screening of features in omics experiments.

Tropospheric ozone (O 3 ) is among the most damaging air pollutant to plants. Plants alter the atmospheric O 3 concentration in two distinct ways: (i) by the emission of volatile organic compounds (VOCs) that are precursors of O 3; and (ii) by dry deposition, which includes diffusion of O 3 into vegetation through stomata and destruction by nonstomatal pathways. Isoprene, monoterpenes, and higher terpenoids are emitted by plants in quantities that alter tropospheric O 3 . Deposition of O 3 into vegetation is related to stomatal conductance, leaf structural traits, and the detoxification capacity of the apoplast. The biochemical fate of O 3 once it enters leaves and reacts with aqueous surfaces is largely unknown, but new techniques for the tracking and identification of initial products have the potential to open the black box. Tropospheric O 3 formation O 3 (see Glossary) in the stratosphere filters UV radiation, but in the troposphere O 3 is a damaging air pollutant to human and plant health [Environmental Protection Agency (EPA): https://www.epa.gov/ground-level-ozone-pollution; Box 1]. Tropospheric O 3 (trioxygen) is an allotrope of oxygen that forms through chemical reactions with two chemically distinct precursors: nitrogen oxides (NO x = NO + NO 2 ) and reactive carbon molecules including carbon monoxide (CO), methane (CH 4 ), and VOCs (Figure 1) [1]. Rates of O 3 formation depend on sunlight and the relative concentrations of NO x and reactive carbon molecules; namely, methane and VOCs [1]. The reaction of nitric oxide (NO) with the peroxy radical (R 2 ) is the central reaction for the formation of O 3 in the troposphere [2]. In this reaction, NO is converted to NO 2 (Figure 1), which is rapidly photolyzed to form O 3 and recycle NO. The efficiency with which O 3 is produced from NO x pollution varies with the location and time of emissions. For example, in the polluted regions at the Earth's surface, NO x rapidly reacts to form HNO 3 , which serves as a reservoir for NO x [3]. In less polluted areas, NO 2 photolysis competes more effectively with HNO 3 production and more molecules of NO x react with peroxy radicals to form O 3 . In regions where NO x is propelled into the free troposphere, like the tropics, O 3 production is especially efficient [1,4]. Additionally, the VOC:NO x ratio determines the O 3 concentration [5]. In urban areas with elevated NO x due to high emissions, O 3 formation is limited by VOCs, leading to locally suppressed O 3 concentrations. NO x transported away from urban centers can mix with VOCs, resulting in greater O 3 concentrations in suburban areas [5].Global O 3 production in the troposphere is estimated to be between 4960 and 5530 Tg year −1 (Figure 1), with most O 3 produced from chemical reactions and a smaller amount exchanged with the stratosphere [6]. Although most of the O 3 produced in the troposphere is lost by chemical conversions, dry deposition of O 3 to the terrestrial biosphere accounts for nearly 20% of O 3 removal from the troposphere [7]. Temperature also in...

Tropospheric ozone is a major air pollutant that significantly damages crop production. Crop metabolic responses to rising chronic ozone stress have not been well studied in the field, especially in C 4 crops. In this study, we investigated the metabolomic profile of leaves from two diverse maize ( Zea mays ) inbred lines and the hybrid cross during exposure to season‐long elevated ozone (~100 nl L −1 ) in the field using free air concentration enrichment (FACE) to identify key biochemical responses of maize to elevated ozone. Senescence, measured by loss of chlorophyll content, was accelerated in the hybrid line, B73 × Mo17, but not in either inbred line (B73 or Mo17). Untargeted metabolomic profiling further revealed that inbred and hybrid lines of maize differed in metabolic responses to ozone. A significant difference in the metabolite profile of hybrid leaves exposed to elevated ozone occurred as leaves aged, but no age‐dependent difference in leaf metabolite profiles between ozone conditions was measured in the inbred lines. Phytosterols and α‐tocopherol levels increased in B73 × Mo17 leaves as they aged, and to a significantly greater degree in elevated ozone stress. These metabolites are involved in membrane stabilization and chloroplast reactive oxygen species (ROS) quenching. The hybrid line also showed significant yield loss at elevated ozone, which the inbred lines did not. This suggests that the hybrid maize line was more sensitive to ozone exposure than the inbred lines, and up‐regulated metabolic pathways to stabilize membranes and quench ROS in response to chronic ozone stress.