Plant biochemistry influences tropospheric ozone formation
Received: 06-May-2022 / Manuscript No. bcp-22-63545 / Editor assigned: 09-May-2022 / PreQC No. bcp-22-63545 / Reviewed: 14-May-2022 / QC No. bcp-22-63545 / Revised: 20-May-2022 / Manuscript No. bcp-22-63545 / Published Date: 27-May-2022 DOI: 10.4172/2168-9652.1000377
Abstract
Tropospheric ozone (O3) is among the most damaging air pollutant to plants. Plants alter the atmospheric O3 concentration in two distinct ways: (i) by the emission of volatile organic compounds (VOCs) that are precursors of O3; and (ii) by dry deposition, which includes diffusion of O3 into vegetation through stomata and destruction by nonstomatal pathways. Isoprene, monoterpenes, and higher terpenoids are emitted by plants in quantities that alter tropospheric O3. Deposition of O3 into vegetation is related to stomatal conductance, leaf structural traits, and the detoxification capacity of the apoplast. The biochemical fate of O3 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.
Keywords
Introduction
Tropospheric O3 formation
O3 in the stratosphere filters UV radiation, but in the troposphere O3 is a damaging air pollutant to human and plant health [Environmental Protection Agency EPA. Tropospheric O3 (trioxygen) is an allotrope of oxygen that forms through chemical reactions with two chemically distinct precursors: nitrogen oxides (NOx = NO + NO2) and reactive carbon molecules including carbon monoxide (CO), methane (CH4), and VOCs Rates of O3 formation depend on sunlight and the relative concentrations of NOx and reactive carbon molecules; namely, methane and VOCs [ ]. The reaction of nitric oxide (NO) with the peroxy radical (RȮ2) is the central reaction for the formation of O3 in the troposphere. In this reaction, NO is converted to NO2 which is rapidly photolyzed to form O3 and recycle NO. The efficiency with which O3 is produced from NOx pollution varies with the location and time of emissions. For example, in the polluted regions at the Earth’s surface, NOx rapidly reacts to form HNO3, which serves as a reservoir for NOx. In less polluted areas, NO2 photolysis competes more effectively with HNO3 production and more molecules of NOx react with peroxy radicals to form O3. In regions where NOx is propelled into the free troposphere, like the tropics, O3 production is especially efficient. Additionally, the VOC:NOx ratio determines the O3 concentration. In urban areas with elevated NOx due to high emissions, O3 formation is limited by VOCs, leading to locally suppressed O3 concentrations. NOx transported away from urban centers can mix with VOCs, resulting in greater O3 concentrations in suburban areas [1].
VOCs
Plants produce a vast diversity of biogenic VOCs, including isoprene, monoterpenes, and higher terpenoids .Terrestrial vegetation emits isoprene at high levels (~400–600 Tg C year− ) and isoprene has high chemical reactivity in the troposphere .The tropospheric lifetime of isoprene is only ~ –2 h and it is rapidly oxidized by hydroxyl radicals, O3, and nitrate radicals (NO3). The degradation of VOCs leads to the formation of peroxyl radicals. Those react with NO to form NO2, which then photolyzes to form O3 In areas with very low NOx, peroxy radicals formed from isoprene oxidation react with each other or O3, resulting in net O3 destruction. Globally, modeling studies estimate that forestemitted isoprene increases the tropospheric O3 concentration by 5–8% .Isoprene oxidation can also produce peroxyacylnitrates (PANs), which can be transported long distances under cool, high-altitude conditions. The long-distance transport of PANs can contribute to O3 formation far from the pollutant source. Thus, globally, biogenic VOCs contribute to O3 formation in the troposphere, although there is significant variation in isoprene emissions among ecosystems and species .For example, broadleaf forests have average isoprene emissions of 2.6 mg m-2 h- , needle-leaf evergreen trees emit 2.0 mg m-2 h- , and crops emit very little, only 0.09 mg m-2 h-; . This variation in emission led to concerns that increasing the planting of isoprene-emitting bioenergy species will increase O3 stress, leading to crop yield loss and increased human mortality [2].
Stomatal control of O3 deposition
Deposition of O3 to terrestrial ecosystems is a significant sink for O3, and understanding variation among ecosystems and species in O3 uptake is needed for accurate prediction of tropospheric O3 concentrations .Dry deposition occurs when atmospheric turbulence transports O3 close to a surface and then O3 moves through a boundary layer around a surface. O3 dry deposition occurs through stomata as well as other, non-stomatal pathways including uptake by leaf cuticles, soil, water, snow, and manmade surfaces. A synthesis of observation studies found that stomatal uptake accounts for 45% of O3 deposition on average across ecosystems. This percentage varies with season and ecosystem, but given a prominent role of stomata in O3 deposition, understanding O3 flux through stomata is a major research focus. To estimate O3 diffusion through stomata, the resistance of stomata to water vapor is multiplied by the ratio of the diffusivity of water vapor to that of O3 (.6 ), with the assumption that the water leaving a leaf is proportional to the O3 entering and that O3 reactions in the leaf do not limit stomatal uptake. Both of these assumptions, that water loss is proportional to O3 uptake and that there is negligible resistance to O3 destruction inside the leaf, have been questioned and remain active research areas. Furthermore, long-term exposure to elevated O3 pollution often reduces plant biomass and stomatal conductance, which limits subsequent O3 deposition and can feed forward to increase atmospheric O3 concentration [3].
The rapid response of stomata to O3
Greater stomatal conductance tends to lead to more sensitivity to O3, often attributable to greater O3 uptake and subsequent oxidative damage. In the model species arabidopsis, natural variation in O3 sensitivity, measured as ion leakage, was correlated with wholerosette conductance. Additionally, the greater O3 sensitivity of the Cape Verde island accession has been linked to constitutively high stomatal conductance caused by impaired function of mitogenactivated protein kinase [4]. Thus, stomatal closure is a direct way to reduce O3 uptake by leaves and alleviate oxidative damage. Stomatal pores close rapidly in response to acute O3 exposure, followed by reopening, which depends on the O3 treatment concentration and duration . Some low-level O3 exposure may also allow a faster response to higher doses of O3 and therefore provide protection against greater O3-induced injury, a process known as priming. In an experiment with common bean, exposure of leaves to 30 min of 200-ppb O3 before a greater, 600-ppb treatment resulted in greater stomatal closure and lower VOC emissions compared with the 600-ppb treatment alone. The correlation of sensitivity to O3 stress with stomatal conductance, and the fact that stomata close in response to O3, suggest that greater O3 tolerance could be engineered by altering stomatal conductance. Stomata are the entry points for the CO2 used for photosynthesis and so reducing stomatal conductance might also reduce CO2 entry into the leaf and compromise productivity. However, recent work has demonstrated that genetic manipulations to reduce stomatal density only moderately reduced stomatal conductance and did not change photosynthesis, suggesting that there is room to optimize stomatal density to atmospheric conditions [5].
Conclusion
Identifying the extent to which plant biochemistry and physiology contribute to tropospheric O3 formation, destruction, and deposition will help in understanding the mechanisms that underpin plant O3 sensitivity and improve predictions of global tropospheric O3 concentration. Plant species release more than 30 000 different biogenic VOCs, including reactive classes of non-methane biogenic VOCs such as isoprene, which are emitted in large enough quantities to impact tropospheric O3 concentrations. While isoprene can increase O3 concentrations locally, monoterpenes and higher terpenoid compounds also rapidly react with O3 in the leaf boundary layer and can protect plants from oxidative stress. Deposition of O3 into vegetation is related to stomatal conductance and leaf structural traits. While there is evidence that antioxidants quench ROS within leaves, variation in detoxification capacity among different species is significant and the biochemical fate of O3 once it enters leaves and reacts with aqueous surfaces remains largely unknown New techniques for the tracking and identification of initial products have the potential to shed light on that question and could improve the identification of targets to increase O3 tolerance.
Acknowledgement
None
Conflict of Interest
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Citation: Wasylka P (2022) Plant biochemistry influences tropospheric ozone formation. Biochem Physiol 11: 377. DOI: 10.4172/2168-9652.1000377
Copyright: &Copy; 2022 Wasylka P. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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