Ellen – Excellent work. Very good summary and synthesis of high quality papers on a complex topic. Written thoughtfully and professionally. Your English can be improved (and will improve with more practice), but otherwise you make your points clear and organized your thoughts well. I liked your Conclusion section very much, because it showed how well you had digested the material. Good work. See below for more comments. Critical Scientific Chemical Logical Written Late Total (20) thinking rigor (4) content (4) flow (2) quality submission (4) (6) (%) 4 4 4 2 5 0 19 CHEM 380 Fall 2014 Yilin Lu Review: Impacts of Increasing Tropospheric Ozone on Plants 1. Introduction: As more and more people pay attention to air quality, people have begun to notice that CO2 is not the only byproduct produced during industrial processes. More pollutants are also generated, such as volatile organic compounds (VOCs), NOx (mainly NO and NO2) along with particulate matter. While increasing CO2 is responsible for enhanced global warming, providing high temperature and favoring the chemical reaction to produce a secondary pollutant: tropospheric ozone (incomplete sentence). As CO2, ozone can be removed by plants, which are considered as natural cleaners. Yet, ozone is also harmful to both animals and plants due to its high activity. Damaged plants will not function well and if the damage is too severe to cure, plants could die releasing all stored C, N, and H2O back to the atmosphere and soil. As a result, atmospheric CO2 could be increased, which could cause more enhanced global warming. So, we want to keep plants healthy since they are critical links in ecological cycles and in global climate management. Tomoko Komada 12/24/14 10:25 PM
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Deleted: with This paper will mainly focus on how ozone can cause damage to the plants and, on the other hand, how plants will react back, including how they can resist against ozone at some point. Good introduction. 2.1. Oxidative Damage Associated with High Ozone Concentration One possible impacts of tropospheric ozone taken up by plants through leaf is that it can react with chemical substances inside of leaf, damaging leaf structure and breaking down the redox balance of leaf to an oxidative environment (Bortolin et al. 2014). In the experiment preformed by Bortonlin and his partners, for the purpose of determining what impacts of long-­‐term exposure to high concentration ozone could be on the pepper plants, red pepper plants were exposed to O3 at an average concentration around171.5ug/m3 (about 171.6 ppb at 15 oC at mean sea-­‐level) for six hours per day and the experiment lasted more than two month. They find that ozone is highly active with lipids to produce reactive oxygen species (ROS), such as H2O2, and thiobarbituric acid reactive substances (TBARS)—
malondialdehyde — in cell membranes while these ROS could further damage proteins by reacting with amino acids, increasing carbonyl groups. Thus, damages on lipids and proteins are represented as increase in TBARS (about 57% of the control samples, which are not exposed into the high-­‐level ozone) and in carbonyl groups content (about 160%), respectively, in the experiment. Losing lipids and proteins, plants are losing stored energy as well as nutrition. Furthermore, some proteins act as enzymes. Damage in proteins during long-­‐term exposure could be linked to the evidence found in the experiment, which shows that several antioxidant enzymes decreased. Compared to the control group, superoxide dismutase (SOD) dropped 36%, catalase (CAT) decreased by 43% while ascorbate peroxidase (APX) was 38% less than the control samples (Bortolin et al. 2014). Antioxidant enzymes are critical to prevent plants against oxidative damage. Yet, under the condition of long-­‐term exposure to high ozone concentration, the enzymes themselves are inactive by ROS, leading to a more oxidative environment and thus more harm for leaf structure. Not only reducing proteins, ROS are also associated with chlorophyll reduction – you mean lowering chlorophyll content? (Bortolin et al. 2014). According to the authors, there are two hypotheses to result in this reduction: first, the high concentration of ozone combined with long period exposure has damage on chlorophyll, which might be due to the reaction with ROS; secondly, ozone could act as catalyst to speed up the breakdown process of chlorophyll during leaves fading. Either one of the two, or both of them, could be linked to decrease in the chlorophyll content in leaves as observed in the experiment – both chlorophyll a and chlorophyll b decreased while chlorophyll a decreased more than chlorophyll b (by 42% and by 25%, respectively). As chlorophyll plays an important role in photosynthesis process – absorbing certain wavelength of light for the reaction, providing election and producing ATP during electron transport chain, decreasing chlorophyll will certainly limit photosynthetic productivity, decreasing plants’ abilities of transforming energy and processing carbon fixation. As a result, plants will have less energy to maintain normal functions. Tomoko Komada 12/24/14 10:29 PM
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Comment [1]: Here, by reducing, you mean decreasing in content, correct? Following decrease in stored energy and damage to enzymes caused by ozone exposure, decrease in total polyphenol content by around 38% was also found as one of the results of the experiment. Bortolin et al. point out that, as antioxidant enzymes, polyphenol is expected to protect proteins and mainly lipids from oxidative damages. However, the decrease of polyphenol itself, as they explain, possibly results from either inadequacy of energy produced by photosynthesis or inadequacy of enzymes to maintain the production of polyphenol. Hence, ozone-­‐enrichment could invalidate? the protection to lipids and proteins and enhance injures on plants. All of the above impacts are linked to a long-­‐term exposure to ozone at a high concentration, which could be somehow as an extreme case and might have different outcomes from the experiments with a lower concentration for a less exposure period. Good. But would be stronger if you cited more than just Bortolin et al. (2014) to make the case. 2.2. Decreasing Stomatal Conductance Responsive to Ozone Stress Another impact of tropospheric ozone on plants could be that most plants would decrease stomatal conductance as a means to controlling ozone in-­‐take. However, this reaction could lead to other problems while when? ozone concentration is very high. Pellegrini et al. (YEAR) executed an experiment to examine how reduction of stomatal conductance could lead to negative feedbacks on outcomes of photosynthesis by comparing the control group (Melissa officinalis plants without being treated by ozone) to the ones that were exposed to 200ppb ozone for 5 hours as well as at the 24th hour and the 48th hour after ending the exposure (so-­‐called recovery periods)(2011). They found that both stomatal conductance to water vapor and photosynthetic activities maintained decreasing through 24-­‐hour of the recovery period while intercellular CO2 increased through all 48-­‐hour recovery period. The increase in intercellular CO2 represents decreased in carbon fixation efficiency of plants due to the decrease in photosynthetic efficiency (Pellegrini et al., 2011). As discussed above, one possible cause to the decreased photosynthesis could be the direct or indirect damage to the chlorophyll from ozone. In addition, the other reasonable cause would be linked to excessive stomatal closure (Pellegrini et al., 2011). Stomatal closure would decrease the amount of water evaporating, which is associated with decrease in absorption of water from roots as well as transportation it to leaves (transpiration effect), thus decreasing photosynthesis efficiency by decreasing the amount of one of its required reactants. Interesting. Furthermore, less water transported to leaves could lead to reduction in election transport rate (ETR) in the photosystem II – it remained decreasing during the whole recovery period in the experiment. Meanwhile, ozone exposure resulted in an increase in the amount of electrons needed for the carbon fixation (Pellegrini et al., 2011). This means there is more demand of electrons than supply and, as a result, the ability of carbon fixation is decreased, explaining the increase in intercellular CO2. This is another way how excessive stomatal closure negatively impact on the photosynthesis effect. Moreover, stoma closure could be further related to decrease in transportation of nutrition absorbed by roots (transpiration effect) as well as decrease in respiration, which provides energy for plants to remain basic functions. These consequences might also be the factors that cause damage to plants’ health (still citing Pellegrini et al.?). Tomoko Komada 12/24/14 10:35 PM
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Comment [4]: grammar – do you mean “it continued to decrease”? Tomoko Komada 12/24/14 10:40 PM
Deleted: . 2.3. Plants’ Resistance to Ozone When ozone at high concentrations has general negative impacts on plants, plants themselves have certain coping ability against ozone at lower concentrations. First, as mentioned, decreasing stomatal conductance is a technique that most plants use as a response of ozone stress. An experiment was executed, on two soybean species – Sambaiba and Tracaja, to assess how stomatal closure responds to ozone as a kind of damage resistance for different species while each of them was exposed to filtered air as control group and to filtered air with 40 and 80 ppb ozone respectively as two experimental groups for 30 hours in all within 5 days (Bulbovas et al., 2014) this sentence is too long; break it up. It follows that Sambaiba, with almost no control in stomatal conductance (gs – what is this? Shorthand for stomatal conductance?), has no resistance for ozone damage, which was measured by visible leaf injury (VLI), at both concentrations. While gs remained about 0.2 mol H2O/(m2*s) for all three groups tested, VLI increased 5% under 40ppb ozone and around 24% under 80ppb ozone, which are compared to non-­‐injury control group. Meanwhile, Tracaja, which has ability to close stoma to avoid ozone uptake, has certain ability to prevent ozone damage under 40ppb ozone, but also get damaged under 80ppb ozone. Yet, the damage to Tracaja still seems less than the one to Sambaiba even under 80ppb ozone. Gs of Tracaja decreased about 24% under 40ppb ozone and 57% under 80ppb as VLI was only 0.9% under 40ppb ozone and suddenly rose to 20.5% under 80ppb ozone. The experiment shows that, although excessive stomatal closure under ozone-­‐rich environment could still lead to plant injury as mentioned in the section 2.2, stomatal closure still is an efficient way to coping against low ozone stress. Also, not all plants have this ability, and those who don’t have would be more sensitive to ozone corresponding to the direct damage caused by ozone as discussed in the section 2.1. Besides, plants usually contain antioxidant substances that protect them from certain amount of oxidative damage in spite of the fact that they could be destroyed by ROS when plants are exposed under ozone-­‐rich air for a long time. The examples of these antioxidant substances are β-­‐carotene, anthocyanin, ascorbate (ASA), reduced glutathione (GSH) and CAT (Pellegrini et al., 2011; Bortolin et al. 2014; and Bulbovas et al., 2014). Different antioxidant substances react differently to ozone stress. β-­‐carotene decreases under ozone stress (Pellegrini et al., 2011). In contrast, anthocyanin, ascorbate (ASA), reduced glutathione (GSH) and CAT will increase (Bortolin et al. 2014 and Bulbovas et al., 2014). Last but not the least, there could be some unknown factors that contribute different resistance of different species against ozone damage. Some species have a clear linear relationship between damage due to ozone and concentrations of ozone with a positive slope, such as soybean (Sun et al. 2014); higher ozone concentrations, more damage caused. However, when observing other plants (including not only crops but also grasses, temperature deciduous tree, tropical tree and etc.), the relationship becomes unclear, that is, ozone damage to plants such as negative impacts on photosynthesis does not necessary increase with increasing ozone density (Lombardozzi et al., 2013). What makes different species have different responses to ozone exposure could be due to different ozone resistances that different plants might have. Understanding what factors associated with the resistances could be helpful to develop a plan related to natural cleaners of ozone in urban environment. Tomoko Komada 12/24/14 10:41 PM
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Comment [5]: Not a good word to start a new paragraph with. Better transition here could be “In addition to potential stomatal control,…” Tomoko Komada 12/24/14 10:48 PM
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Deleted: to 3. Conclusion: Overall, plants themselves have some abilities to resist against ozone damage under a low density of ozone. Therefore, they can help to “clean” ozone from the environment by in-­‐
taking it, just as how they can do for CO2. Yet, these abilities won’t work under an ozone-­‐
rich environment due to either oxidative damage or being force to limit the size of stomata too much under ozone stress. The basic functions of photosynthesis, transpiration as well as respiration may be also limited as a consequence, reducing the amount of energy that can support plants and, therefore, harming the plants. However, we don’t really know which density of ozone could be consider as low or high and the answer to this question could also be different for different pants. Also, we are not sure what makes stomata of different plants act differently against ozone. Furthermore, what really makes different plants have different relationship with ozone concentration (different abilities of resistance) is another question we don’t have an answer for yet. In future research, they could be a good direction for us to study more about interactions between ozone and plants. After all, ozone pollutions (photochemical smog and atmospheric brown clouds) are regional problems due to the short lifetime of ozone – about 2 weeks in summer and 2 months in winter. It is possible that we can use this feature combined with future researches on different resistance of plants against ozone to make a plan about removing ozone from atmosphere by those natural cleaners at high-­‐risk locations. References: Bortolin R. C., Caregnato F. F., Divan A. M. Jr., Reginatto F. H., Gelain D. P., and Fonseca Moreira J. C. (2014) Effects of chronic elevated ozone concentration on the redox state and fruit yield of red pepper plant capsicum baccatum. Ecotoxicology and Environmental Safety. 100:114 -­‐ 121 Bulbova P., Souza S.R., Esposito J. B. N., Moraes R.M., Alves E.S. Domingo M. and Azevedo R. A. (2014) Assessment of the ozone tolerance of two soybean cultivars (Glycine mad cv. Sambaiba and Tracaja cultivated in Amazonian areas. Environmental Science and pollution Research 21:10514 -­‐ 10524 Lombardozzi D., Sparks J.P., and Bonan G. (2013) Integrating O3 influences on terrestrial processes: photosynthetic and stomatal response data available for regional and global modeling. Biogeosciences. 10: 6815-­‐6831 Pellegrini E., Carucci M. G., Campanella A., Lorenzini G., and Nail C. (2011) Ozone stress in Melissa officinalis plants assessed by photosynthei function. Environmental and Experimental Botany. 73:94 – 101 Tomoko Komada 12/24/14 10:49 PM
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Sun J., Feng Z., and Ort D.R. (2014) Impacts of rising tropospheric ozone on photosynthesis and metabolite levels on field grown soybean. Plant Science. 226: 147-­‐161