Short-term physiological and biometrical responses of Lepidium sativum seedlings exposed to PET-made microplastics and acid rain.

Plastics enter in terrestrial natural system primarily by agricultural purposes, while acid rain is the result of anthropogenic activities. The synergistic effects of microplastics and acid rain on plant growth are not known. In this study, different sizes of polyethylene terephthalate (PET) and acid rain are tested on Lepidium sativum, in two separate experimental sets. In the first one we treated plants only with PET, in the second one we used PET and acid rain together. In both experimentations we analyzed: i) plant biometrical parameters (shoot height, leaf number, percentage inhibition of seed germination, fresh biomass), and ii) oxidative stress responses (hydrogen peroxide; ascorbic acid and glutathione). Results carried out from our experiments highlighted that different sizes of polyethylene terephthalate are able to affect plant growth and physiological responses, with or without acid rain supplied during acute toxicity (6 days). SHORT DESCRIPTION: This study showed that different sizes of PET microplastics affect physiological and biometrical responses of Lepidum sativum seedlings, with or without acid rain; roots and leaves responded differently.

Physiological and biochemical responses of tea seedlings (Camellia sinensis) to simulated acid rain conditions.

Tea (Camellia sinensis), widely planted in the south of China, and often exposed to acid rain. However, research concerning the impacts of acid rain on physiology and biochemistry of tea plants is still scarce. In this study, we investigated the influence of simulated acid rain (SAR) on plant height, root length, photosynthetic pigment, Fv/Fm, proline, malondialdehyde, antioxidant enzyme activity, total nitrogen, caffeine, catechins, and free amino acids. Our results showed that SAR at pH 4.5 did not hinder plant development because growth characteristics, photosynthesis, and ascorbate peroxidase and catalase activities did not decrease at this pH compared to those at the other investigated pH values. However, at pH 3.5 and pH 2.5, the activities of antioxidase and concentrations of malondialdehyde and proline increased significantly in response to the decrease of photosynthetic pigments and Fv/Fm. In addition, the increase in acidity increased total nitrogen, certain amino acid content (theanine, cysteine), and decreased catechin and caffeine contents, resulting in an imbalance of the carbon and nitrogen metabolisms. Our results indicated that SAR at pH 3.5 and pH 2.5 could restrict photosynthesis and the antioxidant defense system, causing metabolic disorders and ultimately affecting plant development and growth, but SAR at pH 4.5 had no toxic effects on tea seedlings when no other stress factors are involved.

Ecophysiological responses of Jatropha curcas L. seedlings to simulated acid rain under different soil types.

Acid rain is a global environmental problem. Acid rain can affect plants directly by damaging the leaves and indirectly by soil acidifying. Many studies have been conducted to investigate the impacts of acid rain on plant under a single soil type. However, there is little information on the effect of acid rain on plant under different soil types. Jatropha curcas L. is an energy plant widely distributed in acid rain pollution area with various soil types. In this study, we investigated the effects of acid rain (pH2.5, pH3.5, pH4.5, pH5.6) on the growth, physiology, nutrient elements and bacterial community of J. curcas seedlings under different soil types [Red soils (RS), Yellow soils (YS), Yellow-brown soils (YBS), and Purplish soils (PS)]. Acid rain and soil types significantly influence the growth of J. curcas seedlings, and there was a significant interaction between acid rain and soil types. Acid rain (pH 4.5) was beneficial to the growth of J. curcas seedlings, whereas acid rain (pH 2.5 or 3.5) inhibited growth of J. curcas seedlings. The growth of J. curcas seedlings could resist the stress of acid rain by scavenging and detoxification of active oxygen species in leaves. Combined with the increase in relative growth rate of seedlings treated with simulated acid rain at pH 4.5, we inferred that K can stimulate the growth of seedlings. The lower soil pH, cation exchange capacity and base saturation had stronger inhibitory effects on growth of J. curcas seedlings. YBS and PS were beneficial for growth of J. curcas seedlings by higher buffering capacity under acid rain treatments. The phylum Proteobacteria was found to predominate in rhizosphere soils. YBS was favorable to support Proteobacteria growth and reproduction. The redundancy analysis showed that the Cyanobacteria were favorable to growth of J. curcas seedlings.

Effects of sulfuric, nitric, and mixed acid rain on Chinese fir sapling growth in Southern China.

The influence of acid rain on plant growth includes direct effects on foliage as well as indirect soil-mediated effects that cause a reduction in root growth. In addition, the concentration of NO3- in acid rain increases along with the rapid growth of nitrogen deposition. In this study, we investigated the impact of simulated acid rain with different SO42-/NO3- (S/N) ratios, which were 1:0, 5:1, 1:1, 1:5 and 0:1, on Chinese fir sapling growth from March 2015 to April 2016. Results showed that Chinese fir sapling height growth rate (HGR) and basal diameter growth rate (DGR) decreased as acid rain pH decreased, and also decreased as the percentage of NO3- increased in acid rain. Acid rain pH significantly decreased the Chlorophyll a (Chla) and Chlorophyll b (Chlb) content, and Chla and Chlb contents with acid rain S/N 1:5 were significantly lower than those with S/N 1:0 at pH 2.5. The chlorophyll fluorescence parameters, maximal efficiency of Photosystem II photochemistry (Fv/Fm) and non-photochemical quenching coefficient (NPQ), with most acid rain treatments were significantly lower than those with CK treatments. Root activities first increased and then decreased as acid rain pH decreased, when acid rain S/N ratios were 1:1, 1:5 and 0:1. Redundancy discriminant analysis (RDA) showed that the Chinese fir DGR and HGR had positive correlations with Chla, Chlb, Fv/Fm ratio, root activity, catalase and superoxide dismutase activities in roots under the stress of acid rain with different pH and S/N ratios. The structural equation modelling (SEM) results showed that acid rain NO3- concentration and pH had stronger direct effects on Chinese fir sapling HGR and DGR, and the direct effects of acid rain NO3- concentration and pH on HGR were lower than those on DGR. Our results suggest that the ratio of SO42- to NO3- in acid rain is an important factor which could affect the sustainable development of monoculture Chinese fir plantations in southern China.

Effect of exogenous calcium on growth, nutrients uptake and plasma membrane H+-ATPase and Ca2+-ATPase activities in soybean (Glycine max) seedlings under simulated acid rain stress.

Calcium (Ca) is one of essential elements for plant growth and development, and also plays a role in regulating plant cell physiology and cellular response to the environment. Here, we studied whether calcium played a role in enhancing tolerance of plants to acid rain stress by hydroponics and simulating acid rain stress. Our results show that acid rain (pH 4.5/pH 3.0) caused decreases in dry weight biomass, chlorophyll content and uptake of nutrients elements (NO3-, P, K, Mg, Zn and Mo) and an increase in membrane permeability of root. However, all parameters in soybean treated with exogenous calcium (5 mM) and acid rain at pH 4.5 were closed to the control levels. In addition, exogenous calcium (5 mM) alleviated the inhibition induced by pH 3.0 acid rain on the activity of plasma membranes H+-ATPase and the expression of GmPHA1 at transcriptional level, being benefiting to maintaining uptake of nutrients (NO3-, P, K, Mg, and Zn), and then lower the decrease in dry weight biomass and chlorophyll content. After a 5-day recovery (without acid rain stress), all parameters in soybean treated with acid rain at pH 3.0 and exogenous calcium were still worse than those of the control, but obviously better than those treated with acid rain at pH 3.0. Higher activity of plasma membrane H+-ATPase in soybean treated with acid rain at pH 3.0 and exogenous calcium was good to uptake of nutrients and promoted the recovery of soybean growth, compared with soybean treated with acid rain at pH 3.0. In conclusion, exogenous calcium could alleviate the inhibition caused by acid rain on soybean growth by increasing the activity of plasma membrane H+-ATPase for providing driving force to nutrient absorption, and its regulating effect was limited by intensity of acid rain. Furthermore, the application of exogenous calcium can be one of ways to alleviate the damage caused by acid rain to plants.

Hydrolytic enzymes mediated lipid-DNA catabolism and altered gene expression of antioxidants under combined application of lead and simulated acid rain in Fenugreek (Trigonella foenum graecum L.) seedlings.

Understanding ill effects of simultaneous existence of various abiotic stresses, commonly observed due to various anthropogenic activities and global climate change these days, over plants growth, metabolic activity and yield responses are important for continued agricultural productivity and food security. In the present study, seedlings of Fenugreek (Trigonella foenum graecum L.) were subjected to lead (Pb, 1200 ppm) and/or simulated acid rain (SAR, pH 3.5) for 30 days, and were then analysed. The results revealed reduced growth, and total lipid and DNA contents, while enhanced Pb accumulation, biological concentration factor, biological accumulation coefficient, translocation factor, lipase activity, and levels of free fatty acid, conjugated diene, lipid hydroperoxide, DNA oxidation and DNase activity under Pb and/or SAR exposure. Additionally, activities and gene expression levels of antioxidants (superoxide dismutase, catalase, guaiacol peroxidase and ascorbate peroxidase) were enhanced in response to applied treatments. The results also suggested that inhibitions/ accelerations determined under joint addition of Pb and SAR were comparatively more profound than those measured under their single application. Additionally, root was more sensitive to Pb treatment, compared to both leaf and shoot. Hence, under simultaneous presence of two or more number of abiotic stresses, the strategy opted by plants for survival is chiefly governed by the interaction between prevailing stressors, which is then conceived by plants as a new state of stress.

Exogenous Melatonin Mitigates Acid Rain Stress to Tomato Plants through Modulation of Leaf Ultrastructure, Photosynthesis and Antioxidant Potential.

Acid rain (AR) is a serious global environmental issue causing physio-morphological changes in plants. Melatonin, as an indoleamine molecule, has been known to mediate many physiological processes in plants under different kinds of environmental stress. However, the role of melatonin in acid rain stress tolerance remains inexpressible. This study investigated the possible role of melatonin on different physiological responses involving reactive oxygen species (ROS) metabolism in tomato plants under simulated acid rain (SAR) stress. SAR stress caused the inhibition of growth, damaged the grana lamella of the chloroplast, photosynthesis, and increased accumulation of ROS and lipid peroxidation in tomato plants. To cope the detrimental effect of SAR stress, plants under SAR condition had increased both enzymatic and nonenzymatic antioxidant substances compared with control plants. But such an increase in the antioxidant activities were incapable of inhibiting the destructive effect of SAR stress. Meanwhile, melatonin treatment increased SAR-stress tolerance by repairing the grana lamella of the chloroplast, improving photosynthesis and antioxidant activities compared with those in SAR-stressed plants. However, these possible effects of melatonin are dependent on concentration. Moreover, our study suggests that 100-µM melatonin treatment improved the SAR-stress tolerance by increasing photosynthesis and ROS scavenging antioxidant activities in tomato plants.

Alterations in cytosol free calcium in horseradish roots simultaneously exposed to lanthanum(III) and acid rain.

The extensive use of rare earth elements (REEs) has increased their environmental levels. REE pollution concomitant with acid rain in many agricultural regions can affect crop growth. Cytosol free calcium ions (Ca(2+)) play an important role in almost all cellular activities. However, no data have been reported regarding the role of cytosol free Ca(2+) in plant roots simultaneously exposed to REE and acid rain. In this study, the effects of exposures to lanthanum(III) and acid rain, independently and in combination, on cytosol free Ca(2+) levels, root activity, metal contents, biomass, cytosol pH and La contents in horseradish roots were investigated. The simultaneous exposures to La(III) and acid rain increased or decreased the cytosol free Ca(2+) levels, depending on the concentration of La(III), and these effects were more evident than independent exposure to La(III) or acid rain. In combined exposures, cytosol free Ca(2+) played an important role in the regulation of root activity, metal contents and biomass. These roles were closely related to La(III) dose, acid rain strength and treatment mode (independent exposure or simultaneous exposure). A low concentration of La(III) (20 mg L(-1)) could alleviate the adverse effects on the roots caused by acid rain, and the combined exposures at higher concentrations of La(III) and acid rain had synergic effects on the roots.

[THE EFFECT OF ACID RAIN ON ULTRASTRUCTURE AND FUNCTIONAL PARAMETERS OF PHOTOSYNTHETIC APPARATUS OF PEA LEAVES].

The effects of simulated acid rain (SAR) on the ultrastructure and functional parameters of the photosynthetic apparatus were studied using 14-day-old pea leaves as test system. Pea plants were sprayed with an aqueous solution containing NaNO3(0.2 mM) and Na2SO4(0.2 mM) (pH 5.6, a control variant), or with the same solution, which was acidified to pH 2.5 (acid variant). Functional characteristics were determined by chlorophyll fluorescence analysis. Acid rain application caused reduction in the efficiency of the photosynthetic electron transport by 25%, which was accompanied by an increase by 85% in the quantum yield of thermal dissipation of excess light quanta. Ultrastructural changes in chloroplast were registered by transmission electron microscopy (TEM) after two days of the SAR-treatment of pea leaves. In this case, the changes in the structure of grana, heterogeneity of thylakoids packaging in granum, namely, the increase of intra-thylakoid gaps and thickness of granal thylakoids compared to the control were found. The migration of protein complexes in thylakoid membranes of chloroplasts isolated from leaves treated with SAR was suppressed. It was shown also that carbonic anhydrase activity was inhibited in chloroplast preparations isolated from SAR-treated pea leaves. We proposed a hypothesis on the possible inactivation of thylakoid carbonic anhydrase under SAR and its involvement in the inhibition of photochemical activity of chloroplasts. The data obtained allows to suggest that acid rains negatively affect the photosynthetic apparatus disrupting the membrane system of chloroplast.

Evidence for episodic acidification effects on migrating Atlantic salmon Salmo salar smolts.

Field studies were conducted to determine levels of gill aluminium as an index of acidification effects on migrating Atlantic salmon Salmo salar smolts in the north-eastern U.S.A. along mainstem river migration corridors in several major river basins. Smolts emigrating from the Connecticut River, where most (but not all) tributaries were well buffered, had low or undetectable levels of gill aluminium and high gill Na(+) /K(+) -ATPase (NKA) activity. In contrast, smolts emigrating from the upper Merrimack River basin where most tributaries are characterized by low pH and high inorganic aluminium had consistently elevated gill aluminium and lower gill NKA activity, which may explain the low adult return rates of S. salar stocked into the upper Merrimack catchment. In the Sheepscot, Narraguagus and Penobscot Rivers in Maine, river and year-specific effects on gill aluminium were detected that appeared to be driven by underlying geology and high spring discharge. The results indicate that episodic acidification is affecting S. salar smolts in poorly buffered streams in New England and may help explain variation in S. salar survival and abundance among rivers and among years, with implications for the conservation and recovery of S. salar in the north-eastern U.S.A. These results suggest that the physiological condition of outmigrating smolts may serve as a large-scale sentinel of landscape-level recovery of atmospheric pollution in this and other parts of the North Atlantic region.

Effects of simulated acid rain on the morphology, phenology and dry biomass of a local variety of maize (Suwan-1) in Southwestern Nigeria.

Effects of acid rain on the morphology, phenology and dry biomass of maize (Suwan-1 variety) were investigated. The maize seedlings were subjected to different pH treatments (1.0, 2.0, 3.0, 4.0, 5.0 and 6.0) of simulated acid rain (SAR) with pH 7.0 as the control for a period of 90 days. The common morphological defects due to SAR application were necrosis and chlorosis. It was observed that necrosis increased in severity as the acidity increased whilst chlorosis was dominant as the acidity decreased. SAR encouraged rapid floral and cob growth but with the consequence of poor floral and cob development in pH 1.0 to 3.0 treatments. The result for the dry biomass indicates that pH treatments 2.0 to 7.0 for total plant biomass were not significantly different (P > 0.05) from one another, but were all significantly higher (P < 0.05) than pH 1.0. Therefore, it may be deduced that Suwan-1 has the potential to withstand acid rain but with pronounced morphological and phenological defects which, however, have the capacity to reduce drastically the market value of the crop. Therefore, it may be concluded that Suwan-1 tolerated acid rain in terms of the parameters studied at pH 4.0 to 7.0 which makes it a suitable crop in acid rain-stricken climes. This research could also serve as a good reference for further SAR studies on maize or other important cereals.

A transcriptomic study reveals differentially expressed genes and pathways respond to simulated acid rain in Arabidopsis thaliana.

Acid rain, as a worldwide environmental issue, can cause serious damage to plants. In this study, we provided the first case study on the systematic responses of arabidopsis (Arabidopsis thaliana (L.) Heynh.) to simulated acid rain (SiAR) by transcriptome approach. Transcriptomic analysis revealed that the expression of a set of genes related to primary metabolisms, including nitrogen, sulfur, amino acid, photosynthesis, and reactive oxygen species metabolism, were altered under SiAR. In addition, transport and signal transduction related pathways, especially calcium-related signaling pathways, were found to play important roles in the response of arabidopsis to SiAR stress. Further, we compared our data set with previously published data sets on arabidopsis transcriptome subjected to various stresses, including wound, salt, light, heavy metal, karrikin, temperature, osmosis, etc. The results showed that many genes were overlapped in several stresses, suggesting that plant response to SiAR is a complex process, which may require the participation of multiple defense-signaling pathways. The results of this study will help us gain further insights into the response mechanisms of plants to acid rain stress.

Interactive effects of cadmium and acid rain on photosynthetic light reaction in soybean seedlings.

Interactive effects of cadmium (Cd(2+)) and acid rain on photosynthetic light reaction in soybean seedlings were investigated under hydroponic conditions. Single treatment with Cd(2+) or acid rain and the combined treatment decreased the content of chlorophyll, Hill reaction rate, the activity of Mg(2+)-ATPase, maximal photochemical efficiency and maximal quantum yield, increased initial fluorescence and damaged the chloroplast structure in soybean seedlings. In the combined treatment, the change in the photosynthetic parameters and the damage of chloroplast structure were stronger than those of any single pollution. Meanwhile, Cd(2+) and acid rain had the interactive effects on the test indices in soybean seedlings. The results indicated that the combined pollution of Cd(2+) and acid rain aggravated the toxic effect of the single pollution of Cd(2+) or acid rain on the photosynthetic parameters due to the serious damage to the chloroplast structure.

Comparative proteomic analysis of proteins in response to simulated acid rain in Arabidopsis.

A proteomic study using 2-D gel electrophoresis and MALDI-TOF MS was performed to characterize the responses of Arabidopsis thaliana plants to simulated acid rain (SiAR) stress, which is a global environmental problem and has become a serious issue in China in recent years. The emphasis of the present study was to investigate the overall protein expression changes when exposed to SiAR. Out of over 1000 protein spots reproducibly resolved, 50 of them changed their abundance by at least 2-fold. Analysis of protein expression patterns revealed that a set of proteins associated with energy production, metabolism, cell rescue, cell defense and protein folding, etc., could play important roles in mediating plant response to SiAR. In addition to this, some proteins involved in stress responses and jasmonic acid pathway are also involved in plant response to SiAR. More interestingly, the expression of several ubiquitination-related proteins changed dramatically after 32-h SiAR treatment, suggesting that they may act as a molecular marker for the injury phenotype caused by SiAR. Based on our results, we proposed a schematic model to explain the mechanisms associated with the systematic response of Arabidopsis plants to SiAR.

Impacts of simulated acid rain on soil enzyme activities in a latosol.

Acid rain pollution is a serious environmental problem in the world. This study investigated impacts of simulated acid rain (SAR) upon four types of soil enzymes, namely the catalase, acid phosphatase, urease, and amylase, in a latosol. Latosol is an acidic red soil and forms in the tropical rainforest biome. Laboratory experiments were performed by spraying the soil columns with the SAR at pH levels of 2.5, 3.0, 3.5., 4.0, 4.5, 5.0, and 7.0 (control) over a 20-day period. Mixed results were obtained in enzyme activities for different kinds of enzymes under the influences of the SAR. The catalase activities increased rapidly from day 0 to 5, then decreased slightly from day 5 to 15, and finally decreased sharply to the end of the experiments, whereas the acid phosphatase activities decreased rapidly from day 0 to 5, then increased slightly from day 5 to 15, and finally decreased dramatically to the end of the experiments. A decrease in urease activities was observed at all of the SAR pH levels for the entire experimental period, while an increase from day 0 to 5 and then a decrease from day 5 to 20 in amylase activities were observed at all of the SAR pH levels. In general, the catalase, acid phosphatase, and urease activities increased with the SAR pH levels. However, the maximum amylase activity was found at pH 4.0 and decreased as the SAR pH increased from 4.0 to 5.0 or decreased from 4.0 to 2.5. It is apparent that acid rain had adverse environmental impacts on soil enzyme activities in the latosol. Our study further revealed that impacts of the SAR upon soil enzyme activities were in the following order: amylase>catalase>acid phosphatase>urease. These findings provide useful information on better understanding and managing soil biological processes in the nature under the influence of acid rains.

Recovery of benthic invertebrate communities from acidification in Killarney Park lakes.

Using a reference-condition comparison, recovery of benthic invertebrate communities from acidification was assessed in three lakes in Killarney Wilderness Park approximately 40-60 km from the massive metal smelters in Sudbury, Canada. Test site analyses (TSAs) were used to compare the park lakes to 20 reference lakes near Dorset Ontario, 200 km to the east. An extension of a previous survey (1997-2001) of two sensitive mayfly species (Stenonema femoratum and Stenacron interpunctatum) was conducted in one of the lakes. TSA results indicate that the three Killarney lakes remain significantly different from reference condition due primarily to higher abundances of a few acid-tolerant families and the presence of some less abundant sensitive families. Colonization rates differ greatly between the two mayfly species presumably because of competition for available habitat. Overall, this study suggests that early colonizers will gain an advantage to out-compete subsequent arrivals, and these competitive interactions will delay the return of communities to reference condition.

Physiological Responses of Earthworm Under Acid Rain Stress.

Acid rain has become one of the major global environmental problems, and some researches reported that acid rain may have a certain inhibition on soil biodiversity. Besides this, it is well known that earthworm (Eisenia fetida) plays an important role in the functioning of soil ecosystems. For this point, we conducted a series of experiments to investigate whether acid rain would take effects on earthworms. In the present study, the earthworms were incubated on filter paper and in soil under acid rain stress. The mortality and behavior of earthworms were recorded, and epidermal damage and the activity of the CYP3A4 enzyme were measured for the tested earthworms. Our experimental results showed that the earthworms could not survive in the acid rain stress of pH below 2.5, and acid rain with weak acidity (i.e., 4.0 ≤ pH ≤ 5.5) promoted the activity of the CYP3A4 enzyme in the earthworms, while acid rain with strong acidity (i.e., 3.0 ≤ pH ≤ 3.5) inhibited it. Moreover, the degree of damage in sensitive parts of the earthworms increased with the decrease of pH value. This study suggests that acid rain can cause discomfort response and the direct epidermal damage of earthworms, and even kill them.

Photosynthesis and oxidative stress in the restinga plant species Eugenia uniflora L. exposed to simulated acid rain and iron ore dust deposition: potential use in environmental risk assessment.

The Brazilian sandy coastal plain named restinga is frequently subjected to particulate and gaseous emissions from iron ore factories. These gases may come into contact with atmospheric moisture and produce acid rain. The effects of the acid rain on vegetation, combined with iron excess in the soil, can lead to the disappearance of sensitive species and decrease restinga biodiversity. The effects of iron ore dust deposition and simulated acid rain on photosynthesis and on antioxidant enzymes were investigated in Eugenia uniflora, a representative shrub species of the restinga. This study aimed to determine the possible utility of this species in environmental risk assessment. After the application of iron ore dust as iron solid particulate matter (SPM(Fe)) and simulated acid rain (pH 3.1), the 18-month old plants displayed brown spots and necrosis, typical symptoms of iron toxicity and injuries caused by acid rain, respectively. The acidity of the rain intensified leaf iron accumulation, which reached phytotoxic levels, mainly in plants exposed to iron ore dust. These plants showed the lowest values for net photosynthesis, stomatal conductance, transpiration, chlorophyll a content and electron transport rate through photosystem II (PSII). Catalase and superoxide dismutase activities were decreased by simulated acid rain. Peroxidase activity and membrane injury increased following exposure to acid rain and simultaneous SPM(Fe) application. Eugenia uniflora exhibited impaired photosynthetic and antioxidative metabolism in response to combined iron and acid rain stresses. This species could become a valuable tool in environmental risk assessment in restinga areas near iron ore pelletizing factories. Non-invasive evaluations of visual injuries, photosynthesis and chlorophyll a fluorescence, as well as invasive biochemical analysis could be used as markers.

Growth and physiological response of an endangered tree, Horsfieldia hainanensis merr., to simulated sulfuric and nitric acid rain in southern China.

As nitrogen deposition increases, acid rain is gradually shifting from sulfuric acid rain (SAR) to nitric acid rain (NAR). Acid rain can severely affect plant growth, damage ecosystems, and reduce biodiversity. Thus, a shift in acid rain type presents another challenge to the conservation of endangered plant species. We investigated the effect of three acid rain types (SAR, mixed acid rain [MAR], and NAR) and pH on the growth of an endangered Chinese endemic tree, Horsfieldia hainanensis Merr., using simulated rain in a greenhouse environment. Over nine months, growth indices, chlorophyll content, antioxidant enzyme activity, malondialdehyde content, and chlorophyll fluorescence parameters were investigated for treated and control saplings. The results indicated that at a pH of 5.6, H. hainanensis could adapt to SAR and MAR, but NAR inhibited below-ground growth. At a pH of 2.5 and 4.0, SAR inhibited stem and leaf biomass accumulation, whereas NAR inhibited root biomass accumulation and altered root morphology. MAR had intermediary effects between those of SAR and NAR. Adverse effects on leaf physiology were reduced as the rain type shifted from SAR to NAR; however, roots were increasingly adversely affected. Our results suggest that conservation efforts for H. hainanensis should shift from an above-ground to a below-ground focus as acid rain transitions toward NAR.

Effects of lanthanum and acid rain stress on the bio-sequestration of lanthanum in phytoliths in germinated rice seeds.

REEs in the environment can be absorbed by plants and sequestered by plant phytoliths. Acid rain can directly or indirectly affect plant physiological functions. Currently, the effects of REEs and acid rain on phytolith-REEs complex in plants are not yet fully understood. In this study, a high-silicon accumulation crop, rice (Oryza sativa L.), was selected as a representative of plants, and orthogonal experiments were conducted under various levels of lanthanum [La(III)] and pH. The results showed that various La(III) concentrations could significantly improve the efficiency and sequestration of phytolith La(III) in germinated rice seeds. A pH of 4.5 promoted phytolith La(III) sequestration, while a pH of 3.5 inhibited sequestration. Compared with the single treatment with La(III), the combination of La(III) and acid rain inhibited the efficiency and sequestration of phytolith La(III). Correlation analysis showed that the efficiency of phytolith La(III) sequestration had no correlation with the production of phytolith but was closely correlated with the sequestration of phytolith La(III) and the physiological changes of germinated rice seeds. Phytolith morphology was an important factor affecting phytolith La(III) sequestration in germinated rice seeds, and the effect of tubes on sequestration was more significant than that of dumbbells. This study demonstrated that the formation of the phytolith and La(III) complex could be affected by exogenous La(III) and acid rain in germinated rice seeds.