Non-linear direct effects of acid rain on leaf photosynthetic rate of terrestrial plants

The impact of acid rain on cadmium phytoremediation in sunflower (Helianthus annuus L.)

Sunflower is an ideal crop for phytoremediation of cadmium-contaminated farmland, as it brings economic benefits while conducting soil remediation. Due to industrial emissions and car exhaust, Cd contaminated areas are often accompanied by acid rain. However, the impact of acid rain on the Cd remediation capacity of sunflowers and its potential influencing factors are unclear. An experiment was manipulated to elucidate the effects of Cd concentration (0,10,50,100 μmol/L) and acid rain (pH 4.0) on the phytoremediation ability of sunflowers, in which the properties of them were explored. The results indicated that Cd stress is the main factor affecting the growth of sunflowers. Without AR, Cd treatment decreased sunflower biomass by 67.5-85.6%. Under AR, Cd treatment decreased sunflower biomass 53.9-86.4%. Compared without AR, the relative chlorophyll content with AR increased by 22.3-23.1%, while the YII with AR decreased by 6.5-20.0%. There was an interaction between acid rain and Cd stress on antioxidant enzyme activity. With AR, CAT activity at 0 μmol/L Cd treatment increased by 25.6%, compared without AR. Whether there is acid rain or not, the POD and SOD activities were increased at 10, 50 μmol/L Cd treatment, but they were decreased at 100 μmol/L Cd treatment. Among them, acid rain exacerbated the impact of POD activity (decreased by 31.4%) at 100 μmol/L Cd treatment and SOD activity (decreased by 15.1%) at 50 μmol/L Cd treatment, compared without AR. In this experiment, the phytoremediation capacity of sunflowers mainly depended on the concentration of Ca in the leaves and their antioxidant capacity. Acid rain enhanced 77.5% the total Cd accumulation at 10 μmol/L Cd treatment, compared without AR. Acid rain exacerbated the damage of Cd to the chloroplast structure of sunflowers, and reduced the accumulation of starch particles. The study findings may be useful for improving the phytoremediation of Cd-contaminated soil.

Response and driving factors of soil enzyme activity related to acid rain: a meta-analysis

As a global pollution, acid rain can significantly alter soil physicochemical and biochemical processes, but our knowledge of how acid rain affects soil enzyme activity is still limited. To quantify the overall magnitude and direction of the response of soil enzyme activity to acid rain, we conducted a linear mixed model-based meta-analysis of 40 articles. Our analysis revealed that acid rain decreased enzyme activity by an average of 4.87%. Soil dehydrogenase and protease activities were particularly sensitive to acid rain, with significant inhibitions observed. The effect of acid rain was moderated by acid rain intensity (i.e., H+ addition rate, total H+ added, and acid rain pH) and soil fraction (i.e., rhizosphere and bulk soil). Structural equation modelling further revealed that acid rain suppressed soil microbial biomass by acidifying the soil and that the reduction in microbial biomass directly led to the inhibition of enzyme activity in bulk soil. However, the enzyme activity in the rhizosphere soil was not affected by acid rain due to the rhizosphere effect, which was also not impacted by the decreased soil pH induced by acid rain in rhizosphere. Our study gives an insight into how bulk soil enzyme activity is impacted by acid rain and highlights the need to incorporate rhizosphere processes into acid rain-terrestrial ecosystem models.

An overview of the direct and indirect effects of acid rain on plants: Relationships among acid rain, soil, microorganisms, and plants

Acid rain (AR) causes numerous environmental problems and complex negative effects on plants globally. Many studies have previously reported on direct effects of AR or its depositional substances on plant injury and performance. However, few studies have addressed the indirect effects of AR on plants as mediated by soil microorganisms and the abiotic environment of the soil rhizosphere. The indirect effects (e.g., AR → soil microorganisms→plants) need greater attention, because acidic deposition not only affects the distribution, composition, abundance, function, and activity of plant-associated microorganisms, but also influences the dynamics of some substances in the soil in a way that may be harmful to plants. Therefore, this review not only focused on the direct effects of AR on plant performance, growth, and biomass allocations from a whole-plant perspective, but also addressed the pathway of AR-soil chemical characteristics-plants, which explains how soil solute leaching and acidification by AR will reduce the availability of essential nutrients and increase the availability of heavy metals for plants, affecting carbon and nitrogen cycles. Mainly, we evaluated the AR-soil microorganisms-plants pathway by: 1) synthesizing the potential roles of soil microbes in alleviating soil acidic stress on plants and the adverse effects of AR on plant-associated soil microorganisms; 2) exploring how plant mycorrhizal types affect the detection of AR effect on plants. The meta-analysis showed that the effects of AR-induced pH on leaf chlorophyll content, plant height, and plant root biomass were dependent on plant mycorrhizal types. Some possible reasons for different synergy between mycorrhizal symbiotic types and plants were discussed. Future research relating to the effects of AR on plants should focus on the combined direct and indirect effects to evaluate how AR affects plant performance comprehensively.

Changes in the composition of rhizosphere bacterial communities in response to soil types and acid rain

It is widely known how acid rain negatively impacts plant physiology. However, the magnitude of these effects may depend on soil types. Although the response of aboveground parts has received much attention, the effects of soil types and acid rain on underground processes are yet to be studied, specifically with respect to the composition and diversity of bacterial communities in the rhizosphere. Based on a high throughput sequencing approach, this study examined how different soil types, acid rain of different pH, and interactions between the two factors influenced the growth and rhizosphere bacterial communities of Jatropha curcas L. The present study pointed out that the soil pH, total nitrogen (TN), total phosphorus (TP), total potassium (TK), and total organic carbon/total nitrogen (C/N) were more related to soil type than to acid rain. The growth of J. curcas aboveground was mainly affected by acid rain, while the underground growth was mainly influenced by soil type. Changes in bacterial abundance indicated that the genera (Burkholderia-Paraburkholde, Bryobacter, Cupriavidus, Mycobacterium, and Leptospirillu) and phyla (Acidobacteria and Actinobacteria) could likely resist acid rain to some extent, with Acidobacteria, Gemmatimonadetes and Proteobacteria being well adapted to the copiotrophic environments. Results of correlational analyses between Firmicutes and soil properties (pH, TN, TK) further indicated that this phylum was also well adapted to a nutrient-deficient habitat of low pH. Finally, while Mycobacterium and Bradyrhizobium could adapt to low pH, high soil TK contents were not conducive to their enrichment. The results also showed that acid rain shifted the bacterial groups from fast-growing copiotrophic populations to slow-growing oligotrophic ones. The RDA analysis, and Pearson's rank correlation coefficients indicated that soil pH and TK were the main factors influencing bacterial richness.

Environmental drivers of the leaf nitrogen and phosphorus stoichiometry characteristics of critically endangered Acer catalpifolium

Acer catalpifolium is a perennial deciduous broad-leaved woody plant, listed in the second-class protection program in China mainly distributed on the northwest edge of Chengdu plain. However, extensive anthropogenic disturbances and pollutants emissions (such as SO₂, NH₃ and NO_X) in this area have created a heterogeneous habitat for this species and its impacts have not been systematically studied. In this study, we investigated the leaf nitrogen (N) and phosphorus (P) content of A. catalpifolium in the natural distribution areas, and a series of simulation experiments (e.g., various water and light supply regimes, different acid and N deposition levels, reintroduction management) were conducted to analyze responses of N and P stoichiometric characteristics to environmental changes. The results showed that leaf nitrogen content (LNC) was 14.49 ~ 25.44 mg g^-1, leaf phosphorus content (LPC) was 1.29~3.81 mg g^-1 and the N/P ratio of the leaf (L-N/P) was 4.87~13.93. As per the simulation experiments, LNC of A. catalpifolium is found to be relatively high at strong light conditions (80% of full light), high N deposition (100 and 150 kg N ha^-1), low acidity rainwater, reintroduction to understory area or N fertilizer applications. A high level of LPC was found when applied with 80% of full light and moderate N deposition (100 kg N ha^-1). L-N/P was high under severe shade (8% of full light), severe N deposition (200 kg N ha^-1), and reintroduction to gap and undergrowth habitat; however, low L-N/P was observed at low acidity rainwater or P fertilizer application. The nutrient supply facilitates corresponding elements uptake, shade tends to induce P limitation and soil acidification shows N limitation. Our results provide theoretical guidance for field management and nutrient supply regimes for future protection, population rejuvenation of this species and provide guidelines for conservation and nutrient management strategies for the endangered species.

Evaluation of the economic characteristics of the fruit of 45 superior Camellia weiningensis Y.K. Li. trees

Reports related to Camellia weiningensis Y.K. Li. are rare. We evaluated the economic characteristics of the mature fruit of 45 superior C. weiningensis trees using principal component analysis (PCA) and gray correlation analysis, and identified excellent germplasms according to performance. PCA was employed to reduce the dimensions. PCA was performed for the original 15 indices of fruit diameter, fruit length, fruit shape, single-fruit weight, pericarp thickness, oil yield, fresh seed rate, dry seed rate, dry kernel rate and palmitic acid, stearic acid, linolenic acid, oleic acid, linoleic acid and arachidonic acid contents. According to the requirements of a cumulative contribution rate ≥ 80% and an eigenvector value > 1, six principle components were selected. These indices underwent weighted summation to establish a function model for comprehensive evaluation. Finally, the comprehensive rankings of the cultivars according to PCA were compared with those according to gray correlation analysis. The genetic variation coefficients of the 15 parameters ranged from 2.24% (oleic acid content) to 22.70% (single-fruit weight, with a range of 21.34 g). The top ten excellent cultivars with the highest comprehensive scores according to PCA and those according to gray correlation analysis were compared. According to PCA, oleic acid content, fruit diameter, fruit length, pericarp thickness, arachidonic acid content and dry seed rate can serve as representative evaluation indicators of C. weiningensis. The outcomes obtained based on PCA were basically consistent with those obtained based on gray correlation analysis. Finally, nine excellent cultivars were finally determined, i.e., WY-1, WY-6, WY-8, WY-25, WY-27, WY-30, WY-33, WY-35, WY-38 and WY-44. The results obtained in terms of crown yield were basically consistent with the outcomes of the comprehensive assessments, which indicates the reliability of the assessment methods used in this study.

Effects of acid rain on plant growth: A meta-analysis

Anthropogenic driven acid gases emission has caused acid rain in many regions globally. Although efforts have been made to assess the effects of acid rain on terrestrial ecosystems, a systematic assessment of growth-related traits across plant aboveground and belowground is lacking. Hence, we performed a phylogenetically controlled meta-analysis of 755 observations from 69 independent studies to quantify the effects of acid rain on six growth-related traits of plant. We estimated the inhibitory effects of acid rain on plant growth in general and found that aboveground and belowground plant parts responded differently. The acidity of acid rain and acid rain interval had direct modulation effects on plant growth. We also found that there were interactions between acid rain pH and other acid rain characteristics (i.e., acid rain interval, mole ratio of S:N, and acid rain rate) and experimental characteristics (i.e., initial soil pH and plant exposure part), indicating that there were pH-dependent interaction patterns. Thus, an effective approach to evaluate and predict the effects of acid rain on plant growth is to fully consider the direct effects of acid rain pH and the interactions between acid rain pH and other factors.

Photosynthetic rate and chlorophyll fluorescence of barley exposed to simulated acid rain

Acid rain is considered one of the most serious plant abiotic stresses. Photosynthesis is the basis of crop growth and development. The effect of acid rain on barley photosynthesis remains unclear. A glasshouse experiment was conducted, and the photosynthetic rate, chlorophyll (Chl) fluorescence, and pigment content of barley were measured in simulated acid rain (SAR) under pH 6.5, 5.5, 4.5, and 3.5. The results showed that net photosynthetic rate, maximal photosynthetic rate, and light saturation point decreased and the light compensation point, and dark respiration rate increased with increasing acidity. The results suggested that photosynthesis in barley plants was inhibited by SAR stress. The Chl content and stomatal conductance declined in parallel with the reduced net photosynthetic rate when barley plants were under SAR stress conditions. This indicated that non-stomatal factors may contribute to reduced photosynthesis under acid rain stress. Acid rain had greater effects on the photosynthesis of the acid rain-sensitive plant Zhepi 33 than on non-sensitive Kunlun 12. A significant difference in parameters such as the maximal fluorescence, variable fluorescence, and active PSII reaction centers was found among the SAR treatments and may be used to evaluate the sensitivity of plants to acid rain stress. The visualization model showed that the photosynthetic reaction centers were inactivated in acid rain stressed barley plants. These findings are valuable for the evaluation of the plant sensitivity to acid rain stress and may be used for the detection and monitoring of acid rain effects on plants in the future.

The abatement of acid rain in Guizhou province, southwestern China: Implication from sulfur and oxygen isotopes

The high frequency of acid rain in southern China has captured public and official concern since 1980s. Subsequently, gas emission reduction measures have been implemented to improve the air quality. Variations in SO₂ emission intensities can influence the sulfur and oxygen isotopic compositions of sulfate in rainwater, since atmospheric sulfate is mainly formed via the oxidation of sulfur gases from natural and anthropogenic sources. To evaluate the impacts of emission reduction measures on atmospheric sulfate, the seasonal and long-term trends in stable isotopic compositions of sulfate in rainwater in Guizhou province, southwestern China have been investigated based on rainwater samples collected from June 2016 to June 2018 and literature investigation (2000-2010).The results reveal that coal combustion remains a major contributor to sulfate in rainwater, although its SO₂ emission has significantly decreased over the past two decades. The δ³⁴S_sulfate and δ¹⁸O_sulfate values in rainwater are negatively correlated and have significant seasonal changes. The seasonality in δ³⁴S_sulfate has been interpreted as due to the changes in contributions of dimethyl sulfide and coal combustion, while the seasonal pattern of δ¹⁸O_sulfate is consistent with that of δ¹⁸O_water values, indicating sulfate in rainwater is mainly formed by heterogeneous oxidation of SO₂. Combined with the data from previous studies (Xiao and Liu, 2002; Liu, 2007; Xiao et al., 2009; Xiao et al., 2014), we found that the volume weighted mean δ³⁴S values of sulfate in rainwater in Guizhou province show a marked increase between 2001 and 2018, indicating that the ³⁴S-depleted SO₂ emission from coal combustion has declined during this period. Furthermore, the synchronous changes in δ³⁴S values, sulfate concentration and pH values of rainwater suggest that the frequency of acid rain in Guizhou province has dropped over the past two decades, which is likely to result from the emission reduction measures taken in Guizhou province.

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.

Elevated nitrogen metabolism and nitric oxide production are involved in Arabidopsis resistance to acid rain

Acid rain (AR) can induce great damages to plants and could be classified into different types according to the different SO₄^2-/NO₃^- ratio. However, the mechanism of plants' responding to different types of AR has not been elucidated clearly. Here, we found that nitric-rich simulated AR (N-SiAR) induced less leaves injury as lower necrosis percentage, better physiological parameters and reduced oxidative damage in the leaves of N-SiAR treated Arabidopsis thaliana compared with sulfate and nitrate mixed (SN-SiAR) or sulfuric-rich (S-SiAR) simulated AR treated ones. Of these three types of SiAR, N-SiAR treated Arabidopsis maintained the highest of nitrogen (N) content, nitrate reductase (NR) and nitrite reductase (NiR) activity as well as N metabolism related genes expression level. Nitric oxide (NO) content showed that N-SiAR treated seedlings had a higher NO level compared to SN-SiAR or S-SiAR treated ones. A series of NO production and elimination related reagents and three NO production-related mutants were used to further confirm the role of NO in regulating acid rain resistance in N-SiAR treated Arabidopsis seedlings. Taken together, we concluded that an elevated N metabolism and enhanced NO production are involved in the tolerance to different types of AR in Arabidopsis.