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Qualitative questions are exploratory and are open-ended. A well-formulated study question forms the basis for developing a protocol, guides the selection of design, and data collection methods. Qualitative research questions generally involve two parts, a central question and related subquestions. The central question is directed towards the primary phenomenon under study, whereas the subquestions explore the subareas of focus. It is advised not to have more than five to seven subquestions. A commonly used framework for designing a qualitative research question is the "PCO framework" wherein, P stands for the population under study, C stands for the context of exploration, and O stands for the outcome/s of interest.[12] The PCO framework guides researchers in crafting a focused study question.

Purposive or purposeful sampling is a widely used sampling technique.[35] It involves identifying a population based on already established sampling criteria and then selecting subjects who fulfill that criteria to increase the credibility. However, choosing information-rich cases is the key to determine the power and logic of purposive sampling in a qualitative study.[1]

13. Carter SM, Little M. Justifying knowledge, justifying method, taking action: Epistemologies, methodologies, and methods in qualitative research. Qual Health Res.2007;17:1316–28. [PubMed]

44. Tong A, Sainsbury P, Craig J. Consolidated criteria for reporting qualitative research (COREQ): A 32-item checklist for interviews and focus groups. Int J Qual Health Care.2007;19:349–57. [PubMed]

Deng Xiaopingparamount leader of the People"s Republic of China (PRC) from December 1978 to November 1989. After Chinese Communist Party chairman Mao Zedong"s death in 1976, Deng gradually rose to supreme power and led China through a series of far-reaching market-economy reforms earning him the reputation as the "Architect of Modern China".world"s second largest economy by GDP nominal in 2010.

Although Deng got involved in the Marxist revolutionary movement in China, the historian Mobo Gao has argued that "Deng Xiaoping and many like him [in the Chinese Communist Party] were not really Marxists, but basically revolutionary nationalists who wanted to see China standing on equal terms with the great global powers. They were primarily nationalists and they participated in the Communist revolution because that was the only viable route they could find to Chinese nationalism."

The successes of the Soviet in Jiangxi made the party leaders decide to move to Jiangxi from Shanghai. The confrontation among Mao, the party leaders, and their Soviet advisers was increasingly tense and the struggle for power between the two factions led to the removal of Deng, who favored the ideas of Mao, from his position in the propaganda department. Despite the strife within the party, the Jiangxi Soviet became the first successful experiment of communist rule in rural China. It even issued stamps and paper money under the letterhead of the Soviet Republic of China, and the army of Chiang Kai-shek finally decided to attack the communist area.

Surrounded by the more powerful nationalist army, the Communists fled Jiangxi in October 1934. Thus began the epic movement that would mark a turning point in the development of Chinese communism. The evacuation was difficult because the Army of the Republic had taken positions in all areas occupied by the Communists. Advancing through remote and mountainous terrain, some 100,000 men managed to escape Jiangxi, starting a long strategic retreat through the interior of China, which ended one year later when between 8,000 and 9,000 survivors reached the northern province of Shaanxi.

During the Zunyi Conference at the beginning of the Long March, the so-called 28 Bolsheviks, led by Bo Gu and Wang Ming, were ousted from power and Mao Zedong, to the dismay of the Soviet Union, became the new leader of the Chinese Communist Party. The pro-Soviet Chinese Communist Party had ended and a new rural-inspired party emerged under the leadership of Mao. Deng had once again become a leading figure in the party.

In the final phase of the war, Deng again exercised a key role as political leader and propaganda master as Political Commissar of the 2nd Field Army commanded by Liu Bocheng where he was instrumental in the PLA"s march into Tibet. He also participated in disseminating the ideas of Mao Zedong, which turned into the ideological foundation of the Communist Party. His political and ideological work, along with his status as a veteran of the Long March, placed him in a privileged position within the party to occupy positions of power after the Communist Party managed to defeat Chiang Kai-shek and founded the People"s Republic of China.

On 1 October 1949, Deng attended the proclamation of the People"s Republic of China in Beijing. At that time, the Communist Party controlled the entire north, but there were still parts of the south held by the Kuomintang regime. He became responsible for leading the pacification of southwest China, in his capacity as the first secretary of the Department of the Southwest. This organization had the task of managing the final takeover of that part of the country still held by the Kuomintang; Tibet remained independent for another year.

Under the political control of Deng, the Communist army took over Chongqing in late November 1949 and entered Chengdu, the last bastion of power of Chiang Kai-shek, a few days later. At that time Deng became mayor of Chongqing, while he simultaneously was the leader of the Communist Party in the southwest, where the Communist army, now proclaiming itself the People"s Liberation Army, suppressed resistance loyal to the old Kuomintang regime. In 1950, the Communist Party-ruled state also seized control over Tibet.

In 1963, Deng traveled to Moscow to lead a meeting of the Chinese delegation with Stalin"s successor, Nikita Khrushchev. Relations between the People"s Republic of China and the Soviet Union had worsened since the death of Stalin. After this meeting, no agreement was reached and the Sino–Soviet split was consummated; there was an almost total suspension of relations between the two major communist powers of the time.

The Cultural Revolution was not yet over, and a radical leftist political group known as the Gang of Four, led by Mao"s wife Jiang Qing, competed for power within the Party. The Gang saw Deng as their greatest challenge to power.self-criticisms. Although he admitted to having taken an "inappropriate ideological perspective" while dealing with state and party affairs, he was reluctant to admit that his policies were wrong in essence. His antagonism with the Gang of Four became increasingly clear, and Mao seemed to lean in the Gang"s favour. Mao refused to accept Deng"s self-criticisms and asked the party"s Central Committee to "discuss Deng"s mistakes thoroughly".

On 2 February 1976, the Central Committee issued a Top-Priority Directive, officially transferring Deng to work on "external affairs" and thus removing Deng from the party"s power apparatus. Deng stayed at home for several months, awaiting his fate. The Political Research Office was promptly dissolved, and Deng"s advisers such as Yu Guangyuan suspended. As a result, the political turmoil halted the economic progress Deng had labored for in the past year.Cultural Revolution and specifically pointed to Deng as an internal, rather than external, problem. This was followed by a Central Committee directive issued to all local party organs to study Mao"s directive and criticize Deng.

Deng"s reputation as a reformer suffered a severe blow when the Qingming Festival, after the mass public mourning of Zhou on a traditional Chinese holiday, culminated in the Tiananmen Incident on 5 April 1976, an event the Gang of Four branded as counter-revolutionary and threatening to their power. Furthermore, the Gang deemed Deng the mastermind behind the incident, and Mao himself wrote that "the nature of things has changed".

Deng"s elevation to China"s new number-one figure meant that the historical and ideological questions around Mao Zedong had to be addressed properly. Because Deng wished to pursue deep reforms, it was not possible for him to continue Mao"s hard-line "class struggle" policies and mass public campaigns. In 1982 the Central Committee of the Communist Party released a document entitled On the Various Historical Issues since the Founding of the People"s Republic of China. Mao retained his status as a "great Marxist, proletarian revolutionary, militarist, and general", and the undisputed founder and pioneer of the country and the People"s Liberation Army. "His accomplishments must be considered before his mistakes", the document declared. Deng personally commented that Mao was "seven parts good, three parts bad". The document also steered the prime responsibility of the Cultural Revolution away from Mao (although it did state that "Mao mistakenly began the Cultural Revolution") to the "counter-revolutionary cliques" of the Gang of Four and Lin Biao.

In early 1979, Deng undertook an official visit to the United States, meeting President Jimmy Carter in Washington as well as several Congressmen. The Chinese insisted that former President Richard Nixon be invited to the formal White House reception, a symbolic indication of their assertiveness on the one hand, and their desire to continue with the Nixon initiatives on the other. As part of the discussions with Carter, Deng sought United States approval for China"s contemplated invasion of Vietnam in the Sino-Vietnamese war.Zbigniew Brzezinski, Carter reserved judgment, an action which Chinese diplomats interpreted as tacit approval, and China launched the invasion shortly after Deng"s return.

Deng initially continued to adhere to the Maoist line of the Sino–Soviet split era that the Soviet Union was a superpower as "hegemonic" as the United States, but even more threatening to China because of its close proximity.Mikhail Gorbachev took over the Kremlin in 1985, and formal relations between the two countries were finally restored at the 1989 Sino-Soviet Summit.

Deng quoted the old proverb "it doesn"t matter whether a cat is black or white, if it catches mice it is a good cat." The point was that capitalistic methods worked.Zhao Ziyang, who in 1980 replaced Hua Guofeng as premier, and Hu Yaobang, who in 1981 did the same with the post of party chairman. Deng thus took the reins of power and began to emphasize the goals of "four modernizations" (economy, agriculture, scientific and technological development and national defense). He announced an ambitious plan of opening and liberalizing the economy.Chairman and made the General Secretary the ex officio leader of the party.

The last position of power retained by Hua Guofeng, chairman of the Central Military Commission, was taken by Deng in 1981. However, progress toward military modernization went slowly. A border war with Vietnam in 1977–1979 made major changes unwise. The war puzzled outside observers, but Xiaoming Zhang argues that Deng had multiple goals: stopping Soviet expansion in the region, obtain American support for his four modernizations, and mobilizing China for reform and integration into the world economy. Deng also sought to strengthen his control of the PLA, and demonstrate to the world that China was capable of fighting a real war. Zhang thinks punishment of Vietnam for its invasion of Cambodia was a minor factor.military exercise necessary for the PLA, and in September, the North China Military Exercise took place, becoming the largest exercise conducted by the PLA since the founding of the People"s Republic. Moreover, Deng initiated the modernization of the PLA and decided that China first had to develop an advanced civilian scientific infrastructure before it could hope to build modern weapons. He therefore concentrated on downsizing the military, cutting 1 million troops in 1985 (百万大裁军),

Because of the 1989 Tiananmen Square protests, Deng"s power had been significantly weakened and there was a growing formalist faction opposed to Deng"s reforms within the Communist Party. To reassert his economic agenda, in the spring of 1992, Deng made a tour of southern China, visiting Guangzhou, Shenzhen, Zhuhai and spending the New Year in Shanghai, using his travels as a method of reasserting his economic policy after his retirement from office.modern history of China, as it saved the Chinese economic reform and preserved the stability of the society.

Deng is remembered primarily for the economic reforms he initiated while paramount leader of the People"s Republic of China, which pivoted China towards a market economy, led to high economic growth, increased standards of living of hundreds of millions,Nobel Peace Prize.Mao Zedong and with bringing an end to the chaotic era of the Cultural Revolution.Communist power of the time, the Soviet Union, which collapsed in 1991.

1975–1976 and 1977–1980, Europa Publications (2002) "The People"s Republic of Chine: Introductory Survey" The Europa World Year Book 2003 volume 1, (44th edition) Europa Publications, London, p. 1075, col. 1, ISBN 1-85743-227-4; and Bo, Zhiyue (2007) China"s Elite Politics: Political Transition and Power Balancing World Scientific, Hackensack, New Jersey, p. 59, ISBN 981-270-041-2

Yang, Benjamin (1998). Deng: A Political Biography. ISBN 9781134964765. Archived from the original on 1 December 2021. Retrieved 20 September 2019.; focus on rise to power, with brief coverage of actions in power.

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Phytoremediation is a plant-based approach, which involves the use of plants to extract and remove elemental pollutants or lower their bioavailability in soil (Berti and Cunningham, 2000). Plants have the abilities to absorb ionic compounds in the soil even at low concentrations through their root system. Plants extend their root system into the soil matrix and establish rhizosphere ecosystem to accumulate heavy metals and modulate their bioavailability, thereby reclaiming the polluted soil and stabilizing soil fertility (Ali et al., 2013; Jacob et al., 2018; DalCorso et al., 2019). There are advantages of using phytoremediation, which include: (i) economically feasible—phytoremediation is an autotrophic system powered by solar energy, therefore, simple to manage, and the cost of installation and maintenance is low, (ii) environment and eco-friendly—it can reduce exposure of the pollutants to the environment and ecosystem, (iii) applicability—it can be applied over a large-scale field and can easily be disposed, (iv) it prevents erosion and metal leaching through stabilizing heavy metals, reducing the risk of spreading of contaminants, (v) it can also improve soil fertility by releasing various organic matters to the soil (Aken et al., 2009; Wuana and Okieimen, 2011; Jacob et al., 2018). During the past decades, numerous studies have been conducted to understand the molecular mechanisms underlying heavy metal tolerance and to develop techniques to improve phytoremediation efficiency. In the current review, the mechanisms of how heavy metals are taken up and translocated in plants are described, and the detoxification strategies (avoidance and tolerance) adopted by plants in response to heavy metal have been discussed. The main objective is to overview the recent advances in developing phytoremediation techniques, including the strategies to improve heavy metal bioavailability, tolerance, and accumulation. This review also highlights the application of genetic engineering to improve plant performance during phytoremediation.

After entering into root cells, heavy metal ions can form complexes with various chelators, such as organic acids. These formed complexes including carbonate, sulfate, and phosphate precipitate, are then immobilized in the extracellular space (apoplastic cellular walls) or intracellular spaces (symplastic compartments, such as vacuoles) (Ali et al., 2013). The metal ions sequestered inside the vacuoles may transport into the stele and enter into the xylem stream via the root symplasm (Thakur et al., 2016) and subsequently are translocated to the shoots through xylem vessels. Through apoplast or symplast, they are transported and distributed in leaves, where the ions are sequestered in extracellular compartments (cell walls) or plant vacuole, thereby preventing accumulation of free metal ions in cytosol (Tong et al., 2004).

Avoidance strategy refers to the ability of plants to limit the uptake of heavy metals and restrict their movement into plant tissues through root cells (Dalvi and Bhalerao, 2013). It works as the first line of defense at extracellular level through a range of mechanisms such as root sorption, metal ion precipitation, and metal exclusion (Dalvi and Bhalerao, 2013). Upon exposure to heavy metals, plants first try to immobilize them either through root sorption or by modifying metal ions. A variety of root exudates, such as organic acids and amino acids, act as a heavy metal ligand to form stable heavy metal complexes in the rhizosphere (Dalvi and Bhalerao, 2013). Some root exudates can change the pH of rhizosphere, which lead to precipitation of heavy metals, thereby limiting their bioavailability and lessening the toxicity (Dalvi and Bhalerao, 2013). Through metal exclusion mechanism, exclusion barriers exist between the root system and the shoot system to limit the access of heavy metals from soil only to roots; the uptake and root-to-shoot transport is restricted to protect aerial parts against harmful heavy metals. Moreover, arbuscular mycorrhizas can restrict the entry of heavy metals into the root by absorption, adsorption, or chelation of heavy metals in the rhizosphere, thus, working as an exclusion barrier for heavy metal uptake (Hall, 2002). Embedding the heavy metals in the plant cell walls is another mechanism of heavy metal avoidance (Memon and Schröder, 2009). Cell wall pectins consist of carboxylic groups of polygalacturonic acids, which are negatively charged and able to bind heavy metals. Therefore, cell wall acts as a cation exchanger to restrict entry of free heavy metal ions into the cells (Ernst et al., 1992).

After chelation, the complexes of ligands with heavy metals are actively transported from the cytosol into inactive compartments, such as vacuole where the complexes are stored without toxicity (Tong et al., 2004). Sequestration and vacuolar compartmentalization provide an effective protection against the detrimental effects of heavy metals by removing toxic heavy metal ions from sensitive sites of the cell where cell division and respiration occur, thereby reducing the interactions between heavy metal ions and cellular metabolic processes and avoiding damages to cell functions (Sheoran et al., 2011). The uptake, translocation, and detoxification of heavy metals in plants are illustrated in Figure 1.

The process of phytoextraction of heavy metals includes a few steps: (i) mobilization of heavy metals in rhizosphere, (ii) uptake of heavy metals by plant roots, (iii) translocation of heavy metal ions from roots to aerial parts of plant, (iv) sequestration and compartmentation of heavy metal ions in plant tissues (Ali et al., 2013). The efficiency of phytoextraction relies on a few factors such as plant selection, plant performance, heavy metal bioavailability, soil, and rhizosphere properties. Therefore, the strategies to improve phytoextraction efficiency are developed in light of those aspects and are discussed below.

Appropriate selection of the plant species is vital for effective phytoextraction. The plant species for phytoextraction should possess the following characteristics: (i) high tolerance to the toxic effects of heavy metals, (ii) high extraction ability with accumulation of high levels of heavy metals in aboveground parts, (iii) fast growing with high biomass production, (iv) abundant shoots and extensive root system, (v) good adaptation to prevailing environment, strong ability to grow in poor soils, easy cultivation and harvest, (vi) highly resistant to pathogens and pests, be repulsive to herbivores to avoid heavy metals entering into the food chain (Seth, 2012; Ali et al., 2013).

Among these characteristics, metal-accumulating capacities and aboveground biomass are the key factors that determine the phytoextraction potential of a plant species. Therefore, two different strategies for plant selection are being employed: (i) the use of hyperaccumulator plants, which can accumulate heavy metals in aboveground parts to a greater extent and (ii) the use of plants with high aboveground biomass production, which may have lower metal-accumulating capacities, but overall accumulation of heavy metals is comparable to that of hyperaccumulators (Robinson et al., 1998; Salt et al., 1998; Ali et al., 2013).

Generally, hyperaccumulators are plant species capable of accumulating very high levels of heavy metals in their aboveground parts without phytotoxicity symptoms (Rascio and Navari-Izzo, 2011; van der Ent et al., 2013). The naturally occurring heavy metal hyperaccumulator can accumulate metals at levels 100-fold greater than common non-hyperaccumulating species under the same conditions (Rascio and Navari-Izzo, 2011). Strictly, the definition of hyperaccumulator should meet the following criteria: (1) the shoot-to-root ratio of heavy metal concentration is greater than 1, which is a sign of efficient ability to transport metals from roots to shoots (McGrath and Zhao, 2003; Marques et al., 2009); (2) the shoot-to-soil ratio of heavy metal concentration is greater than 1, indicating a higher capability to take up heavy metals from soil (McGrath and Zhao, 2003); and (3) the concentration of the metal in the shoot is higher than 10 mg/kg for Hg, 100 mg/kg for Cd and Se, 1,000 mg/kg for Co, Cu, Cr, Ni, and Pb, and 10,000 mg/kg for Zn and Mn (Baker and Brooks, 1989).

Searching for effective hyperaccumulators is a key and the most straightforward strategy for successful phytoremediation of heavy metals. Currently, more than 450 plant species from at least 45 angiosperm families have been identified as metal hyperaccumulators so far (Suman et al., 2018), ranging from annual herbs to perennial shrubs and trees, such as Brassicaceae, Fabaceae, Euphorbiaceae, Asterraceae, Lamiaceae, and Scrophulariaceae families (Salt et al., 1998; Dushenkov, 2003). Some species can even accumulate more than two elements, such as Sedum alfredii, which can hyperaccumulate Zn, Pb, and Cd (He et al., 2002; Yang et al., 2002, 2004). A list of some plants, which show high capacity of heavy metal accumulation is given in Table 1. However, using edible crops for phytoremediation should be avoided as heavy metals can accumulate in edible parts of the plant and thus enter into the food chain by human or animal consumption, raising concerns on human health. Hence, selection of the non-edible hyperaccumulators is a key for efficient and safe phytoremediation of heavy metals.

Heavy metal pollution is a vital issue for agricultural production and food health due to the toxic effects and rapid accumulation in the environment. To prevent or mitigate heavy metal contamination and revegetate the contaminated soil, a variety of techniques have been developed. Phytoremediation has been proven to be a promising technique for revegetation of heavy metal-polluted soil with a good public acceptance and shows a variety of advantages compared with other physicochemical techniques. The application of heavy metal hyperaccumulators is the most straightforward approach for phytoremediation, and hundreds of hyperaccumulator plants have been identified so far. However, phytoremediation with these natural hyperaccumulators still suffers from a few limitations, as it is a time-consuming process, which takes a very long time to clean-up heavy metal-contaminated soil, particularly in moderately and highly contaminated sites. This may partially be due to slow growth rate and low biomass production of these hyperaccumulators. Therefore, improving plant performance is a critical step for developing high effective phytoremediation. Fortunately, genetic engineering approach has been emerging as a powerful tool to modify plants with desired traits such as fast grow, high biomass production, high heavy metal tolerance and accumulation, and good adaption to various climatic and geological conditions. Hence, good understanding of the mechanisms of heavy metal uptake, translocation, and detoxification in plants, and identification and characterization of different molecules and signaling pathway, will be of great importance for the design of ideal plant species for phytoremediation via genetic engineering. Genes involved in heavy metal uptake, translocation, sequestration, and tolerance can be manipulated to improve either heavy metal accumulation or tolerance in plants. In addition, chelating agents and microorganisms can be used either to increase heavy metal bioavailability, which facilitates heavy metal accumulation in plants, or to improve soil health and further promote plant growth and fitness.

Buendía-González, L., Orozco-Villafuerte, J., Cruz-Sosa, F., Barrera-Díaz, C., and Vernon-Carter, E. (2010). Prosopis laevigata a potential chromium (VI) and cadmium (II) hyperaccumulator desert plant. Bioresour. Technol. 101, 5862–5867. doi: 10.1016/j.biortech.2010.03.027

Tong, Y.-P., Kneer, R., and Zhu, Y.-G. (2004). Vacuolar compartmentalization: a second-generation approach to engineering plants for phytoremediation. Trends Plant Sci. 9, 7–9. doi: 10.1016/j.tplants.2003.11.009