# Lead Tolerance and Enrichment Characteristics of Three Hydroponically Grown Ormamental Plants
## 1 Introduction  
Lead is a heavy metal element that has poor mobility on the surface of soil and is difficult to degrade [1]. With the rapid development of China's industrialization, urbanization, and agricultural modernization, human activities have released a large amount of Pb into the environment. What is even scarier is that Pb can enter the human body through the food chain or skin contact and damage human health [2, 3]. Therefore, the identification of optimal methods to remove the heavy metal Pb from soil and reduce its toxicity represents an urgent need within current economic and social development [4, 5].  
Against the background of China's promotion of constructing an ecological civilization, phytoremediation technology has become the first choice for Pb-contaminated soil due to its economic, green, and sustainable development advantages [6, 7]. Phytoremediation involves the use of plant roots, which absorb heavy metals and then transfer them to aerial parts. Plants can communicate and interact with the soil microbiome and other plants through their roots [8]. Roots show high developmental plasticity and are frequentlyadapted to their environment. Finally, the shoot parts are harvested and treated to remove, degrade, or fix heavy metals [9]. Therefore, plant roots are the first tissues and organs to make contact with pollutants [10], and studies have found that the root architecture of plants has an important relationship with the absorption and accumulation of heavy metals [11]. With the development of science and technology, the use of plant genetic engineering could greatly promote the process of heavy metal pollution remediation [12]. The use of ornamental plants to remediate heavy metal pollution in soil could not only reduce the content of heavy metals in environmental soil but also achieve the purpose of beautifying the environment [13]. The screening of heavy metal hyperaccumulator plant varieties has always been an important method to promote heavy metal pollution remediation [14]. However, it is very difficult to find a plant species that meets the criteria for hyperaccumulation plants.  
Moreover, the shortcomings of hyperaccumulator plants, such as small biomass and slow growth, had become the key factors limiting the widespread application of phytoremediation of heavy metal-contaminated soils [15]. Therefore, the selection of ornamental plants with fast growth rates, large biomass, and potential for hyper-enrichment of heavy metals for pollution remediation had also become one of the feasible measures [16]. Previous studies have reported that _T. patula, S. nigrum_, and _M. jalapa_ were ornamental plants with a high capacity to enrich Pb [3; 17; 18], and had been selected as the key candidates for remediation of Pb-contaminated soil.  
More than 450 species of heavy metal hyperaccumulating and tolerant plants are known in tropical and temperate regions, most of which were nickel hyperaccumulators, while lead hyperaccumulation occurs in a few species [19]. Currently, there are also a large number of reports on the screening of ornamental plants with a high capacity of enriching Pb [13]. However, most of the current studies have focused on the biomass and enrichment coefficients of ornamental plants [5; 14; 20]. Studies on how ornamental plant root morphology responds to Pb stress and the correlation between root morphology and Pb tolerance, uptake, and enrichment in ornamental plants are lacking. The aim of this study is to determine the response of root morphology of test ornamental plants to Pb stresses and to study the relationship between the root architecture of ornamental plants and their ability to tolerate and enrich the heavy metal Pb through hydroponic experiments. The research results provide an important scientific basis for accelerating the large-scale popularization and application of flowers and the phytoremediation of heavy metal and Pb-contaminated environments.