Uptake of NPs by the root has been observed at primary and lateral root junction as well according to the transverse slice. Figure 2a is one transverse slice localized at the arrow in Fig. 3c showing the junction between primary root and lateral root. We found that the yttria NPs were absorbed by the lateral roots, and particulates began to accumulate along the outer epidermis of primary roots with limited entrance into the vascular tissue of the primary root. It might happen that endodermal cell walls were blocking the entrance of aggregated yttria NPs into vascular tissue. This is shown in the upper section of the 3D visualization where no yttria NPs were observed above the root system. Besides the full view of the translocation in the cabbage root system, the distribution of yttria NPs at the micro-scale within a lateral root was detected and investigated . Figure 4a shows the localization of the micro-scale lateral root visualization. The 3D visualization of micro-scale was built by the segmented transverse reconstructed slices, and the red regions were localized yttria NPs . It is clear that roots are able to uptake the yttria NPs in ground tissue ,stacking flower pot tower which appear to accumulate in the root with limited entrance of yttria NPs into vascular tissue being transported through the xylem.
Xylem vessels are small with diameters usually smaller than 1 μm in vegetables like cabbage plants to over 100 μm in vessels found in trunks of large trees . Vessels allow nutrients contained in water to be distributed throughout the plant. For NPs, however, if they aggregate, the blockage is expected, that is what we have observed in this study. Long term studies might show that yttria NPs might provide more negative than positive effects on plant growth and development as found with other NPs. Using K-edge subtraction image technique with dualenergy X-ray scanning, the concentration of target NPs can be calculated. This method has been discussed elsewhere. Cadmium is not essential but highly toxic for higher plants even at a trace level, with the clear exception of certain hyper accumulator plant species . There is an increasing body of evidence showing that Si has many direct and indirect beneficial effects on the growth of plants subjected to various forms of abiotic stress including Cd stress . So far, numerous studies have demonstrated that Si can enhance resistance or tolerance to Al , Mn and salt toxicity in plants . By contrast, less work has been done about possible roles of Si in Cd tolerance, although Chen et al. reported that application of silicon-containing steel sludge and furnace slag could decrease Cd uptake by wetland rice. More recently, we have shown that Si-mediated alleviation of Cd toxicity in pakchoi could be attributed to Si-suppressed Cd uptake and root-to-shoot transport . However, the underlying mechanisms are still poorly understood.
More importantly, studies have been focused mainly on the roles of Si in alleviating heavy metal toxicity in Si-accumulating graminaceous plant species such as rice and maize , while less work has been done on the possible roles of Si in dicots such as pakchoi, bean and strawberry that do not accumulate much amount of Si in their tissues . In this study, we show that Si was able to mitigate Cd toxicity in roots of pakchoi, with respect to antioxidant enzymes and non-enzymatic antioxidants, and histochemical characterization using two contrasting pakchoi cultivars that differ greatly in response to Cd exposure. To confirm Si-mediated roles in antioxidative defense system, we performed histochemical staining experiments with Evans blue and Schiff’s reagent . The Evans blue was applied to determine the loss of plasma membrane integrity and the Schiff’s reagent to determine the degree of peroxidation of membrane lipids . For the Cd sensitive cultivar , the roots treated with both concentrations of Cd alone were stained to different extents, and under the higher Cd treatment, the roots were stained extensively. And the roots became lighter straining in the treatment with Cd plus Si compared with the Cd treatment alone. For example, root tips were more heavily stained in the Cd1 treatment than in the Cd1Si treatment. For the Cd-tolerant HYD, very similar changes were also observed in the roots . Furthermore, the Si beneficial effects on the protection of cell membrane against Cd-induced oxidative damage were more significant in the Cd-tolerant plant roots than in the Cd-sensitive plant roots. For the Cd-sensitive cultivar , addition of Cd significantly decreased SOD activities in roots compared with the control, which was intensified with increasing Cd concentrations . The activity of SOD was increased by 47.3%, 12.0% and 9.6% in the plants treated with Cd plus Si compared with the corresponding Cd treatments without Si, respectively .
For the Cd-tolerant cultivar , very similar changes were noted in SOD activity in the Cd treatments with or without Si added, with an exception that no significant differences in SOD were found between the Cd1 treatment alone and the control . For the sensitive cultivar , CAT activity in the Cd treatment significantly decreased with increasing Cd concentrations compared with the control. Addition of Si significantly increased CAT activity in Cd-stressed pakchoi roots compared with Cd treatment alone throughout the whole experiment . For example, addition of Si increased CAT activities by 3.7%, 28.4% and 25.7%, respectively, at 0, 0.5 and 5.0 mg L-1 Cd, compared with the corresponding Cd treatments alone. For the Cd-tolerant cultivar , very similar results were obtained of CAT activities in the Cd treatments with or without Si, with an exception that addition of Si did not result in significant differences in CAT activities between the lower and the higher Cd treatments . For the Cd-sensitive cultivar, addition of Si significantly increased APX activities in roots by 55.1% compared with the control. The activity of APX was 16.7% higher in the Cd1 plus Si treatment than in the Cd1 treatment alone, compared to 11.4% at the Cd2 level . For the Cd-tolerant cultivar, very similar changes were observed in APX activities in the Cd treatments with or without Si, with an exception that significant increases in APX activity were found between the Cd plus Si treatment and the Cd treatment alone .While agriculture in wealthy nations has entered the domain of mass market and industrialization, subsistence farming is still practiced in vast swaths of the developing world. In this model of agriculture, the output of a family’s farm is primarily used to feed that family. This agricultural context can have profound consequences for human health, as such limited diets contribute to micro-nutrient malnutrition. Iron deficiency is the most common deficiency, affecting over 2 billion people including many women of childbearing age and children . The associated costs are immense in terms of both human life and economic output . Because communities are often constrained to specific crop species by environmental and cultural factors, it is necessary to pursue local crop improvement in addition to larger scale efforts. Natural variation within closely related species provides the means to manipulate agronomically important traits, iron uptake among these,danish trolley reducing the severity of iron deficiency in subsistence cultures. Staple cereals including rice and wheat have a wide range of variation in iron uptake and spatial allocation among their cultivars; this has been leveraged previously to enhance their iron uptake properties . Foxtail millet is a critical subsistence crop in Northern China, India, and parts of Africa . Additionally, S. italica and its wild ancestor, S. viridis, are emerging model organisms. S. italica is a relatively recent domesticate, and can be considered a subspecies of Setaria viridis in spite of their phenotypic differences .
Indeed, the two species are capable of cross pollination and exhibit a continuous spectrum of morphological traits. In light of this, they are often denoted Setaria italica subsp. viridis and Setaria italica subsp. italica. Throughout this thesis, groups of individuals containing both subspecies will be referred to as either Setaria or the Setaria species complex. Due to its close relationship with economically important crops like maize and wheat , small stature, relatively short life span, status as a C4 plant, and sequenced genome, S. italica subsp. viridis is becoming a preferred subject for genetic studies . Iron acquisition does not occur in a vacuum; ionic homeostasis is intricately connected . The concentration of iron plays a role in influencing the concentration of other ions through its control over plant height. Ions such as zinc, copper, and manganese share enough properties with iron to be chelated by PS and taken up by the same transporters , though the transporters do seem to be exclusively regulated by iron status. In rice, iron deficiency associated upregulation of the iron transporters OsIRT1 and OsIRT2 contributes to cadmium uptake . Ion interactions with the soil solution also impact the bio-availability of other, complementary ionic species. If there are multiple ions held very tightly to a given colloid within the soil, they may work to ‘shield’ nearby ions from plant roots, thereby decreasing the availability of an ion that is typically fairly accessible . The interconnection of ions in the grasses is not entirely dependent upon iron; magnesium and cadmium compete for translocation in barley , calcium impacts concentrations of manganese, potassium, strontium, rubidium, and zinc . It is clear, then, that the relative concentration of one ion in the soil can impact the ultimate concentration of different ions in the plant, and that any attempt to understand the genetics underlying the homeostasis of one ion must necessarily address the complex dynamics of the complete ionic milieu. The interconnection of the concentration of various ions has led researchers to the concept of the ionome of an organism; that is, its mineral nutrient and trace element composition . The ionome is essentially the inorganic component of an organic system. The study of the ionome is referred to as ionomics. This process relies on the quantitative measurement of the ionic composition of a living organism as it varies with genetic background, developmental time points, and physiological conditions . Ionomics relies on the simultaneous sampling of the entire ionome; this sampling is often conducted with the use of Inductively Coupled Plasma-Mass Spectroscopy . Because iron is central to the growth, development, and ultimate reproductive capacity of plants, the different availabilities of this nutrient in different soils provides a strongly variable selective pressure. Though S. italica appears to have been domesticated from S. viridis in Gansu Province, Northwestern China in around 5900 BP , it has since been cultivated in many diverse regions, including ‘Southeast Europe, South and Central China, the Far East, the Americas, Australia’, and parts of Africa . Given this wide pattern of cultivation, it is likely that S. italica has been cultivated in many different soil types. Indeed, the USDA’s Germplasm Resources Information Network contains 772 S. italica accessions at time of writing. The documentation for these accessions contains information such as the exact coordinates of collection, which indicate the diversity of soils in which S. italica has been cultivated during the last century. The diversity of soil characteristics is commonly defined by the USDA’s 12 order system . These orders differ in a wide variety of properties, many of which are important to ionic homeostasis in the plants that live upon the soil. Among these characteristics are the percentage of organic matter, mean annual soil temperature, base saturation, pH and cation exchange capacity .CEC is largely dependent upon pH: the number of exchangeable cations increases in the soil solution as pH increases . Phytosiderophore uptake in the grasses also depends upon pH, and therefore on soil type, with lower pH contributing to increased PS uptake . There is evidence that soil type influences PS exudation as well: the Tamaro wheat cultivar appears to exude different levels of PS in different soils . This is consistent with the differential induction of PS in media with different iron availabilities . Plants working to obtain mineral nutrients not only face the obstacle of extracting insoluble iron from the soil, they also face significant competition from organisms in the rhizosphere for these resources.