However, due to the short lifetimes of PET radioisotopes, only short biological processes, such as photosynthesis, may be imaged. In contrast, single gamma-ray emitting radio tracers used in single photon emission computed tomography are typically metals and do not easily label organic molecules. However, many trace element metals are essential to a plant’s survival . An active area of plant research is studying hyper accumulation of metals in plants using a radioisotope of that metal; commonly studied metals include: Cd, Zn, Mn, Co, and Ni . Other potential applications for SPECT imaging include: studying plant ion transport in xylem , studying metabolic processes such as tracers for phloem transport , and studying signaling by labeled exogenous peptides or proteins . One further advantage of imaging systems based on gamma-ray detection is the possibility of detecting the interactions of multiple radioisotopes simultaneously as the gamma-rays that they emit have distinct energies that can be distinguished from each other by the detector. For example, simultaneous imaging of 65Zn and 109Cd would enable teasing apart the competition dynamics in their uptake. However, there is an inherent trade off in increased sensitivity of the UCD-SPI system with spatial resolution. Spatial resolution at the mm-scale could be obtained using a collimator to better define the spatial origin of detected gamma rays, but this would lead to a greatly reduced event rate in the system. For the high-energy gamma rays of 65Zn, collimation is a particular challenge. Given the hours-long time scale of the transport studied here, it is possible that the choice of a collimator could have provided improved event positioning while preserving a usable event rate. A possible hybrid approach could have included using an insertable/removable collimator to acquire an alternating combination of two types of images: high sensitivity-low spatial resolution without the collimator; and low sensitivity-higher spatial resolution with a collimator. However, led grow lights the use of a collimator for this system is unexplored thus far.Zn uptake into the symplast in the outer root layers and loading into the apoplastic xylem stream are well understood on molecular level.

However, the dynamics of symplastic movement and patterning of the radial transport have thus far only been modeled to elucidate the timescales of these events . After xylem loading, the mass flow-mediated movement of Zn into the shoot inside the xylem is expected to occur within 30 min in Arabidopsis, as previously shown for water and Cd in xylem sap . From previous SPECT imaging, we have shown that a pulse of radiolabelled pertechnetate moving in the xylem stream reaches the shoot apical meristem of a 2 week old sunflower already in 5 min . The rate-limiting step for root-to-shoot translocation of Zn was proposed to be xylem loading involving HMA4 transporters in both A. halleri and A. thaliana . The dynamics of root-to-shoot Zn flux, however, have so far remained unclear in different species and transgenic lines. Estimates of Zn translocation rates from root to shoot were first obtained by spectroscopy methods of ashed shoot tissues. Early work with metal hyper accumulator Noccaea caerulescens suggested that the speed of root-to-shoot Zn transport was between 20 and 60 hr . Recently, positron imaging of Zn uptake estimated the time for Zn root-to-panicle transport in dwarfed mature rice to be 5.3 hr . Here, we have produced the first Zn root-to-shoot imaging data for A. halleri using UCD-SPI. Zn accumulates within the shoot of A. halleri, consistent with its ability to hyperaccumulate Zn, different from the HMA4 RNAi line. The speed of Zn transport into the shoot in our data as observed with the smoothed standard error show clear shoot accumulation within 5–7 hr, respectively . These results are in line with previous reports for rice . This contrasts strongly with the faster speed of the other xylem-transported compounds, such as water in A. thaliana , Cd in A. halleriand pertechnetate in sunflower , all measured to reach the shoot in 30 min. It should be noted, however, that the experiments demonstrating water transport and Cd transport were carried out using decapitated stems and are thus destructive in nature, but also far more sensitive to small quantities than the method used here. The slower speed of Zn transport indicates that Zn loading into the xylem by HMA4 is slow and under tight control even in the metal hyper accumulator A. halleri. Modelling the radial transport of Zn uptake has indeed indicated that HMA concentration is one of the key determinants of the uptake dynamics . The HMA4 transporter pumps Zn2+ from the root symplasm into the apoplastic xylem sap of A. thaliana . Strongly elevated expression of A. halleri HMA4 was suggested to be responsible for the increased in root-to-shoot translocation of Zn in A. halleri relative to A. thaliana . This conclusion was drawn based on the quantification of shoot Zn concentrations after long-term growth in HMA4-RNAi lines and wild-type A. halleri and in A. thaliana Col-0 . In the same experiment, root Zn concentration was elevated in some A. halleri HMA4 RNAi lines relative to A. halleri wild-type plants and even relative to A. thaliana . HMA4 is critical to the ability of A. halleri to hyper accumulate Zn.

We tested the functional role of HMA4 for A. halleri Zn translocation from root to shoot by imaging the Zn uptake dynamics of A. halleri HMA4-RNAi line relative to A. halleri. We found that the Zn signal in the shoot of HMA4-RNAi line did not increase over our 40-hr imaging period, but conversely, we saw a continuous decrease in shoot Zn signal with significant differences observable at 3 hr . The lack of an increase in shoot Zn confirms that Zn loading into the xylem is abolished in the HMA4-RNAi plants . The continuous decrease in the Zn signal in the shoot ROI seems to reflect bleeding of the strong early Zn signal from the root ROI into the shoot ROI. The dissipating signal through the A. halleri HMA4- RNAi time course could be due to apoplastic 65Zn adsorbed to the cell walls of outer root layers during the 65Zn pulse and not removed by the triple rinsing with Hoagland solution. This cell wall-adsorbed 65Zn would be desorbed into the growth medium during the imaging period by diffusion. The influx of Zn into the root symplasm is very tightly and rapidly regulated in Zn-concentration dependent fashion . Without the loading of Zn into the xylem, Zn builds up in the root symplasm. In the case of A. halleri HMA4-RNAi, the symplasm could be saturated with Zn at 3 hr after the resupply, leading to prevention of further uptake of the cell wall-adsorbed 65Zn and thus higher Zn desorption than Zn uptake into the symplasm. Finally, we compared the dynamics of Zn movement in the Zn hyper accumulator A. halleri with those in the related species A. thaliana, a non-metal hyper accumulator. Based on previous studies comparing Zndeficient to Zn-sufficient plants of A. thaliana and/or A. halleri, the Zn concentrations in our hydroponic solutions can be estimated to result in moderate Zn deficiency . The net concentration of 65Zn in the resupply media over the 24 hr period showed a net decrease, suggesting that Zn was taken up into the shoot, although these levels are variable. We found that Zn resupply after Zn deprivation in A. thaliana did not lead to detectable uptake or change of Zn in the shoot or the root ROI. It is possible that the small size and flat rosette growth habit of A. thaliana affected our ability to detect Zn dynamics. Also, low abundance of HMA4 transporters in A. thaliana roots may lead to much slower dynamics that we were unable to capture. In the absence of quantification of 65Zn levels in the shoot, it is possible, although unlikely, that Zn was not translocated in A. thaliana.Although most plants prevent the accumulation of heavy metals so as to avert toxicity, metal hyper accumulators selectively extract high concentrations of metals from the soil into their shoots without incurring symptoms of toxicity . By using the heavy metal radio label 65Zn and the UCD-SPI imaging system, we gained a more detailed spatiotemporal understanding of the dynamics of metal movement into plants, which may be a path toward the use and understanding of metal hyper accumulating plants for such advantageous applications.Reading comprehension is a complex process that requires the coordination and integration of a number of component cognitive skills. The ability to read single words in isolation is widely accepted as one skill critical to comprehension, but successful reading comprehension does not always directly stem from adequate word identification skills. Some individuals who are skilled word readers are not skilled passage comprehenders , supporting the idea that reading comprehension requires processes above and beyond single word reading. Theoretical models of reading comprehension propose that successful comprehension requires a reader to draw on both text-based information and prior knowledge in order to build a coherent and meaningful mental representation of the text . This mental representation is the reader’s understanding of the text’s deeper meaning; it consists of ideas from the text,strawberry gutter system relevant background knowledge, and inferences the reader makes about things not explicitly stated in the text . Building this mental representation is a dynamic process because cognitive demands change over time. For example, readers are known to spend more time processing words and sentences at the beginning of a text relative to later points.

This could be due to the fact that, without context or relevant background knowledge activated to facilitate comprehension, comprehension necessitates more effortful attention to the initial construction of a mental representation . Conversely, later stages of comprehension processes are facilitated by an increasing semantic contextualization . A number of imaging studies have examined the neurobiological correlates of reading comprehension . Patterns of activation emerge when processing discourse that cannot be predicted from models of reading single words, or even single sentences, in isolation . Areas that consistently appear to be unique to processing narrative texts include the dorsal medial prefrontal cortex and bilateral temporal parietal junction, often attributed to social cognition required in story comprehension, bilateral temporal poles , which play a role in generating specific semantic associations in connected text, and posterior medial structures, including posterior cingulate cortex and precuneus , which have been associated with updates in and integration of the reader’s mental model . This demonstrates that reading connected text involves additional processes beyond the phonological, orthographic, semantic, and syntactic processes seen at the word and sentence level. Still, many questions regarding how readers form a coherent text representation remain unanswered. Only a handful of studies have examined how the neural correlates of discourse processing change over the temporal progression of the discourse . Of the few, Xu et al. used fMRI to compare the activation associated with reading the beginning of a story with the activation associated with reading the end of the story . They found that processing the story’s setting and initiating events resulted in strongly left lateralized activation, while processing the story’s outcome resulted in increased activation in right hemisphere perisylvian and extrasylvian regions thought to contribute to inference and contextualization of narrative .These right hemisphere regions have since been related to social cognition processes that may be narrative-specific . This study provides evidence that reading comprehension not only involves processes distinct from those required in single word reading, but also that comprehension demands can vary from point to point within a given text. Similarly, by modifying the cohesiveness of text Yarkoni et al. identified neural regions that showed linear increases in activation as a function of reading time. More specifically, they compared construction processes with maintenance processes . They found that regions in the posterior parietal cortex associated with visuospatial updating and attention are involved in the construction of a reader’s mental model, while perisylvian language areas were more involved in its maintenance. These studies support theoretical models that suggest that building a mental representation of text is a dynamic process in which the cognitive demands shift from one point in the text to the next. Nevertheless, it is important to note that all of the aforementioned fMRI studies on discourse processing have exclusively examined narrative texts; none to date have examined expository texts . However, event-related potential and behavioral studies suggest such genre distinctions are important. For example, Baretta et al. used ERP to distinguish between narrative and expository texts.