Month: January 2023

Total soil C and N were strongly associated with EOC and EON, the soil C:N ratio, and POXC. These variables had negative values along axis 2 and thus contrasted with the pattern of soil inorganic N. Weak loading of AMT1.1, NRT2.1, Nii, and GS2 on the first two principal components reflects the lack of association of expression levels of these genes with biogeochemical and plant variables. Non-overlapping confidence ellipses for seven out of 13 fields on the PCA biplot indicated distinct N cycling patterns . Fields 1 and 2, with the highest values along axis 1, had low values of all variables included in the analysis. Field 4 had the highest values along axis 2 corresponding with higher soil NH4 + and NO3 – . Fields 10, 11, 12, and 13 were associated with high values of labile and total soil C and N. Overlapping confidence ellipses of fields 3, 5, 6, 7, 8, and 9 close to the origin indicate similar, moderate values of this suite of variables for these fields. Three groups of fields were identified by k-means cluster analysis of the same 28 variables included in the PCA . Group 1 included fields 1 and 2, which had low mean values for yield , the lowest mean soil C and N and soil inorganic N pools , and the lowest mean value of GS1 relative expression in roots. Groups 2 and 3 had similarly higher mean yield , shoot N, and petiole NO3 – than group 1, but these two groups differed substantially in their soil C and N pools. Group 2 had higher soil NH4 + and NO3 – pools as well as root expression of AMT1.2 while group 3 had higher total and labile soil C pools. Expression of GS1 was similar in both groups. Based on the relative magnitude of F-statistics calculated for each variable,vertical indoor hydroponic system soil C and N, EOC, EON, shoot N, and soil NO3 – at transplant and anthesis were most strongly differentiated across the three groups.

The high F-statistics of AMT1.2 and GS1 relative to other N metabolism genes indicate that root expression of these genes are most responsive to soil N cycling.This study confirms that working organic farms can produce high yields with tightly-coupled N cycling that minimizes the potential for N losses. Such farms had the highest soil C and N and used high C:N organic matter inputs coupled with labile N inputs that resulted in high soil biological activity, low soil inorganic N pools, high expression for a root N assimilation gene, adequate plant N, and high yields. Organic systems trials have previously shown crop N deficiencies that lead to less-than-ideal crop productivity; losses of N when Navail ability is poorly synchronized with crop N demand; or alternatively, that organic production can reduce N losses. But how working organic farms achieve yields competitive with high-input conventional production with low potential for N losses has not been demonstrated. Elevated expression of a key gene involved in root N assimilation, cytosolic glutamine synthetase GS1, in fields with tightly coupled N cycling confirmed that plant N assimilation was high when plant-soil-microbe N cycling was rapid and inorganic N pools were low, thus showing potential as a novel indicator of N availability to plants. Improving biologically-based farming systems will benefit from research that uses novel tools to uncover innovations happening on farms, especially if the research process helps facilitate knowledge exchange among farmers and researchers.To characterize the substantial variation in crop yield, plant-soil N cycling, and root gene expression across 13 fields growing the same crop on similar soil types, we propose three N cycling scenarios: “tightly-coupled N cycling”, “N surplus”, and “N deficient”.

Values of indicator variables suggest differing levels of provisioning, regulating, and supporting ecosystem services in each scenario . Fields in group 3 show evidence of tightly-coupled plant-soil N cycling, a desirable scenario in which crop productivity is supported by adequate N availability but low potential for N loss. Despite consistently low soil NO3 – pools in these fields, well below the critical mid-season level for conventional processing tomatoes in California, total above ground N concentrations were very close to or only slightly below the critical N concentration for processing tomatoes. Tomato yields were also above the county average . This discrepancy between low soil inorganic N pool sizes and adequate tomato N status is due to N pools that were turning over rapidly as a result of efficient N management, high soil microbial activity, and rapid plant N uptake. Composted yard waste inputs with relatively high C:N ratios in concert with limited use of labile organic fertilizers applied during peak plant N demand provided organic matter inputs with a range of N availability. A companion study showed how high potential activities of N-cycling soil enzymes but lower activities of C-cycling enzymes in this set of fields reflect an abundant supply of C but N limitation for the microbial community, thus stimulating production of microbial enzymes to mineralize N. Plant roots can effectively compete with microbes for this mineralized N, especially over time and when plant N demand is high. High root expression of GS1 in these fields indicates that root N assimilation was elevated and thus actual plant N availability and uptake was higher than low inorganic N pools would suggest . Fields from group 2 demonstrated N surplus, showing similar yields to group 3 but with lower total and labile soil C and N and a higher potential for N losses, given much higher soil inorganic N . While actual N losses depend on a host of factors , high soil NO3 – is considered an indicator for N loss potential. Results from a companion study support the idea that soil microbes were C rather than N-limited in these fields, showing higher potential activities of C-cycling soil enzymes but low activities of N-cycling soil enzymes, the inverse of group 3 .

An alternative multivariate clustering approach based on an artificial neural network suggests multiple potential drivers of higher inorganic N pools in these fields, including both management factors and soil characteristics . For instance, field 4 had strong indications of surplus N driven at least in part by a large application of seabird guano , a readily-mineralizable organic N fertilizer, at tomato transplanting when plant N demand is low. In contrast, higher inorganic N in field 8 was likely driven by low plant N demand based on very low soil P availability, which resulted in plant P limitation. These site-specific problems were identifiable due to the focus on variability across similar organic fields and illustrate the need for site-specific approaches to reduce N losses. Finally, the two fields included in group 1 were exemplary of N deficiency, in which low N availability compromises crop productivity but also likely limits N losses within the growing season. While low soil NH4 + and NO3 – concentrations were similar to group 3,vertical farming tower for sale low total and labile soil organic matter and poorly-timed organic matter inputs compromised microbial activity and likely limited N mineralization.Cytosolic glutamine synthetase GS1 encodes for the enzyme that catalyzes the addition of NH4 + to glutamate, the former resulting from either direct uptake of NH4 + from soil or reduction of NO3 – in roots. GS1 is thus the gateway for N assimilation in roots and is upregulated to increase root N assimilation capacity. Similar levels of GS1 expression in groups 2 and 3, in spite of large differences in soil NH4 + and NO3 – concentrations at the anthesis sampling, suggests that plant N availability is indeed higher in group 3 fields than would be expected based on measurement of inorganic N pools alone. The low levels of GS1 expression found in fields with clear N deficiency supports this idea. These results complement recent experimental approaches that showed rapidly increased expression of GS1 in tomato roots in response to a pulse of 15NH4 + -N on an organic farm soil, which was linked to subsequent increases in root and shoot 15N content, even when this pulse did not significantly change soil inorganic N pools. GS1 transcripts and glutamine synthetase enzyme activity also increased with increasing NH4 + and NO3 – availability in sorghum roots, suggesting this response may be widespread among plant species.

Interestingly, inclusion of soil GWC in multiple linear regression models increased the proportion of GS1 expression variability explained to nearly 30% ; soil water content increases microbial activity as well as the mass flow and diffusion of inorganic N to roots. Further research will undoubtedly show how other factors like crop physiological N demand relative to C fixation and P availability increase the interpretability of N uptake and assimilation gene expression in roots.The N cycling scenarios identified on this set of organic fields corresponded at least in part with landscape clusters based on landscape and soil characteristics . Fields that balanced high yields with low potential for N loss and high internal N cycling capacity were part of PAM cluster 1, which had the highest productive capacity rating . Landscape clusters encompassing more marginal soils included both low-yielding fields exhibiting N deficiency or high-yielding fields that used inputs of highly available N like seabird guano to alleviate N deficiency . But these inputs led to the highest soil NO3 – levels and thus came at the cost of higher potential for N loss. Long-term efforts to increase internal soil N cycling capacity would help alleviate both N deficiency and the need for such large inputs of labile N. Whether farmers are willing to invest in management to increase soil N cycling capacity depends in part on how likely they perceive the benefits to be, especially on marginal soils. The discussions that we had with each farmer in this study indicated genuine interest in adaptive management to further tighten plant-soil N cycling, but this may not always be the case. Indeed, the proportion of management vs. inherent soil characteristics responsible for driving differences in N cycling is challenging to untangle. Farmers may allocate more resources to more productive land and likewise fewer resources to more marginal land, or may selectively transition more marginal land to organic management. Documenting the multiple services provided by increases in soil quality and facilitating information exchange among organic growers such as through the landscape approach used here may help build momentum for efforts to improve soil quality and plant-soil-microbe N cycling.The so-called chelation strategy , which is mainly found in graminaceous plants, is based on the excretion of phytosideropores to the rhizosphere. Phytosideropores rapidly chelate Fe, to form Fe-PS chelates that are subsequently transported into the root cells through a specific transporter. The socalled reduction strategy relies on the coordinated action of a membrane bound Fe reductase, that reduces Fe to Fe, an Fe uptake transporter and an H+ -ATPase that lowers the pH of the rhizosphere, is mainly used by non graminaceous plants, including Beta vulgaris. The reduction strategy includes root morphological, physiological and biochemical changes that lead to an increased capacity for Fe uptake. Morphological changes include root tip swelling, development of transfer cells and an increase in the number of lateral roots, leading to an increase in the root surface in contact with the medium. Some plants are able to accumulate and/or release both reducing and chelating substances, such as phenolics and flavins, which may have a role in Fe acquisition. Iron has been shown to down-regulate riboflavin synthesis in flavinogenic yeast strains and some bacteria. In plants, Rbfl and derivatives are accumulated and/ or excreted in Fe-deficient roots and could act as a redox bridge for electron transport to the Fe reductase. Moreover, FRO2 belongs to a super family of flavocytochrome oxidoreductases, and a recent study confirmed that the FRO2 protein contains FAD sequence motifs on the inside of the membrane. Also, a connection between Fe deficiency perception and Rbfl excretion has been described to occur through basic helixloop-helix transcription factors in Arabidopsis thaliana. At the metabolic level, increases in the activity of phosphoenolpyruvate carboxylase and several enzymes of the glycolytic pathway and the tricarboxylic acid cycle have been found in different plant species grown under Fe deficiency.

Through mechanical weathering processes, these clay fragments become incorporated into the soil and may provide a long-term source of PAH contamination in the environment . Polycyclic aromatic hydrocarbons are ubiquitous environmental contaminants and 16 PAHs are considered priority pollutants by the U.S. .There have been a few ecotoxicological evaluations concerning the large PAH concentrations from clay target fragments, but these studies have reported that the PAHs elicit low toxicity in aquatic organisms . This low toxicity was determined to be primarily due to the low bio-availability of PAHs resulting from the process of making clay targets in which the PAHs in the binding agent are bound under heat and pressure with dolomitic limestone . In addition, due to their aromatic nature and hydrophobicity, PAHs typically bind to nonpolar soil domains such as organic matter, further decreasing their bio-availability . However, a recent clay target ecotoxicity study using Eisenia andrei showed that the content of clay fragments in soils was correlated with PAH bio-accumulation in the terrestrial soil organism, suggesting that direct ingestion can be a more important route of exposure and potentially explain the lack of toxicity in exposed aquatic organisms . Clay-target contaminated site evaluations have also concluded that the elevated PAH concentrations in the soil from the clay target fragments pose an unacceptable level of risk to future potential residents and current site workers.

There are several PAH remediation strategies involving physical, chemical, biological,stackable flower pots and thermal technologies; however, conventional PAH removal methods such as incineration, excavation, and land filling are expensive and inefficient . Because of these issues, biological remediation practices such as bio-augmentation and phytoremediation have become preferred in situ treatment technologies as they are considered to be cost-effective and more environmentally friendly for the cleanup of PAH-contaminated soils . However, biological remediation operations can also be ineffective due to the limited PAH soil bio-availability that is a consequence of the clay target manufacturing process and the physicochemical properties of these compounds, which can be further exacerbated by the aging effect in field-contaminated soils . These PAH bio-availability limitations can be overcome through the use of surfactants that increase the desorption of PAHs from the soil to the aqueous phase, thus increasing their bio-availability to the degrading soil microbes . Bio-surfactants such as rhamnolipids or glycolipids offer an environmentally-friendly alternative to synthetic surfactants and are becoming more economically-feasible through the use of low-cost substrates and offer distinct advantages to synthetic surfactants such as reduced toxicity, high biodegradability, and greater stability under different temperature, pH, or salinity conditions . In practical surfactant-enhanced PAH contaminated soil remediation applications, mixtures of surfactants are commonly used to take advantage of the potential synergistic effects that can result in increased solubilization at a reduced effective surfactant concentrations . The bioaugmentation of biosurfactant-producing soil microbes has also been shown to be an effective strategy for the remediation of PAH-contaminated soils. For example, M. vanbaalenii PYR-1, a glycolipid-producing microorganism isolated from an oilcontaminated estuary near the Gulf of Mexico, has been shown to enhance PAHsolubility and degradation in PAH-contaminated soils .

Another in situ biological remediation treatment commonly used to increase PAH bio-availability is phytoremediation, or the use of plants and the associated rhizosphere to restore contaminated sites . Phytoremediation is considered to be an effective, low-cost alternative to cleanup large contaminated sites . The PAH bio-availability is enhanced in the plant rhizosphere, as plant roots secrete root exudates that promote PAH desorption from the soil matrix . In addition, plant roots may release enzymes that play a key role in the degradation of PAHs including oxygenases, dehydrogenases, phosphatases, and lignolytic enzymes . Finally, plant roots also provide easily degradable carbon sources and other nutrients that increases microbial biomass, diversity, and activity, contributing to enhanced PAH degradation through direct metabolism or co-metabolism . Because of the numerous benefits provided by the rhizosphere, grass species are recommended for phytoremediation treatments due to their extensive fibrous root systems and large root surface area, and hence more extensive interactions between PAHs and the rhizosphere microbial community . Typically, the primary contaminants of concern during the remediation of outdoor shooting range soils are heavy metals from ammunition; however, large concentrations of PAHs from the clay target fragments remain in the contaminated soil and could possibly become more bioavailable during the remediation of the metals.Brij-35 nonionic surfactant and sodium dodecyl sulfate anionic surfactant were purchased from Sigma-Aldrich. Rhamnolipid biosurfactant isolated from P. aeruginosa NY3 was purchased from AGAE Technologies .Diatomaceous earth, Ottawa sand, and all GC-MS grade solvents used in this study were purchased from Thermo Fisher Scientific . All substrates utilized for soil enzymatic analyses were purchased from Tokyo Chemical Industry Co., .

Bermudagrass, switch grass, and lettuce [Lactuca sativa] seeds were purchased from Lowe’s.A Vista coarse sandy loam was collected manually using a shovel from the 0-15 cm soil depth of an abandoned shooting range located near Lake Elsinore, California that was littered with clay target fragments with no prior soil remediation or waste removal from the site. The collected soil was air-dried for 5 d at approximately 23 °C and sieved through a 2-mm stainless-steel mesh screen.The soil pH and electrical conductivity were determined potentiometrically in a 1:2 soil-to-water suspension . Total metal analysis was carried out using an Optima 7300 DV inductively coupled, argon-plasma optical emission spectrometer following U.S. EPA Method 3050B after a 6-h digestion in a mixture of nitric acid, hydrogen peroxide, and hydrochloric acid at 95 °C .Mycobacterium vanbaalenii PYR-1 was stored at -80 °C in a 30% glycerol stock and the inoculum was prepared according to a previous method in MBS solution amended with pyrene as a carbon source . The CMC of Brij-35 and rhamnolipid biosurfactant was determined previously . The CMC of the Brij-35/SDS surfactant mixture was determined by measuring the surface tension of surfactant solutions over a concentration range using a Du Noüy ring-tensiometer and using the inflection in the plot of surface tension against surfactant concentration. The CMC was determined to be 0.099 mM at 0.5/0.5 molar fraction, which was similar to a previous study .After the soil was thoroughly mixed, 1 kg soil was placed in a stainless-steel bowl and 150 mL of distilled water was added and mixed to achieve a soil water potential of approximately -33 kPa determined by a soil tensiometer. For the M. vanbaalenii PYR-1 bioaugmented treatments, 15 mL M. vanbaalenii PYR-1-MBS solution was added to yield approximately 106 CFU/g soil and thoroughly mixed . The same procedure using only the MBS solution was added to the non-inoculated, or native, soil as the control. The PYR-1-MBS solution was reapplied every 2 months by adding the inoculum solution into the soil rhizosphere 5 cm below the soil surface . Once the soil treatments were prepared, the soil was added to the phytoremediation sample containers,flower pots for sale which consisted of 800-mL glass jars that were first painted on the outside with black paint, followed by aluminum enamel to prevent exposure to light . The pots contained approximately 50 g of 2-cm diameter gravel at the bottom to allow for accumulation of any excess soil water . Bermudagrass and switch grass seeds were surface-sterilized by three sequential washings in 0.1% sodium hypochlorite, followed by two rinses with sterile distilled water . Bermudagrass and switch grass seeds were planted at a rate of 20 seeds/pot and sealed with plastic wrap for 1 week for optimal seedling emergence conditions. After 2 weeks, plants were thinned to 8 plants/pot and amended with a commercial fertilizer for bermuda grass establishment. Treatments were then fertilized monthly with 100 mg/kg-N as urea, and 12.5 mg/kg-P as monobasic potassium phosphate . Due to the potential toxicity of surfactants to emerging plant seedling , surfactant addition at 50 mg/kg was initiated 1 week after plant thinning and initial fertilizer application. Since rhamnolipid biosurfactants have been previously shown to be degraded by the soil microbial community and are considered more biodegradable than the synthetic surfactants used in this study, surfactants at the initial rate were reapplied to the soil surface every 40 d .

Each pot was placed randomly in one of four blocks, each containing one replication of all treatment combinations in a climate-controlled growth chamber . The PAH phytoremediation experiment was continued for 8 months in the growth chamber under a 12/12 hour day/night period at 23±1/19±1 °C and 40% relative humidity. The average light intensity was obtained through fluorescent and incandescent lighting in the growth camber . Each pot was weighed daily for 8 months and the soil moisture was gravimetrically adjusted to 20% by application of distilled water . The quantity of distilled water added to the soil to achieve proper soil moisture was not adjusted for vegetation biomass produced during the study. Plant shoots were trimmed to a height of 5 cm every 3 months in order to stimulate continuous plant growth . At the end of the 8-month phytoremediation experiment, plant shoots and roots were separated from the soil as described in section 2.7. Once the vegetation was removed, the soil was sieved to pass through a 2-mm sieve and separated into two subsamples. The first soil subsample was air-dried for 7 d at approximately 23 °C in the dark and used for PAH analysis and toxicity assay . The second soil subsample was used for soil enzyme analysis and kept at fieldmoist conditions and analyzed within 1 week after the termination of the experiment. set at 60 °C and then raised at 5°C/min to 280°C . Quantification of PAHs was performed using an internal standard-normalized calibration curve and coefficients of determination for all calibration curves fulfilled the requirement of R2 ≥ 0.99. Soil dehydrogenase soil activity was analyzed by the use of 2–3- -5-phenyl tetrazoliumchloride as a substrate . A 1.0-g soil aliquot was mixed with Tris buffer and INT substrate in a stoppered 100-mL Erlenmeyer flask, and the mixture was incubated for 2 h at 40 °C in the dark. After incubation, the mixture was extracted using 10 mL N,Ndimethylformamide:ethanol mixture for 1 h at 23 °C in the dark and shaken every 20 min. Immediately after filtration, iodonitrotetrazolium formazan formation was measured colorimetrically at 464 nm against the reagent blank using a UV-Visible spectrophotometer . Soil dehydrogenase activity was expressed as µg INTF produced/g dry soil 2h. Soil polyphenol oxidase activity was measured by the utilization of pyrogallic acid as a substrate to form purpurogallin . Ten mL of 1.0% pyrogallol was added to 1.0 g soil sample and incubated at 30 °C for 2 h at 200 rpm. Afterwards, 5 mL of citrate-phosphate buffer was added to the treatment to stop the reaction, followed by the addition of 35 mL ether and shaking for 30 min at 200 rpm. The colored ether with dissolved purple gallic prime was measured colorimetrically at 430 nm on a UV-Visible spectrophotometer.The polyphenol oxidase activity was expressed as mg purpurogallin produced/g dry soil 2h. Control assays for each soil enzyme activity included autoclaved soil treatments, assays without soil, and assays without substrate addition during incubation . Results of soil enzyme activities are reported on an oven-dry-weight basis.At the end of the phytoremediation experiment, plant shoots were cut at the soil surface and rinsed with distilled water to remove any adhering soil. Approximately 4 g shoot subsample was taken and freeze-dried for PAH extraction and the remaining shoots were dried to a constant weight at 55 °C and weighed to determine total shoot biomass. The freeze-dried plant shoots were ground to pass a 2-mm, stainless-steel mesh screen using a Wiley Mill Grinder and 2 g was used to determine PAH shoot concentrations using procedures similar to those for soil PAH extraction . Plant roots were manually collected from the soil using forceps, placed on a 500-µm stainless-steel sieve, and thoroughly rinsed with distilled water to remove any adhering soil particles. Approximately 2 g root subsample was taken and freeze-dried and 1 g was used to determine PAH root concentrations similar to shoot analysis. The remaining plant roots were dried to a constant weight at 55 °C and weighed to determine total root biomass. The lettuce seed toxicity assay was performed to evaluate changes in phytotoxicity before and after remediation treatments by following a method in Cofield et al. . Briefly, 100 g soil at 85% water-holding capacity was placed in a 150 mm ´ 15 mm Petri dish and 40 lettuce seeds were evenly distributed and pressed into the soil.

The respiration rate of shade grown segments that were shifted to high light became significantly more negative four days after transplant . The shade grown segments exposed continuously to low light showed small changes in Afull sun and Rd relative to the SH-SU segments. Rd acclimation in SU-SH segments took about 13 days, with an initial rapid change followed by a more gradual change .The intercellular CO2 concentration remained constant among the treatments , and the changes in Afull sun were attributable to shifts in photosynthetic capacity rather than CO2 supply. Stomatal conductance paralleled the changes in Afull sun . We did not find evidence of changing patterns of stomatal control, and the simplest explanation is that conductance simply responded to Afull sun acclimation. Leaf acclimation was highly localized; individual leaf segments acclimated to local light autonomously from the rest of the leaf. Previous studies of leaf acclimation have focused on entire leaves, and acclimation by segments of mature leaves has received less attention . The ability for mature leaf acclimation varies among species, and has been reported in several herbaceous and a few woody species . Our findings of segmented acclimation are most closely related to those of Prioul et al. , who found that Afull sun, chlorophyll content,fodder growing system and Rubisco activity changed markedly from the base to tip of Lolium multiflorum leaves. Likewise, reciprocal transplants to contrasting light conditions inL. multiflorum showed the capability for rapid photosynthetic reacclimation to high and low light along the leaf, even in fully expanded leaves .

Detailed investigations of leaf anatomy and biochemistry are beyond the scope of this study, but our observations provide evidence of the mechanisms responsible for acclimation. Photosynthetic acclimation in Typha appears to result from biochemical or cellular changes, and a general up or down regulation of metabolic activity within individual leaf segments.Acclimation did not involve a significant change in nitrogen content on either a mass or area basis. We did not find evidence that a morphological change in leaf thickness, or a net movement of nitrogen into or out of a leaf segment, were required for photosynthetic acclimation.Our results confirm previous reports that species from highly variable light environments have a strong capacity for photosynthetic acclimation. In the case of T. latifolia, light heterogeneity is created by the combination of a basal meristem and a dense canopy of live leaves and litter . Typha leaves are exposed to markedly different light environments as they grow and individual segments are pushed upward . The upper segments of leaves in the field, which occurred in a brighter environment, had higher rates of CO2 uptake . Previous field studies on T. latifolia have also reported large CO2 assimilation and gs gradients along leaves . We hypothesize that the patterns of leaf photosynthesis and conductance in Typha reflect four properties. Mature Typha leaf segments are morphologically preformed to function in high light and allow high rates of Afull sun, regardless of the current or growth environment. Mature Typha leaf segments contain sufficient amounts of nitrogen to support high rates of Afull sun, regardless of the current or growth environment. Mature Typha leaf segments rapidly reallocate nitrogen between active and inactive pools in response to local light availability; acclimation occurs at a local level and does not require nitrogen translocation into or out of a leaf segment.

The controls on stomatal conductance remain constant over time; the patterns of conductance can be explained based on simple, short-term adjustments that act to maintain a nearly constant Ci concentration despite the changes in Afull sun and the physical environment. We interpret these patterns as a highly plastic strategy that maximizes carbon gain by a monocot growing in a vertically heterogeneous light environment. The construction of leaves that are morphologically capable of high rates of Afull sun is a simple consequence of the spatial decoupling of the growth environment fromdegradation of cellular components, such as Rubisco, cytochrome f, and chloroplast ATPase . The amount of nitrogen in leaf segments remained nearly constant over time, leading us to hypothesize a fraction of the nitrogen in shaded segments is stored in inactive pools and is rapidly activated following transfer to high light. These changes may include adjustments in partitioning among carboxylation, electron transport and light harvesting, chloroplast ultrastructure, volume, and orientation . The high N content of shaded segments should not be viewed as wasteful. These nutrients can be reabsorbed and reallocated to the rhizome during senescence; a high reabsorption efficiency of P and N has been reported for Typha dominguensis . Moreover, this strategy allows a leaf segment to rapidly and autonomously respond to a change in light availability, without importing or exporting nitrogen to or from other leaf segments or organs.Polycyclic aromatic hydrocarbons are a group of persistent organic pollutants that are composed of two or more fused aromatic rings in linear, angular, or cluster arrangements . Depending upon the structure of the rings, PAHs are classified as either alternant or non-alternant.

Alternant PAHs contain only fused six membered rings , while non-alternant PAHs contain four- or five-membered rings in addition to the six-membered rings . The aromatic structure of PAHs results in increased thermodynamic and chemical stability due to electron delocalization in the π orbitals, which plays a critical role in the environmental fate and toxicity of these contaminants . There are 16 PAHs designated as priority pollutants by the United States Environmental Protection Agency due to their occurrence in the environment and toxicity. The physicochemical properties of the 16 priority PAHs are detailed in Table 1.1 . These compounds are all hydrophobic, as demonstrated by their relatively high octanol-water partition coefficients and low solubility in water . The impact of the PAH structure on its chemical behavior is primarily dependent upon molecular size and angularity. Typically, an increase in the number of rings and angularity results in increased electrochemical stability and hydrophobicity . For example, low-molecular-weight PAHs are considerably more water soluble and volatile than high-molecular-weight PAHs . In addition to increases in hydrophobicity and environmental persistence with increasing PAH molecular size, PAH genotoxicity generally increases and toxicological concern shifts towards chronic toxicity, primarily carcinogenesis . Numerous studies have indicated that LMW PAHs exhibit acute toxicity to humans, whereas HMW PAHs exhibit chronic effects such as genotoxicity . The acute effects of PAHs on human health such as nausea, vomiting, and respiratory and skin irritation depend primarily on the extent of exposure, the route of exposure , and the concentration and toxicity of the individual PAHs . Polycyclic aromatic hydrocarbons can be widely distributed throughout the human body and have been detected in almost all internal organs, especially adipose tissues due to their lipophilicity . Once they enter the body, PAHs undergo metabolism primarily through the cytochrome P450 mixed-function oxidase system. This metabolic pathway transforms PAHs into polar epoxide intermediates that are further converted to dihydrodiol derivatives and phenols, which then form glucuronide and sulfate conjugates that are finally excreted in the bile and urine . However, this metabolic transformation can also result in the formation of electrophiles that elicit deleterious human health effects . Because of this, PAHs are considered procarcinogens because they do not directly induce DNA damage, but require metabolic activation to exert their genotoxic, mutagenic, or carcinogenic effects . There are three major pathways for PAH carcinogenic activation: the bay region dihydrodiol epoxide pathway, the radical cation pathway, and the o-quinone pathway,which result in the formation of radical cations, diol epoxides, and electrophilic and redox-active o-quinones, respectively, all of which may react with DNA to produce DNA adducts . Following extensive and systemic studies on the toxic effects of individual PAH metabolites in animals, it has been determined that the vicinal or bay-region diol epoxides are considered the ultimate mutagenic and carcinogenic species of PAHs .The International Agency for Research on Cancer classifies numerous PAHs as known, probably,chicken fodder system or possibly carcinogenic to humans . The IARC has determined that benzo[a]pyrene is one of the most potent carcinogenic PAHs, benzo[a,h]anthracene is a probable human carcinogen, and naphthalene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, and indeno[1,2,3-c,d]pyrene are possible human carcinogens . Altogether, PAHs rank as #9 on the Agency for Toxic Substances and Disease Registry’s Substance Priority List , which ranks contaminants based on a combination of their toxicity, frequency, and potential for human exposure at National Priority List sites . Of the approximately 1,400 NPL sites that are targeted for remediation by the U.S. EPA, more than 700 sites are contaminated with PAHs.

Polycyclic aromatic hydrocarbons are formed primarily during the incomplete thermal decomposition of organic substances and their subsequent recombination . Thus, the composition of the PAHs formed is dependent upon the temperature and the starting organic material . Polycyclic aromatic hydrocarbons occur as complex mixtures in the environment instead of single compounds due to their differing physicochemical properties during the incomplete combustion process . There are three primary sources of PAHs in the environment: pyrogenic, petrogenic, and diagenetic/biogenic. Pyrogenic PAHs areproduced from the rapid oxygen-depleted, high-temperature incomplete combustion of fossil fuels and organic materials . These pyrogenic PAHs are formed from the breakdown of organic matter to LMW radicals during pyrolysis, which is then followed by rapid reassembly into PAH structures . Pyrogenic PAHs are typically found at greater concentrations in urban areas because the major sources of pyrogenic PAHs are the incomplete combustion of gasoline and diesel in vehicles, the production and use of coal tar and asphalt, heat and power generation, and discharges from aluminum smelters and manufactured gas plants . The most abundant pyrogenic PAHs are typically fluoranthene and pyrene . Petrogenic PAHs originate from diagenetic processes at relatively low temperatures over a long duration, leading to the formation of petroleum and other fossil fuels containing PAHs . Petrogenic PAHs are introduced into the environment through accidental oil spills, discharge from tanker operations, and underground and aboveground storage tank leakage . Diagenetic/biogenic PAHs are produced from biogenic precursors by plants, algae/phytoplankton, and microorganisms . For example, concentrations of perylene, naphthalene, and phenanthrene concentrations have been found in hydromorphic soils, Magnolia flowers, and Coptotermes formosanus termite nests . While diagenetic/biogenic PAHs are often found at background levels in recent sediments, they are frequently the primary PAHs in older sediments deposited before increased industrial activity . The environmental ubiquity of PAHs is due to their chemical stability and numerous natural and anthropogenic sources. Natural sources of PAHs include volcanic eruptions, forest and prairie fires, and seeps of crude oil deposits . Anthropogenic sources, which contribute the vast majority of PAH contamination in the environment, include the production and use of fossil fuels such as coal, oil, and natural gas, refinement of crude oil, heat and power generation, wood treatment preservation processes, landfills, residential wood burning, and improper industrial waste disposal or spillage . Over the past century, there has been a substantial increase in environmental concentrations of PAHs following increased anthropogenic sources from industrialization, which can be demonstrated by PAH levels being the greatest in urban areas followed by agricultural and rural environments . Even the lowest PAH concentrations in temperate soils are approximately 10 times greater than PAH concentrations assumed to have been present before global industrialization . Once PAHs are emitted to the environment primarily through the combustion of fossil fuels, they are distributed atmospherically and deposited onto terrestrial, lacustrine, and marine surfaces . However, unlike most persistent organic pollutants like polychlorinated biphenyls that follow the global distillation transportation effect, PAH concentrations generally decrease as the distance from the initial source increases . Atmospheric PAHs are generally more abundant at night than daytime, and during the winter months compared to the summer months due to greater deposition at lower temperatures and increased coal combustion for heating . Polycyclic aromatic hydrocarbons are semi-volatile organic compounds; therefore, PAHs can be found in both vapor and particle phases depending on the vapor pressure of the PAH, temperature, and size and surface area of suspended particles . However, given the physicochemical properties of PAHs, they tend to more readily sorb to atmospheric particulates than be present in the gas phase . Because PAHs are commonly adsorbed onto atmospheric particulates, PAH transformation and degradation by thermal or photodecomposition and reactions with O3, SO2, NOx, or OH radicals are reduced and can even be completely inhibited .