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 .