Mannuronate and guluronate blocks of the alginate have the capability for Mn2+ binding . However, at high alginate treatment the accumulation of Mn in roots was similar to the accumulation obtained with NPs alone. More experiments are needed to explain these results. In the case of Zn, the medium alginate treatment increased its accumulation in roots by 52% compared to control. The increase of elements in roots by CeO2 NPs may be due to the enhancement of soil cation exchange capacity produced by the CeO2 NPs. A small experiment was performed to determine if CeO2 NPs increased soil CEC. Results showed that the CEC in soil spiked with 400 mg/kg CeO2 NPs was 6× compared to control. Similar to soil natural colloids, e.g. kaolin and NOM, CeO2 NPs have a large surface area, with negative surface charge , that supplies many exchange sites for cation adsorption . to CeO The concentration of Na in roots of plants exposed 2 NPs alone decreased by 10.5% compared to control. In the CeO2 NP-alginate treatments, the reduction of Na in roots was higher . This could be associated with the increase in K absorption . As a C4 plant, corn needs Na to regenerate phosphorenolpyruvate,dutch bucket hydroponic the substrate for carboxylation . Thus, reduction of Na absorption could represent a toxicity pathway of CeO2 NPs to corn plants.Concentration of macro and micro-elements in corn shoots treated with CeO2 NPs and alginate are shown in Fig. 3.
As one can see in this figure, only the concentration of K was significantly affected by the NP treatments . All the CeO2 NP-containing treatments increased the concentration of K in corn shoots. However, there were no differences between the alginate and non-alginate treatments. An outward rectifying channel mediates potassium release into the xylem, which is controlled by the stress hormone abscisic acid . Perhaps the presence of NPs up regulates the production of abscisic acid, which in turn, increases the uptake of K. The fact that higher concentrations of Al, Fe, and Mn in roots, compared to control, were not observed in shoots could indicate that these elements were bound to NPs and stuck on the surface of the roots. Divalent cations have shown to bind with alginate.Chlorophyll and other leaf pigments are related to stress response.To elucidate if the CeO2 NPs coupled to alginate impacted the photosynthesis machinery in young corn plants , chlorophyll concentration in fresh leaves of one month-old plants was determined . As seen in this figure, none of the treatments affected Chl-b fluorescence. Chl-a was not affected by CeO2NPs alone; however, the combination of CeO2 NPs with alginate, at all concentrations, significantly reduced Chl-a fluorescence, compared to control. The CeO2 NPs plus alginate at low and medium concentration reduced Chl- a fluorescence by 16.5% and 18.4%, respectively.The soil was treated with CeO2 NPs with different concentration of alginate. Error bars stand for standard deviation. an indicative of stress . There exists the possibility that the CeO2 NP treatments affected the concentration of nitrogen or silicon in the corn plants, which are related to chlorophyll production or degradation.
Heat-shock proteins are general stress related proteins involved in the protection, restoration, and degradation of damaged cell components, especially proteins, during most abiotic stresses . Western-blot analytical technique was used to determine the content of HSP 70 in fresh leaves of one month-old corn plants. As seen in Fig. 5, HSP 70 was apparently over produced in the CeO2 NP treatments including medium and high amount of alginate. Khodakovskaya et al. discovered that multi-walled carbon nanotubes induce changes in gene expression in tomato leaves and roots, up-regulating the stress-related genes, such as heat shock protein 90. The present study showed that in corn plants, the HSP 70 increased expression in response to CeO2 NPs-alginate effect. However, the specific functions or structures protected by HSPs remain unknown. Heckathorn et al. reported that the chloroplast small HSP, which function is to protect photosynthesis during heavy metal stress in corn leaves , were trigged by heavy metal. Previous studies have shown that most of the CeO2 NPs taken by plants remain in the nanopariculate form; thus, it is possible that some of the NPs present in corn leaves reached the chloroplast affecting the HSP70 expression. More studies are needed in order to decipher these uncertainties. Land-based ecosystems in the northern hemisphere appear to remove, at least temporarily, a substantial portion of anthropogenic CO# from the atmosphere . The mechanisms behind this C sink are not well understood, even though knowledge of these processes is vital to predict and interpret the responses of ecosystems to global change .
Changes in plant productivity due to CO# enrichment , nitrogen deposition , land use change , and climatic effects have been investigated as potential components . However, the response of microbial communities to these perturbations, and their potential influence on C cycling, have received scarce attention. Mycorrhizal fungi in particular might play an important role in the sequestration of C in soil under elevated CO# and N deposition. This group, which symbiotically colonizes plant roots, forms associations with 80% of plant species and is found in nearly every habitat in the world . Plants allocate an estimated 10–20% of net photosynthate to mycorrhizal fungi, although this number can range from 5 to 85% among systems . A substantial amount of C allotted to mycorrhizal tissues could be long-lived in the soil. Chitin, which is not readily decomposed , can constitute up to 60% of fungal cell walls . Arbuscular mycorrhizal fungi are also the sole producers of glomalin, a potentially recalcitrant glycoprotein . AM hyphae in the absorptive hyphal network have lifespans of only 5–7 d , and with each cycle residual hyphal C should remain in the soil. Furthermore, some micro-arthropods prefer to graze on nonmycorrhizal fungi rather than on a variety of AM fungi , and therefore might not necessarily speed up tissue turnover significantly. As a result, glomalin alone can account for 30–60% of C in undisturbed soils , assuming that the protein is 30% C by weight M. C. Rillig, pers. comm. Likewise, portions of ectomycorrhizal biomass were responsible for approx. 15% of soil organic matter in two hardwood forests . Carbon derived from mycorrhizal tissue can account for a significantly sized pool within ecosystems and globally. Because mycorrhizal fungi acquire most or all their C directly from living plants, the nutrient status of foliage strongly affects mycorrhizal growth. As elevated CO# generally increases plant growth and root-to-shoot ratio , greater allocation of C to mycorrhizal structures might follow . Effects of elevated CO# on mycorrhizal growth have been reviewed by O’Neill , Diaz , Hodge , and Staddon & Fitter , with an emphasis on changes in percentage root length colonized and total root length colonized per plant. These reviews indicate that the percentage of roots with mycorrhizal structures might not necessarily change under elevated CO# . However,dutch buckets system as root biomass tends to rise, total mycorrhizal biomass per plant might do so as well. This response varies among systems and does not necessarily occur universally. By contrast, increases in N availability through deposition or fertilization tend to reduce root colonization and fruit body production by ECM fungi . Effects of CO# and N availability on the biomass or production of extraradical hyphae have been less intensively studied or summarized. In this review, we address the current state of knowledge regarding the potential for mycorrhizal tissue to form a sink or source of C in response to elevated CO# or N deposition. First, we present an overview of processes and pools involved in the cycling of mycorrhizal C and the relevance of various measures of mycorrhizal dynamics . Interand intraspecific variations in traits that could affect C dynamics are considered. Second, we discuss known effects of CO# concentration on hyphal biomass, turnover, tissue quality and community composition. Next, we focus on the influence of N availability on these same factors, and finally we address potential interactions between elevated CO# and N availability.Processes involved in the cycling of mycorrhizal C include production, survivorship and decomposition rates of tissue. As mycorrhizal tissue grows, C is transferred from the atmosphere via plants to the pool of live hyphae. Micro-arthropods might graze a fraction of live hyphae, but grazing on AM hyphae should be low, as in feeding trials mites and collembola appear to prefer nonmycorrhizal fungi.
When grazing of AM fungal hyphae does occur, animals often only clip the hyphae, severing connections to the root but not ingesting mycelial mass . Thus changes in micro-arthropod numbers might not have a major impact on C flux from AM hyphae to soil organic matter. Instead, death rates of live hyphae determine the flflux of C from the live to the dead hyphal pool. At this point, dead tissue is distributed between active and slow soil organic matter pools as a function of tissue quality . Active soil organic matter includes sugars and other metabolites that are processed relatively quickly by decomposers; slow soil organic matter consists of recalcitrant components such as chitin and glomalin, and might last from years to decades in the soil. In plant tissues, higher N content generally speeds decomposition rates . Finally, as soil organic matter decomposes, a portion of C remains in decomposer tissues, and the rest returns to the atmosphere. Each of these fluxes and pools might be affected directly by elevated CO# and N deposition, or indirectly through changes in the composition of the mycorrhizal community.Groups of mycorrhizal fungi differ in several factors, including growth rate, that could influence C cycling. For example, isolates of ECM fungi vary markedly in productivity, both within species and among species . In AM fungi, Sanders et al. observed significant differences in hyphal biomass among three Glomus species. In addition, after 16 wk growth, total hyphal lengths of Acaulospora denticulata and Scutellospora calospora were significantly greater than those of two Glomus species in a glasshouse experiment with Artemisia tridentata . If mycorrhizal communities are altered by climate change, then variation in growth and biomass among groups could affect the amount of atmospheric C that is initially drawn into the pool of live hyphae. Mycorrhizal groups also differ in tissue qualities that might affect the rate at which this mycorrhizal C is returned to the atmosphere. Wallander et al. found that five morphotypes of ECM fungi on field-grown Pinus sylvestris varied more than twofold in chitin concentration. Likewise, glomalin content in AM hyphae differed between Gigaspora and Glomus , and significantly between Glomus caledonium and Glomus intraradices . In addition, mean N concentrations in the hyphae of four isolates of Paxillus involutusranged from 5 to 9% when grown in culture.These results suggest that the identity, as well as the amount, of mycorrhizal fungi might be important in soil C dynamics.As most mycorrhizal structures are relatively delicate and often below ground, measurements of mycorrhizal biomass, growth rate or turnover present some challenges. Most mycorrhizal studies under elevated CO# or N deposition have quantified changes in mycorrhizal colonization . This measure might be an appropriate index for nutrient transfer to the host plant . However, because extraradical hyphae account for a large portion of fungal biomass , direct measures of hyphal length are a valuable indicator of the mycorrhizal C pool . Furthermore, root colonization does not necessarily increase linearly with hyphal biomass, and environmental changes might alter relationships between the two variables. For instance, the ratio of AM hyphal length : total root length colonized by AM varied nearly twofold among CO# and N treatments in Gutierrezia sarothrae , and was nearly three times greater under ambient versus elevated CO# in a serpentine grassland . In an additional study, Staddon et al. noted a decrease in this ratio with elevated CO# in Plantago lanceolata and Trifolium repens. For this reason we focus primarily on hyphal length or biomass in this review. We emphasize that hyphal length per unit soil area is a particularly meaningful variable in field studies, and might be used eventually to scale biomass to the ecosystem or regional level. The life stage of hyphae is also an important consideration when measuring fungal productivity. Few CO# or N studies have made the distinction between live and dead hyphae when determining hyphal length.