Internal transport of gases is crucial for vascular plants inhabiting aquatic, wetland or flood-prone environments. Diffusivity of gases in water is approximately 10 000 times slower than in air; thus direct exchange of gases between submerged tissues and the environment is strongly impeded. Aerenchyma provides a low-resistance internal pathway for gas transport between shoot and root extremities. By this pathway, O2 is supplied to the roots and rhizosphere, while CO2, ethylene, and methane move from the soil to the shoots and atmosphere. Diffusion is the mechanism by which gases move within roots of all plant species, but significant pressurized through-flow occurs in stems and rhizomes of several emergent and floating-leaved wetland plants. Through-flows can raise O2 concentrations in the rhizomes close to ambient levels. In general, rates of flow are determined by plant characteristics such as capacity to generate positive pressures in shoot tissues, and resistance to flow in the aerenchyma, as well as environmental conditions affecting leaf-to-air gradients in humidity and temperature. O2 diffusion in roots is influenced by anatomical, morphological and physiological characteristics, and environmental conditions. Roots of many (but not all) wetland species contain large volumes of aerenchyma (e.g. root porosity can reach 55%), while a barrier impermeable to radial O2 loss (ROL) often occurs in basal zones. These traits act synergistically to enhance the amount of O2 diffusing to the root apex and enable the development of an aerobic rhizosphere around the root tip, which enhances root penetration into anaerobic substrates. The barrier to ROL in roots of some species is induced by growth in stagnant conditions, whereas it is constitutive in others. An inducible change in the resistance to O2 across the hypodermis/exodermis is hypothesized to be of adaptive significance to plants inhabiting transiently waterlogged soils. Knowledge on the anatomical basis of the barrier to ROL in various species is scant. Nevertheless, it has been suggested that the barrier may also impede influx of: (i) soil-derived gases, such as CO2, methane, and ethylene; (ii) potentially toxic substances (e.g. reduced metal ions) often present in waterlogged soils; and (iii) nutrients and water. Lateral roots, that remain permeable to O2, may be the main surface for exchange of substances between the roots and rhizosphere in wetland species. Further work is required to determine whether diversity in structure and function in roots of wetland species can be related to various niche habitats.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2003.00846.x

Long-distance transport of gases in plants: a perspective on internal aeration and radial oxygen loss from roots

http://dzumenvis.nic.in/microbes and metals interaction/pdf/metal uptake transport and release.pdf

Metal uptake, transport and release by wetland plants: implications for phytoremediation and restoration.

The aerenchyma (air-space) tissue in the wetland macrophyte Spartina alterniflora conveys sufficient oxygen to roots for predominately aerobic respiration in moderately, but not highly, reduced substrates. Continuously flooded plants survive by respiring anaerobically, although growth is decreased. Two metabolic adaptations to flooding are displayed in this species, depending on the degree of soil reduction.

https://science.sciencemag.org/content/sci/214/4519/439.full.pdf

Oxygen Deficiency in Spartina alterniflora Roots: Metabolic Adaptation to Anoxia

Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.

Characterization of Fe Plaque and Associated Metals on the Roots of Mine-Waste Impacted Aquatic Plants

Methane emission from natural wetlands: interplay between emergent macrophytes and soil microbial processes. A mini-review

Spartina alterniflora, salt marsh cordgrass, is the dominant angiosperm of a large majority of regularly flooded marshes of the Atlantic and Gulf coasts of the United States. In Louisiana, this species often occurs in two distinct zones: a more productive streamside site (adjacent to tidal creeks), and a less productive and sparsely populated inland area. Reddish-brown deposits are present on the roots of streamside Spartina and visually absent from the roots of inland plants. A study of streamside roots using scanning electron microscope and energy dispersive X-ray microanalysis demonstrated that the coatings are restricted to the outer cell wall of the epidermis and are composed primarily of iron. Roots of inland plants have minor iron deposits. Citrate-dithionite extraction of the coatings and subsequent atomic absorption spectrophotometric analysis confirmed these deposits to be iron, although some manganese was present. Approximately 50 times more iron was found on streamside roots compared to roots from inland plants. These results indicate a better developed oxidized rhizosphere associated with streamside Spartina roots than the inland and, hence, the potential for a more favorable environment in which nutrient uptake may proceed. SALT MARSH GRASSES, as indicated by their high productivity (Turner, 1976), are well adapted to anaerobic substrates. Spartina alterniflora Loisel., the dominant angiosperm of a large majority of regularly flooded salt marshes along the Atlantic and Gulf coasts of the United States, possesses a well developed aerenchyma system (Anderson, 1974) which serves as a pathway for the diffusion of atmospheric oxygen from the leaves to the roots (Teal and Kanwisher, 1966) and provides the oxygen needed by the roots for metabolic processes. The oxygen not consumed in the roots during respiration diffuses into the anoxic mud surrounding the roots and forms an oxidized rhizosphere, i.e., a relatively aerobic microzone around the root (Armstrong, 1970). The oxidized rhizosphere of wetland plants has been hypothesized to act in excluding from the plant soil toxins which may accumulate in anaerobic soils (Armstrong, 1978). Potential phytotoxins such as Fe 2, Mn 2, H2S, ethylene, and certain organic acids, may be oxidized I Received for publication 9 March 1981; revision accepted 30 May 1981. This study was supported by funds from the Louisiana Sea Grant College Program, a part of the National Sea Grant Program maintained by the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The authors would like to thank the Geology Department of Louisiana State University for the use of the analytical scanning electron microscope and Ms. Rita Strate for her typing expertise. to harmless compounds upon contact with the oxidized rhizosphere. Bartlett (1961) has demonstrated this process in rice roots where potentially toxic soluble ferrous iron is oxidized to ferric iron precipitates. Spartina alterniflora often occurs as relatively distinct height forms along a complex gradient of environmental factors landward from tidal creek banks. Along the Louisiana Gulf Coast this gradient in Spartina productivity is identified by two forms: high vigor streamside (= 1 m high) and low vigor inland (-70 cm high) (Kirby and Gosselink, 1976). Streamside habitats are approximately 10 cm higher in elevation than inland areas as a result of greater vertical accretion rates in the former (DeLaune, Patrick and Buresh, 1978). This difference in surface elevation between the two sites results in greater soil waterlogging in the inland zone compared to the streamside (Mendelssohn, McKee and Patrick, 1981). The objectives of the present study were: 1) to evaluate the extent to which the rhizosphere of Spartina alterniflora is important in precipitating iron and manganese, and 2) to determine any difference in the intensity of precipitation as a function of location (streamside vs. inland). MATERIALS AND METHODS-Field sampling-Living, i.e., turgid and structurally intact, roots of Spartina alterniflora were collected from a monospecific salt marsh located

Elemental analysis of deposits on the roots of spartina alterniflora loisel.

Spartina alterniflora, salt marsh cordgrass, is the dominant angiosperm of a large majority of regularly flooded marshes of the Atlantic and Gulf coasts of the United States. In Louisiana, this species often occurs in two distinct zones: a more productive streamside site (adjacent to tidal creeks), and a less productive and sparsely populated inland area. Reddish-brown deposits are present on the roots of streamside Spartina and visually absent from the roots of inland plants. A study of streamside roots using scanning electron microscope and energy dispersive X-ray microanalysis demonstrated that the coatings are restricted to the outer cell wall of the epidermis and are composed primarily of iron. Roots of inland plants have minor iron deposits. Citrate-dithionite extraction of the coatings and subsequent atomic absorption spectrophotometric analysis confirmed these deposits to be iron, although some manganese was present. Approximately 50 times more iron was found on streamside roots compared to roots from inland plants. These results indicate a better developed oxidized rhizosphere associated with streamside Spartina roots than the inland and, hence, the potential for a more favorable environment in which nutrient uptake may proceed.

Elemental analysis of deposits on the roots of

SUMMARY

The ligand osmium binding technique utilizing thiocarbohydrazide has been a successful alternative to evaporation or sputter coating of botanical specimens. A modification to this technique is presented which we have found successful in our laboratory for the preparation of a variety of specimens. This technique permits an overall reduction in the quantity of osmium necessary to provide conductivity. Further, dehydration time and specimen handling is reduced by the complete chemical dehydration of the tissue utilizing 2-2-dimethoxypropane.

Thiocarbohydrazide binding for botanical specimens for scanning electron microscopy: a modification

The ontogeny and ultrastructure of secretory oil cells in the leaves of Magnolia grandiflora were studied with light microscopy, scanning electron microscopy, and transmission electron microscopy. Oil cells are initiated by anticlinal division in ground meristem cells throughout the leaf blade. The oil cell increases greatly in size, and oil fills a large central vacuole. Although both lysigenous development and a stalk or cupule attaching the oil sac to the wall have been reported for secretory oil cells, neither was found in oil cells of M. grandiflora. The suberin layer in the wall of these cells forms late in development, is part of a bipartite rather than a tripartite wall, and is not continuous within the oil cell.

Ontogeny and ultrastructure of secretory oil cells in Magnolia grandiflora L.

Specimens of Synedra goulardi Breb. from samples taken in Costa Rica were acid cleaned and examined by light and scanning electron microscopy. This linear-lanceolate diatom displays an unusual pattern of markings in its central area which have been mistaken as striae but are actually internal costae. As is typical for members of its genus and that of many other genera in the Fragilariaceae, S. goulardi possesses non-stalked labiate processes, apical pore fields and a lack of marginal spines. There is a great deal of variability in the striae of the genus, ranging from single rows of puncta to areolate striae. Synedra goulardi has double punctate striae. The presence of fossil forms (Neogene) with single punctate striae suggest this may be a primitive condition in the genus.

Valve morphology of synedra goulardi (bacillariophyceae)1

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