Rhizome phyllosphere oxygenation in Phragmites and other species in relation to redox potential, convective gas flow, submergence and aeration pathways

Underground rhizomes of emergent aquatic macrophytes are important for perennation, vegetative spread, competition and anchorage. In four species we examined the potential for the development of oxidized phyllospheres around rhizome apical buds, similar to the protective oxygenated rhizospheres around roots. Redox potentials and polarographic measurements of radial oxygen loss were recorded using platinum cathodes around the apical buds. The aeration pathway from atmosphere to phyllosphere was investigated anatomically and by applied pressurized gas flow. Redox potentials increased by +400, +45, +200 and +340 mV around rhizome apices of Phragmites australis, Oryza rhizomatis, Carex rostrata and Glyceria maxima, respectively. Radial oxygen loss from rhizome apices of Phragmites was increased by convective gas flow through the rhizome and by shoot de-submergence, and decreased by resistances applied within the aeration pathway and by shoot submergence. We conclude that oxygen passes via internal gas-space connections between aerial shoot, rhizome and underground buds and into the phyllosphere regions via scale-leaf stomata and surfaces on the buds. We suggest that oxidized phyllospheres may protect rhizome apices against phytotoxins in waterlogged soils, just as oxidized rhizospheres protect roots.     

Effect of broken dead culms of Phragmites australis on radial oxygen loss in relation to radiation and temperature

The amount of oxygen released from the roots of Phragmites australis was quantified to examine the effects of airflow through dead culms, radiation, and temperature on radial oxygen loss (ROL). To investigate the effect of dead culms on ROL quantitatively, the ROL of individual plants with open dead culms was compared to that of plants with sealed dead culms as a function of light intensity and temperature. The relationship between ROL and plant morphology (aboveground biomass, shoot diameter, shoot height) was investigated. When exposed to 300, 600, and 900 μmol m⁻² s⁻¹ light, the ROL was 15.6, 22.5, and 30.9 μmol O₂ g⁻¹ dry root day⁻¹, respectively, from plants with open dead culms and 11.0, 16.4, and 23.3 μmol O₂ g⁻¹ dry root day⁻¹, respectively, from plants with sealed dead culms. The ROL from plants with open dead culms was obviously higher than that from plants with sealed dead culms in every condition. The ROL from plants with open culms was 37% and 30% higher than that from plants with sealed culms at 20°C and 30°C, respectively. The effects of plant-specific parameters such as leaf area and shoot diameter on radial oxygen loss were evident. From the point of view of rhizosphere oxidation during the growing season, the existence of open dead culms should be taken into consideration for optimal plant management in constructed wetlands. This study provides a theoretical understanding of the effects of open dead culms, light conditions, and temperature on radial oxygen loss. Handling editor: S. M. Thomaz     

Rice: sulfide-induced barriers to root radial oxygen loss, Fe2+ and water uptake, and lateral root emergence

BACKGROUND AND AIMS: Akagare and Akiochi are diseases of rice associated with sulfide toxicity. This study investigates the possibility that rice reacts to sulfide by producing impermeable barriers in roots. METHODS: Root systems of rice, Oryza sativa cv. Norin 36, were subjected to short-term exposure to 0.174 mm sulfide (5.6 ppm) in stagnant solution. Root growth was monitored; root permeability was investigated in terms of polarographic determinations of oxygen efflux from fine laterals and the apices of adventitious roots, water uptake, anatomy and permeability to Fe2+ using potassium ferricyanide. KEY RESULTS: Both types of root responded rapidly to the sulfide with immediate cessation of growth, decreased radial oxygen loss (ROL) to the rhizospheres and reduced water uptake. Profiles of ROL measured from apex to basal regions of adventitious roots indicated that more intense barriers to ROL than normal were formed around the apices. Absorption of Fe2+ appeared to be impeded in sulfide-treated roots. In adventitious roots, deposition of lipid material (suberisation) and thickenings of walls within the superficial cell layers were obvious within a week after lifting the treatment and could prevent the emergence of laterals and commonly result in their upward longitudinal growth within the cortex. Death of laterals sometimes occurred prior to emergence; emergent laterals eventually died. In adventitious roots, blockages formed within the vascular and aeration systems in response to the sulfide. CONCLUSIONS: In both adventitious and lateral roots, sulfide-induced cell wall suberization and thickening of the superficial layers were correlated with reduced permeability to O2, water and Fe2+. This study sheds light on some of the symptoms of diseases such as Akiochi. The results correlate with the authors' previous findings on the effects on roots of sulfide and lower organic acids in Phragmites and of acetic acid in rice.     

Effects of photoperiod on growth of and denitrification by Pseudomonas chlororaphis in the root zone of Glyceria maxima,studied in a gnotobiotic microcosm

The emergent macrophyte Glyceria maxima was subjected to different photoperiods and grown with ammonium or nitrate as nitrogen source in presterilized microcosms with spatially separated root and non-root compartments. The microcosms were inoculated with the denitrifying bacterium Pseudomonas chlororaphis. The effect of the plant and the photoperiod on growth and denitrification by P. chlororaphis was assessed. The plant had a strong positive effect on the growth of the bacteria. The bacterial numbers in the root compartment of the planted microcosms were 19-32 times higher than found in the non-root sediment of the unplanted systems. Lengthening the photoperiod resulted in elevated bacterial numbers due to the higher carbon exudation of the plant. This effect was greater still with the nitrate-fed plants, where additional P. chlororaphis growth could proceed via denitrification, indicating oxygen-limiting conditions in the microcosms. Higher porewater N₂O concentrations in the root compartments as compared to the non-root compartments, which were highest for the long photoperiod, were also indicative of a plant-induced stimulation of denitrification. An effect of a diurnal oxygen release pattern of G. maxima on denitrification could not be detected. The gnotobiotic microcosm used in this study represents a potential system for the study of the behaviour and interactions of important bacterial groups, such as nitrifying and denitrifying bacteria where plant roots drive bacterial activity.     

Aluminium and phosphate uptake by Phragmites australis: the role of Fe,Mn and Al root plaques

Aluminium, a potentially phytotoxic metal, is an important constituent of many mine water discharges but has largely been neglected in the literature. The behaviour of this element in the rhizosphere of the wetland plant Phragmites australis was investigated in the laboratory in the presence and absence of Mn and Fe root plaques. Electron microscopy and chemical extraction techniques were utilized to determine the physico-chemical properties of the plaques and any association of Al. Both Mn and Fe plaques occurred as amorphous coatings on root surfaces with uneven distributions. Al was not adsorbed onto the surface of either plaque type but formed a separate phosphate deposit closely resembling the Fe and Mn plaques. Phosphorus was also found to be adsorbed to the surface of the Fe plaques (but not the Mn plaques). Both mechanisms were found to immobilize P at the root surface but this did not significantly reduce the concentration of P in aerial plant tissues that was sufficient to ensure adequate growth.     

Changes in growth,porosity, and radial oxygen loss from adventitious roots of selected mono‐ and dicotyledonous wetland species with contrasting types of aerenchyma

Growth in stagnant, oxygen‐deficient nutrient solution increased porosity in adventitious roots of two monocotyledonous (Carex acuta and Juncus effusus) and three dicotyledonous species (Caltha palustris, Ranunculus sceleratus and Rumex palustris) wetland species from 10 to 30% under aerated conditions to 20–45%. The spatial patterns of radial oxygen loss (ROL), determined with root‐sleeving oxygen electrodes, indicated a strong constitutive ‘barrier’ to ROL in the basal root zones of the two monocotyledonous species. In contrast, roots of the dicotyledonous species showed no significant ‘barrier’ to ROL when grown in aerated solution, and only a partial ‘barrier’ when grown in stagnant conditions. This partial ‘barrier’ was strongest in C. palustris, so that ROL from basal zones of roots of R. sceleratus and R. palustris was substantial when compared to the monocotyledonous species. ROL from the basal zones would decrease longitudinal diffusion of oxygen to the root apex, and therefore limit the maximum penetration depth of these roots into anaerobic soil. Further studies of a larger number of dicotyledonous wetland species from a range of substrates are required to elucidate the ecophysiological consequences of developing a partial, rather than a strong, ‘barrier’ to ROL.     

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

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.     

Rice and Phragmites: effects of organic acids on growth, root permeability, and radial oxygen loss to the rhizosphere

Young Phragmites plants were grown in two cocktails of monocarboxylic acids (C(1)-C(5)) at pH 6, where the concentration of each acid was innocuous and the total undissociated (potentially toxic) concentrations were 0.35 mmol/L and 0.42 mmol/L. Rice plants were subjected to 1.5 mmol/L acetic acid at pH 4.5 (undissociated concentration = 1.05 mmol/L). In Phragmites, each cocktail curtailed root growth especially and induced premature shoot senescence. In both species, after 3-5 d of treatment, radial oxygen loss (ROL) from apical regions of adventitious roots, and from Phragmites laterals, was reduced to very low values and associated with cell wall lignification and suberization in the surface cell layers. At later stages of treatment, rice responded to acetic acid in similar ways to Phragmites, with the development of intercellular and callus type occlusions in the gas space system, vascular blockages, and the failure of laterals to emerge. The results are relevant to the supply of oxygen from Phragmites roots to sediments for the phytopurification of waste waters, to the efflux of methane and carbon dioxide from wetlands, and to rice cultivation.     

Seasonal fluxes of O2, DIC and CH4 in sediments with Vallisneria spiralis: indications for radial oxygen loss

Seasonal fluxes of dissolved oxygen, inorganic carbon and methane were measured in microcosms containing vegetated (Vallisneria spiralis L.) and unvegetated sediments under controlled laboratory conditions. We tested if measured fluxes were affected by a moderate (6% as loss on ignition, LOI) and an elevated (10%) organic matter content (OM) in sediments. Microcosms were set up with plants and sediments collected from two riverine sites, upstream (moderate OM load) and downstream (elevated OM load) of a wastewater treatment plant. Light and dark fluxes were measured and V. spiralis net primary production and respiration rates were calculated. Unvegetated sediments were always net heterotrophic and behaved as methane sources to the water column, with significantly higher CH₄ release during summer from sediment with elevated OM load. Vegetated sediments were always net autotrophic with attenuated or negative CH₄ fluxes, suggesting the occurrence of processes within the rhizosphere that inhibit methane production or favor its oxidation. Vegetated sediments had an unbalanced O₂ to DIC stoichiometry, with average photosynthetic quotients varying between 0.30 and 0.68, significantly below one. The missing oxygen amount varied seasonally, with a minimum in the summer coinciding with the highest water temperature, but was not dependent upon the two OM levels. Overall these results suggest that V. spiralis is likely to transport a significant proportion of photosynthetically produced oxygen to the rhizosphere.     

Winter limnology: a comparison of physical,chemical and biological characteristics in two temperate lakes during ice cover

The winter dynamics of several chemical, physical, and biological variables of a shallow, polymictic lake (Opinicon) are compared to those of a deep, nearby dimictic lake (Upper Rock) during ice cover (January to early April) in 1990 and 1991. Both lakes were weakly inversely thermally stratified. Dissolved oxygen concentration was at saturation (11–15 mg l⁻¹) in the top 3 m layer, but declined to near anoxic levels near the sediments. Dissolved oxygen concentrations in the deep lake were at saturation in most of the water column and approached anoxic levels near the sediments only. Nutrient concentrations in both lakes were fairly high, and similar in both lakes during ice cover. Total phosphorus concentrations generally ranged between 10–20 μg l⁻¹, NH₄-N between 16–100 μg l⁻¹, and DSi between 0.9–1.9 mg l⁻¹; these concentrations fell within summer ranges. NO₃-N concentrations were between 51–135 μg l⁻¹ during ice cover, but occurred at trace concentrations (<0.002 μg l⁻¹) during the summer. The winter phytoplankton community of both lakes was dominated by flagellates (cryptophytes, chrysophytes) and occasionally diatoms. Dinoflagellates, Cyanobacteria and green algae were poorly represented. Cryptophytes often occurred in fairly high proportions (20–80%) throughout the water column, whereas chrysophytes were more abundant just beneath the ice. Zooplankton population densities were extremely low during ice cover (compared to maximum densities measured in spring or summer) in both lakes, and were comprised largely of copepods.     

Waterlogging tolerance in the tribe Triticeae: the adventitious roots of Critesion marinum have a relatively high porosity and a barrier to radial oxygen loss

Nine species from the tribe Triticeae – three crop, three pasture and three ‘wild’ wetland species – were evaluated for tolerance to growth in stagnant deoxygenated nutrient solution and also for traits that enhance longitudinal O₂ movement within the roots. Critesion marinum (syn. Hordeum marinum) was the only species evaluated that had a strong barrier to radial O₂ loss (ROL) in the basal regions of its adventitious roots. Barriers to ROL have previously been documented in roots of several wetland species, although not in any close relatives of dryland crop species. Moreover, the porosity in adventitious roots of C. marinum was relatively high: 14% and 25% in plants grown in aerated and stagnant solutions, respectively. The porosity of C. marinum roots in the aerated solution was 1·8–5·4‐fold greater, and in the stagnant solution 1·2–2·8‐fold greater, than in the eight other species when grown under the same conditions. These traits presumably contributed to C. marinum having a 1·4–3 times greater adventitious root length than the other species when grown in deoxygenated stagnant nutrient solution or in waterlogged soil. The length of the adventitious roots and ROL profiles of C. marinum grown in waterlogged soil were comparable to those of the extremely waterlogging‐tolerant species Echinochloa crus‐galli L. (P. Beauv.). The superior tolerance of C. marinum, as compared to Hordeum vulgare (the closest cultivated relative), was confirmed in pots of soil waterlogged for 21 d; H. vulgare suffered severe reductions in shoot and adventitious root dry mass (81% and 67%, respectively), whereas C. marinum shoot mass was only reduced by 38% and adventitious root mass was not affected.     

Comparative morphology and ecology of two species of Litomosoides (Nematoda: Filarioidea) of rodents in Florida, with a key to the species of Litomosoides Chandler, 1931

From April 1970 to August 1971, 235 cotton rats (Sigmodon hispidus) and 281 rice rats (Oryzomys palustris) were trapped in salt and fresh water marshes in north-central Florida and examined for natural infections of filarial worms, Litomosoides spp. Cotton rats from both types of marshes were infected (28–44 per cent prevalence), whereas only rice rats from the salt marsh were infected (56 per cent prevalence). Older animals were more commonly infected than younger ones. In cotton rats the worms were located in the pleural cavity, whereas in rice rats the worms were located primarily in the abdominal cavity. Filarial worms from rice rats were transmitted experimentally to laboratory-reared rice rats and cotton rats, but worms from cotton rats were transmitted only to cotton rats. Morphological studies on adult forms and microfilariae indicated that the worms in rice rats were distinct from those in cotton rats and are therefore described as Litomosoides scotti sp. n. The cotton rat filariids were referable to Litomosoides carinii (Travassos, 1919) Vaz, 1934. L. scotti differs from L. carinii in the ratio of the spicules, in the shape of the distal end of the right spicule and in having shorter microfilariae.     

Photosynthetic acclimation to light in woody and herbaceous species: a comparison of leaf structure, pigment content and chlorophyll fluorescence characteristics measured in the field

Acclimation of foliage photosynthetic properties occurs with varying time kinetics, but structural, chemical and physiological factors controlling the kinetics of acclimation are poorly understood, especially in field environments. We measured chlorophyll fluorescence characteristics, leaf total carotenoid (Car), chlorophyll (Chl) and nitrogen (N) content and leaf dry mass per area (LMA) along vertical light gradients in natural canopies of the herb species, Inula salicina and Centaurea jacea, and tree species, Populus tremula and Tilia cordata, in the middle of the growing season. Presence of stress was assessed on the basis of night measurements of chlorophyll fluorescence. Our aim was to compare the light acclimation of leaf traits, which respond to light availability at long (LMA and N), medium (Chl a/b ratio, Car/Chl ratio) and short time scales (fluorescence characteristics). We found that light acclimation of nitrogen content per unit leaf area (N(area)), chlorophyll content per unit dry mass (Chl(mass)) and Chl/N ratio were related to modifications in LMA. The maximum PSII quantum yield (F(v) /F(m)) increased with increasing growth irradiance in I. salicina and P. tremula but decreased in T. cordata. Leaf growth irradiance, N content and plant species explained the majority of variability in chlorophyll fluorescence characteristics, up to 90% for steady-state fluorescence yield, while the contribution of leaf total carotenoid content was generally not significant. Chlorophyll fluorescence characteristics did not differ strongly between growth forms, but differed among species within a given growth form. These data highlight that foliage acclimation to light is driven by interactions between traits with varying time kinetics.     

Parasite loss and introduced species: a comparison of the parasites of the Puerto Rican tree frog, (<Emphasis Type="Italic">Eleutherodactylus coqui</Emphasis>), in its native and introduced ranges

The Puerto Rican frog, Eleutherodactylus coqui has invaded Hawaii and reached densities far exceeding those in their native range. One possible explanation for the success of E. coqui in its introduced range is that it lost its co-evolved parasites in the process of the invasion. We compared the parasites of E. coqui in its native versus introduced range. We collected parasite data on 160 individual coqui frogs collected during January-April 2006 from eight populations in Puerto Rico and Hawaii. Puerto Rican coqui frogs had higher species richness of parasites than Hawaiian coqui frogs. Parasite prevalence and intensity were significantly higher in Hawaii, however this was likely a product of the life history of the dominant parasite and its minimal harm to the host. This suggests that the scarcity of parasites may be a factor contributing to the success of Eleutherodactylus coqui in Hawaii.     

Review of Australian Asperoseius Chant, Euseius Wainstein, Okiseius Ehara and Phytoscutus Muma (Acari: Mesostigmata: Phytoseiidae) with a key to the genera of Australian Amblyseiinae and descriptions of two new species

Previous workers have used a conservative generic classification of the subfamily Amblyseiinae that treated most Australian Phytoseiidae as species in the genus Amblyseius Berlese. However, this lumping masks the occurrence of a number of genera recognised overseas, including some that contain important biological control agents. In this review, a key to the genera of Australian Amblyseiinae is provided and the Australian species in four amblyseiine genera are reviewed and keyed. Asperoseius Chant is represented by a single known Australian species, A. australiensis Fain and Krantz collected from a Culicoides midge in South Australia. Six species of Euseius Wainstein are recognised in Australia, including four endemics: E. dowdi (Schicha), E. elinae (Schicha), E. neovictoriensis (Schicha), and E. victoriensis (Schicha), and two Asia–Pacific species: E. noumeae (Schicha) and E. ovalis (Evans). Okiseius Ehara is represented in Australia by the previously described O. morenoi Schicha and O. domatorum (Schicha); the Asian species O. subtropicus Ehara (new record); and two new species, O. tribulation and O. cowbay described from tropical rainforest in Far North Queensland. Phytoscutus acaridophagus Collyer, a mite described from New Zealand, is reported in association with acarid mites ( Neotropacarus sp.) in Victoria and in Queensland.