Root Oxygen Research Articles

Growth and root oxygen release by Typha latifolia and its effects on sediment methanogenesis

Growth of Typha latifolia L. and its effects on sediment methanogenesis were examined in a natural organic sediment and a sediment enriched with acetate to a concentration of 25 mM in the interstitial water. The lower redox potential and higher oxygen demand of the acetate-enriched sediment did not significantly impede growth of T. latifolia despite some differences in growth pattern and root morphology. Plants grown in acetate-enriched sediment were ca. 15% shorter than plants grown in natural sediment, but the former produced more secondary shoots at earlier stages, which resulted in similar total biomasses after 7 weeks of growth in the two sediment types. Plants grown in acetate-enriched sediment had thicker and much shorter roots than plants grown in natural sediment. This difference did not significantly affect the release of oxygen from the roots when measured under laboratory conditions, which was 0.12–0.20 mmol O 2 g −1 DW h −1. Enrichment with acetate resulted in much higher sediment methanogenesis rates (643 vs. 90 nmol CH 4 g −1 sediment DW h −1). Growth of T. latifolia significantly reduced methanogenesis in both types of sediment, but the effect was twice as marked in the natural sediment (34%) as in the acetate-enriched sediment (18%), although in absolute terms the reduction was higher in the enriched sediment. The data suggest that this effect of plant growth was via root oxygen release and its effect on redox conditions. In the natural sediment, oxygen release resulted in a significantly higher redox potential and lower sediment oxygen demand, whereas there were no significant changes in the acetate-enriched sediment. The very high oxygen demand of this sediment probably masked the effect of root oxygen release so that a significant reduction in methanogenesis occurred without any significant increase in the redox potential. This demonstrates how root oxygen release from plants like T. latifolia can significantly alter rates of biogeochemical processes such as methanogenesis, even in sediments with high oxygen demands where this is not evident from instantaneous parameters such as redox potential.

Formation of root plaques and their influence on tissue phosphorus content in Lobelia dortmanna

Oxygen release from the roots of the freshwater isoetid, Lobelia dortmanna L., influences redox related processes of iron (Fe) and manganese (Mn) in the sediments. Because oxidized species of Fe and Mn have high capacity for phosphorus (P) binding, the availability of sediment P to aquatic plants is influenced by root oxygen release. We investigated 30 sediment cores vegetated with L. dortmanna from oligotrophic Mirror Lake to analyse sediment variables responsible for precipitation of Fe and Mn on root surfaces (plaques), and P content of plant tissue. Close negative correlations between redox potential and concentrations of Fe ( r=−0.924) and Mn plaques ( r=−0.897) were found. Also, correlations between exchangeable Fe and Fe plaques ( r=0.716) and between exchangeable Mn and Mn plaques ( r=0.783) were found indicating that plaques are formed in reduced sediments with high concentrations of reduced Fe and Mn. For plants with Fe plaque concentrations lower than 30 μmol g −1 there were significant positive correlations between the tissue P content and the concentration of the less-plant-available P fractions in the sediment. In contrast, plants with Fe plaque concentrations higher than 30 μmol g −1 showed low tissue-P content (about 20 μmol g −1) independent of the P concentration in the sediment indicating that P uptake by L. dortmanna is reduced by Fe plaques. L. dortmanna with low tissue P content (<30 μmol g −1) showed significantly higher mycorrhizal infection than plants with high tissue P content (>30 μmol g −1) indicating that fungal association may be important in P assimilation by rooted aquatic plants in habitats with low P availability.

Sulfate reduction in lake sediments inhabited by the isoetid macrophytes Littorella uniflora and Isoetes lacustris

Sulfur cycling was examined in sediments inhabited with the isoetids Littorella uniflora and Isoetes lacustris in the oligotrophic soft-water Lake Kalgaard, Denmark. Based on short-term tracer incubations sulfate reduction was measured along a transect from the shore (0.6 m) to profundal sediments (4.6 m). The sulfate reduction rates were low (0.008–0.8 mmol m −2 d −1) in the sandy shallow sediments with low organic content (<1.3 mmol C g −1 sed DW) and high redox potentials (>100 mV), whereas sulfate reduction was higher at the deeper sites (2.7–4.6 mmol m −2 d −1) with high organic content (max. 11.5 mmol C g −1 sed DW) and lower redox potentials (<100 mV). High concentrations of dissolved organic carbon (DOC) were found in the low particulate organic sediments (up to 18.4 mM), and most of the DOC pool consisted of acetate (40–77%). Reoxidation of sulfides due to root oxygen release was probably important at all sites and a positive efflux of sulfate across the sediment–water interface was measured, attaining rates (up to 4.8 mmol m −2 d −1) similar to the sulfate reduction rates. Reoxidation of sulfides was also manifested by high fraction (>80%) of reduced sulfides being accumulated as elemental sulfur or pyrite (chromium reducible sulfur, CRS). The largest pools of CRS were found in high organic sediment with vertical distributions resembling those of the sulfate reduction rates. The overall effect of isoetid growth on sulfur cycling in the rhizosphere is a suppression of sulfate reduction in low organic sediments and the governing of sulfide reoxidation in sediments with higher organic content.

Ammonia Removal Processes for Urine in an Upflow Macrophyte System

Constructed wetlands represent an alternative technology for the removal of nitrogen from wastewater, although removal mechanisms are poorly understood. Detailed studies of ammonia removal from diluted urine in a vertical upflow macrophyte system were made in a column fitted with sampling ports, filled with sand, and planted with common reed (Phragmites australis). Variations in profiles for ammonia, nitrate, and dissolved oxygen were interpreted for a range of initial ammonia concentrations and loadings. The profiles demonstrate that the previously proposed mech anism of root oxygenation promoting bacterial nitrification/denitrification may have been overemphasized. In systems with sufficient root density and an upflow regime, luxury plant uptake with rhizome storage dominates N removal. Plant uptake of ammonia regulates oxygen demand and provides suitable conditions, both in terms of oxygen and ammonia concentrations, for additional bacterial nitrification. Consequently, higher concentrations and loading rates of ammonia can be treated by an upflow macrophyte system as compared to a bacterial system alone.

Effects of different submersed macrophytes on sediment biogeochemistry

Porewater phosphate levels in submersed macrophyte grassbeds varied among years in the upper Chesapeake Bay (Maryland, USA) coincident with macrophyte species variation during these same years (1990, 1993, 1995). When native, deep-rooted Vallisneria americana Michx. was a codominant in the grassbed, the porewater phosphate concentrations were significantly lower ( P < 0.001) than concentrations when the exotic, shallow-rooted species Hydrilla verticillata (L.f.) Royle and Myriophyllum spicatum L. were codominants. There were significant relationships ( P < 0.001) between solid-phase inorganic phosphorus and reactive metals (Fe, Mn) in both native and exotic grassbeds. However, the slopes of the regression relationships between years were significantly different ( P < 0.001), suggesting greater retention of inorganic phosphorus in sediments when V. americana was a codominant at the site. In addition, significant relationships between reactive manganese and iron in the sediments were observed, but the coefficient of determination was statistically greater ( P < 0.001) when V. americana was a codominant at the site. Furthermore, plant cores of V. americana and H. verticillata had noticeably different sediment redox profiles, with the oxidation-reduction status of V. americana sediments being more oxidized in the root zone (i.e., +125 mV vs −5 mV at 4 cm depth). These data suggest that macrophyte species composition can alter sediment biogeochemistry resulting in varying porewater phosphate and solid-phase phosphorus and metal levels. Possible explanations for these biogeochemical differences may be attributed to morphological differences among macrophyte species (i.e., root/shoot ratio, canopy type, growth form) and differences in root oxygenation capabilities.

Growth and physiological responses of neotropical mangrove seedlings to root zone hypoxia.

Seedlings of Rhizophora mangle L., Avicennia germinans (L.) Stearn., and Laguncularia racemosa (L.) Gaertn. f. were cultured in aerated or N(2)-purged solution for 12 weeks to assess their relative responses to low oxygen tensions. All three species responded to low oxygen treatment by modifying physiological and morphological patterns to decrease carbon loss by root respiration. However, the extent to which seedling physiology and morphology were altered by low oxygen treatment differed among species. Maintenance of root oxygen concentrations, root respiration rates and root extension rates by R. mangle demonstrated an ability to avoid low oxygen stress with minimal changes in root morphology and physiology. In contrast, oxygen concentrations in A. germinans and L. racemosa roots declined from 16 to 5% or lower within 6 h of treatment. Root hypoxia led to significant decreases in respiration rates of intact root systems (31 and 53% below controls) and root extension rates (38 and 76% below controls) by A. germinans and L. racemosa, respectively, indicating a greater vulnerability of these species to low oxygen tensions in the root zone compared with R. mangle. I conclude that the relative performance of mangrove seedlings growing in anaerobic soils is influenced by interspecific differences in root aeration and concomitant effects on root morphology and physiology.

Microelectrode and Modelling Study of Oxygen Distribution in Roots

The use of polarographic microelectrodes to explore the radial distribution of oxygen in maize roots aerated chiefly via the shoot is described. Also given is a brief description of a mathematical model of root aeration which introduces two simultaneous oxygen sources (the shoot and the rooting medium), and mimicks more realistically than hitherto the diffusive characteristics of the non-porous epidermal/hypodermal shell.In keeping with modelling predictions, steep radial diffusion gradients were a characteristic of the non-porous epidermal/hypodermal shell and stele, while shallow profiles generally characterised the cortex. Stelar anoxia, previously forecast by earlier mathematical modelling, was inducible by manipulating the oxygen concentrations around the shoot.The stelar diffusion gradients always indicated an inward diffusion from the cortex. However, two-way gradients were sometimes evident in the epidermal/hypodermal annulus, and were also found in the sheathing extensions of the root cap. These indicated diffusion into these annuli simultaneously from both the cortex and the rooting medium. Elsewhere the gradients in the epidermis/hypodermal cylinder were either solely positive or negative depending upon the oxygen concentrations within the root cortex and surrounding agar. Examples of two-way and one-way diffusion in the epidermal/hypodermal cylinder were created satisfactorily by the model.Finally, the results indicate that with roots in stagnant anaerobic media, the bulk of the root cap proper is likely to be anoxic, and that some anoxia may be the norm within the basipetal extensions of the cap which ensheath the root apex.

Evaluation of root oxygenation and growth in baldcypress in response to short-term soil hypoxia

Seedlings of baldcypress (Taxodiumdistichum (L.) Rich. var. distichum) were grown under laboratory and greenhouse conditions to determine the extent to which short-term soil hypoxia influences root aerenchyma–air space formation (expressed as a percentage of total root volume) and concomitant radial oxygen loss. Subsequent photosynthesis and growth responses were also determined. A colorimetric technique involving the use of Ti3+-citrate, a strong reducing compound, was used to quantify radial oxygen loss from whole root system. Soil redox potential of −250 ± 10 mV resulted in enhancement of both root porosity and radial oxygen loss as much as 3-fold compared with plants under well aerated conditions (515 ± 25 mV). The mean oxygen loss from roots was 1.4 mmol O2•g−1•day−1 in drained plants and 4.6 mmol O2•g−1•d−1 in flooded plants. Mean root porosity was 13.3 and 41.4% in drained and flooded plants, respectively. Stomatal conductance, net photosynthesis, and height growth were adversely affected by reduced soil conditions. Baldcypress exhibited an avoidance mechanism under reduced soil conditions by increasing aerenchyma formation and rhizosphere oxygenation at young ages. This may explain the significance of flooding episodes encountered in young stages in enabling baldcypress saplings and trees to tolerate flooding in later stages of the life cycle.

On the Difficulties of Measuring Oxygen Release by Root Systems of Wetland Plants

The suitability of bathing whole root systems in nutrient solutions for measuring root oxygen release was re-examined. The rate of oxygen release from the roots of the wetland plant Juncus ingens was measured in oxygen-depleted solution (oxygen concentration < 15 μMOL L −1 ) and in a titanium (III) citrate buffer with a low redox potential similar to that of natural sediments (E H < −200 mV). When incubated in a solution containing 15 μmol O 2 l −1 in a closed chamber, J. ingens roots consumed oxygen until all the oxygen in the chamber was exhausted and there was zero net oxygen exchange between the roots and the solution (...)

Effects of oxygen and nitrate on ammonium uptake kinetics and adenylate pools in Phalaris arundinacea L. and Glyceria maxima (Hartm.) Holmb

SynopsisWe studied the effects of oxygen (aerated versus O2 depleted ∼0.5 mg 1−1 O2) and nitrate (none versus 10 μmol 1−1) on the ammonium uptake kinetics and adenylate pools in two wetland plants differing in their degree of flood tolerance (Phalaris arundinacea L. and Glyceria maxima (Hartm.) Holmb.). The study was performed as a random block design in a growth chamber. The -uptake kinetics were estimated by using a computerised nonlinear parameter estimation procedure to fit the differential form of a modified Michaelis–Menten model to solution depletion curves. The uptake kinetics for differed between the two species: Vmax was significantly higher for P. arundinacea (24.7 to 29.6 μmol h−1 g−1 root dry weight) than for G. maxima (4.6–10.3 μmol h−1 g−1 root dry weight). The concentration at which uptake ceases (Cmin) was 0.2 to 0.5 μmol 1−1 for P. arundinacea and significant higher (1.1–2.7 μmol 1−1) for G. maxima.Km varied between 3.1 and 6.2 μmol 1−1 for P. arundinacea, and 1.6 and 3.0 μmol 1−1 for G. maxima. The different uptake kinetics of the two species reflect the different structure of their root systems: P. arundinacea has an extensive root system consisting of many thin roots whereas G. maxima has fewer but thicker roots. The uptake kinetics also suggest that P. arundinacea is adapted to growing at lower ambient concentrations than G. maxima. Oxygen had no consistent effect on uptake kinetics. However, the plants that had in the nutrient solution as well as had slightly higher Vmax values and lower Cmin and Km values than those without . Thus, both species were able to sustain their uptake characteristics at low external O2 concentrations, probably because of internal aeration through the air-space tissue of the plants. Nitrate deprivation also lowered the energy charge ratio and adenine nucleotide content in roots. The roots recovered quickly from deprivation once was resupplied. The stresses imposed by partially O2-depleted conditions and lack of nitrate were therefore relatively mild and reversible. It seems that the inherent aerenchyma development under aerated conditions in these species is sufficient to maintain adequate root oxygenation under partially O2-depleted conditions.

Soil Physicochemical Patterns and Mangrove Species Distribution--Reciprocal Effects?

1 The distributions of two dominant mangrove species in a neotropical forest were associated with spatial variation in soil redox potentials (Eh) and interstitial water sulphide concentrations. Mangrove zones dominated by Rhizophora mangle L. alone or in combination with Avicennia germinans (L.) L. were characterized by moderately reducing soils (Eh = 100-300 mV) with low sulphide concentrations (< 0.3 gM), whereas a zone dominated by A. germinans had strongly reducing soils (Eh < -100 mV) with high sulphide (2-4 mM). 2 Less reducing soil conditions near the aerial roots of both mangrove species indicated that the relationship between soil physicochemical patterns and mangrove zonation reflected not only effects of soil factors on the vegetation, but effects of the adult root systems on soil Eh and sulphide. 3 A glasshouse experiment with seedlings demonstrated that although R. mangle and A. germinans are both flood tolerant, the species respond differently to flooding in terms of growth and biomass partitioning, particularly when the soil is strongly reducing and sulphide is present. Flooding under reducing soil conditions resulted in a 20-40% decrease in total biomass of A. germinans seedlings (relative to drained controls), whereas seedling biomass was increased 9-24 % above drained controls in R. mangle. 4 A significant decrease in root oxygen concentrations in response to reducing soil conditions and sulphide was observed in A. germinans, but not R. mangle, and indicated that differential flood tolerance of seedlings is partly due to differences in root aeration. 5 The implication of these findings is that mangrove seedling dynamics are influenced by spatial and temporal variation in soil Eh and sulphide levels, but that these soil factors are also modified by the root systems of adult trees. These reciprocal processes may play an important role in the development of spatial organization in this intertidal community.

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and 1·6-fold larger in NH+4-fed than in NO-3-fed wheat (Triticum aestivum L.) and maize (Zea mays L.) plants respectively. Higher root oxygen consumption in NH+4-fed plants than in NO-3-fed plants was associated with higher total nitrogen contents in NH+4-fed plants. Root oxygen consumption was, however, not correlated with growth rates or shoot:root ratios. Carbon dioxide release was 1·4- and 1·2-fold larger in NO+3-fed than in NH+4-fed wheat and maize plants respectively. Differences in oxygen and carbon dioxide gas exchange rates resulted in the gas exchange quotients of NH-4-fed plants (wheat, 0·5; maize, 0·6) being greatly reduced compared with those of NO-3-fed plants (wheat, 1·0; maize, 1·1). Measured rates of HCO-3 assimilation by PEPc in roots were considerably larger in 4 mM NH+4-fed than in 4 NO-3 plants (wheat, 2·6-fold; maize, 8·3-fold). These differences were, however, insufficient to account for the observed differences in root carbon dioxide flux and it is probable that HCO-3 uptake is also important in determining carbon dioxide fluxes.Thus reduced root extension in NH+4-fed compared with NO-3-fed wheat plants could not be ascribed to differences in carbon dioxide losses from roots.

Depletion of Oxygen in Roots of Wheat and Maize by Respiration Following Encasement in Olive Oil

The extent to which isolated root systems of 14-d-old seedlings of wheat (Triticum aestivum) and maize (Zea mays) were depleted of oxygen by respiration was measured after immersing them in outgassed olive oil to exclude oxygen entry from the air. At intervals over 45 min, gas from the roots was removed under partial vacuum and oxygen partial pressures measured by gas chromatography. Contrary to earlier findings (Erdmann and Wiedenroth, 1988), roots were able to utilize almost all of their oxygen within 20 min at 25°C, including that dissolved in the thin water covering interposed between roots and oil. The rate of aerobic respiration could be estimated readily from the time course of oxygen depletion.

Physiological adaptation of waxapple to waterlogging

ABSTRACTWaxapple (Syzygium samarangense Men. et. Perry) plants receiving up to 40d of continuous flooding treatment showed no symptoms of physiological disorder, but the treatment resulted in early flowering. In this report, several physiological parameters of flooded plants are compared with those of nonflooded plants. Both control plants and 9‐d‐flooding‐treated plants exhibited aerenchyma formation in the cortex tissue beginning 5 cm from the root tip. After 7d flooding treatment, the oxygen consumption rate of the root section was only 20% of that of the controls. Following flooding treatment, the roots showed an increase in alcohol dehydrogenase activity as well as an increase in three isozymes. However, malate dehydrogenase activity was decreased, and no significant change of NADP‐malic enzyme activity was observed. There were no significant differences in levels of ethylene, 1‐aminocyclopropane‐1‐carboxylic acid and 1‐(malonylamino) cyclopropane‐1‐carboxylic acid in petiole and roots of flooded and non‐flooded plants during the stage examined. It is inferred that the presence of aerenchyma in the root cortex allows a higher level of internal gas exchange, and thus, makes waxapple surprisingly flood tolerant. However, reduced root oxygen consumption rate may have limited root respiration rate and vegetative growth.

An oxidation-reduction buffer for evaluating the physiological response of plants to root oxygen stress

Zea mays and Spartina patens were grown in nutrient solution containing either an oxidized (+4) or a reduced (+3) form of titanium citrate. Low oxidation-reduction conditions in the nutrient solution as a result of titanium (+3) citrate reduced photosynthetic activity of Zea mays. Photosynthetic activity of flood-tolerant S. patens was initially reduced by the addition of titanium (+3) citrate but subsequently increased, indicating the existence of adaptation mechanisms in S. patens. Titanium citrate was non-toxic since titanium (+4) citrate (oxidized form) added to rooting medium resulted in no reduction in photosynthetic activity of either species. Titanium (+3) citrate may be an excellent non-toxic oxidation-reduction buffering system for evaluating wetland plant response to root oxygen stress.

Light-enhanced convective throughflow increases oxygenation in rhizomes and rhizosphere of Phragmites australis (Cav.) Trin. ex Steud.

The convective throughflow of gases found recently in Phragmites australis increased with increasing fluxes of photosynthetically active radiation. Despite this, the oxygen in the venting gases remained lower than in air and the convection is thought to depend mainly on a humidity-induced diffusion of atmospheric gases across the leaf sheaths. When convection was prevented, and aeration was dependent entirely upon diffusion, no photosynthetic enhancement of rhizome and root aeration could be detected. In terms of oxygen transport, rapid rates of convection were much more effective than diffusion and substantially enhanced the entry of oxygen into the rhizome system. This resulted in large increases in the efflux of oxygen into the rhizospheres of basal laterals and apical and sub-apical regions of the adventitious roots. This effect was attributed to a greater diffusion of oxygen from rhizome to root. Substantial diurnal fluctuations in rhizome oxygen regimes and in root oxygen efflux occurred. It is estimated that because of convection and diffusion, up to 0.31 kg of oxygen d-1 m-1 may enter the rhizome system of vigorous stands of Phragmites during summer months. By comparison, diffusion alone could be expected to supply less than 0.01 kg d-1 . Scanning electron micrographs of root-shoot junctions showed that the resistance to gaseous diffusion from rhizome to roots must be especially small in Phragmites.

Growth and Survival Responses ofRumexSpecies to Flooded and Submerged Conditions: The Importance of Shoot Elongation, Underwater Photosynthesis and Reserve Carbohydrates

Plants of Rumex thyrsiflorus Fingerh., R. crispus L. and R. maritimus L., which are zoned along a gradient of elevation in a river foreland ecosystem, and differ in their flood-tolerance, were subjected to different flooding levels. Under conditions of soil flooding, the growth rates of the flood-tolerant R. crispus and R. maritimus were as high as under drained conditions, but that of the flood-intolerant R. thyrsiflorus was halved. Upon submergence, the low elevation species R. maritimus showed rapid shoot elongation; when elongation resulted in a protrusion of leaves above the water surface, the plants survived. Alternatively, underwater photosynthesis also led to a 100% survival of submerged R. maritimus plants, provided that enough inorganic carbon was made available in the water. This could be attributed in part to the use of photosynthetically-derived oxygen for root respiration; in a hydroculture experiment, with 5 0 mM C02 in the water in the shoot environment, photosynthetically-derived oxygen contributed more than 50% to root oxygen consumption at low oxygen concentrations in the root environment. The intermediately elevated species R. crispus appeared to be much more tolerant towards conditions of prolonged total submergence: older plants survived eight weeks submergence in the, dark. This response was explicable in terms of a dormancy strategy, which is characterized by a slow consumption of carbohydrates stored in the tap-root; The differential responses of R. maritimus and R. crispus to total submergence reveal the limitations, of flood-tbleranee arid reflect the different life-histories of the species.

Effect of long-term flooding on root metabolic response in five freshwater marsh plant species

Five freshwater marsh plant species exhibited different root metabolic responses when flooded to three water depths in field macrocosms. The capacity for alcoholic fermentation (as indicated by alcohol dehydrogenase activity) increased and remained at a relatively high level in the roots of the least flood-tolerant species, Scolochloa festucacea, but was not stimulated significantly or only temporarily in the more tolerant species, Scirpus acutus, Scirpus validus, Typha glauca, and Phragmites australis. During the first month of flooding, alcohol dehydrogenase activity showed a positive relationship with flooding depth and a negative relationship with soil redox potential. Malate accumulated in the roots of S. acutus, S. validus, and to a lesser extent in P. australis in response to flooding; concentrations showed a significant positive relationship with water depth and a significant negative relationship with soil redox potential during the first month of flooding. Differences in root metabolism among the five species were still evident after 1 year of continual flooding. Root specific gravities and air space cross-sectional volumes demonstrated potential species differences in root resistance to oxygen movement and root oxygen volume, respectively. The results suggest that the observed metabolic response reflected differential aeration of the roots resulting from differences in root structure and soil oxygen demand (reducing power).

Effect of fluctuating rhizosphere redox potential on carbon assimilation ofSpartina alterniflora

Spartina alterniflora Lois. plants from a Louisiana salt marsh were subjected to fluctuating levels of soil redox potential under controlled environmental conditions. The experiment was designed to examine the changes in carbon assimilation rates in response to the change in rhizosphere sediment redox condition representing a broad range of reduction normally associated with oxygen deficient environments. Variation in sediment redox potential is frequently encountered by this species in its natural environment in Louisiana's Gulf Coast marshes as a result of tidal patterns. Results indicated some adverse effects of extreme anoxic conditions on carbon assimilation ofS. alterniflora, a possible reflection of this species limited ability for maintaining root oxygenation under rapid, intense reduction in soil redox potential. It was also demonstrated that gas exchange limitations may be temporary and apparently may follow by some recovery. Carbon assimilation rates declined 15 to 21% when soil redox level decreased rapidly to below-200 mV which was followed by substantial recovery. A system for accurate control and measurement of rhizosphere redox potential and simultaneous measurement of plant photosynthetic activity is described.

Gas composition and respiration of water oak (Quercus nigra L.) and green ash (Fraxinus pennsylvanica Marsh.) roots after prolonged flooding

We compared the effects of 9.5 months of a continuous flooding treatment with a drained control treatment on one year-old seedlings of green ash (Fraxinus pennsylvanica Marsh.) and water oak (Quercus nigra L.), two tree species common to the bottomland-hardwood forests of eastern North America. The internal root gas composition of the more flood tolerant species, green ash, maintained higher oxygen and lower carbon dioxide concentrations under the flooding treatment than water oak. This apparently resulted in differences in rhizosphere oxidation. The amounts of Fe and Mn and the Fe/Mn ratio of the root coating extracted from trees in reduced soil conditions were much higher for the green ash than the water oak. It is argued that this reflects differences in the ability of these two species to maintain rhizosphere oxidation under prolonged periods of flooding and to prevent the accumulation of reduced potentially phytotoxic compounds. Alcohol dehydrogenase activity increased in the green ash and decreased in the water oak in the flooded treatment. This indicated that the better adapted species was able to rely upon increased anaerobic respiration in order to compensate for the decreased root oxygen supply, but the water oak was unable to maintain previous levels of respiration, probably as the result of sulfide toxicity.