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Keywords:

  • Juncus effusus;
  • Juncus inflexus;
  • aeration;
  • diffusion;
  • oxygen;
  • roots

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Radial oxygen loss (ROL) from the roots of two semiaquatic rushes, Juncus effusus L. and Juncus inflexus L., was studied in reducing titanium citrate buffer, using both closed incubations and a flow-through, titrimetric system. In closed experiments, roots released oxygen at a constant rate over a wide range of external oxygen demands, with the ROL rate only depending on sink strength at low demands, and no oxygen release into oxidized solutions. In the titrimetric experiments, roots continued to release oxygen at constant rates when provided with a constant external oxygen demand. ROL was higher in J. effusus (9·5 ± 1 × 10−7 mol O2 h−1 root−1) than in J. inflexus (4·5 ± 0·5 × 10−7 mol O2 h−1 root−1). Light and dark changes around the shoots did not affect the ROL rate in J. inflexus, whereas in J. effusus ROL was ≈ 1·75 times higher in the light than in the dark, presumably due to changes in stomatal aperture. These results suggest that ROL is controlled by the external oxygen demand at low to moderate reducing intensities, but that structural limitations to oxygen diffusion rates prevent ROL from continuing to increase at higher external oxygen demands.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The rates of oxygen diffusion through waterlogged soils are usually too slow to satisfy their biological and chemical oxygen demand, causing them to become oxygen-depleted and anaerobic ( Ponnamperuma 1984; Gambrell, DeLaune & Patrick 1991). Under these conditions, the roots of most plants cannot obtain enough oxygen to satisfy respiratory needs, especially for mitosis in the apical meristem, and quickly die. Although many rhizomes have metabolic strategies for tolerating complete anoxia for several weeks or months ( Crawford 1989), long-term survival of roots in flooded soils is normally only possible by internal transport of oxygen from the atmosphere to the roots ( Armstrong et al. 1991 ). Roots of all plants have some intercellular airspaces, which can be important for internal aeration even in unsaturated soils, but only a minority of hydrophytic species have internal gas transport processes that are efficient enough to allow indefinite survival in waterlogged soils. Efficient internal oxygen transport in roots is usually conferred by a combination of high internal porosity, which minimizes internal resistances to diffusion and respiratory oxygen demand ( Armstrong 1979; Drew, Saglio & Pradet 1985), and reduced epidermal and hypodermal permeability, which conserves oxygen internally, restricting loss into the soil ( Smits et al. 1990 ; Armstrong et al. 1991 ; Sorrell 1994).

All roots nevertheless lose some oxygen to reducing soils. Some root surfaces must remain permeable in even the most specialized aquatic plants if nutrient and water uptake are to continue, and sediments are a competitive sink for the oxygen transported internally in roots ( Armstrong & Beckett 1987). This radial oxygen loss (ROL) may even be beneficial, for it allows roots to create a protective rhizosphere adjacent to their more permeable surfaces, where toxic products of anaerobic microbial metabolism can be oxidized before entering the root ( Armstrong, Armstrong & Beckett 1992). The key to long-term root survival in flooded soils with high phytotoxin concentrations and high oxygen demands is therefore the production of roots that have high internal oxygen fluxes, and hence can satisfy their internal oxygen demands while simultaneously maintaining high fluxes into the rhizosphere.

The transport of oxygen in individual roots is easily measured and is now well-understood ( Armstrong et al. 1991 ). It occurs by axial diffusion in gas-filled spaces and aerenchyma of the cortex, with radial diffusion into nonporous tissues such as the stele, and into the soil ( Armstrong et al. 1991 ; Sorrell 1994). However, plants usually bear many roots of differing lengths, branching and thickness, and the interactions between the total oxygen transport in an entire root system and the oxygen demand of the surrounding soil are less well-understood. The role of the external oxygen demand in root aeration and ROL has until recently received little interest, but its intensity now appears to be an important control of the relative partitioning of oxygen between sinks within the root and outside it ( Sorrell & Armstrong 1994; Brix & Sorrell 1996). As sediments become more reducing, the competition for oxygen induced by their high oxygen demand stresses plants, impairing growth and survival ( Kludze & DeLaune 1994; 1996). Little is known, however, of the nature of the response of ROL to external sink strength.

This study therefore examined the way in which ROL from roots of two Juncus species is affected by the intensity of the external oxygen demand. Two hypotheses suggested by theoretical models of root aeration ( Armstrong et al. 1991 ; Sorrell & Armstrong 1994) were considered. First, that ROL is driven by the external oxygen demand but cannot increase indefinitely and will ultimately be limited by internal resistances to diffusion in the roots. Second, that the ROL rate will be constant at any given external demand. ROL was measured in titanium (III) citrate buffers, a technique that has recently been used for measuring ROL in several emergent plants ( Sorrell, Brix & Orr 1993; Kludze et al. 1994 ). Previous applications of the titanium citrate method have used closed, fixed volumes of buffer, which are convenient for measurements but necessarily require experiments to begin with relatively high Ti3+ concentrations that the plants reduce during the experiment. In this study, I have measured ROL using both this design and a flow-through titrimetric system, where ROL can be measured with a constant oxygen demand around the roots.

MATERIALS AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Plant material

Juncus effusus and Juncus inflexus were grown from rhizome cuttings in a growth chamber (temperature = 15 °C, relative humidity = 80%, light intensity = 3·0 × 10−4 mol (photons) m−2 s−1), in the hydroponic media described by Brix et al. (1994) . The solution around the roots was stagnant, with oxygen concentrations < 10% saturation. Plants were used in experiments after 2–3 weeks growth, when their primary roots were up to 0·2 m long. The ROL apparatus was in the same growth cabinet as the hydroponic growth system, so that plants were under the same light, temperature and humidity conditions during measurements as they had experienced during growth. After experiments, the lengths and diameters of all roots and shoots were measured, root branching patterns recorded, and root porosity estimated nondestructively by weighing underwater ( Brix & Sorrell 1996). Dry weights were determined by drying at 105 °C for 12 h.

Preparation of titanium (III) citrate solutions

Stock solutions of the reduced titanium (III) citrate buffer were prepared in a nitrogen-purged glove bag, using commercially available titanium (III) chloride in HCl solution (Aldrich Chemical Co., Milwaukee, WI, USA), as described by Zehnder & Wuhrmann (1976) and Kludze et al. (1994) . TiCl3 was added to de-oxygenated 200 mol m−3 sodium citrate (1 : 10 dilution), and the pH of the titanium citrate adjusted to 5·0 with saturated sodium carbonate (≈ 1 : 2 dilution). Various quantities of the stocks were then added to a pH 6·0 nutrient solution, prepared as described by Brix et al. (1994) , to produce a range of titrant solutions with different redox potentials (Eh). The solutions had slightly different final Ti3+ concentrations, and required independent calibration by addition of known amounts of oxygen ( Kludze et al. 1994 ).

Apparatus

The apparatus used for measuring root oxygen release is shown in Fig. 1, demonstrating the principle of the auto-titration with Ti citrate. A root chamber (750 cm3), filled with the Ti citrate nutrient solution, was immersed in a water bath for temperature control, and additional titrant pumped from a reservoir to maintain a constant redox potential in the chamber. Avoiding contamination of the solution by external oxygen is always of prime importance in studies of oxygen transport by plants, so the perspex root chamber was fully immersed in the water bath, and the reservoir was glass. All tubing was of high-density cross-linked polyethylene, except for a short Tygon section in the pump. Particular care was taken to avoid leaks through the seal in the collar of the chamber where plants were mounted. The shoot-root junction was wrapped with Parafilm and enclosed in a block of sponge plastic to protect it from mechanical damage, this then being wrapped in Parafilm and sealed into the collar with Terostat sealing compound. Preliminary tests showed that there was no significant oxygen leakage through this seal, and transpiration by the plants (< 5 cm3 per plant per day) had no effect on the redox potential of the solutions.

image

Figure 1. Schematic diagram of the titration system used for ROL measurement. 1, titrant reservoir containing titanium citrate solution; 2, peristaltic pump; 3, acrylic root chamber mixed by stirring bar (4) with vent (5) to equalise pressure; 6, platinum and calomel electrodes; 7, redox stat; 8, collar and seal (plant not shown).

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The Eh of the solution in the chamber was measured with a Radiometer P1040 platinum electrode and K4040 calomel reference electrode, calibrated in pH 4·0 and 7·0 quinhydrone buffers. The signal from these electrodes was monitored by a Knick 70 process pH/mV meter, which also controlled the titration via a Struer peristaltic pump. The signal from the electrodes and the pump were recorded on a personal computer with Labtech Notebook datalogging software (Laboratory Technologies Corporation, Andover, MA, USA). The pump delivered a signal of 0 V when inactive and 7·6 V when active, and the software averaged the signals every 50 s.

Operation of the chamber

ROL was measured in this apparatus in two ways: closed experiments, and open titration. The two designs are complementary, providing different types of information about ROL. Each plant had its ROL measured first in a closed experiment, and then by open titration. In both cases the solution in the chamber was gently mixed throughout to prevent gradients in Ti3+ concentration developing around the roots. The consequent minimization of unstirred layers outside the roots means that the ROL rates measured here may be somewhat higher than in soil, although probably by no more than ≈ 10% in light of earlier comparisons of ROL between stirred and unstirred conditions ( Caffrey & Kemp 1991). In closed experiments, the peristaltic pump was switched off, the volume of nutrient solution in contact with the roots was constant, and ROL was determined from the rate of change of the Eh reading (mV h−1) as the roots oxidized the solution. This was converted to an oxygen release rate from calibrations performed by injecting known volumes of oxygen-saturated water (1·42 mol O2 m−3) into aliquots of the experimental solution and measuring the subsequent increase in Eh ( Sorrell et al. 1993 ). ROL was measured in solutions with initial Eh values at c.– 200 mV, and was followed until all the Ti3+ ions had been oxidized (Eh = + 250 to + 300 mV). In open titration the system operated as a redox-stat, with the pump delivering fresh Ti citrate from the reservoir to maintain a constant set redox potential in the chamber. The set redox potentials in these experiments were either Eh = − 30 mV or − 40 mV, chosen to avoid the very high Ti3+ concentrations required for closed experiments. The Ti3+ ion reacts with oxygen by a first-order reaction ( Zehnder & Wuhrmann 1976), so it was necessary to program the process controller with time lags and to only pump for short periods, to avoid over-titration. These periods varied depending on the strength of the titrant and the redox end-point set in the chamber.

Data manipulation and calculation of radial oxygen loss rates

In closed measurements, time courses of redox potential versus time allowed ROL to be calculated from the known strength of the titanium citrate buffer and its response to oxygen ( Sorrell et al. 1993 ). In open titration, ROL was calculated from the record of pumping activity. This appeared as a saw-tooth profile ( Fig. 2), due to the pump being programmed to avoid over-titration. The amplitude of the saw-tooth movements in Fig. 2 is an artefact of the number of readings averaged by the datalogger; the actual amount of titrant delivered is given by the mean pump activity, which was proportional to ROL only when titrating near the end-point under steady-state conditions. For any particular setting of the process meter, this mean voltage was proportional to the rate of titrant delivery, which in turn was proportional to ROL.

image

Figure 2. Example of the saw-tooth profile recorded when the peristaltic pump is operating at constant maximum activity (control experiment, titrant containing 1·7 mol Ti3+ m−3 and pumping into a de-oxygenated nutrient solution with Eh set point = − 40 mV). Rate of titrant delivery is calculated from the mean pumping activity (heavy line, a moving average of the profile).

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RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Control experiments

In closed measurements, the redox potential in the chamber remained stable during control experiments without plants, once the titanium citrate buffer had equilibrated with the nutrient solution, indicating that oxygen leakage into the chamber was not a problem. In control experiments with flow-through titration, the pump delivered titanium citrate into the chamber until the set-point had been reached, and Eh then remained stable with no further pumping activity.

Closed measurements

Plants released oxygen into the titanium citrate buffer in the closed experiments, oxidizing the Ti3+ ions so that Eh increased over time ( Fig. 3). After an initial lag phase of 0·5–1 h (omitted from Fig. 3 for clarity), rates of Eh increase in the buffer remained constant over most of the Eh range examined. The only significant deviations from linearity occurred when most of the Ti3+ ions had been oxidized and the oxygen demand of the solution was relatively low. In all experiments the rate of Eh increase, equivalent to ROL, began decreasing as the Eh reading came within 100–150 mV of full oxidation, which was between Eh = + 200 – + 300 mV. Hence, ROL in this solution was largely unaffected by the sink strength of the external solution over moderate to strong oxygen demands, becoming saturated at quite low oxygen demands. Roots did not release oxygen in fully oxidized buffers with all the Ti3+ converted to Ti4+, as the roots were the only oxygen sink in these solutions. Mean ROL values for three plants, measured over the linear part of the traces, were 18·9 ± 4·7 × 10−6 mol O2 h−1 plant−1 for J. effusus and 8·0 ± 2·3 × 10−6 mol O2 h−1 plant−1 for J. inflexus.

image

Figure 3. Time course of closed measurement of ROL from a J. inflexus plant, based on the increase in Eh over time. Dotted lines used to calculate ROL rate as the solution becomes fully oxidized (+ 255 mV). Shoots in the light throughout.

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Titrimetric measurements

The flow-through titration system was developed to allow measurements of ROL under a constant oxygen demand, rather than the continuously increasing Eh of the closed measurements. Some preliminary experiments were begun with de-oxygenated nutrient solution in the chamber, and the titrator then operated to gradually reduce Eh until it reached the end-point, where a stable reading was achieved in equilibrium with the titration ( Fig. 4). In these experiments the initial period where the pump operated at its maximum programmed activity represents the gradual increase in the Ti3+ concentration of the chamber, until the set point Eh was reached (– 40 mV in Fig. 4). However, because of the long time needed to achieve equilibration, in most experiments an initial aliquot of stock titanium citrate was added to the chamber at time 0. The subsequent lower pumping activity is proportional to ROL, the mean pumping activity representing the integration of saw-tooth profiles such as that shown in Fig. 2.

image

Figure 4. Gradual titration (heavy line) of Eh (fine line) down to set-point (− 40 mV) at maximum pumping activity (28 cm3 titrant h−1), and subsequent steady-state titration of ROL for a J. inflexus plant. Mean titration calculated from saw-tooth profile as shown in Figure 2. Shoots in the light throughout, ROL = 8·3 × 10−6 mol O2 h−1 plant−1 at Eh = − 40 mV.

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In all experiments with plants in the chamber there was a positive pumping activity under steady-state conditions due to ROL. Ideally one would hope to have a precisely stable Eh reading at the set point and all variations in pumping activity to be responses to ROL. In practice, the pumping delays introduced by the need to avoid over-titration allowed Eh to fluctuate slightly, increasing a little due to ROL and then decreasing again once the pump responded and delivered more Ti3+ into the chamber ( Fig. 5). Eh therefore remained slightly higher than the set point during most experiments, fluctuating ±≈ 5 mV under steady-state conditions, although short-lived larger deviations could occur when conditions were changed (e.g. in the dark-light-dark transitions shown in Fig. 5). Apart from these small fluctuations, plants continued to release oxygen at extremely constant rates in this apparatus for several hours. ROL rates measured in the flow-through apparatus were similar to those in the closed experiments ( Table 1).

image

Figure 5. Steady-state radial oxygen loss from a J. inflexus plant, showing response to illumination and darkening of the shoots. Eh set point = − 30 mV, titration of ROL (heavy line) in dynamic equilibrium with Eh (fine line). Mean titration calculated from saw-tooth profile as shown in Figure 2.

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Table 1.  Steady-state radial oxygen loss by roots of the two Juncus species in titrimetric assays, with the end point Eh set at − 30 mV and the shoots in the light. Mean values of 3 plants, SEM always < 20% of mean. In all cases, the species differ significantly at P = 0·05 (t-test, P = 0·05)
Radial oxygen loss (10−6 mol h−1) J. effususJ. inflexus
Total release (plant−1) 16·16·8
Per root (root−1) 0·950·45
Root dry weight basis (g−1) 14·58·1

The stability of steady-state readings in the flow-through apparatus allowed ready identification of changes in ROL due to external conditions, and revealed differences in the responses of the two species. In J. inflexus, illuminating and darkening the shoots had little overall effect on ROL, apart from the short-lived transients discussed above, and there was no significant difference between the two treatments under steady-state conditions. In J. effusus, however, ROL was higher in the light than in the dark ( Fig. 6).

image

Figure 6. Steady-state radial oxygen loss from a J. effusus plant, showing response to illumination and darkening of the shoots. Eh set point = − 30 mV, titration of ROL (heavy line) in dynamic equilibrium with Eh (fine line). Mean titration calculated from saw-tooth profile as shown in Figure 2.

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ROL and root morphology

Table 1 shows how J. effusus had a higher ROL rate than J. inflexus, whether expressed per plant, per root, or per unit dry weight. Both species produced similar numbers of roots during growth ( Table 2), confirming that individual J. effusus roots had higher ROL rates than individual J. inflexus roots. Explaining this difference conclusively is not possible without more detailed analyses of the arrangement and intensity of internal respiratory sinks and permeability of the tissues. However, Table 2 shows how J. inflexus roots were shorter than those of J. effusus, but were also thicker and more porous and hence had a lower resistance to diffusion. As there was very little lateral root production in either species, total oxygen fluxes would be expected to be greater in J. inflexus than J. effusus. This comparison suggests that J. inflexus is better able to conserve oxygen internally than J. effusus, while the latter releases a greater proportion of the oxygen it transports.

Table 2.  Morphology of roots from plants used in ROL assays, including relationship between weight and length. Mean values of 3 plants ± SEM. † = species significantly different (t-test, P = 0·05)
 J. effususJ. inflexus
Number of roots per plant17 ± 515 ± 3
Root length (mm)†103 ± 4460 ± 26
Root diameter (mm)† 1·1 2·15
Total root dry weight (g)† 0·111 0·203
Weight length−1 (mg mm−1)† 1·08 3·38
mg dry wt root−1 6·613·5
Root porosity (%)†39 + 451 ± 1

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The roots of the two Juncus species studied here showed ROL responses supporting the first hypothesis guiding this study. ROL increased as the external oxygen demand increased, but this response was quickly saturated at higher demands. These results are consistent with differences in ROL between plants grown in sediments with different oxygen demands. For example, in Spartina patens (Aiton) Muhlenb., plants grown at Eh = + 200 mV have much lower ROL than plants grown at Eh = − 200 mV, but there is little difference in ROL between plants grown at Eh = − 200 and − 300 mV ( Kludze & DeLaune 1994). This may be partly due to plants grown in aerated conditions having less aerenchyma and therefore more living cells, favouring internal consumption ( Justin & Armstrong 1987). However, a similar response is seen in individual plants, where ROL rates are constant over a wide Eh range, only responding to Eh at relatively low external demands ( Sorrell et al. 1993 ; Sorrell & Armstrong 1994; Fig. 3 of this study). The most likely explanation of this saturation of ROL is that the root surface area available for diffusion quickly becomes the factor limiting the flux, rather than the external demand. In many wetland plants, the root surface area that is permeable to oxygen is only a very small proportion of the whole, due to the extensive epidermal and hypodermal lignification that conserves oxygen internally and directs it toward the root tip ( Armstrong & Armstrong 1988; Sorrell 1994).

It is therefore not surprising that roots do not continue to increase their ROL rate indefinitely as the external demand increases. The two Juncus species studied here have prominent lignification of their stelar tissues and endodermis as well as the hypodermis and epidermis, all of which conserve oxygen internally and limit the sites of oxygen release ( Justin & Armstrong 1987), consistent with their ROL being rapidly saturated by the external demand. By the same argument, ROL in species not capable of hypodermal and epidermal lignification ought to con- tinue increasing over a wider range of external demands, although this has not been directly tested. However, in Rumex species, which lose oxygen indiscriminately over the entire root surface, ROL is higher in flood-tolerant taxa than flood-intolerant taxa ( Laan et al. 1989 ). In this case, the lower internal resistances of the roots of the flood- tolerant species to oxygen diffusion may follow directly through to ROL, in contrast to plants that can control the location of their oxygen loss, which may have high internal diffusion rates and efficient aeration, but maintain a low ROL ( Smits et al. 1990 ).

Another implication of the control of root wall permeability is that any expression of ROL rates on area, dry wt., or length bases is probably a rather coarse way of comparing entire root systems. The most appropriate expression will depend on the aim of the comparison and the morphology of the species under consideration. The release per root or the total release from similar-sized plants may be the most useful way of making interspecific comparisons in rhizosphere oxidation capacity. The rate of release per root may be the expression that is most directly determined by the root attributes that control internal diffusion rates (diameter, porosity, permeability and respiratory demand), as these are the anatomical and metabolic parameters that vary in response to growth conditions ( Penhale & Wetzel 1983; Laan, Clement & Blom 1991; van Wijck, de Groot & Grillas 1992).

Although this study has shown that increasing the external oxygen demand from moderate to very high intensities does not greatly increase overall ROL rates, it could still damage roots in other ways. High external demands may divert internal oxygen fluxes away from the primary apex to laterals, leading to oxygen stress in the distal tissues, or from stelar tissues. This is likely to be responsible for the anaerobic metabolism common in roots of many wetland plants ( Burdick & Mendelssohn 1990; Pezeshki & DeLaune 1990) and, if conditions are intensely reducing, can deprive the apical meristem of oxygen and kill the root (e.g. Armstrong, Armstrong & van der Putten 1996). High external demands may also consume the oxygen that is released more quickly, inhibiting the ability of ROL to form an oxidized rhizosphere ( Flessa & Fischer 1992), and making roots more susceptible to soil phytotoxins such as sulphides and organic acids. Entry of phytotoxins into roots in turn further impedes aeration and oxygen transport as it induces internal callus growth ( Armstrong et al. 1996 ), all of which underlines how intimately aeration and ROL are linked to survival and growth of roots.

The advantage of the flow-through system used here was its ability to produce prolonged ROL measurements at a constant external oxygen demand, where effects such as light versus dark treatments could be examined without Eh increasing in the solution during the experiment. Another advantage was that the long-term measurements could be made in solutions that were only slightly reducing, avoiding any possible stress that plants could suffer in the necessarily stronger solutions used in closed measurements. However, it proved less useful for examining the effect of different external oxygen demands, for the times needed for re-equilibration to a different Eh set point were impractically long if over-titration was to be avoided. The flow-through and closed experiments were therefore complementary in revealing how ROL is controlled in plants.

The flow-through experiments also supported the second hypothesis of this study, that roots continue releasing oxygen at a constant rate when the external demand is constant. Although titanium citrate and similar in vitro methods can never be a perfect sediment analogue, lacking as they do the microbial and chemical complexity of soils ( Kludze & DeLaune 1996), they do allow the role of oxygen transport and ROL to be isolated and identified. The implication of these titrimetric measurements is that roots continue to release oxygen into the surrounding sediment provided an external sink persists. Although the formation of the oxidized rhizosphere by roots as they grow through sediment may result in a shallower gradient immediately outside the root surface, the infinite sink of the bulk sediment will still allow oxygen release to persist indefinitely, albeit more slowly. Under natural conditions, sediment respiration will therefore continue to account for a considerable proportion of the oxygen transported in roots, because of the sink effect ( Howes & Teal 1994; Sorrell & Armstrong 1994). The importance of this root oxygen release for nutrient cycling in flooded sediments is evident in the high activity of aerobic biogeochemical processes in rhizospheres of aquatic plants ( Bosse & Frenzel 1997; Lombardi, Epp & Chanton 1997), including J. effusus ( Roden & Wetzel 1996).

The titrimetric method was also particularly useful for identifying differences in oxygen transport physiology between the two species, such as the presence of light-dark differences in ROL in J. effusus but not in J. inflexus that are probably related to stomatal behaviour. Both species are well-adapted for oxygen transport, with high root porosities that are constitutive, rather than environmentally induced, and low permeabilities to oxygen over much of their root surface ( Justin & Armstrong 1987). This suggests that the longer root lengths produced by J. effusus in this study are probably not per se the explanation for its higher ROL. A greater proportion of its root surface may be more oxygen-permeable than J. inflexus, which is consistent with the latter apparently being capable of longer root growth in flooded soils ( Justin & Armstrong 1987).

This study has provided an alternative method for measuring ROL by plant roots and provided new insights to the control of ROL in the atmosphere-plant-sediment continuum. The titrimetric approach is complementary to closed-volume measurements of earlier studies, providing different information about ROL. The results are consistent with earlier findings of significant oxygen releases from roots into reducing rhizospheres, and support the concept that the external oxygen demand is important for providing the driving force for ROL ( Armstrong & Beckett 1987; Sorrell & Armstrong 1994).

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The author thanks Bent Lorenzen for technical assistance in assembling the titrimetric system. This study was financed by the Danish Natural Science Research Council, Project no. 11–0074–1 ‘Ecophysiology of Aquatic Macrophytes’. H. Brix and B.J. Biggs kindly provided useful comments on an earlier draft of the manuscript.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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  • Justin S.H.F.W. & Armstrong W. (1987) The anatomical characteristics of roots and plant response to soil flooding. New Phytologist 106, 465 495.
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  • Kludze H. & DeLaune R.D. (1996) Soil redox intensity effects on oxygen exchange and growth of cattail and sawgrass. Soil Science Society of America Journal 60, 616 621.
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