268
Jens Borum, Kaj Sand-Jensen, Thomas Binzer, Ole Pedersen and Tina Maria Greve
to conduct and interpret due to the existence of several sources and sinks of oxygen driving internal
transport at different rates depending on conditions
in the water column and sediment. Oxygen release
and transport in seagrasses have mostly been assessed by measuring oxygen changes in incubation
chambers, but oxygen variability within the plants
can be assessed at much higher spatial and temporal
resolution using microelectrodes. Also planar optodes could be applied to describe oxygen release
to the rhizosphere at higher spatial and temporal
resolution.
Quantitatively, photosynthetic oxygen evolution
is the most important source of oxygen for internal
transport and aerobic metabolism, but passive diffusion of oxygen from the water column to leaves
and below-ground tissues during darkness is also
important, and this source is necessary for maintaining the oxygen supply to roots and rhizomes during
dark periods of more than 1–2 h. The largest loss
of oxygen from seagrasses is from leaves to water
column during periods of high light and photosynthesis, but the continuous leakage of oxygen from
roots and rhizomes to the anoxic sediment both during light and dark periods also represents a major
sink to plant oxygen. During high photosynthesis
and active plant growth, the respiratory oxygen consumption is much lower than oxygen release to the
external media. However, the relative importance
of respiration increases markedly with increasing
temperature or at times of low photosynthetic rates.
The internal transport of oxygen between leaves and
below-ground tissues is most likely to be primarily
driven by passive diffusion within the air-filled lacunae. Pressurization, however, does occur and may account for some internal oxygen transport especially
during transient shifts between light and darkness.
There is a need for more direct measurements of
oxygen traveling velocities to elucidate the roles of
passive diffusion vs. pressurization under different
environmental conditions, and the possible coupling
between internal oxygen dynamics and seagrass dieoffs deserves further investigation.
Acknowledgments
This work has been partially funded by the EU
(EVK3-CT-2000-00044) and the Danish Research
Agency (&41-00-49.66). We thank Peter Larsen for
allowing us to present unpublished data on the thickness of diffusive boundary layers. We wish to thank
the reviewers, W. Armstrong, W.M. Kemp and B.
Sorrell, for their helpful comments and valuable suggestions.
References
Angelstein U (1910) Uber die Kohlens¨ aureassimilation submerser Wasserpflanzen in Bikarbonat- und Karbonatl¨ osungen.
Beitrage Biologie Pflanzen 10: 87–117
Armstrong W (1971) Radial oxygen losses from intact rice roots
as affected by distance from the apex, respiration and water
logging. Physiologia Plantarum 25: 192–197
Armstrong W (1979) Aeration in higher plants. Adv Botanical
Res 7: 225–332
Armstrong J and Armstrong W (1990) Light-enhanced convective through flow increases oxygenation in rhizomes and rhizosphere of Phragmites australis (Cav.) Trin. Ex Steud. New
Phytol 114: 121–128
Armstrong J, Armstrong W and Beckett PM (1992) Phragmites
australis: Venturi- and humidity-induced convections enhance
rhizome aeration and rhizosphere oxidation. New Phytol 120:
197–207
Armstrong J, Armstrong W, Beckett PM, Halder JE, Lythe S,
Holt R and Sinclair A (1996) Pathways of aeration and the
mechanisms and beneficial effects of humidity- and venturiinduced convections in Phragmitis australis (Cav.) Trin. ex
Steud. Aquat Bot 54: 177–198
Armstrong W, Cousins D, Armstrong J, Turner DW and Beckett
PM (2000) Oxygen distribution in wetland plant roots and
permeability barriers to gas-exchange with the rhizosphere: A
microelectrode and modelling study with Phragmitis australis.
Ann Bot 86: 687–703
Armstrong W, Strange ME, Cringle S and Beckett PM (1994)
Microelectrode and modelling study of oxygen distribution in
roots. Ann Bot 74: 287–299
Borum J (1985) Development of epiphytic communities on eelgrass (Zostera marina) along a nutrient gradient in a Danish
estuary. Mar Biol 87: 211–218
Borum J, Pedersen O, Greve TM, Frankovich TA, Zieman JC,
Fourqurean JW and Madden C (2005) The potential role
of plant oxygen and sulphide dynamics in die-off events
in tropical seagrass, Thalassia testudinum. J Ecol 93: 148–
158
Bowes G (1985) Pathways of CO 2 fixation by aquatic organisms.
In: Lucas WJ and Berry JA (eds) Inorganic Carbon Uptake by
Aquatic Photosynthetic Organisms, pp 187–210. American
Society of Plant Physiology, Rockville, MD, USA
Bowling DJF (1973) Measurement of gradient of oxygen partial
pressure across the intact root. Planta 111: 323–328
Brix H, Sorrell BK and Orr PT (1992) Internal pressurization and
convective gas flow in some emergent freshwater macrophytes.
Limnology Oceanography 37: 1420–1433
Caffrey JM and Kemp WM (1990) Nitrogen cycling in sediments with estuarine populations of Potamogeton perfoliatus
and Zostera marina. Mar Ecol Prog Ser 66: 147–160
Caffrey JM and Kemp WM (1991) Seasonal and spatial patterns
of oxygen production, respiration and root-rhizome release in
Jens Borum, Kaj Sand-Jensen, Thomas Binzer, Ole Pedersen and Tina Maria Greve
to conduct and interpret due to the existence of several sources and sinks of oxygen driving internal
transport at different rates depending on conditions
in the water column and sediment. Oxygen release
and transport in seagrasses have mostly been assessed by measuring oxygen changes in incubation
chambers, but oxygen variability within the plants
can be assessed at much higher spatial and temporal
resolution using microelectrodes. Also planar optodes could be applied to describe oxygen release
to the rhizosphere at higher spatial and temporal
resolution.
Quantitatively, photosynthetic oxygen evolution
is the most important source of oxygen for internal
transport and aerobic metabolism, but passive diffusion of oxygen from the water column to leaves
and below-ground tissues during darkness is also
important, and this source is necessary for maintaining the oxygen supply to roots and rhizomes during
dark periods of more than 1–2 h. The largest loss
of oxygen from seagrasses is from leaves to water
column during periods of high light and photosynthesis, but the continuous leakage of oxygen from
roots and rhizomes to the anoxic sediment both during light and dark periods also represents a major
sink to plant oxygen. During high photosynthesis
and active plant growth, the respiratory oxygen consumption is much lower than oxygen release to the
external media. However, the relative importance
of respiration increases markedly with increasing
temperature or at times of low photosynthetic rates.
The internal transport of oxygen between leaves and
below-ground tissues is most likely to be primarily
driven by passive diffusion within the air-filled lacunae. Pressurization, however, does occur and may account for some internal oxygen transport especially
during transient shifts between light and darkness.
There is a need for more direct measurements of
oxygen traveling velocities to elucidate the roles of
passive diffusion vs. pressurization under different
environmental conditions, and the possible coupling
between internal oxygen dynamics and seagrass dieoffs deserves further investigation.
Acknowledgments
This work has been partially funded by the EU
(EVK3-CT-2000-00044) and the Danish Research
Agency (&41-00-49.66). We thank Peter Larsen for
allowing us to present unpublished data on the thickness of diffusive boundary layers. We wish to thank
the reviewers, W. Armstrong, W.M. Kemp and B.
Sorrell, for their helpful comments and valuable suggestions.
References
Angelstein U (1910) Uber die Kohlens¨ aureassimilation submerser Wasserpflanzen in Bikarbonat- und Karbonatl¨ osungen.
Beitrage Biologie Pflanzen 10: 87–117
Armstrong W (1971) Radial oxygen losses from intact rice roots
as affected by distance from the apex, respiration and water
logging. Physiologia Plantarum 25: 192–197
Armstrong W (1979) Aeration in higher plants. Adv Botanical
Res 7: 225–332
Armstrong J and Armstrong W (1990) Light-enhanced convective through flow increases oxygenation in rhizomes and rhizosphere of Phragmites australis (Cav.) Trin. Ex Steud. New
Phytol 114: 121–128
Armstrong J, Armstrong W and Beckett PM (1992) Phragmites
australis: Venturi- and humidity-induced convections enhance
rhizome aeration and rhizosphere oxidation. New Phytol 120:
197–207
Armstrong J, Armstrong W, Beckett PM, Halder JE, Lythe S,
Holt R and Sinclair A (1996) Pathways of aeration and the
mechanisms and beneficial effects of humidity- and venturiinduced convections in Phragmitis australis (Cav.) Trin. ex
Steud. Aquat Bot 54: 177–198
Armstrong W, Cousins D, Armstrong J, Turner DW and Beckett
PM (2000) Oxygen distribution in wetland plant roots and
permeability barriers to gas-exchange with the rhizosphere: A
microelectrode and modelling study with Phragmitis australis.
Ann Bot 86: 687–703
Armstrong W, Strange ME, Cringle S and Beckett PM (1994)
Microelectrode and modelling study of oxygen distribution in
roots. Ann Bot 74: 287–299
Borum J (1985) Development of epiphytic communities on eelgrass (Zostera marina) along a nutrient gradient in a Danish
estuary. Mar Biol 87: 211–218
Borum J, Pedersen O, Greve TM, Frankovich TA, Zieman JC,
Fourqurean JW and Madden C (2005) The potential role
of plant oxygen and sulphide dynamics in die-off events
in tropical seagrass, Thalassia testudinum. J Ecol 93: 148–
158
Bowes G (1985) Pathways of CO 2 fixation by aquatic organisms.
In: Lucas WJ and Berry JA (eds) Inorganic Carbon Uptake by
Aquatic Photosynthetic Organisms, pp 187–210. American
Society of Plant Physiology, Rockville, MD, USA
Bowling DJF (1973) Measurement of gradient of oxygen partial
pressure across the intact root. Planta 111: 323–328
Brix H, Sorrell BK and Orr PT (1992) Internal pressurization and
convective gas flow in some emergent freshwater macrophytes.
Limnology Oceanography 37: 1420–1433
Caffrey JM and Kemp WM (1990) Nitrogen cycling in sediments with estuarine populations of Potamogeton perfoliatus
and Zostera marina. Mar Ecol Prog Ser 66: 147–160
Caffrey JM and Kemp WM (1991) Seasonal and spatial patterns
of oxygen production, respiration and root-rhizome release in