330
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
Classically, in this process, phosphoglycolate is produced by the competitive reaction of O 2 with RuDP
catalysed by Rubisco (Lorimer, 1981). From there
glycolate is formed in the chloroplast, by dephosphorylation, and is transported to peroxisomes where it
is oxidised by glycolate oxidase (consuming molecular oxygen), and transaminated to form glycine. Then
with the help of mitochondria the glycine is decarboxylated (with the formation of carbon dioxide) and
the carbon skeleton recycled through the peroxisome
and the chloroplast, finally, being converted to glycerate and glycerate-1-phosphate, which can re-enter
the Calvin-Benson Cycle. A key product is therefore the presence of glycolate. Glycolate is certainly
found in seagrasses using
14 C techniques (Larkum,
unpublished). The presence of peroxisome-like bodies has also been documented (Kuo, 1989). However,
direct quantitative data on the activity of photorespiration in seagrasses is lacking. In theory, if this process is very active it could consume a large amount of
the oxygen produced in photosynthesis. This would
constitute a futile cycle with no competitive advantage, yet it might be an unavoidable process due to
the high O 2 levels in epidermal cells caused by the
leaf anatomy and the surrounding DBL (see Section
III). On the other hand some of the excess O 2 might
be consumed under these circumstances by the operation of a Mehler Ascorbate Peroxidase (MAP) pathway (Fig. 2; Miyake & Asada, 2003). In this process
a Mehler reaction intercepts the electrons coming out
of the reducing side of PSI (which would normally
be delivered to NADP
+ ) as follows
2O 2 + 2e
−
→ 2O
−
2 (non-catalyzed reaction
at reduced ferredoxin)
2O
−
2 + 2H
+
→ O 2 + H 2 O 2
(catalyzed by Fe superoxide dismutase)
The hydrogen peroxide produced is then detoxified
with ascorbate peroxidase:
2H 2 O 2 + ascorbate (oxidised)
→ 2H 2 O + ascorbate (reduced) + O 2
The reduced ascorbate is re-oxidised by a glutathione/NADPH system. Overall, 1 O 2 is reduced
and 2 H 2 O are produced by PS I while in photosynthesis 2 H 2 O are consumed and 1 O 2 are produced by PS II, i.e. there is no net O 2 exchange. Thus
there is no net build up of oxygen. If such a pathway were to be up-regulated during periods of high
electron transport rate (ETR), i.e. under high PAR,
then excess concentrations of O 2 could be avoided.
Whether such a process actually takes place is not
known. There is evidence for such processes in algae (Miyake & Asada, 2003), but an investigation
of the role of competing processes in Ulva lactuca
(Longstaff et al., 2002), under high light, concluded
that this pathway could account for only a small fraction of the flow of electrons and that cycling around
PSII was a more substantive process in discharging
excess energy from absorbed photons. A diagram
to illustrate the competing pathways for photosynthetically generated electrons and energy is given in
Fig. 2.
From this section it can be seen that there is much
still to be done on the photosynthetic metabolism
of seagrasses before we know the various processes
in detail. Work on freshwater aquatics has advanced
much further in the last decade than in seagrasses
(see, eg., Bowes et al., 2002; Maberly and Madsen,
2002).
VIII. Fluorescence Studies
Chlorophyll a fluorescence techniques provide a
very powerful tool to understand a wide range of
photokinetic processes associated with photosynthesis in seagrasses. Several types of fluorometer are
available; these include the Pulse Amplitude Modulated (PAM; Schreiber et al., 1986; Schreiber, in
press), non-modulated PEA (Plant Efficiency Analyser; Strasser et al., 2000) system and the Fast
Repetition Rate (FRR) fluorometer (Gorbunov et al.,
2000). To date, the majority of seagrass fluorescence research has been performed with the DivingPAM and current research is examining such topics as extremes of light adaptation and acclimation
(specifically addressing the questions: how can seagrasses tolerate extended periods of low light and
how can intertidal seagrasses survive photodamage
from excess irradiance) and effects of pollution. Seagrasses generally function as shade-adapted plants.
However, tropical seagrasses can be exposed on a
daily basis to photoinhibitory levels of light (i.e. in
excess of requirements), while for some temperate
seagrasses, usually growing in light-limited environments, there may be severe stress when they are
exposed intertidally on cloudless days in summer.
The use of fluorescence techniques to monitor
anthropogenic stress is dealt with in Ralph et al.,
Chapter 24.
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
Classically, in this process, phosphoglycolate is produced by the competitive reaction of O 2 with RuDP
catalysed by Rubisco (Lorimer, 1981). From there
glycolate is formed in the chloroplast, by dephosphorylation, and is transported to peroxisomes where it
is oxidised by glycolate oxidase (consuming molecular oxygen), and transaminated to form glycine. Then
with the help of mitochondria the glycine is decarboxylated (with the formation of carbon dioxide) and
the carbon skeleton recycled through the peroxisome
and the chloroplast, finally, being converted to glycerate and glycerate-1-phosphate, which can re-enter
the Calvin-Benson Cycle. A key product is therefore the presence of glycolate. Glycolate is certainly
found in seagrasses using
14 C techniques (Larkum,
unpublished). The presence of peroxisome-like bodies has also been documented (Kuo, 1989). However,
direct quantitative data on the activity of photorespiration in seagrasses is lacking. In theory, if this process is very active it could consume a large amount of
the oxygen produced in photosynthesis. This would
constitute a futile cycle with no competitive advantage, yet it might be an unavoidable process due to
the high O 2 levels in epidermal cells caused by the
leaf anatomy and the surrounding DBL (see Section
III). On the other hand some of the excess O 2 might
be consumed under these circumstances by the operation of a Mehler Ascorbate Peroxidase (MAP) pathway (Fig. 2; Miyake & Asada, 2003). In this process
a Mehler reaction intercepts the electrons coming out
of the reducing side of PSI (which would normally
be delivered to NADP
+ ) as follows
2O 2 + 2e
−
→ 2O
−
2 (non-catalyzed reaction
at reduced ferredoxin)
2O
−
2 + 2H
+
→ O 2 + H 2 O 2
(catalyzed by Fe superoxide dismutase)
The hydrogen peroxide produced is then detoxified
with ascorbate peroxidase:
2H 2 O 2 + ascorbate (oxidised)
→ 2H 2 O + ascorbate (reduced) + O 2
The reduced ascorbate is re-oxidised by a glutathione/NADPH system. Overall, 1 O 2 is reduced
and 2 H 2 O are produced by PS I while in photosynthesis 2 H 2 O are consumed and 1 O 2 are produced by PS II, i.e. there is no net O 2 exchange. Thus
there is no net build up of oxygen. If such a pathway were to be up-regulated during periods of high
electron transport rate (ETR), i.e. under high PAR,
then excess concentrations of O 2 could be avoided.
Whether such a process actually takes place is not
known. There is evidence for such processes in algae (Miyake & Asada, 2003), but an investigation
of the role of competing processes in Ulva lactuca
(Longstaff et al., 2002), under high light, concluded
that this pathway could account for only a small fraction of the flow of electrons and that cycling around
PSII was a more substantive process in discharging
excess energy from absorbed photons. A diagram
to illustrate the competing pathways for photosynthetically generated electrons and energy is given in
Fig. 2.
From this section it can be seen that there is much
still to be done on the photosynthetic metabolism
of seagrasses before we know the various processes
in detail. Work on freshwater aquatics has advanced
much further in the last decade than in seagrasses
(see, eg., Bowes et al., 2002; Maberly and Madsen,
2002).
VIII. Fluorescence Studies
Chlorophyll a fluorescence techniques provide a
very powerful tool to understand a wide range of
photokinetic processes associated with photosynthesis in seagrasses. Several types of fluorometer are
available; these include the Pulse Amplitude Modulated (PAM; Schreiber et al., 1986; Schreiber, in
press), non-modulated PEA (Plant Efficiency Analyser; Strasser et al., 2000) system and the Fast
Repetition Rate (FRR) fluorometer (Gorbunov et al.,
2000). To date, the majority of seagrass fluorescence research has been performed with the DivingPAM and current research is examining such topics as extremes of light adaptation and acclimation
(specifically addressing the questions: how can seagrasses tolerate extended periods of low light and
how can intertidal seagrasses survive photodamage
from excess irradiance) and effects of pollution. Seagrasses generally function as shade-adapted plants.
However, tropical seagrasses can be exposed on a
daily basis to photoinhibitory levels of light (i.e. in
excess of requirements), while for some temperate
seagrasses, usually growing in light-limited environments, there may be severe stress when they are
exposed intertidally on cloudless days in summer.
The use of fluorescence techniques to monitor
anthropogenic stress is dealt with in Ralph et al.,
Chapter 24.