336
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
of xanthophyll and carotenoid pigment; whereas, for
T. testudinum the F v /F m ratio declines at a rate of
1.5% cm
−1 along the blade (Enr´ ıquez et al. (2002)).
This gradation was attributed to down-regulation
occurring in the 4–8 cm region, whilst photodamage (photoinhibition) occurred in section >11 cm
from the meristem. Treatment with chloramphenicol confirmed that the apical region had limited
capacity for protein synthesis during high light
exposure.
Another way to study photosynthetic effects by
fluorescence, apart from PAM fluorometry, is to
study the so-called O-J-I-P fluorescence rise kinetics, i.e. the initial rise in fluorescence upon turning
on actinic light. In theory, the O-J-I-P polyphasic
transients can be used to follow the sequential reduction of PSII internal electron acceptors. Enr´ ıquez
et al. (2002) studied fluorescence rise kinetics in T.
testudinum using the Hansatech PEA fluorometer.
It was found that apical regions had an elevated J
transient, which was taken to indicate a decrease in
the Q
−
A reoxidation capacity of these older sections
of the leaf. This is one characteristic of photoinhibition, where, in contrast to UV-B inhibition, the
acceptor side of PSII is inhibited; however, it could
also be linked to non-photochemical quenching (a
photoprotective mechanism). The powerful tool of
fluorescence induction will undoubtedly provide insights into the dynamic processes of photosynthesis.
However, more work is needed to address its use in
seagrasses.
I. Comparison of Fluorescence and O 2
Evolution-Derived Rates of Photosynthesis
Fluorescence measures only photon-driven electron
transport through PSII, which should be linearly
correlated with O 2 evolution. It should also be directly correlated with the rate of photosynthesis,
since NADP
+ reduction depends on linear electron
transport through PSII and then PSI. However, both
electron transport and oxygen exchange are complicated by several competing processes, including
cyclic electron transport within PSII, photorespiration and the Mehler Ascorbate Peroxidase (MAP;
O 2 -consuming) pathway (Fig. 2). Therefore, chlorophyll a fluorescence techniques cannot easily be
compared with O 2 evolution. C. nodosa and H.
ovalis, have been shown to have a linear relationship between ETR and O 2 evolution (Beer et al.,
1998, Beer and Bj¨ ork, 2000). However, it is more
common for a curvilinear relationship to exist between photosynthesis as estimated by fluorescence
and oxygen-evolution techniques (Beer et al. 1998;
Ralph and Burchett, 1995; Beer and Bj¨ ork, 2000).
In algae, where these two techniques have been
widely compared, it has been shown that there is
probably a cycle of electrons around PSII under high
irradiances (Franklin and Badger, 2001; Longstaff
et al., 1999) and therefore PAM-derived ETRs only
correlate with O 2 -derived rates at low to moderate
irradiances (Longstaff et al., 2002). The relationship becomes increasingly non-linear at irradiances
above saturation. Above saturation, a range of possible uncoupling processes may be influencing this
response such as photorespiration, MAP pathway
reactions and photoinhibition (Fig. 2). Photorespiration has been suggested as causing the divergence
from linearity for several species H. stipulacea and
Z. marina. On the other hand C. nodosa showed a
linear response and it was suggested that this seagrass may have developed a carbon concentration
mechanism (Beer et al. 1998). However, it likely
that most seagrasses have a CCM mechanism (Section IV) and thus some other factor would have
to be invoked. The MAP pathway is largely unexplored in seagrasses (see section VII) which is regrettable since, with the high O 2 levels generated
due to the deep diffusion boundary layers at the
seagrass leaf surface, high activity of this pathway
might be predicted (Miyake and Asada 2003). Thus,
the results from seagrasses appear to mirror those
found for algae (above) indicating more research
is needed to detail the mechanisms involved in the
non-linearity between oxygen and fluorescence in
seagrasses.
Perhaps the critical limitation of correlating fluorescence and oxygen evolution is the assessment of
the light absorption by the leaf, which is fundamental
in deriving ETR from PAM data. Beer et al. (1998)
recommended that a more accurate method of estimating the absorptance should be found. Numerous
attempts have been made to estimate the absorption
of light (Enr´ ıquez, et al., 1992) (as well as reflection
and transmission); however, until recently no single method has provided an adequate estimate (eg.
see Beer and Bj¨ ork, 2000). Schwarz and Hellblom
(2002) recently demonstrated that the fraction of absorbed light varies with depth, therefore questioning the merit of accurately determining the absorption for each species if it also needs to correct for
depth. A new instrument called the Imaging-PAM
(Walz Effeltrich, Germany) has the capacity to measure absorbtivity, where the relative absorption of red
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
of xanthophyll and carotenoid pigment; whereas, for
T. testudinum the F v /F m ratio declines at a rate of
1.5% cm
−1 along the blade (Enr´ ıquez et al. (2002)).
This gradation was attributed to down-regulation
occurring in the 4–8 cm region, whilst photodamage (photoinhibition) occurred in section >11 cm
from the meristem. Treatment with chloramphenicol confirmed that the apical region had limited
capacity for protein synthesis during high light
exposure.
Another way to study photosynthetic effects by
fluorescence, apart from PAM fluorometry, is to
study the so-called O-J-I-P fluorescence rise kinetics, i.e. the initial rise in fluorescence upon turning
on actinic light. In theory, the O-J-I-P polyphasic
transients can be used to follow the sequential reduction of PSII internal electron acceptors. Enr´ ıquez
et al. (2002) studied fluorescence rise kinetics in T.
testudinum using the Hansatech PEA fluorometer.
It was found that apical regions had an elevated J
transient, which was taken to indicate a decrease in
the Q
−
A reoxidation capacity of these older sections
of the leaf. This is one characteristic of photoinhibition, where, in contrast to UV-B inhibition, the
acceptor side of PSII is inhibited; however, it could
also be linked to non-photochemical quenching (a
photoprotective mechanism). The powerful tool of
fluorescence induction will undoubtedly provide insights into the dynamic processes of photosynthesis.
However, more work is needed to address its use in
seagrasses.
I. Comparison of Fluorescence and O 2
Evolution-Derived Rates of Photosynthesis
Fluorescence measures only photon-driven electron
transport through PSII, which should be linearly
correlated with O 2 evolution. It should also be directly correlated with the rate of photosynthesis,
since NADP
+ reduction depends on linear electron
transport through PSII and then PSI. However, both
electron transport and oxygen exchange are complicated by several competing processes, including
cyclic electron transport within PSII, photorespiration and the Mehler Ascorbate Peroxidase (MAP;
O 2 -consuming) pathway (Fig. 2). Therefore, chlorophyll a fluorescence techniques cannot easily be
compared with O 2 evolution. C. nodosa and H.
ovalis, have been shown to have a linear relationship between ETR and O 2 evolution (Beer et al.,
1998, Beer and Bj¨ ork, 2000). However, it is more
common for a curvilinear relationship to exist between photosynthesis as estimated by fluorescence
and oxygen-evolution techniques (Beer et al. 1998;
Ralph and Burchett, 1995; Beer and Bj¨ ork, 2000).
In algae, where these two techniques have been
widely compared, it has been shown that there is
probably a cycle of electrons around PSII under high
irradiances (Franklin and Badger, 2001; Longstaff
et al., 1999) and therefore PAM-derived ETRs only
correlate with O 2 -derived rates at low to moderate
irradiances (Longstaff et al., 2002). The relationship becomes increasingly non-linear at irradiances
above saturation. Above saturation, a range of possible uncoupling processes may be influencing this
response such as photorespiration, MAP pathway
reactions and photoinhibition (Fig. 2). Photorespiration has been suggested as causing the divergence
from linearity for several species H. stipulacea and
Z. marina. On the other hand C. nodosa showed a
linear response and it was suggested that this seagrass may have developed a carbon concentration
mechanism (Beer et al. 1998). However, it likely
that most seagrasses have a CCM mechanism (Section IV) and thus some other factor would have
to be invoked. The MAP pathway is largely unexplored in seagrasses (see section VII) which is regrettable since, with the high O 2 levels generated
due to the deep diffusion boundary layers at the
seagrass leaf surface, high activity of this pathway
might be predicted (Miyake and Asada 2003). Thus,
the results from seagrasses appear to mirror those
found for algae (above) indicating more research
is needed to detail the mechanisms involved in the
non-linearity between oxygen and fluorescence in
seagrasses.
Perhaps the critical limitation of correlating fluorescence and oxygen evolution is the assessment of
the light absorption by the leaf, which is fundamental
in deriving ETR from PAM data. Beer et al. (1998)
recommended that a more accurate method of estimating the absorptance should be found. Numerous
attempts have been made to estimate the absorption
of light (Enr´ ıquez, et al., 1992) (as well as reflection
and transmission); however, until recently no single method has provided an adequate estimate (eg.
see Beer and Bj¨ ork, 2000). Schwarz and Hellblom
(2002) recently demonstrated that the fraction of absorbed light varies with depth, therefore questioning the merit of accurately determining the absorption for each species if it also needs to correct for
depth. A new instrument called the Imaging-PAM
(Walz Effeltrich, Germany) has the capacity to measure absorbtivity, where the relative absorption of red