全 文 ：Received 1 Dec. 2003 Accepted 17 Mar. 2004
Supported by the State Key Basic Research and Development Plan of China (1998010100) and the National Natural Science Foundation of
China (39870604).
* Author for correspondence. E-mail: .
http://www.chineseplantscience.com
Acta Botanica Sinica
植 物 学 报 2004, 46 (7): 811-818
Enhancement of the Mehler-peroxidase Reaction in
Salt-stressed Rumex K-1 Leaves
CHEN Hua-Xin1, AN Sha-Zhou2, LI Wei-Jun2, GAO Hui-Yuan1*, ZOU Qi1
(1. College of Life Sciences, Shandong Agricultural University, Tai’an 271018, China;
2. Department of Grassland Sciences, Xinjiang Agricultural University, Urumqi 830052, China)
Abstract: The effects of salt stress on photosynthesis, Mehler-peroxidase reaction (MPR) and the
susceptibility of PSⅡ to photoinhibition were investigated in Rumex K-1 leaves. Salt stress resulted in
dramatic decrease in photosynthesis, but had no significant effect on maximal photochemistry of PSⅡ
(Fv/Fm). During photosynthetic induction, a considerable electron flow was transported to oxygen in MPR
both in control and salt-stressed leaves. Under steady state photosynthesis, enhanced electron flow to
oxygen in MPR was observed only in salt-stressed leaves. The enhanced MPR in salt-stressed leave was
accompanied by enhanced activities of scavenging enzymes, i.e. superoxidase dismutase (SOD) and ascor-
bate peroxidase (APX). In the presence of saturating CO2, decreasing oxygen concentration from 21% to
2% did not affect the susceptibility to photoinhibition in control leaves, but largely increased the suscep-
tibility to photoinhibition in salt-stressed leaves. Based on these results, it is concluded that the enhanced
MPR in salt-stressed Rumex leaves serves as a sink to drain the excess electrons off the electron chain
and thus mitigates photoinhibition.
Key words: Rumex ; salt stress; photoinhibition; Mehler-peroxidase reaction; chlorophyll fluorescence
Salt stress is a major environmental factor that limits
crop production. One of the typical responses of a plant to
salt stress is the decrease in photosynthesis. Some studies
have shown that the decrease in photosynthesis is corre-
lated with inhibited PSⅡ activity and electron transport
(Krist, 1990; Mishra et al.,1991), whereas some other stud-
ies have shown that the decrease in photosynthesis is due
to the partial stomatal or the non-stomatal limitation which
is involved in the dark enzymatic processes of CO2
assimilation, such as the decrease in the activity and con-
tent of Rubisco, RuBP or Pi regeneration capacity (Ziska et
al., 1990; Brugnoli and Björkamn, 1992; Delfine et al., 1998).
It is still a matter of debate how salt stress inhibits
photosynthesis.
The decrease in photosynthesis implies decreased de-
mand for reducing equivalent (ATP and NADPH).
Consequently, the absorption of light energy in salt-stressed
leaves exceeds what is required to drive CO2 fixation. Pho-
tosynthetic apparatus might be damaged even under mod-
erate light if the excess light energy can not be dissipated
timely. Protection from photodamage can be accomplished
by a reversible decrease in PSⅡ efficiency as a result of an
increased conversion of excitation energy into heat (Weis
and Berry, 1987; Zhao and Wang, 2002). This process,
termed as non-radiative energy dissipation, presumably
occurs in the antennae and can be measured by non-pho-
tochemical quenching of chlorophyll fluorescence (NPQ).
According to its relaxing upon transferring of illuminated
leaves into darkness, NPQ can be resolved into three
components, i.e. qE, qT and qI (Krause and Weis, 1991).
qE, energy-dependent quenching, accounts for most of
NPQ only under extremely challenging conditions. Its es-
tablishment requires a large transmembrane proton gradi-
ent and is strongly associated with the de-epoxidation state
of xanthophyll cycle (Müller et al., 2001).
The Mehler-peroxidase reaction (MPR) consists of the
Mehler reaction, which is the photoreduction of oxygen by
PSⅠ to a superoxide anion radical, followed by the
dismutation of this superoxide anion radical by superoxidase
dismutase (SOD) to hydrogen peroxide and oxygen. Hy-
drogen peroxide is then reduced by ascorbate peroxidase
(APX) to water, followed by the regeneration of ascorbate
by direct reduction of monodehydroascorbate reductase
(Asada, 1999). The Mehler-peroxidase reaction results in
electron flow from PSⅡ to PSⅠ with no net oxygen
evolution. The Mehler-peroxidase reaction is suggested to
dissipate excess electrons under excess light (Osmond and
Grace, 1995; Asada, 1999). In addition to acting as a sink for
Acta Botanica Sinica 植物学报 Vol.46 No.7 2004812
excess electron, Mehler-peroxidase reaction was also sup-
posed to contribute to generation of transmembrane
gradient, which would initiate NPQ and dissipate excess
energy as heat (Schreiber and Neubauer, 1990; Neubauer
and Yamamoto, 1992). Mehler-peroxidase reaction thus
could decrease the excitation pressure over electron trans-
port chain and minimize the risk of photoinhibition and
photodamage. However, so far there is little evidence ob-
tained for a significant electron flow in MPR in vivo
conditions. Whether or not MPR contribute to generation
of transmembrane proton gradient in vivo is not clear yet.
Rumex K-1, a hybrid of Rumex patientia × R.
tianschaious, is a salt-tolerant fodder crop with a high con-
tent of leaf protein. In northwest of China, this crop is used
in the reclamation of drought and saline soil where plants
experience salinity and high light during the summer. In the
present study, chlorophyll fluorescence, gas exchange and
active oxygen species scavenging enzymes were analyzed
in the leaves of Rumex K-1 seedlings. The aim of this work
is to examine whether or not salt stress led to an increased
electron flow to oxygen in MPR. In addition, if MPR in-
creased considerably in salt-stressed leaves, does it miti-
gate photoinhibition under high light?
1 Materials and Methods
1.1 Plant materials and salt treatment
Seeds of Rumex K-1 were germinated in experimental
field. Seedlings with 3-4 leaves were then transplanted to
pots (15 cm in diameter and in height) filled with sand and
watered daily with 1/2 Hoagland nutrient solution. The
plants were grown in a greenhouse with PPFD of about 800
mmol.m-2.s-1 and temperature of about 25 ℃ during
daytime. A week later, the seedlings were subjected to salt
treatment. Salt concentrations were stepped up in 50 mmol/
L NaCl every 2 d until final concentration (200 mmol/L)
were achieved. NaCl was dissolved in Hoagland nutrient
solution. All measurements were made on the latest fully
expanded leaves two weeks after the final concentration
was achieved.
1.2 Measurement of chlorophyll fluorescence and gas
exchange
Measurement of gas exchange and chlorophyll fluores-
cence were carried out simultaneously. Analysis of gas ex-
change was carried out by a portable photosynthesis sys-
tem (CIRAS-1, PP System, UK). Leaf temperature was kept
at 25 ℃ via temperature control device of the CIRAS-1.
Irradiance was provided by halogen lamp and was adjusted
to 800 mmol. m-2.s-1 via automatic light unit of the CIRAS-
1. Chlorophyll fluorescence was measured with a portable
pulse-modulated fluorometer (FMS2, Hansatech, UK) by
placing optic detecting head in the automatic leaf cuvette
of the CIRAS-1 with an angle of 45°, according to the meth-
ods of Lu and Zhang (1998a). The minimal fluorescence
(Fo) was determined by a weak modulated measuring light.
The maximal fluorescence (Fm) with all PSⅡ reaction cen-
ters closed was determined by a 0.8-s saturating light (7 000
mmol. m-2.s-1). The steady-state fluorescence (Fs) was
measured continuously and saturating light was given at
an interval of 120 s to determine maximal fluorescence in
the light-adapted state (Fm). The quantum yield of PSⅡ
electron transport (PSⅡ) was calculated as: PSⅡ = (Fm-
Fs)/Fm according to Genty et al. (1989). qE, the fast relax-
ing component of NPQ, was resolved by extrapolation in
semi-logarithmic plots of the maximum fluorescence versus
time as described by Johnson et al. (1993).
1.3 Estimation of electron transport rate
The electron transport rate through PSⅡ (Jf) was esti-
mated from the fluorescence data according to the follow-
ing equation:
Jf = PPFD×PSⅡ×a
where PPFD is photosynthetically active flux density,
PSⅡ is the quantum yield of PSⅡ electron transport
(Genty et al., 1989), and a is a constant that depends on the
molar ratio of PSⅡ to PSⅠ and the efficiency of absorp-
tion of light by the leaves. a was determined according to
the method of Miyake and Yokota (2000). a was determined
to be 0.38 and 0.43 for control and salt-stressed leaves of
Rumex K-1 seedlings, respectively.
The rate of electron transport required to maintain pho-
tosynthetic carbon reduction cycle (PCR) and
photorespiratory carbon oxidation cycle (PCO) was calcu-
lated from gas exchange according to Von Caemmerer and
Farquhar (1981).
Jg = (A+Rd) ×(4Cc+8G*)/(Cc-G*)
where A is net photosynthetic rate, Rd is a rate of mito-
chondrial respiration in the light, Cc is the pressure of CO2
at site of carboxylation, and G* is the partial pressure of
CO2 at which the rate of carboxylation of RuBP equals to
the rate of photorespiratory evolution of CO2. Rd and G*
were determined according to the methods of Brooks and
Farquhar (1985). Cc was determined by equation: Cc = Ci-
A/gm, where gm is mesophyll conductance to CO2 and was
determined according to Harley et al. (1992). In Rumex K-1
leaves, G* was determined to be 43.2 mbar and 35.6 mbar,
and gm was determined to be 0.72 mmol. m-2.s-1 and 1.21
mmol. m-2.s-1, for control and salt-stressed leaves,
respectively.
CHEN Hua-Xin et al.: Enhancement of the Mehler-peroxidase Reaction in Salt-stressed Rumex Leaves 813
1.4 Biochemical assays
Leaf tissues were homogenized with 0.1 mol/L sodium
phosphate buffer (pH 6.8) in a chilled pestle and mortar.
The homogenate was centrifuged at 12 000g for 15 min and
the resulting supernatant was used for determination of
enzyme activities, SOD activity was determined according
to Paoletti et al. (1986). APX activity was determined ac-
cording to the method of Nakano and Asada (1981); the
content of protein was determined as described by Bradford
(1976). Chlorophylls and carotenoids were extracted with
80% acetone and the extracts were analyzed by UV-1201
(Shimadzu, Japan) according to the method of Arnon (1949).
1.5 Photoinhibitory treatment and recovery
Rumex K-1 leaves were illuminated by a high PPFD of
1 600 mmol. m-2.s-1 in 21% O2 or 2% O2 at 25 ℃,
respectively. To suppress photorespiration, both treat-
ments were made in the presence of saturating CO2 (5 000
mmol/mol). Photoinhibition was assessed by decrease in
the Fv/Fm. Fv/Fm was measured in 20 min dark-adapted
leaves. After photoinhibition, the leaves were recovered at
a PPFD of about 20 mmol. m-2.s-1 in ambient O2 and CO2.
2 Results
2.1 Effects of salt stress on photosynthesis, PSⅡ photo-
chemistry and chlorophyll content
Table 1 showed the effects of salt stress on
photosynthesis, maximal PSⅡ photochemistry (Fv/Fm)
and chlorophyll content. Salt stress did not induce decrease
in Fv/Fm, indicating that salt stress had no effects on maxi-
mal photochemistry of PSⅡ. However, salt stress caused
increase in the content of chlorophyll and intercellular car-
bon dioxide concentration. By comparison, photosynthe-
sis (Pn) and stomatal conductance (Gs) decreased signifi-
cantly under salt stress.
2.2 Electron transport rate during photosynthetic induc-
tion and under steady state photosynthesis
By simultaneous measurement of chlorophyll fluores-
cence and gas exchange, the electron transport rate through
PSⅡ (Jf) and the electron transport rate required to main-
tain PCR and PCO (Jg) were calculated (Fig.1). It was ob-
served that Jf was much higher than Jg, especially during
the early stage of photosynthetic induction. The ratio of
Jf/Jg was at its maximum at the start time of illumination
both in control and salt-stressed leaves. When steady state
photosynthesis was approached, Jf and Jg almost converged
in control leaves whereas in salt-stressed leaves Jf was still
much higher than Jg. The large difference between Jf and Jg
during photosynthetic induction indicated that alternative
electron sink was operating in salt-stressed leaves. It was
shown that this alternative electron transport depended on
oxygen (Fig.1). In the presence of 2% O2, the difference
between Jf and Jg in salt-stressed leaves was largely
diminished, indicating that the alternative electron sink was
restricted to Mehler-peroxidase reaction.
Figure 2 shows quantum yield of PSⅡ electron trans-
port (PSⅡ) and the rate of electron transport through
PSⅡ (Jf) in the presence of saturating CO2, and 21 % or 2 %
O2 under steady state photosynthesis. In control leaves,
PSⅡ was insensitive to oxygen concentration, therefore Jf
was almost the same in 21% O2 and 2% O2. By comparison,
PSⅡ was found to be sensitive to oxygen concentration in
salt-stressed leaves. Decrease of oxygen concentration from
21 % to 2 % resulted in a large decrease in PSⅡ and in Jf
when PPFD was above 200 mmol. m-2.s-1. It was important
Table 1 Effects of salt stress on net photosynthetic rate (mmol.m-2.s-1), maximal photochemistry of PSⅡ (Fv/Fm), chlorophyll
content (mg/dm2), stomatal conductance (Gs, mmol.m-2.s-1), and intercellular carbon dioxide concentration (Ci，mmol/mol). Photo-
synthetic gas exchange was measured under a PPFD of 800 mmol.m-2.s-1 and at a temperature of about 25 ℃
NaCl Pn Fv/Fm Chl Gs Ci
CK 12.1±1.26 0.832±0.005 1.829±0.191 221±23.7 156±16
200 (mmol/L) 5.5±0.83 0.824±0.003 2.246±0.236 65±5.5 253±22
Fig.1. Electron transport rate calculated from gas exchange (Jg,
open symbol) and fluorescence (Jf, closed symbol) in control
(circles) and salt-stressed leaves (triangles), in 21% O2 (A) and in
2% O2 (B), and the ratio of Jf to Jg in 21 % O2 (C) and in 2 % O2
(D) during photosynthetic induction. Gas exchange and chloro-
phyll fluorescence were recorded simultaneously when dark-
adapted leaves were exposed to a PPFD of 800 mmol.m-2.s-1.
CO2 concentration was kept at 380 mmol/mol and temperature at
25 ℃ via automatic leaf curvette of CIRAS-1.
Acta Botanica Sinica 植物学报 Vol.46 No.7 2004814
to note that the data were obtained in the presence of satu-
rating CO2. Under such conditions, the oxygenase activity
of Rubisco was largely inhibited. Therefore, the oxygen-
sensitive electron flow in salt-stressed leaves represented
the electron flow to oxygen in Mehler-peroxidase reaction.
The data demonstrated that under steady state
photosynthesis, a considerable electron flow to oxygen in
the Mehler-peroxidase reaction occurred in salt-stressed
leaves.
2.3 Energy-dependent chlorophyll fluorescence quench-
ing
Figure 3 shows energy-dependent quenching (qE)
under a range of PPFDs. It was observed that qE was small
in the low light. qE increased rapidly with increasing PPFDs
when light was above 250 mmol. m-2.s-1 both in control
and salt-stressed leaves. At high light intensity, qE was
higher in salt-stressed leaves than that in control leaves.
Reducing oxygen concentration from 21% to 2% caused
only a slight increase in qE in control leaves. However, a
significant increase in qE was observed in salt-stressed
leaves when oxygen concentration was reduced to 2%.
2.4 Effects of O2 on the susceptibility of PSⅡ to
photoinhibition
Figure 4 shows the effect of O2 on the susceptibility of
PSⅡ to photoinhibition. A significant decrease in Fv/Fm
was observed in both control and salt-stressed leaves, indi-
cating the occurrence of photoinhibition. However, the de-
crease was more pronounced in salt-stressed leaves than
in control leaves, suggesting that salt stress increased the
susceptibility of PSⅡ to photoinhibition. Decreasing oxy-
gen concentration from 21% to 2% did not affect the sus-
ceptibility in control leaves but markedly increased the
susceptibility in salt-stressed leaves. During the period
of Fv/Fm recovery, control leaves exhibited a fast recovery,
which was similar in 21% O2 and 2% O2. By comparison,
salt-stressed leaves exhibited a slower recovery.
Additionally, the recovery was slower in 2% O2 than in
21% O2. Obviously, O2 mitigated photoinhibition in salt-
stressed leaves. It has been well known that photorespira-
tion and the Mehler-peroxidase reaction are oxygen-depen-
Fig.2. Quantum yield of PSⅡelectron transport, PSⅡ (A, B),
and PSⅡ electron transport rate, Jf (C, D) in control leaves (A,
C) and salt-stressed leaves (B, D) in the presence of saturating
CO2 (5 000 mmol/mol), and 21% O2 (closed circles) and 2% O2
(open circles). Measurement was made at temperature of 25 ℃.
Each point represents the mean ± SE of five individual experiments.
Fig.3. The responses of energy-dependent quenching (qE) to
PPFD in control (circles) and salt-stressed leaves (triangles) in
the presence of saturating CO2 (5 000 mmol/mol), and 21% O2
(open symbol) or 2% O2 (closed symbol). Each point represents
the means ± SE of three individual experiments.
Fig.4. Changes in maximal photochemistry of PSⅡ in control
leaves (circles) and in salt-stressed leaves (triangles) in 21 % O2
(closed symbols) or 2% O2 (open symbols) during photoinhibitory
treatment and subsequent recovery. In the period of
photoinhibitory treatment, leaves were exposed to a high PPFD
of 1 600 mmol.m-2.s-1 under saturating CO2. During subsequent
recovery, leaves were kept at dim light of about 20 mmol.m-2.s-1.
The white and diagonal areas indicate the period of photoinhibition
and the subsequent recovery, respectively. Values are mean ± SE
of five replicates.
CHEN Hua-Xin et al.: Enhancement of the Mehler-peroxidase Reaction in Salt-stressed Rumex Leaves 815
3 Discussion
The results showed that salt stress resulted in a dra-
matic decrease in photosynthesis (Table 1). The PSⅡ
activity, reflected by the maximal efficiency of PSⅡ photo-
chemistry (Fv/Fm), was not affected by salt stress, sug-
gesting that PSⅡ was rather tolerant to salt stress. In
addition, the chlorophyll content in Rumex K-1 leaves in-
creased under salt stress. These results suggested that the
decrease in photosynthesis was not due to limitation of
photosynthetic electron transport but due to the slow-down
of the dark reaction in the Calvin cycle. A significant in-
crease in intercellular CO2 concentration (Ci) observed in
salt-stressed leaves (Table 1), indicating that the decrease
of photosynthesis was not caused by decrease of stomata
conductance either. The slow-down of dark reaction might
be resulted from the decrease in activity and content of
Rubisco, RuBP or Pi regeneration capacity (Brugnoli and
Björkman, 1992; Lu and Zhang, 1998b).
CO2 assimilation serves as the major consumer for light
energy absorbed by chlorophyll antennae. Salt stress had
no effect on the maximal efficiency of PSⅡ photochemis-
try but induced dramatic decrease in photosynthesis (Table
1). Consequently, much of excess electrons would be accu-
mulated in photosynthetic chain when the salt-stressed
leaves were exposed to high light. Salt stress thus could
lead to increased susceptibility of PSⅡ to photoinhibition
at high light if the excess light energy could not be dissi-
pated safely. It was well established that non-radiative dis-
sipation of excess light energy, measured as non-photo-
chemical quenching of chlorophyll fluorescence, was the
most important process to dissipate excess light energy
(Müller et al., 2001). The significant increase in NPQ would
certainly contribute to dissipate the excess light energy.
When leaves were treated with DTT, an inhibitor of xan-
thophyll cycle, the susceptibility of photoinhibition was
much increased (data not shown), clearly demonstrating
the important role of NPQ in photoprotection. However,
the susceptibility of PSⅡ to photoinhibition was still in-
creased in salt-stressed leaves although there was a greater
increase in NPQ in salt-stressed leaves (Figs.3, 4).
During photosynthetic induction, a considerable elec-
tron flow was allocated to oxygen in MPR (Fig.1). Generally,
when dark-adapted leaves were transferred to light, it took
several minutes or even longer for stomata to open. In
addition, several key enzymes in carbon assimilation were
inactivated in the dark, several minutes were also needed
to activate these enzymes (Ruuska et al., 2000). Therefore,
PCR and PCO operated slowly in response to sudden
illumination. This was indicated by the low Jg during the
early phase of photosynthetic induction (Fig.1). However,
much of light energy would be continuously absorbed by
chlorophyll antennae to cause charge separation which
drives electron transport. As a result, the electron flow
through PSⅡ would exceed what was required to maintain
PCR and PCO. Under such conditions, the electron trans-
port chain would become highly reduced. The consider-
able electron flow in MPR would certainly help to drain off
the excess electrons and balance the input of electrons
from PSⅡ and the consumption of the electrons.
Under steady state photosynthesis, a considerable elec-
tron flow was transported to oxygen in the Mehler-peroxi-
dase reaction in salt-stressed leaves (Fig.2). In the Mehler-
peroxidase reaction, there are two sites for electron flow:
one for the photoreduction of O2 at PSⅠ, and the other for
the regeneration of AsA from oxidized AsA. The photore-
duction of monodehydroascorbate follows the photoreduc-
Fig.5. The activities of APX and SOD in control leaves (black)
and salt-stressed leaves (white). The activities are expressed on a
protein basis. Values are mean±SE of three individual
measurements.
dent process which consume the photosynthetic electrons.
Since all photoinhibitory treatments were made under non-
photorespiratory condit ions, the mit igat ion of
photoinhibition by O2 must have been attributed to Mehler-
peroxidase reaction. The results suggested that the elec-
tron flow to oxygen in Mehler-peroxidase reaction played
an import role in photoprotection under salt stress.
2.5 Activities of SOD and APX
SOD and APX are the two key enzymes involved in
active oxygen species scavenging. Their activities were
depicted in Fig.5. It was observed that salt stress increased
the activities of both APX and SOD. Such increase in the
activities of scavenging enzymes may safely eliminate the
harmful oxygen species produced in the Mehler-peroxidase
reaction.
Acta Botanica Sinica 植物学报 Vol.46 No.7 2004816
tion of O2 at PSⅠ. Electrons from water generated in PSⅡ
reduce atmosphere O2 to water in PSⅠ with no net change
of O2 (Asada, 1999). It was therefore suggested that MPR
could provide an important pathway for the consumption
of excess electrons. However, evidence for a significant
electron flow to oxygen in Mehler-peroxidase reaction was
lacking under environmental stresses such as water stress,
low and high temperature (Flexas et al., 1999; Badger et al.,
2000). It was thought that the Mehler-peroxidase reaction
was under strong control in the absence of ATP consump-
tion (Badger et al., 2000). Compared with these earlier
studies, the significant electron flow to oxygen in Mehler-
peroxidase reaction in salt-stressed Rumex K-1 leaves was
quite interesting. Under salt stress, plants generally un-
dergo adaptive responses such as accumulation of some
protective solutes and exclusion of the toxic ions (Hasegawa
and Bressan, 2000; Zhao et al., 2001). All these responses
are the ATP-consuming processes. Possibly, a high de-
mand for ATP was the explanation of the high activity of
the MPR in salt-stressed leaves.
Apart from consumption of excess electrons, the Mehler-
peroxidase reaction was also proposed to be helpful in gen-
eration of the proton gradient across thylakoids membrane.
In isolated chloroplasts, it was shown that electron flow to
oxygen in MPR contributed decisively to membrane
energization (Schreiber and Neubauer, 1990; Neubauer and
Yamamoto, 1992). If this was the case in vivo, it certainly
facilitated the process of thermal energy dissipation in
leaves. However, we observed that the electron flow to
oxygen in MPR did not result in an increase in qE (Fig.3). It
is well known that qE is closely correlated with transmem-
brane proton gradient; a low intrathylakoid pH is required
for the de-epoxidation of violaxanthin to zeaxanthin, which
is believed to be involved in thermal energy dissipation of
excess energy. Therefore, qE can be used as an indicator to
reflect the relative changes in transmembranes proton
gradient. If the electron flow to oxygen in MPR contributed
to generating transmembranes proton gradient, a lower qE
could therefore be expected when MPR was inhibited.
However, in 2% O2 qE was not lower but even higher than
that in 21% O2, especially under high light (Fig.3). This was
argued against a role of MPR in thermal energy dissipation.
The higher qE under 2% O2 was possibly mediated by cy-
clic electron transport around PSⅠ, as suggested by Makino
et al. (2002). Therefore, we concluded that electron flow to
oxygen in the Mehler-peroxidase reaction was not impor-
tant in maintaining a thylakoid proton gradient enough to
initiate thermal energy dissipation.
The reduction of oxygen in the Mehler-peroxidase
reaction, however, will inevitably result in production of
harmful oxygen species, such as superoxide and H2O2
(Asada, 1996). When the rate of electron transport to oxy-
gen in MPR increased under salt stress, the increased for-
mation of active oxygen species (AOS) would potentially
cause deleterious damage on photosynthetic apparatus.
However, in plants effective mechanisms are well devel-
oped to scavenge deleterious AOS (Asada, 1999). The fact
that the increase in MPR was companied by enhanced ac-
tivities of scavenging enzymes under salt stress indicated
that the harmful AOS produced by MPR could be safely
scavenged. We inferred that the benefits of MPR out-
weighed potential deleterious effects caused by AOS in
salt-stressed Rumex K-1 leaves.
To summarize, the results showed that electron flow in
MPR was enhanced in salt-stressed leaves. However, the
enhanced Mehler-peroxidase reaction did not contribute
to the generation of transmembrane proton gradient, rather,
it served as an efficient sink for excess electrons in salt-
stressed Rumex K-1 leaves. We conclude that MPR is an-
other photoprotective mechanism and mitigates
photoinhibition in salt-stressed Rumex K-1 leaves.
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