Tree Physiology 18, 317--324
© 1998 Heron Publishing----Victoria, Canada
Photosynthetic gas exchange and accumulation of phytotoxins in
mangrove seedlings in response to soil physico-chemical characteristics
associated with waterlogging
TAREK YOUSSEF1,2 and PETER SAENGER1
1
Center for Coastal Management, Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia
2
Present address: Biology Department, United Arab Emirates University, P.O. Box 17551, Al-Ain, United Arab Emirates
Received December 1, 1995
Summary We evaluated photosynthetic gas exchange and
the accumulation of iron, manganese and sulfur in seedlings of
five mangrove species (Aegiceras corniculatum (L.) Blanco,
Avicennia marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.)
Lamk., Hibiscus tiliaceus L. and Rhizophora stylosa Griff.)
growing under anoxic soil conditions at low irradiance. Seedlings of the viviparous mangroves showed no significant responses to root anoxia. The presence of ferrous sulfate or
manganous sulfate had a smaller effect on CO2 assimilation,
transpiration rate and stomatal behavior than the presence of
sodium sulfide. Sodium sulfide inhibited photosynthetic gas
exchange and caused complete stomatal closure in all species.
Stomatal closure was probably the result of the damaging effect
of sulfide ions on root cell membranes. Some leaf epinasty and
wilting were also observed in response to the Na2S treatment
in all species. A combination of FeSO4, MnSO4 and Na2S had
a smaller effect on photosynthesis than Na2S alone, especially
for Avicennia marina and Rhizophora stylosa seedlings, which
maintained appreciable rates of CO2 assimilation (2.49 and
3.84 µmol m −2 s −1, respectively) in the presence of all three
phytotoxins.
Roots of phytotoxin-treated seedlings of all species accumulated significant amounts of the corresponding toxin compared
with roots of the control plants. The FeSO4 or MnSO4 treatments had no effect on foliar concentrations of iron or manganese, whereas the Na2S treatment resulted in an accumulation
of S in the leaves of all species. Interactions between Fe2+ and
Mn2+ and sulfide in the culture medium appeared to reduce
their uptake by the seedlings. We conclude that high concentrations of sulfides in mangrove sediments may adversely
affect both growth and survival of mangrove seedlings at low
irradiances.
and seedling survival and growth. Mangrove sediments are
known to range from aerobic through to anoxic and highly
reduced. Mangrove plants growing in waterlogged soils may
be adversely affected by either the strongly reduced conditions
or the accumulation of soluble phytotoxins including reduced
iron, manganese and organic gases, or both (McKee 1993). In
addition, species growing in anaerobic marine sediments must
also cope with toxic concentrations of sulfides (Allam and
Hollis 1972).
In most wetland plants, the first line of defense against these
toxic soluble ions is to render them insoluble at the root surface
by oxidation with air that diffuses to the root from the photosynthesizing shoots (Armstrong et al. 1994). However, Youssef
and Saenger (1996) have shown that, unlike the mature vegetation, roots of mangrove seedling have a very limited capacity
for oxidative detoxification of the rhizosphere under laboratory conditions. Furthermore, detoxification by direct oxidation is even more limited for seedlings under field conditions,
because the seedlings are commonly partially or totally submerged or growing in low irradiances in the understory of the
mature stand.
In the present study, we investigated the effect of three
soluble phytotoxins common in reduced mangrove sediments
(Fe2+, Mn2+ and H2S) singly and in combination on photosynthetic gas exchange of five mangrove species growing at low
irradiances. The distribution of toxins within the seedlings was
also investigated.
Keywords: anoxia, flooding, reduced iron, reduced manganese, sulfides.
Plant materials
Introduction
In mangrove stands, the physico-chemical characteristics of
the substrate can greatly affect both propagule establishment
Materials and methods
Viviparous propagules of Avicennia marina (Forsk.) Vierh.,
Bruguiera gymnorrhiza (L.) Lamk. and Aegiceras corniculatum (L.) Blanco were collected from mature mangrove stands
at the Richmond River Nature Reserve (28°50′ S, 150° E),
NSW, and Rhizophora stylosa Griff. propagules were collected
from Townsville, Queensland, Australia. Propagules of relatively similar dimensions and weight were selected for each
318
YOUSSEF AND SAENGER
species, cultivated on coarse sand beds and irrigated with 25%
seawater (salinity = 8.75) for 4 weeks.
The mangrove associate Hibiscus tiliaceus L. (hereafter, all
taxa are referred to by their generic names) was propagated by
stem cuttings. Commercial rooting hormone containing 4 g l −1
indole butyric acid (Bass Lab., Victoria, Australia) was applied
to the cuttings. Thermally controlled (25 °C), well-drained
sand beds were used to enhance rooting. Cuttings were raised
in a fresh water misting system until roots were developed.
All successfully raised seedlings and cuttings were subsequently transferred to plastic trays containing acid-washed
white sand. Plants were then flooded to 2 cm above the soil
surface with 25% seawater containing 0.1% commercial horticultural nutrient mixture (Aquasol, Hortico, Sydney, Australia) and grown for 4 weeks in a greenhouse in a 12-h
photoperiod. Irradiance was controlled with shade cloth to
about 30% of natural solar radiation (approximately 600 µmol
m −2 s −1).
Experimental design
To simulate conditions that seedlings may experience in the
understory of a mangrove stand, an 18-h photoperiod at a
photosynthetic photon flux density (PPFD) of < 150 µmol
m −2 s −1 was used. Light passed through a diffuser to create a
homogeneous light distribution. Relative humidity was maintained below 50% to create a leaf-to-air vapor pressure difference of about 1.5 to 2.0 kPa. Air temperature was maintained
around 27 ± 0.5 °C.
For each species, 18 seedlings of similar dimensions were
divided into six treatments. Seedlings were randomly placed in
separate hydroponic 1.5-l Perspex (Plexiglas) containers similar to those described by DeLaune et al. (1990). One liter of
1/10 strength Hoagland nutrient solution (Sigma Chemicals,
St. Louis, MO), adjusted with 50 mM NaCl to approximate
10% seawater, was used as the nutrient culture for each container. Solutions were adjusted to pH 6.0. Plants were acclimated to the hydroponic system for 48 h before the 1-week
treatments were begun.
The Eh (redox potential in mV) of each nutrient culture was
monitored with a silver/silver chloride reference electrode and
a platinum electrode connected to a multi-channel circuit.
Containers were sealed with putty (Blu-Tak, Sydney, Australia) and covered with a 3-cm layer of water to ensure an airtight
seal.
Containers were allocated to one of six treatments: aerobic
treatment or control (A), reduced or anoxic treatment (R);
reduced + 2 mM ferrous sulfate (R + Fe); reduced + 2 mM
sodium sulfide (R + S); reduced + 0.1 mM manganous sulfate
(R + Mn); and reduced mixture of the three phytotoxins (R +
Mix). Equal aliquots of reduced or oxidized titanium citrate
buffer, prepared as described by DeLaune et al. (1990), were
injected into the containers assigned to the reduced treatments
to maintain Eh < − 200 mV (to ensure that all phytotoxins
added remained in the reduced form), and > +400 mV for the
containers assigned to the aerobic treatment (control). Induction of ion toxicity in the anoxic treatment was avoided by
using a low concentration of nutrient solution (1/10 strength
Hoagland).
Gas exchange measurements
Photosynthetic gas exchange of intact leaves was measured
with an LI-6200 portable photosynthesis system equipped
with a 1-l leaf chamber (Li-Cor, Inc., Lincoln, NE). Measurements were made 120 h after treatments had begun on at least
two fully expanded leaves of each of three individuals in every
treatment. Water-use efficiency (WUE)----a measure of CO2
fixed per unit of leaf conductance (Pezeshki et al. 1990)----was
calculated as: µmol CO2 µmol −1 H2O kPa −1.
Toxin analysis
After a 1-week treatment, plants were harvested and the roots
washed three times for 10 min each time with equal volumes
of double distilled water, with continuous stirring. There were
no significant iron oxide deposits on the root surface. A black
colloidal precipitation that formed on roots in the mixed phytotoxin treatment was removed by the washing procedure.
Plants were then oven-dried at 80 °C to constant weight (DW).
Plant materials were divided into root, hypocotyl (above the
root--hypocotyl junction) and shoot (including leaves and stem
> 2 cm above the hypocotyl--shoot junction). Known weights
of plant dry material were digested with nitric acid until the
extracts were clear, then made up to known volumes. Total iron
and manganese were estimated by atomic absorption spectrometry (Model GBC-903, GBC Scientific Equipment, Victoria, Australia). Total sulfur in oven-dried material was
determined with an LECO carbon sulfur system CS-244
(LECO Corp., St. Joseph, MI).
Statistical analysis
Data were subjected to two-way analyses of variance,
ANOVA, using the statistical software program Systat 5.2
(SYSTAT, Inc., Evanston, IL).
Results
Photosynthetic CO2 assimilation rate
Under the experimental aerobic conditions, seedlings of all of
the mangrove species had similar but low rates of carbon
assimilation (< 5 µmol m −2 s −1) (Tables 1 and 2). The anoxic
treatment alone had little or no effect on carbon assimilation
rates in most species, although it significantly reduced carbon
assimilation rates in Bruguiera and in Hibiscus (Tables 1
and 2).
Carbon assimilation rate of Rhizophora was not affected by
the presence of reduced iron or manganese (Tables 1 and 2).
Reduced iron significantly decreased assimilation rates in
Avicennia and Bruguiera, and reduced manganese decreased
carbon assimilation rate in Aegiceras. In response to reduced
manganese, Hibiscus showed no detectable photosynthetic
activity and stomata were completely closed at the time of the
measurements.
TREE PHYSIOLOGY VOLUME 18, 1998
RESPONSES OF MANGROVE SEEDLINGS TO SOIL PHYTOTOXINS
319
Table 1. Effect of three soluble phytotoxins on gas exchange characteristics (mean ± SD) of seedlings of five mangrove species grown at low
irradiance. Treatments: A = aerobic; R = anoxic; R + Fe = reduced iron; R + Mn = reduced manganese; R + S = sulfide; and R + Mix = mixture
of the three phytotoxins. Abbreviations: A = carbon assimilation rate (µmol m −2 s −1); E = transpiration rate (mmol m −2 s −1); WUE = water-use
efficiency (µmol CO2 µmol −1 H2O kPa −1); SR = stomatal resistance (s cm −1); and ND = plants did not show any photosynthetic response and
stomata were completely closed. For each treatment, values in columns followed by similar letters are not significantly different.
Treatment
A
E
WUE
SR
Aegiceras
A
R
R + Fe
R + Mn
R+S
R + Mix
HSD
P-value
df
3.47 ± 1.06 a
3.13 ± 0.37 ab
3.42 ± 0.13 a
2.28 ± 0.32 b
ND
ND
0.87
P < 0.05
3 & 20
2.01 ± 0.35 a
2.49 ± 0.15 ab
1.94 ± 0.07 a
3.07 ± 0.52 b
ND
ND
0.65
P < 0.001
3 & 20
1.04 ± 0.192 a
0.84 ± 0.109 ab
1.19 ± 0.063 a
0.59 ± 0.089 b
ND
ND
0.39
P < 0.001
3 & 20
3.59 ± 0.670 a
2.61 ± 0.242 b
3.27 ± 0.203 a
1.91 ± 0.313 b
ND
ND
0.72
P < 0.001
3 & 20
Avicennia
A
R
R + Fe
R + Mn
R+S
R + Mix
HSD
P-value
df
3.78 ± 0.33 a
3.45 ± 0.63 a
2.92 ± 0.39 b
4.11 ± 0.83 a
ND
2.49 ± 0.36 b
0.86
P = 0.001
4 & 25
3.07 ± 0.38 a
1.80 ± 0.17
3.70 ± 1.06 a
3.02 ± 0.13 a
ND
2.87 ± 0.72 a
0.92
P < 0.01
4 & 25
1.00 ± 0.197 a
1.01 ± 0.161 a
0.48 ± 0.063 b
1.15 ± 0.287 a
ND
0.47 ± 0.019 b
0.51
P < 0.001
4 & 25
0.89 ± 0.086 a
4.17 ± 0.485
2.19 ± 0.501
0.76 ± 0.043 a
ND
3.03 ± 0.903
0.83
P < 0.001
4 & 25
Bruguiera
A
R
R + Fe
R + Mn
R+S
R + Mix
HSD
P-value
df
4.97 ± 0.50 a
3.55 ± 0.40 bc
2.85 ± 0.49 c
4.35 ± 1.25 ab
ND
ND
0.99
P < 0.01
3 & 20
3.37 ± 0.18 a
1.92 ± 0.11
3.23 ± 0.52 b
3.24 ± 0.32 ab
ND
ND
0.16
P < 0.001
3 & 20
1.11 ± 0.126 a
0.97 ± 0.134 a
0.49 ± 0.100
1.16 ± 0.282 a
ND
ND
0.47
P < 0.001
3 & 20
1.00 ± 0.106 a
4.20 ± 0.305
2.70 ± 0.730
0.85 ± 0.221 a
ND
ND
0.55
P < 0.001
3 & 20
Hibiscus
A
R
R + Fe
R + Mn
R+S
R + Mix
HSD
P-value
df
3.19 ± 0.98 a
1.67 ± 0.75
4.11 ± 1.17 a
ND
ND
ND
1.07
P < 0.05
2 & 15
2.92 ± 0.62 a
2.70 ± 0.69 a
2.13 ± 0.18 a
ND
ND
ND
0.83
P > 0.05
2 & 15
0.83 ± 0.220 a
0.49 ± 0.291
1.35 ± 0.367 a
ND
ND
ND
0.58
P < 0.01
2 & 15
2.22 ± 0.815 a
2.52 ± 1.068 a
2.79 ± 0.477 a
ND
ND
ND
1.04
P > 0.05
2 & 15
Rhizophora
A
R
R + Fe
R + Mn
R+S
R + Mix
HSD
P-value
df
3.79 ± 0.71 a
3.95 ± 0.81 a
5.14 ± 0.32 a
4.22 ± 0.68 a
ND
3.84 ± 0.60 a
1.35
P > 0.05
4 & 25
2.01 ± 0.10 ab
2.04 ± 0.11 ab
2.57 ± 0.28 a
2.20 ± 0.13 ab
ND
1.92 ± 0.16 b
0.50
P < 0.001
4 & 25
1.15 ± 0.185 a
1.19 ± 0.211 a
1.32 ± 0.099 a
1.29 ± 0.194 a
ND
1.31 ± 0.169 a
0.53
P > 0.05
4 & 25
3.45 ± 0.142 a
3.40 ± 0.146 a
2.67 ± 0.389 a
2.94 ± 0.301 a
ND
3.33 ± 0.055 ab
0.59
P < 0.001
4 & 25
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320
YOUSSEF AND SAENGER
Table 2. Differences in photosynthetic gas exchange (mean ± SD) of seedlings of five mangrove species in response to three phytotoxins under
low light irradiance. Treatments: A = aerobic; R = anoxic; R + Fe = reduced iron; R + Mn = reduced manganese; R + S = sulfide; and R + Mix =
mixture of the three phytotoxins. Abbreviation: ND = plants did not show any photosynthetic response and stomata were completely closed. For
each treatment, values followed by similar letters are not significantly different.
Species
Treatment
Aegiceras
Bruguiera
Hibiscus
Rhizophora
df
HSD
P-value
s )
a
a
a
a
ND
--
a
a
a
a
ND
ND
a
-b
ND
ND
ND
a
-a
a
ND
--
4 & 25
4 & 25
4 & 25
3 & 20
-1 & 10
1.083
0.991
0.930
1.048
-0.634
P > 0.05
P = 0.001
P < 0.001
P < 0.01
-P < 0.01
a
b
-a
ND
--
a
b
-a
ND
ND
a
ab
-ND
ND
ND
b
ab
--ND
--
4 & 25
4 & 25
4 & 25
3 & 20
-1 & 10
0.795
0.714
0.172
0.636
-0.623
P < 0.001
P < 0.01
P < 0.001
P = 0.001
-P < 0.05
Water use efficiency (µmol CO2 µmol −1 H2O kPa −1)
A
-a
R
ab
ab
R + Fe
a
b
R + Mn
-a
R+S
ND
ND
R + Mix
ND
--
a
ab
b
a
ND
ND
a
b
a
ND
ND
ND
a
a
a
a
ND
--
4 & 25
4 & 25
4 & 25
3 & 20
-1 & 10
0.525
0.538
0.508
0.543
-0.352
P > 0.05
P = 0.001
P < 0.001
P = 0.001
-P < 0.001
Stomatal resistance (s cm −1)
A
-R
b
R + Fe
-R + Mn
a
R+S
ND
R + Mix
ND
a
a
a
a
ND
ND
-b
a
ND
ND
ND
-ab
a
-ND
a
4 & 25
4 & 25
4 & 25
3 & 20
-1 & 10
0.892
0.981
0.720
0.576
-0.866
P < 0.001
P < 0.01
P < 0.01
P < 0.001
-P > 0.05
Carbon assimilation rate (µmol m
A
a
R
a
R + Fe
ab
R + Mn
-R+S
ND
R + Mix
ND
Avicennia
−2 −1
Transpiration rate (mmol m −2 s −1)
A
b
R
a
R + Fe
-R + Mn
a
R+S
ND
R + Mix
ND
a
a
a
a
ND
a
Hibiscus, but not the other species, showed some leaf epinasty and wilting soon after exposure to the reduced iron
treatment. All species exhibited leaf epinasty and wilting when
treated with sodium sulfide at low irradiance and these symptoms were more pronounced when seedlings were treated with
a mixture of the phytotoxins.
Sulfide treatment resulted in complete inhibition of photosynthesis and total stomatal closure in all species (Tables 1
and 2). A mixture of the three phytotoxins had a similar effect
to that of sulfide alone in all species except Avicennia, which
maintained an assimilation rate of 2.49 µmol m −2 s −1 and
Rhizophora, which maintained an assimilation rate that was
not significantly different from that of the control (3.84 versus
3.79 µmol m −2 s −1) (Tables 1 and 2).
Transpiration rate and stomatal resistance
All plants in the aerobic treatment showed high rates of transpiration (2 to 3.4 mmol m −2 s −1), although transpiration rates
were significantly lower in Aegiceras and Rhizophora than in
the other species (Tables 1 and 2). In both Avicennia and
Bruguiera, the anoxic treatments significantly reduced transpiration rates compared with the controls. Except for sulfides and
to some extent the mixture of phytotoxins, other treatments
caused only minor variations in transpiration rates of the species (Tables 1 and 2).
Under aerobic conditions, all mangrove seedlings had similar WUE (Tables 1 and 2). In the anoxic treatment, all of the
viviparous species maintained similar WUE, whereas the treatment reduced WUE in Hibiscus. The ferrous-iron-induced
reduction in carbon gain led to a reduction in water-use efficiency (WUE) in Avicennia and Bruguiera, and the reduced
manganese caused a reduction in WUE in Aegiceras and Hibiscus. In the mixed treatments, transpiration rate was significantly higher in Avicennia than in Rhizophora, whereas carbon
assimilation rate was higher in Rhizophora than in Avicennia.
Consequently, in the mixed treatment, WUE was more than
twofold higher in Rhizophora than in Avicennia.
Stomatal resistance followed an opposite pattern to transpiration rate and ranged between 4.2 to 0.76 s cm −1 (Tables 1
and 2). The sulfide treatment caused complete stomatal clo-
TREE PHYSIOLOGY VOLUME 18, 1998
RESPONSES OF MANGROVE SEEDLINGS TO SOIL PHYTOTOXINS
sure in all species, whereas the combined phytotoxin treatment
caused complete stomatal closure in all species except Rhizophora and Avicennia. Stomatal resistance of these two species
was not significantly different. Only Hibiscus showed complete stomatal closure in the reduced manganese treatment.
Differential accumulation of toxins in seedling parts
Differences in iron, manganese and sulfur concentrations
among species were most pronounced in root tissue (Table 3).
The ferrous sulfate treatments resulted in a significant increase
(P < 0.001) in iron concentration in roots of all species (Table 3). The accumulation of iron was greater in the anoxic
treatments than in the aerobic treatments for Aegiceras and
Avicennia (Table 3). In all species, the mixed treatment had a
smaller effect on the accumulation of iron in roots than the
ferrous sulfate treatment. For example, roots of Avicennia
accumulated 9.2 and 4.2 mg gDW−1 of iron in the single and
mixed treatments, respectively.
In the ferrous sulfate treatments, there was a gradual decrease in tissue iron content from the root to the hypocotyl
junction to the shoot in all viviparous species except Avicennia
(Figure 1), such that the total amount of iron in the iron-treated
seedlings was similar to that of seedlings in the aerobic and
mixed toxin treatments (Figure 1).
All species generally followed a similar response to reduced
manganese, except in Bruguiera where the manganese concentration was generally higher in the shoot than in the hypocotyl
area. Hibiscus accumulated larger amounts of manganese in its
roots compared with viviparous species (Figure 1), accounting
for the toxicity symptoms shown by all Hibiscus plants in the
manganous sulfate treatments.
321
The form in which sulfide was applied (singly or mixed)
significantly affected the total sulfur content of roots of all
species. The distribution of sulfide in plant tissues was similar
to that of iron and manganese. Co-occurrence of the three
phytotoxins in the mixed treatment resulted in the formation of
a black colloidal precipitation, presumably of ferrous and
manganous sulfides which, in turn, reduced their availability.
Consequently, the concentration of sulfur was higher in roots
in the sulfide treatment than in roots in the mixed treatment
(Figure 1), except in Bruguiera where the sulfur concentration
of roots was higher in the mixed treatment than in the sulfide
treatment (3.6 versus 2.4 mg gDW−1).
Sulfur concentrations were consistently lower in aerobic
roots than in roots in the sulfide treatment (P > 0.001). In
Avicennia and Aegiceras, sulfur concentrations were lower in
anoxic roots than in control roots, whereas anoxic roots of both
Hibiscus and Rhizophora accumulated more sulfur than control roots.
Although there was a gradual decrease in sulfur concentration from roots to hypocotyls to shoots in all viviparous species, the seedlings treated with sulfide alone had a significantly
higher sulfur concentration in all tissues than control seedlings
(Figure 1). Sulfur concentrations in both shoot and root of
Hibiscus were higher than in all of the viviparous species.
Discussion
The viviparous mangrove species did not respond physiologically to the anoxic treatment, at least in the short term. The
reduced iron and manganese treatments had a smaller effect on
carbon assimilation and stomatal behavior than the sulfide and
mixed phytotoxin treatments. The sulfide treatment com-
Table 3. Total iron, manganese and sulfur concentrations (mean ± SD) in roots of seedlings of five mangrove species in response to the following
treatments: aerobic (A), anaerobic (R), single phytotoxin (RI) and mixture of three soluble phytotoxins (RM). Values followed by the same letter
are not significantly different (P > 0.001) (n = 6).
Treatment
−1
Species
Element (µg gDW )
Aegiceras
Total manganese
Total iron
Total sulfur
Avicennia
Total manganese
Total iron
Total sulfur
20 ± 9
1728 ± 122
554 ± 64
Bruguiera
Total manganese
Total iron
Total sulfur
Hibiscus
Rhizophora
A
R
RI
RM
36 ± 8 a
1762 ± 694
621 ± 189 a
322 ± 43
14607 ± 1843
3330 ± 281
174 ± 8
3139 ± 467
2923 ± 119
37 ± 9
2540 ± 775
341 ± 43
337 ± 22
9188 ± 2346
3804 ± 100
131 ± 11
4156 ± 753
3200 ± 186
67 ± 3
933 ± 142
430 ± 69
94 ± 23
1068 ± 203
681 ± 69
344 ± 101 a
13350 ± 672
2374 ± 967
306 ± 29 a
2571 ± 166
3590 ± 93
Total manganese
Total iron
Total sulfur
168 ± 21
3036 ± 54
638 ± 133
58 ± 6
3753 ± 416
1376 ± 199
742 ± 34
10181 ± 643
6378 ± 237
274 ± 12
7554 ± 62
3060 ± 313
Total manganese
Total iron
Total sulfur
17 ± 3 a
1280 ± 271 a
471 ± 52
28 ± 6 a
1317 ± 226 a
1250 ± 33
366 ± 28
6113 ± 573
2826 ± 99
225 ± 41
5135 ± 559
3883 ± 18
28 ± 2 a
763 ± 66
642 ± 124 a
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322
YOUSSEF AND SAENGER
Figure 1. Total iron, manganese and sulfur concentrations in mangrove seedlings grown at low irradiance in the presence of three soluble
phytotoxins either singly or in combination. Each value is the mean of three individuals ± SD. Roots = black; hypocotyl = shaded; and shoot =
white.
pletely halted photosynthetic gas exchange and most of the
species showed some leaf epinasty and wilting in response to
this treatment.
Hydrogen sulfide is reported to inhibit the activity of cytochrome oxidase----the terminal enzyme in the electron transport chain of aerobic respiration (Allam and Hollis 1972).
Recently, Koch et al. (1990) suggested that, in addition to
inhibiting aerobic respiratory enzymes, hydrogen sulfide may
also affect the alternative anoxic pathways through inhibition
of alcohol dehydrogenase (ADH). There is evidence that sulfide may also inhibit the energy-dependent process of nitrogen
uptake (e.g., Koch et al. 1990, Bradley and Morris 1990). B.F.
Clough (Australian Institute of Marine Science, Townsville,
Australia, personal communication) suggested that, in the absence of rhizosphere oxidation, the damaging effect of sulfides
on the root cell membranes may interfere with the root’s ability
to discriminate against sodium, chloride and other toxic ions.
Our stomatal resistance and carbon assimilation data (Table 1)
appear to support this view.
The treatment containing the three phytotoxins resulted in a
moderation of the toxic effects produced when they were
applied singly. Such moderation was reflected in rates of
carbon assimilation (e.g., Avicennia and Rhizophora maintained rates of carbon assimilation of 2.49 and 3.84 µmol
m −2 s −1, respectively, in the mixed treatment compared with
complete inhibition in the sulfide treatment) and in the relative
amounts of ions accumulated in roots of different species in the
mixed treatment compared with treatments with individual
toxins (Figure 1).
The reduction in accumulation of toxins as a result of a
reduction in toxin availability in the mixed treatment agrees
with the observation that sulfide toxicity only occurs in soils
low in iron (Ponnamperuma 1972). Koch and Mendelssohn
(1989) also noticed that, in sulfide-amended soil cores, roots
TREE PHYSIOLOGY VOLUME 18, 1998
RESPONSES OF MANGROVE SEEDLINGS TO SOIL PHYTOTOXINS
of Spartina alterniflora Loisel. were coated with a black colloidal precipitate. A black precipitate was also observed on the
roots of some marsh plants growing in highly reduced environments in the field (Goodman and Williams 1969, Mendelssohn
and Seneca 1980, Koch and Mendelssohn 1989). Crawford
(1992) reported that, in wetland vegetations, interactions between ions resulted in ferrous iron hindering the absorption of
manganous ions by roots. Manganous ions are most likely to
cause toxicity in soils that are deficient in iron and phosphate.
Accumulation of insoluble sulfides may also function as a sink
for silver, copper, lead, cadmium and iron (Ponnamperuma
1984).
Sulfides may play a significant role in the differential distribution of Rhizophora mangle L. and Avicennia germinans L.
(McKee 1993). Smith et al. (1991) showed that burrowing by
crabs improved soil aeration which, in turn, reduced sulfide
concentrations thereby enhancing productivity and reproductive output in mangrove forests dominated by members of the
family Rhizophoraceae. In addition, loss of the rhizosphere
oxidizing capacity at low irradiances would allow pore-water
sulfide to diffuse into the absorbing roots. The differential
capacity of species to restrict the uptake of sulfides and their
accumulation in foliage could be a significant factor affecting
seedling growth and survival. Snedaker and Lahmann (1988)
suggested that the absence of the understory mangrove is
associated with the effects of shade combined with the persistence of anoxia and the presence of high concentrations of
sulfides in mangrove sediments.
Our data contrast with the observations of Lin and Sternberg
(1992) who noted that high sulfide concentrations (2 mM) did
not reduce carbon assimilation or stomatal conductance in
Rhizophora mangle. Furthermore, McKee (1993) reported that
2 mM sulfide caused an increase in total biomass of seedlings.
We found that 2 mM sodium sulfide halted the photosynthetic
activity of all mangrove seedlings, caused complete stomatal
closure, and resulted in several visible stress symptoms. There
are four explanations that could account for these discrepancies.
First, McKee (1993) showed that loss of sulfides in the form
of H2S resulted in a more than 66% decrease in the initial
sulfide concentration within 48 h of the onset of the experiment. Although McKee (1993) periodically added aliquots of
Na2S to maintain the interstitial concentration of sulfides between 0.5 to 1.0 mM, we conclude that the sulfide concentration to which the plants in McKee’s study were exposed was
lower than sulfide concentrations in mangrove sediments and
beneath the threshold of phytotoxicity.
Second, in stagnant flooded soils, roots of many mangroves
develop a very thin, slightly oxidized zone that can effectively
isolate the actively growing root area from the highly concentrated phytotoxins by oxidative detoxification (Youssef and
Saenger 1996). In our study the roots could not form an
oxidized zone because the nutrient solution was continuously
disturbed by passing nitrogen gas through it.
Third, experiments in which plants are flooded by submerging the soil with water containing certain phytotoxins are not
comparable with studies in hydroponic systems, because pre-
323
existing nutrients in the soil, which may interact with the tested
phytotoxins, are not accounted for. In iron-rich sediments,
sulfide availability to plants is reduced because it is precipitated as FeS (Ponnamperuma 1972, 1984). Sulfide stability is
also controlled by other factors including the Eh and pH of the
sediments (see Gambrell and Patrick 1978, Armstrong 1982,
Ponnamperuma 1984, Gambrell et al. 1991).
Finally, Clarke and Hannon (1970) reported that waterlogging enhanced the growth of Avicennia and Aegiceras seedlings and Lin and Sternberg (1992) reported that 2 mM sulfide
did not reduce carbon assimilation or stomatal conductance in
Rhizophora mangle. We postulate that the discrepancy between these studies and our study can be attributed to the
substrate used. Both of the previous studies used a mixture of
vermiculite and gravel as the substrate, which would keep the
Eh above the value for sulfide production (i.e., > −150 mV
(Gambrell and Patrick 1978, Ponnamperuma 1984). Our suggestion is supported by the study of Pezeshki et al. (1990) in
which Eh was monitored when Avicennia germinans and Rhizophora mangle seedlings were flooded by submerging a similar substrate (sand, vermiculite and peat with water). With
some fluctuation, Eh of the substrate was about (− 92 ± 28 mV)
after 4 weeks of flooding. They found no significant difference
in stomatal conductance or carbon assimilation between
flooded and control plants. We conclude that, under the experimental conditions of these previous studies, the plants only
experienced root anoxia. If this assumption is correct, then our
findings closely match those of the earlier studies. Thus, in our
study, the responses of both Avicennia and Rhizophora to the
anoxic treatment did not differ significantly from those of the
controls.
We conclude that mangroves generally respond not to flooding itself but to the accumulation of soil phytotoxins that
follows flooding. Failure to simulate precisely the conditions
that occur in mangrove sediments following flooding may
minimize or even obscure the actual responses of the seedlings,
because the responses to individual phytotoxins frequently
differ from the responses to combinations of phytotoxins.
Acknowledgments
We are grateful to Dr. B.F. Clough for reviewing the manuscript and
providing the Rhizophora propagules, and Mr. R. Fleetwood for helping with the gas exchange studies.
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