Aquatic Botany, 38 ( 1 9 9 0 ) 3 - 1 7
3
Elsevier Science Publishers B.V., A m s t e r d a m
Root aerenchyma, oxygen leakage patterns and
alcoholic fermentation ability of the roots of some
nymphaeid and isoetid macrophytes in relation to
the sediment type of their habitat
A.J.M. Smits 1, P. Laan 2, R.H.
Thier I and G. van der Velde ~
~Laboratory of Aquatic Ecology and 2Department of Experimental Botany, Catholic University qf
Nijmegen, Toernooiveld, 6525 ED Nijmegen (The Netherlands)
(Accepted for publication 23 March 1990 )
ABSTRACT
Smits, A.J.M., Laan, P., Thier, R.H. and van der Velde, G., 1990. Root aerenchyma, oxygen leakage
patterns and alcoholic fermentation ability of the roots of some nymphaeid and isoetid macrophytes in relation to the sediment type of their habitat. Aquat. Bot., 38: 3-17.
Morphological and metabolic characteristics of the roots of two groups of aquatic macrophytes are
compared. The first group consists of species frequently occurring on organic sediments with a distinct negative redox potential, such as the nymphaeid macrophytes Nyrnphaea alba L., Nuphar lutea
(L.) Sin. and Nymphoides peltata (Gmel.) O. Kuntze. The second group contains species generally
rooting in a mineral sediment with an oxidative character, such as the isoetids Littorella un~llora ( k. )
Ascherson and lsoetes lacustris L., and Luroniurn natans ( L ) Rafinesque.
As a measure for the amount of root aerenchyma, root porosity of these macrophytes was determined. The nymphaeids which were grown in the same type of sediment as the isoetids developed a
larger amount of root aerenchyma. Moreover, the roots of the nymphaeids differed from the roots of
the isoetids by a reduced oxygen permeability of the root wall and a higher ethanol fermentation rate
during anoxic conditions. The root porosity, oxygen leakage patterns and ethanol fermentation ability
of the roots of these two groups of plants are discussed with respect to the sediment typc of their
habitat.
INTRODUCTION
Apart from other numerous environmental factors affecting the growth of
the aquatic plants (Spence, 1967; Wetzel, 1975 ), the organic matter content
of the sediment greatly influences the distribution of aquatic macrophytes
(Sand-Jensen and Sondergaard, 1979; Barko and Smart, 1986).
For example, it is well known that nymphaeid macrophytes occur especially in water bodies with highly organic sediments (Sculthorpe, 1967;
Hutchinson, 1975; Sand-Jensen and Sondergaard, 1979), whereas isoetids
dominate the aquatic vegetation in the littoral zone of soft water bodies with
0304-3770/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
4
A..I.M. SMITS ET AL.
nutrient-poor and sandy sediments (Iversen, 1929; Spence, 1964; Seddon,
1979).
Generally, rooting submersed macrophytes appear to be replaced by floating-leaved and emergent water plants as sediment organic matter increases
(Walker, 1970; Wetzel, 1979; Carpenter, 1981, 1983), but the processes underlying this succession are not clearly understood. Differences in sensitivity
of the root system towards phytotoxins which accumulate in an anoxic organic soil might present possible explanations (Armstrong, 1975; Yoshida,
1975; Drew and Linch, 1980).
Alternatively, morphological and/or metabolic characteristics of plants,
rather than chemical compounds of the sediment, may determine the composition of the aquatic vegetation (Sculthorpe, 1967; Haslam, 1978; Denny,
1980 ). In a sediment with a low redox potential, root vitality may be affected
by inadequate oxygen supply to the root apex or the inability to use appropriate metabolic pathways under anaerobic conditions (Armstrong, 1978;
Crawford, 1982).
The efficiency with which the root apex is provided with oxygen is dependent on the internal porosity of the root and the gas exchange characteristics
between the root and the sediment (Armstrong, 1979; Laan et al., 1989a,b).
The metabolism of roots of wetland and terrestrial plants which endure
root anaerobiosis depends mainly on alcoholic fermentation (cf. Smith and
ap Rees, 1979; Davies, 1980; Roberts et al., 1984). Thus, apart from an efficient oxygen supply to the root apex, alcoholic fermentation of the roots can
be considered as an adaptation to a rhizosphere with a high oxygen demand.
In this paper, some aspects of root aeration and alcoholic fermentation of
the roots of two groups of aquatic macrophytes were studied and compared,
viz. plants which root mainly in organic bottoms and plants which root mainly
in mineral sediments. Macrophytes selected for this study which root mainly
in organic anoxic sediments are the nymphaeid species Nymphaea alba L.,
Nuphar lutea (L.) Sm. and Nymphoidespeltata (Gmel.) O. Kuntze (Van der
Velde et al., 1986 ). Littorella uniflora (L.) Ascherson, Isoetes lacustris L. and
Luronium natans (L.) Rafinesque represent macrophytes which occur mainly
on mineral and oxidative sediments (De Lyon and Roelofs, 1986).
MATERIALS AND METHODS
Plant material
Young and mature plants of Littorella uniflora, Luronium natans and Isoetes lacustris were collected from Staalberg's ven near Oisterwijk, The Netherlands (N 51 ° 34' 28"; E 5 ° 13' 21" ). Seedlings and 1-year-old plants of Nymphaea alba, Nuphar lutea and Nymphoides peltata were cultivated from seeds
collected in two localities both in the surroundings of Nijmegen, The Neth-
ROOT CHARACTERISTICS OF TWO GROUPS OF AQUATIC MACROPHYTES
5
erlands (Oude Waal N 51 ° 51' 14", E 5 ° 53'41 "; Millingerwaard, N
51 o51'53", E 5o59'57" ).
The macrophytes were grown in polyethylene ponds located in a greenhouse in which the temperature was maintained above 10 ° C. All plants were
grown on a mineral sediment (loss on ignition was ,--2%) dredged from
Staalberg's ven.
Root porosity
Root porosity of the various plant species was determined by measurement
of the lacunal cross-sectional area as a proportion of the total cortical root
cross-sectional area (Konings and Verschuren, 1980) and by pycnometry
(Jensen et al., 1969). Both methods are subsequently referred to as crosssectional area measurement and pycnometric measurement. Roots of cultivated macrophytes were carefully washed with tap water to remove adhering
soil particles. Only apical segments of 2-4 cm length were used for both root
porosity determinations.
Oxygen leakage of roots
A slightly modified method of Leyton and Rousseau (1958) was used to
examine the oxygen leakage patterns of the roots. Instead of indigo-carmine,
leuco-methylene blue was used to detect the presence of oxygen. Oxygen leakage of the roots was examined in a solidified agar solution ( 1% w/v ) to which
methylene blue solution (1%) was added (0.5-1.0 ml l - l ) . Methylene blue
is colourless when reduced, but becomes blue by oxidation. Thus, if a root is
submerged in the dye a blue oxidation halo indicates where internally transported oxygen has diffused into the medium.
After immersion of the entire root system in the dye-agar mixture, the glass
tanks or test tubes were carefully filled with tap water. The shoots of the macrophytes were illuminated by halogen light bulbs (light intensity ~ 150/rE
m - 2 s- l near the leaves). In each treatment, the development of oxidation
patterns around the roots during 1.5 h at room temperature ( ~ 2 0 ° C ) was
observed. In a second treatment series, the roots were cooled to 0 ° C to reduce
the effect of respiratory processes on oxygen leakage patterns. Therefore, the
basal ends of the glass tanks or test tubes were partially immersed in melting
ice to within 1 cm of the root base of the plant suspended in the dye-agar
mixture. Oxygen leakage patterns were determined in at least four plants of
each species.
Aerobic root respiration
Whole plants were collected and the roots were washed with tap water. Subsequently, the roots were cut from the shoots and the apical 2-10 cm of healthy
6
A.J.M. SMITS ET AL.
growing roots were put in a 35-ml serum bottle containing an incubation med i u m consisting of a 0.05 m M citrate-0.1 m M phosphate buffer (pH 5.4)
containing 2.5% ( w / v ) glucose. Per serum bottle, 1-1.5 g fresh weight of root
material was used. The bottles were closed airtight with a screw cap and placed
in a waterbath at 20 ° C. Chloramphenicol 0.04% ( w / v ) was added to reduce
the influence of microorganisms on the measurements of metabolic processes.
Root respiration was measured as oxygen uptake by the dissected roots from
an aerated incubation m e d i u m as described previously. The oxygen content
of the samples was determined according to Drew and Robertson (1974).
The measurement ended when ~ 5% of the initial oxygen content of the medium was consumed. In this way, the existence ofhypoxic conditions around
the roots was prevented. Five to six replicates per macrophyte species were
used. After the experiment, the roots were dried at 105 °C for 24 h to constant
weight.
Ethanol and C02 production during root anaerobiosis
Root segments were collected and incubated in a similar way as described
for the root respiration determination. However, the incubation buffer for
this experiment had been pre-saturated with nitrogen gas for 12 h to remove
dissolved oxygen. A rubber sealing pad in the screw cap allowed sampling
with hypodermic syringes. Samples were taken for ethanol and CO2 determinations at regular intervals during the incubation.
Sampling of the incubation m e d i u m was carried out by injecting 0.5 ml of
anoxic buffer from a disposable syringe with a hypodermic needle in the serum
bottle. A second syringe was used to accept the displaced solution. Subsequently, both syringes were alternately emptied in the serum bottle to obtain
a homogeneous mixture. This was repeated four times before a sample of 0.5
ml was withdrawn from the solution. Aliquots of 100/d and 0.3 ml of the
sample were used for the dissolved CO2 and ethanol determination, respectively. For each macrophyte species, CO2 and ethanol measurements were
carried out per 30 min and 2 h, respectively. The experiment lasted 49 h and
was conducted in duplicate. CO2 measurements were carried out using an
Oceanography International Model 0525 HR Infrared Carbon Analyzer; the
CO2 was stripped from the acid media using N2 gas. Ethanol was assayed
enzymatically by the m e t h o d of Bernt and G u t m a n n ( 1974 ). After the experiment, the vitality of the roots was estimated as the capacity to be stained with
a tetrazolium solution (Bertani et al., 1981 ).
RESULTS
Root porosity
The roots of all investigated species are characterized by a well developed
aerenchyma (Fig. 1 ). Typical lysigenous aerenchyma was found in the roots
ROOT CHARACTERISTICS OF TWO GROUPS OF AQUATIC MACROPHYTES
7
Fig. 1. Cross-sectional electron scanning micrographs of the roots of six aquatic macrophytes.
( A ) Isoetes lacustris. ( B ) Littorella uniflora. ( C ) Luronium natans. ( D ) Nymphoides peltata.
(E) Nymphaea alba. (F) Nuphar lutea. Bars represent 100/~m.
of N y m p h o i d e s peltata, L u r o n i u m natans and Littorella uniflora. The roots of
Isoetes lacustris were characterized by a large cortical cavity. The root porosity of all species was high and varied between 30 and 60% (Fig. 2 ). The porosity values of the roots of L u r o n i u m natans and Isoetes lacustris obtained
with the cross-sectional area m e a s u r e m e n t were higher than those obtained
8
A.J.M. SMITS ETAL.
Porosity (Va/Vtot) 1O0
6ol- [ ] Cross sectional area
(N=4)
[ ] Pycnometer measurement (N=5) _z_
L
45
L
._L
30
15
0
ZL
L.u.
L.n.
N.p.
3/.o.
ALL
Fig. 2. Root porosity of the macrophytes used in this study. Porosity is defined as (Va/Vtot) 100,
where Va is the root air volume and Vtot is the total root volume. Bars represent the mean root
porosity values determined according to the cross-sectionalarea measurement or are based on
the pycnometric measurement (vertical lines indicate +1 SE). I.l.=Isoetes lacustris;
L.u. = Littorella uniflora; L.n. = Luronium natans; N.p. = Nymphoides peltata; N.a. = Nymphaea
alba; N.1.= Nuphar lutea.
with the pycnometer measurement. It has to be noted that the aerenchyma o f
the roots o f L u r o n i u m natans was interrupted at regular intervals by tube-like
structures which connected the stele with the cortex. Further, some parts o f
the roots oflsoetes lacustris with a vital appearance were filled with water.
The n y m p h a e i d macrophytes developed more root aerenchyma than the
other three macrophytes (Kruskal-Wallis test; P < 0.05; arcsin transformation o f the porosity values prior to statistical analyses was not necessary because all data were within the 30-70% range; see Sokal and Rohlf, 1981 ).
Both methods for root porosity m e a s u r e m e n t gave similar results ( K r u s k a l Wallis test; P < 0.001 ).
Oxygen leakage o f roots
There was a conspicuous difference between the species with respect to the
pattern of oxidated methylene blue a r o u n d the roots (Fig. 3). Oxygen dif-
ROOT CHARACTERISTICSOF TWO GROUPS OF AQUATICMACROPHYTES
A
9
B
Fig. 3. Examples o f oxygen leakage p a t t e r n s visible by the oxidation of methylene blue. ( A )
Halo f o r m a t i o n along the whole root o f a Isoetes lacustris plant a n d ( B ) confined to ~ 1 cm of
the root apex of a Nuphar lutea seedling. T h e arrows indicate limited halo formation. 1 = root
base ( c o r m ) of lsoetes lacustris; 2 = seed of Nuphar lutea; 3 = oxidation of methylene blue on
the w a t e r - a g a r b o u n d a r y layer.
fused into the m e d i u m along the whole root of Littorella uniflora, Luronium
natans and Isoetes lacustris, whereas oxidation around the roots of Nymphaea
alba, Nuphar lutea and Nymphoides peltata plants was restricted to the root
apex. Within the same species, no difference was found between halo development of young and mature plants. Although the rate of halo development
proceeded faster at 0 °C than at room temperature ( ~ 20 °C), the oxidation
pattern of the roots characteristic to each species remained stable.
Respiration and ethanol fermentation
Aerobic respiration (02 consumption) and anoxic CO2 +ethanol production data of the roots are presented in Table 1. Because of the great similarity
of the ethanol and CO2 production among the replicates, the collected data
per species were pooled. Hence, the CO2 and ethanol production rates per
species were based on 198 and 100 measurements, respectively, and calculated by linear regression (REG procedure, Statistical Analysis Systems
(S.A.S.), 1982).
28.2_+0.4
29.0 -+ 2.0
58.8 -+ 6.9
Nymphoidespeltata
Nymphaea alba
Nuphar lutea*
17.8_+ 1.0
38.8 -+ 0.8
34.5 +_ 1.4
6.5 _+0.2
4.3 _+0.1
11.2 + 0.4
10.6_+0.7
35.1 _+0.7
27.5 +_ 1.6
0.9 +_0.2
4.1 _+0.2
3.3-+ 0.3
Ethanol p r o d u c t i o n
(/~mol g - ~ dry
wt.h -1)
1.3
2.2
3.0
0.6
0.7
1.0
Maximum
CO2 ( m M )
0.4
1.3
1.6
0.1
0.3
0.2
Maximum
ethanol
(mM)
+ +
+ +
+ +
- + + -
Tetrazolium
test
-
The m e a n rate o f oxygen c o n s u m p t i o n o f excised roots (x+_ SE) was m e a s u r e d u n d e r aerobic c o n d i t i o n s . T h e CO2 a n d e t h a n o l p r o d u c t i o n d u r i n g
anaerobiosis was based u p o n 198 CO2 m e a s u r e m e n t s a n d 100 e t h a n o l m e a s u r e m e n t s . " M a x i m u m CO2" a n d " M a x i m u m e t h a n o l " represent the m a x i m u m a c c u m u l a t e d CO2 a n d e t h a n o l c o n c e n t r a t i o n in t h e i n c u b a t i o n m e d i u m to which the roots were subjected d u r i n g t h e course o f the e x p e r i m e n t s .
+ + , + - a n d - - represent decreasing ability to react with t e t r a z o l i u m c o m p a r e d with roots w h i c h h a v e not been subjected to a n a n a e r o b i c incubation;
- = no staining; + - = partly stained; + + = good staining. Statistical analyses were carried o u t using the R E G a n d G L M p r o c e d u r e (S.A.S., 1986).
25.7 -+ 5.0
44.3 _+ 1.7
49.1 +_6.7
C O 2 - p r o d u c t i o n (/tmol
g - ~ dry wt. h - ~)
O 2 - c o n s u m p t i o n (/lmol g - ~
dry wt. h - ~)
n=6;*n=5
Isoetes lacustris
Littorella uniflora
Luronium natans*
Anaerobic incubation
Aerobic i n c u b a t i o n
Oxygen c o n s u m p t i o n , a n d ethanol a n d c a r b o n dioxide p r o d u c t i o n o f excised roots
TABLE 1
t"
-4
k..
11
ROOT CHARACTERISTICS OF TWO G R O U P S OF AQUATIC MACROPHYTES
TABLE 2
S t a t i s t i c a l a n a l y s e s o f t h e CO2 a n d e t h a n o l p r o d u c t i o n r a t e s
Ethanol production
C02 production
1.1.
L.u.
L.n.
k.u.
R.S.
L.n.
N.p.
**
**
**
**
**
N .a.
**
**
**
N.l.
**
**
**
N.a.
N.1.
1.1.
L.u.
L.n.
N.a.
*
*
n.s.
n.s.
n.s.
**
**
**
**
**
**
**
N.l.
**
**
**
**
**
T h e G a u s s i a n l o g i s t i c r e g r e s s i o n ( G L R ) p r o c e d u r e (S.A.S., 1 9 8 6 ) w a s used. * * = s i g n i f i c a n t at the
P < 0 . 0 5 level: * = s i g n i f i c a n t at t h e P < 0 . 1 level; n . s . = n o t s i g n i f i c a n t . I.l.=lsoetes lacustris:
L.u.=Littorella un(flora; L.n.=Luronium
N .p. = 5(vmphoides peltata.
natans: N.a.=Nymphaea
alba; N.l.=Nuphar httea:
It is remarkable that when the aerobic respiration rates of the nymphaeid
macrophytes were compared with the remaining macrophytes, no significant
difference could be observed (Kruskal-Wallis test; P = 0.43 ). In contrast, CO2
production rates of the roots of the nymphaeid macrophytes during anaerobiosis were significantly higher than those of Littorella uniflora, Luronium
natans and Isoetes lacustris (Table 2). With the exception of Nymphoides
peltata, the ethanol production of the roots of Nymphaea alba and Nuphar
lutea under anoxic conditions was also significantly higher than that of the
roots of the other macrophytes. Judging from the tetrazolium staining, the
roots of the macrophytes which produced little or no ethanol during anaerobiosis were less viable than the roots with a high ethanol production rate (see
Table 1 ).
The m a x i m u m ethanol and CO2 concentrations of the incubation media to
which the roots of Littorella uniflora, Isoetes lacustris and Luronium natans
were exposed did not exceed 0.3 and 1 mM, respectively (see Table 1 ).
DISCUSSION
The presence of aerenchyma in the roots of wetland and aquatic plants has
been well documented by a number of authors (Arber, 1920; Sifton, 1945,
1957; Sculthorpe, 1967; Justin and Armstrong, 1987; Laan et al., 1989a, b).
It has been demonstrated that aerenchyma enables aerobic respiration of the
root system in a hypoxic or anoxic rhizosphere (cf. Armstrong, 1979; Laan et
al., 1989a, b).
The cross-sectional area measurement provides a better estimate of the lacunae in the root cortex than the pycnometer measurement because the relative diameter of the stele may vary between species. However, the tube-like
structures in the roots of Luronium natans and the water-filled parts of the
12
A.J.M. SMITS ET AL.
roots of Isoetes lacustris can barely be detected with the cross-sectional area
measurement and can result in an overestimation of the a m o u n t of root
aerenchyma.
Although all plant species were grown on an identical sediment, measurement of the root porosity by both methods revealed that the roots of nymphaeid macrophytes were able to develop more root aerenchyma than the other
three studied species.
Apart from the lacunal system, the root wall permeability is also of considerable importance for the oxygen supply of the root apex. If the gas permeability of the root wall is high, a large part of the oxygen present in the root will
diffuse into the sediment and will subsequently be reduced.
The oxidation reaction of methylene blue provides only qualitative data
about the oxygen leakage from the root, but shows the loci of diffusion quite
well.
In contrast to Littorella uniflora, Luronium natans and Isoetes lacustris, the
oxygen leakage of the nymphaeids is limited to the root apex. This indicates
that the loss of oxygen from the roots of Nymphaea alba, Nuphar &tea and
Nymphoides peltata is limited, and consequently the oxygen supply of the root
apex is more efficient. In all, the well-developed root aerenchyma and the
reduced gas permeability of the root wall of these nymphaeid water plants, in
combination with the internal thermo-osmotic-driven gas transport (Dacey,
1980, 1981; Dacey and Klug, 1982; Schr6der et al., 1986; Grosse and MeviSchiitz, 1987 ), constitute a gas conduction pathway which optimizes the oxygen supply of the apical meristem.
Which function can be assigned to the aerenchyma of the roots of the aquatic
macrophytes which lack a gas-tight root wall? Littorella uniflora, Isoetes lacustris and Luronium natans occur mainly in poorly buffered waters with a
low inorganic carbon content. It is known that the inorganic carbon supply
for photosynthesis of the isoetid macrophytes which occur in these waters
depends on the CO2 produced by decomposition processes in the sediment
(Wium-Andersen, 1971; Sondergaard and Sand-Jensen, 1979; Sand-Jensen
et al., 1982; Richardson et al., 1984).
Considerable assimilation of CO2 originating from the rhizosphere by the
shoots is only feasible when the gas permeability of the whole root system is
high and sufficient aerenchyma is present in the roots and shoots. The high
gas permeability of the roots also implies that apart from CO2 influx, photosynthetic oxygen can diffuse along the whole root to the sediment. This is
confirmed by our observations. The oxidizing effect of the isoetid vegetation
on the rhizosphere is a well known and consistent feature, and stimulates the
CO2-generating decomposition processes in the sediment (cf. Wium-Andersen and Andersen, 1972 ).
The gas exchange characteristics of the roots of isoetids partly explain the
success of these species in waters with a low inorganic carbon content, but
ROOT C H A R A C T E R I S T I C S O F T W O G R O U I ~ ~~)F AQUA~[ ~C MACROPHYTES
13
imply concurrently that this type of root is unsuitable for an environment
with a strong reductive trait. A sediment with a large oxygen d e m a n d will
probably withdraw the oxygen from roots with a high gas permeability before
it can reach the root apex. There is evidence that despite the presence of a
well-developed root aerenchyma, the transport rate of oxygen may not be sufficient to sustain aerobic metabolism in an anoxic rhizosphere (John et al.,
1974; Mendelssohn et al., 1981 ). Therefore, viability of the roots is also dependent on metabolic reactions which proceed under anaerobiosis, and if fermentation is involved, also on the toxicity of the fermentation products. If
the roots are able to continue the processes which are essential to maintain
the cell integrity without the normal oxidative respiration, then they are metabolically adapted to a hypoxic or anoxic rhizosphere.
It has long been known that the underground parts of a number of water
plants produce ethanol during anaerobiosis. Laing (1940) demonstrated that
apart from some helophytes, the rhizomes of Nuphar advena Ait. produced
considerable quantities of ethanol. The roots of the six plant species studied
here produce ethanol under hypoxic conditions. The ethanol fermentation
capacity of the roots of the nymphaeid species was higher than those of the
other species if we assume that ( 1 ) among the species glucose could be utilized equally by the roots and (2) no great differences in ethanol effiux of the
excised roots existed. In spite of the toxic character of ethanol the roots which
produced relatively large amounts of ethanol retained their viability during
the anoxic incubation. Therefore, it is tempting to assume that a considerable
ethanol production during root anaerobiosis is a typical feature of macrophytes which can penetrate an anaerobic sediment to considerable depths (viz.
the nymphaeid macrophytes Nymphaea alba, Nuphar lutea and Nymphoides
peltata).
A number of authors are at issue about the toxicity of the end products of
fermentation (for a list of authors see Raven, 1984 ), but there is no evidence
that ethanol is more toxic than organic acids such as lactic and malic acid.
Drew ( 1979 ) demonstrated that the concentrations of ethanol needed for the
inhibition of root growth (30-170 m M ) are rarely approached, even in longterm experiments or during natural hypoxic conditions. The range of root
ethanol concentrations in the incubation media of Isoetes lacustris, Littorella
uniflora and Luronium natans was lower than that of the roots of the nymphaeid macrophytes, and did not exceed 0.3 m M (Table 1 ). Thus, it seems
unlikely that the reduced viability of Isoetes lacustris, Littorella un(tTora and
Luronium natans roots was caused by ethanol toxicity.
Although ethanol fermentation is thought to be the most important source
of anaerobic energy supply in rhizomes (cf. Cossins, 1978; Braendle and
Crawford, 1987), other metabolic pathways that divert carbon away from
ethanol have been proposed to contribute to anoxia tolerance of the roots of
some plant species. The main alternatives are accumulation of shikimic acid
(Iris pseudacorus L.; Tyler and Crawford, 1970) and amino acids (e.g. Zos-
14
A.J.M. SMITS ET AL.
tera marina L., Pregnall et al., 1984; Hordeum vulgare L., Good and Crosby,
1989).
The role of other metabolic pathways in the roots of the aquatic macrophytes under study needs to be investigated. However, considering the enhanced viability during anaerobiosis of roots with a high ethanol fermentation capability, it seems unlikely that these alternative pathways will overrule
the importance of ethanol fermentation for anoxia tolerance of the roots in
these species.
The authors suggest that morphological characteristics (i.e. well-developed
root aerenchyma and reduced oxygen permeability of the roots), as well as an
extensive alcoholic fermentation ability of the roots of Nymphaea alba and
Nuphar lutea especially, and to a lesser extent Nymphoides peltata, can be
considered essential adaptations to the strong reductive sediments of their
habitats. In addition, the lack of the above-mentioned features in Littorella
uniflora, Luronium natans and Isoetes lacustris roots can partly explain why
the occurrence of these macrophytes is confined to water bodies with a mineral sediment characterized by a small reductive capability.
ACKNOWLEDGEMENTS
We would like to thank Prof. Dr. C. den Hartog who critically read and
improved the manuscript. We also appreciated the stimulating discussions
with C.J. Kok. The drawings were prepared at the Department of Illustration,
Faculty of Science of the Catholic University, Nijmegen.
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