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Journal of Ecology (1989), 77, 693-703
ROOT M ORPH OLOG Y A ND AE REN CH YM A FO RM A TIO N
AS IN DIC ATO R S OF TH E FLO O D-TO LERA NCE
OF RU M EX SPECIES
P. LAAN,* M. J. BERREVOETS,* S. LYTHE,f W. ARMSTRONG!
and C. W P. M. BLOM*
* Department of Experimental Botany, University of Nijmegen, Toernooiveld, 6525 ED
Nijmegen, The Netherlands and t Department of Applied Biology, University of Hull,
Hull, North Humberside HU6 7RX
SUMMARY
(1) Rumex species are zoned along a gradient of elevation in the river ecosystem in The
Netherlands.
(2) Plants of R. thyrsiflorus, R. acetosa, R. obtusifolius, R. crispus, R. conglomerate and
R. maritimus were flooded to identify and quantify any relevant adaptive features and to
test whether their distribution might be caused by a differential response to flooding in the
growing season.
(3) Most Rumex species have a tap-root from which the laterals originate. As a response
to flooding, new laterals are formed.
(4) The number, place of origin, growth direction and formation rate of new laterals
differed between the species.
(5) The number and formation rate of new roots were associated with the elevational
distribution of the species: as a response to flooding, low-elevation species formed more
new roots, and faster, than high-elevation species.
(6) The high-elevation species had root porosity values lower than 10%; the
intermediate- and low-elevation species had values higher than 10%.
(7) Schizogenous aerenchyma was constitutively formed by the low-elevation and flood-
tolerant R. maritimus, and not by the high-elevation and flood-intolerant species R.
thyrsiflorus. In the intermediate-elevation species R. crispus it was induced in stagnant
hypoxic solution cultures.
(8) The results indicate that aercnchyma formation is closely connected with the growth
rate of new roots. It appears that development of aerenchyma in the new roots is the main
determinant in the flood-tolerance of Rumex species.
INTRODUCTION
Flooding induces a number of responses in plant roots, of which aerenchyma formation is
one of the most obviously adaptive (Arber 1920; Sculthorpe 1967; Armstrong 1979;
Konings & Verschuren 1980; Crawford 1982; Jackson & Drew 1984; Justin & Armstrong
1987). Aerenchyma is formed either by some cell wall separation and cell collapse
(lysigeny) or by cell separation without collapse (schizogeny). Both forms result in larger
longitudinal channels in the root cortex. This structure enhances the diffusion of
atmospheric or photosynthetic oxygen via, or from, the shoot to the roots so that aerobic
respiration and growth can be maintained (Armstrong & Gaynard 1976; Lambers,
Steingrover & Smakman 1978; Armstrong 1979; Armstrong & Webb 1985; Drew, Saglio
& Pradet 1985).
Changes in root morphology also occur after flooding and this is true for both wetland
and non-wetland species. There may be: (i) an increase in branching of the roots (Vose
693
694
Flood-tolerance o/’Rumex species
1962; Geisler 1965); (ii) the development of new, adventitious roots (Bergman 1920;
Kramer 1951; Alberda 1953; Arikado & Adachi 1955; Jackson 1955; Drew, Jackson &
Gifford 1979; Wenkert, Fausey & Waters 1981; Hook 1984); and (iii) superficial rooting
(Alberda 1953; Armstrong & Boatman 1967; Sheikh & Rutter 1969; Schat 1984).
In most cases newly formed roots are more porous than roots of the primary root
system (Arikado & Adachi 1955; Luxmoore & Stolzy 1969; Schat 1984; Justin &
Armstrong 1987) and development of new, porous roots is thought to be beneficial to the
whole root system and thus to plant growth and development (Jackson 1955; Stanley,
Kaspar & Taylor 1980). Development of such newly formed roots may be especially
advantageous in situations of transient flooding or in plant species which are unable to
form aerenchyma in the primary root system.
In order to test whether such features might accord with plant distribution within the
genus Rumex, a number of species was subjected to various flooding regimes and the
responses compared. Most of the species used occur as a zonal pattern along an
elevational gradient in the river ecosystem in The Netherlands; this is predominantly
caused by transient flooding in the growing season (Van de Steeg 1984). At the lowest
elevations R. maritimus L. dominates. At slightly higher elevations R. crispus L. and R.
obtusifolius L. are found, whilst R. acetosa L. and R. thyrsiflorus Fingerh. occur above
high flood levels. R . conglomeratus Murr. occurs in periodically wet, low elevation sites
behind the river flood-banks.
All the species except R. acetosa produce a tap-root from which the laterals originate
and, in response to flooding, secondarily formed laterals are developed to different
extents. In R. acetosa new roots are formed on the older primary roots or near the root-
shoot junction.
In this study, the morphology and gas-space development of the root systems are
described and the differences considered in relation to the differential flood-tolerance and
distribution of the species.
MATERIALS AND METHODS
Plant growth
Seeds of Rumex thyrsiflorus, R. acetosa, R. obtusifolius, R. crispus, R. conglomer atus
and R. maritimus were collected from natural populations in the river area near Nijmegen
(The Netherlands).
Sand-culture
Seeds of all species were sown in Petri dishes on wet Whatman filter paper and left to
germinate at 25 °C (day), 15 °C (night), 16 h light at 60-100 /anol m -2 s” 1 PAR, 8 h dark.
After a week a batch of seedlings was transferred to river sand (organic matter content
0-5 + 01 %) in 3-3-litre pots (height 18 cm), and allowed to grow for three to four weeks in
a glasshouse (c. 19 C, relative humidity 70%, 16 h light at minimum 100-150 jumol m ~2
s- 1 PAR, 8 h dark). Half of these plants were then subjected to soil flooding by placing the
pots in 50-litre plastic containers, which were slowly filled with quarter-strength
Hoagland’s solution (Hoagland & Arnon 1950) until the water level was 1-2 cm above the
soil surface. Black polyethylene grains (low density grains, British Petroleum, Grange
mouth, U.K.) floating on the water suppressed growth of algae. The water level was
checked daily. Control, non-waterlogged plants were watered every two or three days
with quarter-strength Hoagland’s solution. At various times during the treatment,
P. L aan et al
695
drained and flooded plants, eight per treatment, were harvested at random and carefully
washed on a sieve to remove the sandy soil. The roots were washed twice to remove the
remainder of the adhering particles, and could then be used to determine root length,
distribution of laterals over the tap-root, and root porosity.
Another batch of seedlings was transferred to vertical PVC-tubes (76 cm long, diameter
7-5 cm) filled with river sand. A metal grid covered with a piece of nylon was mounted on
the bottom of the tubes allowing free contact between the nutrient solution and the
substrate. The plants were allowed to grow for seven weeks, after which the tubes were
flooded to 1-2 cm above the soil surface with quarter-strength Hoagland’s solution. The
water was subsequently maintained at this original level. After three weeks of flooding,
the plants were harvested by carefully pushing out the sand core complete with root
system. Root morphology was described and maximal depth of the different root types
was recorded.
Hydro culture
Five-week-old plants of R. thyrsiflorus, R. crispus and R. mciritimus were grown either
in aerated hydroculture, using quarter-strength Hoagland’s solution (aerated plants), or
in perspex containers with quarter-strength Hoagland’s solution in anaerobic 0*05% agar
(stagnant plants). Stagnant plants developed new lateral roots with a length of 4-8 cm
within one to two weeks. Primary lateral roots of the aerated plants and newly formed
laterals of the stagnant plants were used for examination of root anatomy.
Root porosity by pycnometry
After seven, fourteen and twenty-one days of flooding, three plants were harvested at
random and the root systems carefully washed on a sieve. The apical 20-30 cm of the
different types of new roots were cut off with a razor-blade. Root porosity was measured
with a pycnometer (Jensen et al. 1969); gas was not removed by maceration, but by
evacuation on a freeze-dryer.
Root length
Total root length of secondarily formed roots (i.e. roots formed after flooding) was
measured with a ruler or, when length exceeded 1 m, with a root length scanner (Comair,
accuracy c. 0T m).
Distribution of laterals
Plants which had been flooded for three weeks were separated into shoots and roots.
Tap-roots were divided into segments and, when necessary, the shoot was divided into
segments of about 1 cm. Secondarily formed laterals were counted per segment; they were %
not woody and were thus distinguishable from primary laterals.
Root anatomy
The apical first centimetres of the primary lateral roots and 1-cm segments from the
apical 4-5 cm of the newly formed lateral roots were fixed in 3% (w/v) KMnC>4 (30 min),
washed and stored in distilled water at 3 °C. The segments were dehydrated in an acetone
series (30, 50, 70, 90 and 100%, 10 min each step) and, after rinsing twice in 100% acetone,
placed in propylene oxide for 45 min. The root segments were then embedded in a resin
mixture (Epon 812: Araldite 6005: DDSA = 3:3:8; Frances Allen method, modified from
Coffey, Palevitz & Allen 1972) by gradual replacement of the propylene oxide by the resin
696
FI ood-t olerance o f Rumex species
Rum ex
thyrs/f/orus
Rumex
acetoso
Rumex
obtus/fohus
Rumex
cr/spus
Rumex
cong/omeratus
Rumex
moritimus
0
10
20
30
40
50
60
Root depth (cm)
Fig. 1. Root morphology and root depth of Rumex species after three weeks of flooding in river-
sand (age of the plants at the start of the experiment six weeks; original lateral root system not
shown). Root type: 1, downward-growing laterals; 2, horizontally-growing laterals; 3, thick
downward-growing laterals; 4, adventitious roots.
mixture. The resin was allowed to polymerize for 24 h at 70 C. Sections of 1 ^m from half
way along the blocks were cut with a Cambridge ultra-microtome. The sections were
stained with 1% (v/v) toluidine blue in 1% (w/v) borax and photographed on a Zeiss
photomicroscope. Gas-space cross-sectional area as a proportion of total root cross-
sectional area was determined using an area-meter (MOP-system, Kontron, GMBH).
RESULTS
Changes in root morphology after flooding
The species showed considerable differences in root morphology after three weeks of
flooding in river-sand (Fig. 1). In R. thyrsiflorus the primary laterals died within a week
and few new roots were formed after flooding: some very thin and unbranched
horizontally growing laterals, growing on or just below the water surface, and some
downwardly growing laterals which soon ceased growth. After three weeks the apical 3-5
cm of the longest new downward-growing laterals had a transparent appearance,
indicating that they were fully watersoaked and had ceased to function.
In R. acetosa the primary laterals, as in R. thyrsiflorus, died within one or two weeks and
even fewer new laterals were formed after flooding, most of which were short and
concentrated in the superficial zone. Here too, newly-formed laterals ceased growth and
the tips died after some time, but before this occurred, new superficially growing roots
were formed. Most of these roots originated above the root-shoot junction (adventitious
roots), were very thick and unbranched, and also remained short (type 4; Fig. 1).
In R. ohtusifolius, R. crispus and R. conglomerate the primary laterals did not die when
flooded, but the root system of each of these species was extended considerably by the
formation of different types of new laterals. In particular, large numbers of horizontally
growing laterals were formed on or just below the water surface, and in R. conglomerate
these formed a compact root mat after three weeks (type 2; Fig. 1). All three species also
