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Functional
Ecology 1995
9, 334-339
Contents, uptake rates and reduction of nitrate of
Rumex palustris and Plantago major spp. major grown
on compacted soil
W. M . H. G . E N G E L A A R , E. J. W . V IS S E R . B. W. V E E N * and C . W . P. M . B L O M
Department of Ecology, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen and *Research Institute for
Agrobiology and Soil Fertility (AB-DLO), PO Box 14, 6700 AA Wagen ingen, the Netherlands
Summary
1. Net nitrate-uptake rates, nitrogen contents and nitrate-reduction rates of Rumex
palustris and Plantago major spp. major plants grown on loose or compacted soil
were studied.
2. Compaction lead to a slower growth rate for both species. Accumulation of nitrate
caused increased internal nitrogen concentrations in P. major when grown on com
pacted soil.
3. After transfer to a nutrient solution plants of both species grown on compacted soil
showed faster nitrate-uptake rates, when expressed per unit lateral-root length or
lateral-root surface, than plants grown on loose soil. For Plantago, rates also increased
when expressed per unit shoot or root dry weight.
4. Based on measured nitrate-reductase rates, only R. palustris should be able to
reduce the largest part of the amount of nitrate taken up from the nitrate solution in the
time period it was exposed to this solution.
5. Nitrate-reductase activities of R. palustris and the shoot of P. major correlated well
with the internal nitrate concentrations. Roots of P. major showed no correlation at all.
A possible explanation for this is compartment of nitrate and nitrate-reductase activi
ties in these roots.
6. The accumulation of nitrate in roots of P. major, occurring on compacted soils,
may be beneficial for maintaining the osmotic potential needed to penetrate soils with
a high mechanical resistance. Rumex palustris, a species not occurring on compacted
soils, does not show such an accumulation.
Key-words: Active nitrate uptake, compaction, niche difference, nitrate-reductase activity, osmotic
potential
Functional Ecology (1995) 9, 334-339
Introduction
Soil compaction can be caused by trampling or by
fluctuating water levels. The contribution of large
pores to the total-pore volume decreases (Coulon &
Bruand 1989) and the bulk density and resistance of
the soil to root penetration increase (Vomocil &
Flocker 1961; Kamaruzaman 1988).
The growth of roots is impeded at large bulk
densities (Veen & Boone 1990; Bengough & Mullins
1991) and is restricted to a smaller soil volume (Blom
1978; Engelaar, Jacobs & Blom 1993). Plant roots
may respond to soil compaction in several ways.
When the compaction is local they may concentrate at
less compacted sites (Garcia, Cruse & Blackmer
1988). When a compacted area in the soil can not be
evaded, many roots can extend themselves by alter
nating radial and longitudinal expansion (Hettiaratchi
1990). The osmotic potential of root cells must gener
ate sufficient turgor to overcome the external pres
sures inhibiting growth. Sprent & Thomas (1984)
argued that nitrate may possibly play an important
role in the osmotic driving force for leaf expansion.
Veen & Kleinendorst (1986) found a similar role for
nitrate in the osmoregulation of Italian Ryegrass.
Because the availability of nitrate is closely linked
with the soil water flow (Habib & Lafolie 1991),
nitrate will be readily available to roots even in com
pacted soils, as long as the water flow in the soil is not
inhibited.
Final contents of nitrate in roots will depend on net
uptake rates, reduction of nitrate and transport to the
shoots. These processes are variable within different
plant species and between individuals of one species.
Nitrate could be important in regulating osmotic
potentials of root cells and the penetrating abilities ot
the roots.
335
Nitrate in
plants on
compacted soil
*n this paper, compaction-induced changes in inter
nal concentrations, nitrate-uptake rates and nitrate-
reductase activities of two river foreland species,
which grow on sites with different levels of soil com
paction were compared. The ecological importance of
these changes with respect to the occurrence of the
species on compacted or non-compacted soils is eval
uated. Species were chosen on the basis of their
occurrence in the field: Rumex palustris Sm. from
wet, untrampled sites (Blom et al. 1994) and Plan
tago major spp. major L. growing on relatively dry
places, on and directly along heavily trampled paths
(Haeck 1992).
Materials and methods
PREPARATIONS
Seeds of R. palustris and P. major were collected in
the Rhine delta area, the Netherlands. They were ger
minated on moist filter paper in Petri dishes at a tem
perature of 10°C 12h/25°C 12h. Seedlings with two
fully developed leaves were transferred to pots, each
with a volume of 1 -7 litres, filled with calcareous river
sand. The soils of half the pots were compacted by
hand after saturating the soil with water. Excess water
was removed by leakage and evapotranspiration. The
bulk densities of the uncompacted and compacted
pots ranged from 1 -02 to 1 • 18 g cm 3 and from 1 -34 to
1-41 gem"3 respectively. Total pore volumes were
55-61% and 46-49% for loose and compacted soils
respectively. At the start of the experiment soils were
moistened with tap water to 60% of their water
holding capacity. In the uncompacted series 38—46%
of the total soil volume and in the compacted series
22-28% was occupied by gas-filled pores. A lid was
placed on all the pots, with a hole for the plants in the
centre, confining the sand. The pots with plants were
placed randomly in a greenhouse. Additional Photo-
synthetically Active Radiation (PAR) of 150|imol
m_2s_l was given during a 16h day period by sodium
(Philips Son-T 400 W, Eindhoven, the Netherlands)
and mercury lamps (Philips HLGR, Eindhoven, the
Netherlands) with day and night temperatures of
21-24°C and 18°C respectively. Water losses owing
to evapotranspiration, were compensated for by
adding nutrient solutions to a predetermined weight
every day. This solution which contained 0-5 m M
Mg2+, 3-0 mM K+, 2-0 mM Ca2+, 4-0 mM N 03~, 0-5 mM
H2P04_, l-75mM S042” and traces of FeEDTA, CP,
B3+, Mn:+, Cu2+ and Mof1+ was applied to the bottom
of the pot by means of a syringe.
SAMPLING
Measurements started 9 weeks after potting. The aim
was to measure plants of comparable size and age.
Therefore, every day one plant, the largest at that time
independent of age, was sampled 2-5 h after start of
the day period. Plants and soil were removed care
fully from the pots and the roots were rinsed with tap
water for 3-5 min to remove all the soil. A prelimi
nary experiment showed that this treatment did not
significantly influence the nitrate-uptake rate by
intact plants. Hereafter, the plants were transferred to
the plant compartment of an uptake measurement
system, consisting of two independent closed circuits.
The first had a total internal volume of 600 ml and
consisted of a plant root cuvette, an aeration cuvette
and an electrode cuvette through which a nutrient
solution was pumped, at a rate of 1800 ml min '. The
second circuit contained a thermostatic water-bath
(Haake G, Mess-Technik GmbH, Karlsruhe,
Germany) keeping the temperature of the nutrient
solution at 25 °C. The nutrient solution consisted of
5 m M MES-buffer with the trace elements in the same
concentration as during the growth period, while the
concentration of the macro nutrients was decreased to
one tenth of the concentration applied to the pots. At
this nitrate concentration (400 |JM), the uptake of
nitrate is an active process (Glass et al. 1990) occur
ring at a saturated rate, Vmax (Doddema & Telkamp
1979; De Willigen & Van Noordwijk 1987). The fact
that uptake was an active process was checked by
supplying KCN (final concentration 1 m M ) to addi
tional plants and comparing the uptake rates before
and after this addition. The solution was brought to a
pH of 5-6 with Tris-buffer. Net nitrate consumption
by the plant was continuously measured with a
nitrate-specific electrode (Philips, IS 561-N03-,
Philips Scientific, Cambridge, UK) in combination
with a reference electrode (Yokogawa SR20/AP24,
Electrofact B.V., Amersfoort, the Netherlands), for a
period of 120-160 min starting 20-30 min after trans
fer of the plants. The pH was monitored by a pH elec
trode (Hanna Instruments, HI 1911, Braunschwig,
Amsterdam, the Netherlands) in combination with a
Methrom 654 pH-meter. The nutrient solution and a
Perspex cylinder placed over the shoot were Hushed
with moistened air preventing excessive evaporation.
Four TL-lamps (Philips TLD 18W/84) provided a
PAR of 100 (amol m 2 s~1.
Directly after the nitrate-uptake measurements
nitrate-reductase activity in the roots and shoots of the
plant were determined in duplicate or triplicate by a
modification of the assay described by Jaworski
(1971). For the shoot the youngest two or three fully
developed leaves were cut into segments of
0-5 x 0-5 cm after removal of the veins. Between 100
and 250 mg fresh weight of these segments were put
into 25 ml flasks wrapped with aluminium foil, con
taining 4 ml 0-25 m phosphate buffer (pH = 7-8) with
chloramphenicol (0-5 mg m l 1). After two 1 min peri
ods of vacuum infiltration, 1 ml 0-2 M KN03 solution,
containing 1 -propanol (75 jul ml 1) was added, and the
flasks were closed with a rubber stopper. Samples of
0-4 ml were taken after 30 and 60 min incubation at
30°C in a shaker (60rpm). N 02~ accumulation was
336
W. M. H. G.
Engelaar et al.
Table 1. Free N H / , N 0 3~, organic nitrogen and total nitro
gen concentrations of shoots and roots (nmol g dry wt 1 ) of
Rumex palustris and Piantalo major ssp. major plants
grown on loose (L) or c compacted (C) soil. For each
species and treatment three plants were combined into one
sample
N H / o1
N organic N total
R. palustris
shoots
L 16 34 1920 1970
C 10 19 1905 1934
lateral roots
L 8 53 1074 1 132
C 10 80 1 141 1231
P. major
shoots
L 3 12 1234 1249
C 7 446 1775 2228
roots
L 1 53 771 825
C 6 536 850 1392
measured colorimetrically using a photospectrometer
(Vitratron, Meyvis Co., Bergen op Zoom, the Nether
lands). The same assay was used for 300-500 mg
fresh weight of 10-1-5 cm long root segments but
instead the flasks were Hushed for 1 min with N2-gas
before each incubation period.
After determination of remaining lateral-root
length (Comair rootscanner. Hawker de Havilland
Victoria Ltd, Melbourne, Australia), lateral-root vol
ume. tap-root fresh weight, lateral-root fresh weight
and shoot fresh weight, the dry weight (24 h, 70 °C)
of shoot, tap root and lateral roots were measured and
lateral root surface area was calculated.
For each species and both treatments three addi
tional plants were harvested and their nitrate-reduc-
tase activities were measured as described above.
Hereafter, the three plants of one species and treat
ment were combined into one sample and the internal
nutrient concentrations of these samples were deter
mined according to Troelstra (1983).
STATISTICAL ANALYSES
Differences in plant-growth parameters, uptake rates
and nitrate-reduction rates within one species
between pots with either loose or compacted soil were
analysed by Student’s /-tests. Growth parameters
were log transformed and percentages arcsin trans
formed before analysis. Differences in growing
period between the compacted and loose series of one
species were tested with a Mann-Whitney ¿7-test.
Linear regression lines were calculated for the mean
internal nitrate concentrations versus nitrate-reduc-
tase activities (Sokal & Rohlf 1981). All statistical
analyses were made with the aid of the SAS statistical
package (SAS Institute Inc, Cary, NC).
Results
Plantago plants on compacted soil produced less
shoot dry weight than plants on loose soil
(0-16-0-59 g in 97-103 days compared to 0-68-1-81 g
in 64-81 days) and less lateral-root dry weight
(0-08-0-30g compared to 0-29-0-91 g). For R. palus-
tris plants the growth period on compacted soil, com
pared to loose soil, was also significantly longer
(87-97 days and 67-80 days, respectively) but final
shoot dry weights (0-52-l-70g and 0*90— 1 *61 g for
compacted and loose soil, respectively) and lateral-
root dry weights (0-13-0-47 g compared to
0-27-0-49 g) did not differ significantly. Tap-root dry
weight of R. palustris decreased significantly when
plants were grown on compacted soil: 0-16-0-73 g
and 0-75-1-20g for the compacted and the loose soil,
respectively.
For R. palustris, N-concentrations of plants grown
on loose or compacted soil did not differ (Table 1).
For P. major, plants grown on compacted soil had a
greater total N concentration than those grown on
loose soil, which was the result of a bigger free N03 and organic N concentrations.
For R. palustris plants, soil compaction lead to a
faster net uptake rate when expressed per unit lateral-
root length or unit lateral-root surface area (Table 2).
For P. major plants the uptake rates of plants grown
on compacted soil were faster when expressed per
unit shoot dry weight, lateral-root length, lateral-root
dry weight or lateral-root surface area. Addition of
KCN reduced the uptake rates to 0-5% of the initial
Table 2. Mean nitrate uptake rates (fimol h 1 ± 1 SEM) per plant, per g shoot dry weight (SDW), per m lateral-root length
(LRL), per g lateral-root dry weight (LRDW ) and per 1000 cm2 lateral root surface area (LRSA) of Rumexpalustris and Plun-
tago major ssp. major plants in a nutrient solution after growing in loose (L) or compacted (C) soils. = 4-5
Plant SDW LRL LRDW LRSA
R. palustris L 24-5±3-6 200±3-2 0-35±0-04* 69-1 ±8-3 30-7±4-2*
C 23-4±3-1 21 * 8± 1 - 3 0-82±0-12 86-3±5-4 611 ±8-3
P. major L 24-2±3-3 20-1 ±2-0* 0-28±0-03* 49-3±7-0* 24-8±2-4*
C l9-4±5-9 47-7±8-8 1 -02±0-18 143-7±36-0 73-2±l3-9
*P< 0-05
337
Nitrate in
plants on
compacted soil
rate, tor both species. Rumex palustris shoots reduced
the most of the nitrate taken up, based on the
measured nitrate-reductase activities (Table 3). For P.
major, the contributions of roots and shoots are
approximately equal (Table 3). The relative contribu
tions of roots and shoots, the total amount of nitrate
being reduced and the percentage of nitrate being
taken up which could have been reduced on basis of
the nitrate-reductase activity measurements were not
affected by growth on loose or compacted soil for
either species. For R. palustris, about 80% of the net
nitrate uptake could have been reduced with the
measured nitrate-reductase activities; for P. major the
activities only account for a minor part of the net
nitrate uptake. Figure 1 gives the relation between the
internal nitrate concentrations and nitrate-reductase
activities for roots and shoots. In this figure plants
that were not exposed to the nutrient solution were
also taken into account. There was a clear relation
between the internal nitrate concentrations and
nitrate-reductase activities with the exception of P.
major roots which showed no trend at all.
IcnI CM
O
30FI .palustris
20
T 10JZ£Q
0
01 8
0
0 100 200
P. major ssp. major
Discussion
Although the compaction caused a delay in growth, it
is obvious from the nitrosen concentrations that the
plants did not suffer from nitrogen deficiency (Table 1).
Also for both species shoot/root ratio did not decrease
as is often observed when plants become N-stressed
(Hilbert 1990; Rufty, MacKown & Volk 1990).
The laterals of R. palustris grown on compacted
soil had a lower specific root length (1104 m of root
per g dry weight ± 12-0) than those grown on loose
soil (200-9± 10-8). This implies that these morpho
logically different roots were able to support the same
amount of functional biomass with a less elongated
system compared to plants grown on loose soil (Table
2). It was assumed that the tap root contributed far
less to carbon or mineral nutrient acquisition, as it had
Table 3. Average nitrate reduction rates per plant (jimol h 1), percentages of nitrate
reduction accounted for by roots and shoots and percentage of nitrate taken up, mea
sured with an ion-specific electrode, potentially reduced, based on nitrate reduction
measurements (/i = 4-5, all ±SE). Rumex palustris and Plantago major spp. major
plants were grown on loose (L) or compacted (C) soil before being transferred to a
400 |i m nitrate solution for 3h. No significant differences between the loose and
compacted series within one species were found (^<0-05)
N.R.A. % roots % shoots % converted
R. palustris
L 18-8± 1-13 11 ± 1 -7 89± 1 -7 82±8-4
C 20-3±3-79 12± 1 -9 88± 1 -9 78±4-2
P. major
L 5*34± 1 -34 57± 10 43± 10 26±6-5
C 1 -69±0-51 53±5-9 47±5-9 9-3±20
Fig 1. Nitrate-reductase activities (jimol N 0 2 g dry wt 1
h 1 ) of shoots (open symbols) and roots (closed symbols) of
Rumex palustris and Plantago major ssp. major plants
grown on loose or compacted soil versus their internal
nitrate concentrations. Nitrate concentrations were deter
mined on combined samples of three to five plants of the
same species and treatment. Significant linear regressions
between the two parameters are represented by the lines.
P was 0-0004, 0-0005 and 0-022 for the shoots and roots of
R. palustris and shoots of P. major respectively.
a very small surface area and length compared to the
lateral roots.
The measured uptake rates were the result of an
active uptake process as may be concluded from the
inhibition by KCN. The uptake rates remained con
stant with decreasing nitrate concentrations in the
nutrient solution confirming that Vmax was measured
at substrate concentrations well above the Km value.
The fact that no changes in nitrate concentration of
the nutrient solution were found after addition of
KCN suggests no measurable nitrate efflux from orc c
diffusion into the roots.
For R. palustris, efflux may have been prevented
by the high percentage of nitrate reduction and nitrate
transport to the shoot (Table 3) keeping internal
nitrate concentrations relatively small. Indications for
such a mechanism are the correlations in shoots and
roots between nitrate-reductase activities and internal
nitrate concentrations (Fig. 1), which were also found
in other experiments for different Rumex species
(Langelaan & Troelstra 1992). For P. major, the
uptake rates seem even more independent from inter
nal nitrate concentrations as the plants from com
pacted soil not only show the fastest uptake rates but
0 100 200 300 400 500 600
[NO3] internal
338
W. M. H. G.
Engelaar ct al.
also the greatest internal concentrations (Tables 1 and
2). Compartment of the nitrate within the root as pro
posed by Siddiqi, Glass & Ruth ( 1991 ) could explain
the independency of the nitrate-reductase activity of
the internal nitrate concentration in the roots. A large
part of the nitrate could be non-accessible to the
nitrate reductase by being stored in the vacuole or
alternatively in cells of low nitrate-reductase activity.
Storage of nitrate in, for example, the vacuole would
also reduce the efflux from the root.
Plantcigo major seedlings are very sensitive to a
dry soil in their early growth stages (Blom 1976).
Evapotranspiration dried the top soil in our pots
quickly. In compacted soil with a greater penetration
resistance, Plant ago seedlings, which have a smaller
root than R. palustris at the start, were probably not
able to reach the deeper, wetter soil in a short time,
which would explain their delay in growth.
Both species were able to increase their net nitrate-
uptake rates in response to soil compaction, although
the nitrate taken up was dealt with differently. Plas
ticity in uptake rates might be beneficial to the plant
under circumstances in which the nitrate availability
on the root-soil boundary is temporarily limiting
(Kachi & Rorison 1990), resulting in depletion zones
around the roots (Robinson 1991 ). The occurrence of
P. major ssp. major is largely determined by its ability
to grow in soils with a large mechanical resistance
and its resistance to trampling by cattle or machinery
(Haeck 1992). The high nitrate content of the roots,
resulting from a fast uptake rate in combination with a
slow reduction rate, may be very important in main
taining the osmotic potential the cells need to expand.
Acknowledgements
We would like to thank S. Troelstra and W. Smant, of
the Netherlands Institute of Ecology, and A.G.
Bengough and D. Robinson, of the Scottish Crop
Research Institute, for their practical help and useful
comments.
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Recommended