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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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Received 16 September 1993; revised 22 August 1994; accepted 29 August 1994