Flora 200 (2005) 434–443 A study on nitrate reductase activity (NRA) of geophytes from Mediterranean environment Hu¨lya Arslan , Gu¨rcan Gu¨leryu¨z Department of Biology, Arts and Science Faculty, University of Uludag˘, 16130 Go¨ru¨kle Bursa, Turkey Received 20 June 2004; accepted 25 February 2005 Abstract Nitrate reductase activity (NRA) in different compartments of 14 Mediterranean geophytes (bulbous, tuberous and rhizomatous) and actual mineral nitrogen (NO 3 and NH 4 + ) in their soils were investigated. The nitrate reduction capacities of each species were determined as NRA per total plant material. Differences among compartments for NRA were significant in all species. The highest NRA was found in leaves of tuberous species (Anemone coronaria, Cyclamen coum) and of most bulbous species (Allium flavum, Allium guttatum, Bellevelia sarmatica, Galanthus plicatus, Leucojum aestivum, Ornithogalum nutans, Tulipa sylvestris). Therefore, in this group of species the contribution of the leaves to total plant NRA was the highest. The other bulbous species (Allium scorodoprasum, Crocus chrysanthus, Fritillaria bithynica, Muscari neglectum) and one rhizomatous taxon (Iris suaveolens) have a different NRA distribution within the plants. In these species the highest values of NRA were found in different organs. For example, in Allium scorodoprasum the highest NRA was in tunics, and in flowers in M. neglectum. Although leaves are the main compartments reducing nitrate in most of the studied geophytes, other compartments also contribute to total plant nitrate reduction. Our results show that the nitrate reduction capacity is different among geophyte species. Even if it roughly reflects the nitrogen supply in a habitat, differences in nitrate reduction capacities of different species collected from same sites indicate that the nitrate reducing capacity is species-specific. r 2005 Elsevier GmbH. All rights reserved. Keywords: Geophytes; Nitrate reduction; Nitrate reductase (NR); Mediterranean environment Introduction Nitrogen cycling plays a critical role in the biogeo- chemistry of ecosystems and the nitrogen supply is of great importance for the composition of vegetation. Nitrogen acquisition is dependent on the rate of nitrogen supply and the source of nitrogen available (Ho¨gbom and Ohlson, 1991). The utilisation of nitrate begins with nitrate uptake by roots and involves reduction and distribution among different tissues and cell compartments. Nitrate assimilation can occur in both shoots and roots of most plant (Pate, 1973). The partitioning of NO 3 reduction between shoots and roots varies with plant species, NO 3 supply, growth stage, shoot removal and assay methods (Jiang and Hull, 1999). ARTICLE IN PRESS www.elsevier.de/flora 0367-2530/$ - see front matter r 2005 Elsevier GmbH. All rights reserved. doi:10.1016/j.flora.2005.02.003 Corresponding author. Tel.: +90 224 4429256 1411; fax: +90 224 4428136. E-mail address: [email protected] (H. Arslan).
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ARTICLE IN PRESS
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Flora 200 (2005) 434–443
www.elsevier.de/flora
A study on nitrate reductase activity (NRA) of geophytes from
Mediterranean environment
Hulya Arslan�, Gurcan Guleryuz
Department of Biology, Arts and Science Faculty, University of Uludag, 16130 Gorukle Bursa, Turkey
Received 20 June 2004; accepted 25 February 2005
Abstract
Nitrate reductase activity (NRA) in different compartments of 14 Mediterranean geophytes (bulbous, tuberous andrhizomatous) and actual mineral nitrogen (NO3
� and NH4+) in their soils were investigated. The nitrate reduction
capacities of each species were determined as NRA per total plant material. Differences among compartments forNRA were significant in all species. The highest NRA was found in leaves of tuberous species (Anemone coronaria,Cyclamen coum) and of most bulbous species (Allium flavum, Allium guttatum, Bellevelia sarmatica, Galanthus plicatus,Leucojum aestivum, Ornithogalum nutans, Tulipa sylvestris). Therefore, in this group of species the contribution of theleaves to total plant NRA was the highest. The other bulbous species (Allium scorodoprasum, Crocus chrysanthus,
Fritillaria bithynica, Muscari neglectum) and one rhizomatous taxon (Iris suaveolens) have a different NRA distributionwithin the plants. In these species the highest values of NRA were found in different organs. For example, in Allium
scorodoprasum the highest NRA was in tunics, and in flowers in M. neglectum. Although leaves are the maincompartments reducing nitrate in most of the studied geophytes, other compartments also contribute to total plantnitrate reduction.Our results show that the nitrate reduction capacity is different among geophyte species. Even if it roughly reflects
the nitrogen supply in a habitat, differences in nitrate reduction capacities of different species collected from same sitesindicate that the nitrate reducing capacity is species-specific.r 2005 Elsevier GmbH. All rights reserved.
Nitrogen cycling plays a critical role in the biogeo-chemistry of ecosystems and the nitrogen supply is ofgreat importance for the composition of vegetation.Nitrogen acquisition is dependent on the rate of
e front matter r 2005 Elsevier GmbH. All rights reserved.
nitrogen supply and the source of nitrogen available(Hogbom and Ohlson, 1991). The utilisation of nitratebegins with nitrate uptake by roots and involvesreduction and distribution among different tissues andcell compartments. Nitrate assimilation can occur inboth shoots and roots of most plant (Pate, 1973). Thepartitioning of NO3
� reduction between shoots and rootsvaries with plant species, NO3
� supply, growth stage,shoot removal and assay methods (Jiang and Hull,1999).
ARTICLE IN PRESSH. Arslan, G. Guleryuz / Flora 200 (2005) 434–443 435
The reduction of NO3� catalysed by the enzyme
Nitrate Reductase (NR) is the first and rate-limitingstep of nitrate assimilation by plants (cf. Lee andStewart, 1978). Nitrate reductase activity (NRA) isheavily influenced by the amount of nitrate present andby light (Beevers and Hageman, 1983). Since NR is asubstrate-inducible enzyme (Solomonson and Barber,1990), the NRA of a plant is assumed to reflect the long-term nitrate supply of that plant (Lee and Stewart, 1978;Stadler and Gebauer, 1992). NRA can be accepted as anindicator of nitrate supply in ecological studies (Ge-bauer et al., 1988; Hogberg et al., 1986; Lee and Stewart,1978; Lodhi and Ruess, 1988; Olsson and Falkengren-Grerup, 2003; Stewart et al., 1972).The ability to assimilate different sources of nitrogen
varies among plant species (Lee and Stewart, 1978).Both inter-specific and habitat-related intra-specificdifferences in nitrate assimilation ability have beenreported from a wide range of taxa (Dirr et al., 1973;Gebauer et al., 1988; Guleryuz and Arslan, 1999; Havillet al., 1974; Hogberg et al., 1986, 1990; Lee and Stewart,1978; Osborne and Whittington, 1981; Press and Lee,1982; Woodin et al., 1985). In addition, the differenceamong organs (especially shoot and roots) of plants wasreported by many authors (Andrews, 1986; Gebauer etal., 1984, 1988; Guleryuz and Arslan, 1999; Jiang andHull, 1999; Pate, 1980; Scheurwater et al., 2002). Inprinciple, nitrate reduction can take place in the roots aswell as in the above-ground organs of higher plants.Preferentially nitrate is reduced in the leaves of mostherbaceous plants (Gebauer et al., 1988; Pate, 1980).Although a preferential reduction of nitrate in the rootsof trees was for long times assumed in the literature,Smirnoff et al. (1984) and Al Gharbi and Hipkin (1984)were among the first to show that broadleaf andconiferous trees also have NRA in CO2 assimilatingorgans under field conditions. Moreover, in recentstudies it has been reported that preferential nitratereduction occurs in the above-ground organs (Black etal., 2002; Gebauer and Schulze, 1997; Olsson andFalkengren-Grerup, 2003).Although many investigations of nitrate assimila-
tion were carried out with herbaceous and woodyplants, no attention was paid to geophytes. However,these plants are pioneers of the vegetation seasonand heralds of spring reflecting the status of soil inearly spring.In this study, we aimed to obtain basic information
about the nitrogen metabolism of geophytic species.For this reason, differences among the compartmentsfor NRA values and their contribution rate to totalplant NRA of geophytes were compared. Nitrateassimilation capacity per species was determined.We also investigated the relation between the actualmineral nitrogen of the habitats and the NRA of therespective species.
Material
In this study, 14 geophytic plant species belonging tothe families Liliaceae, Iridaceae, Primulaceae andRanunculaceae, many of which were bulbous (Allium
um, Ornithogalum nutans, and Tulipa sylvestris) wereselected as research material. Tuberous species wereAnemone coronaria and Cyclamen coum. A rhizomatousspecies was Iris suaveolens. General characteristics of thesites are given in Table 1. ‘‘Flora of Turkey and the EastAegean Islands’’ is referred to for the names of taxacited in the text (Davis, 1965–1985).For analysis of nitrogen supply potential of the
sampling sites, soil samples from the root area of eachplant harvested were examined for their actual mineralN and total N content, C/N ratio and pH value.
Methods
Sampling and preparing the material
Plant samples were collected around Bursa City,Turkey. Phytogeographically, Bursa lies in the EasternMediterranean. General characteristics of the sites aregiven in Table 1. Samples were collected between 09.00and 11.00 a.m. Plant samples were taken from plots ofabout 200m2 size. Five samples were taken for eachspecies except of B. sarmatica and A. coronaria. SinceAnemone coronaria was collected from two differentsites, nine samples were taken for this species. Threesamples were harvested of B. sarmatica. Plant sampleswere harvested in the flowering stage.Great care was taken to collect the complete biomass
(above- and belowground) of each plant. Especially,harvesting of complete root systems was difficult underfield conditions.Also, soil samples (about 200–300 g) were taken from
the rooting area of each plant harvested. They weresieved by 4-mm standard sieve and then placed intoplastic bags. Soil samples together with the harvestedplant samples were separately put into plastic bags andtransported in ice boxes to the laboratory.NRA test was applied to different compartments of
plants. The separation of whole plants into compart-ments was different in bulbous, tuberous and rhizoma-tous species due to their morphological properties. Firstof all, below-ground (roots, tubers, bulbs and rhizomes)and above-ground parts (leaves, flowering stem andflowers) were separated. In addition to basic plantcompartments in the tuberous dicot species (Anemone
coronaria and C. coum), NRA of petioles was measured
H. Arslan, G. Guleryuz / Flora 200 (2005) 434–443436
also. In contrast, leaves of bulbous and rhizomatousmonocot species (except of F. bithynica) were separatedinto two parts: light coloured parts (non-green) from thebelowground and coloured parts from the aboveground.In the following, light coloured parts from the basis ofeach leaf are cited as ‘‘leaf base’’ and the other parts arecited as ‘‘leaf’’.
NRA test
NRA of the plant material was determined accordingto an in vivo test described by Hageman and Hucklesby(1971) and Jaworski (1971) and with the modificationgiven by Gebauer et al. (1984). This spectrophotometricmethod is based on the determination of the absorbance
of nitrite (NO2�) which is formed as product of the
reduction of nitrate in the incubation medium.The NRA test was carried out in two steps. In the first
step, whole plants were cleaned with distilled water.Roots, leaves, stems, petioles and flowers were cut intopieces of 8–10mm length. Cubic parts with similarlengths were cut from belowground parts such as bulbs,tubers and rhizomes. The plant material was incubatedfor 2 h at 30 1C in the dark with 5ml of incubationbuffer after vacuum infiltration (6mm Hg; twice for 30 seach) and addition of nitrogen gas. The composition ofthe incubation buffer is 0.08M KNO3, 0.25M KH2PO4
and 1.5% n-propanol, adjusted to pH 7.5 with NaOH(Gebauer et al., 1984). At the end of the incubationperiod, plant materials were removed from the incuba-tion medium. All plant material was dried at 105 1C
ARTICLE IN PRESSH. Arslan, G. Guleryuz / Flora 200 (2005) 434–443 437
up to constant weight and weighed for dry weightdetermination.In the second step of the NRA test, nitrite was
determined colorimetrically at 540 nm by adding 0.3mlof 0.1% N-naphtylethylendiamine HCl solution, 0.3mlof 5% sulphanilamide in 3N HCl, 1ml distilled waterand 1ml of incubation buffer (Gebauer et al., 1984). TheNRA (mmol NO2
� gdw�1 h�1) was calculated by the values
obtained from the two phases of the test.
Soil analyses
Actual soil content of mineral nitrogen was analyzedin subsamples of the fresh soil transported in ice cooledplastic bags to the laboratory. The remaining soilsubsamples were dried in air. The mineral nitrogen ofthe soil was determined by the micro-distillation method(Bremner and Keeney, 1965; Gerlach, 1973). Soilsamples (40 g fresh soil) were put into 500ml Erlenmeyerflasks together with 100ml solution of 1% KAl(SO4)2.This suspension was mechanically shaken with a verticalshaker for 30min and than filtered through Whatmanblack-band (No. 1) filter paper. Some pieces of thymolcrystals (C10H14O) were added to inhibit microbialactivity before storing the extracts in a refrigerator(+4 1C). The mineral N content of the extracts wasdetermined using steam distillation. Aliquots (20ml) ofthe filtered clear solution with added 0.2 g of MgO weresteam distilled in a micro-Kjeldahl apparatus to obtainNH4
+–N. Nitrate–N in the same solution was reduced toNH3 by adding 0.2 g of Devarda’s Alloy. The distillateswere separately collected in 5ml of boric acid (2%),containing 200 ml bromocresol green/methyl red mixedindicator (pH 5.0), and titrated with 0.005N H2SO4.Total nitrogen and organic carbon were analyzed in
air-dried soil samples. Nitrogen was determined by aKjeldahl method using salicylic-sulphuric acid andselenium (Steubing, 1965). Organic carbon content wasdetermined by the incineration method (digestion withconcentrated sulphuric acid and titration by K2Cr2O7)(Steubing, 1965). Total nitrogen and organic C data arepresented as percentage (%) of soil dry weight. C/N wascalculated by dividing organic carbon (%) to totalnitrogen (%). The pH of air-dried soil samples wasmeasured by preparing saturated mud with a soil/waterratio 1:2.5 (Steubing, 1965).
Calculations and statistical analyses
Total dry weight and NRA per unit of respectiveplant compartments (NRA per unit compartment) weredetermined. Total plant biomass was calculated as thesum of dry weights of each compartment. These valueswere used to determine the NRA per unit of therespective compartments and the nitrate reduction
capacities of total plants (NRA per unit of total plantmaterial). NRA per unit of the respective compartments(mmol NO2
�h�1) was calculated by multiplication ofmean NRA per g (mmol NO2
� gdw�1 h�1) with the biomass
(gdw) of each of the respective compartments. Calcula-tion of nitrate reduction capacity (NRA per unit totalplant material) for each species is based on the followingequation (Gebauer and Schulze, 1997):
Nitrate reduction capacity ðNRA per unit total plantÞ ¼
NRAaA þNRAbB þ � � � þNRAxX
A þ B þ � � � þ X,
NRAaA þNRAbB þ � � � þNRAxX : NRA
per unit of respective compartments
A þ B þ � � � þ X : biomass of the respective compartments:
Significant differences among the sampling sites of theplant samples according to some soil parameters andNRA values of plant compartments were tested by one-way ANOVA. We used Tukey’s HSD test to determinethe different groups among samples. Correlationsbetween NRA values in leaves and NO3
� content insoils were tested. All statistical analyses were based on asignificance level of 0.05 (Zar, 1984). Furthermore, thecontributions of individual plant compartments to totalplant NRA were calculated.
Results
Total nitrogen [N%], organic carbon [C%] and actualmineral nitrogen content (NH4
+–N and NO3�–N), C/N
ratios in the soil samples, and pH-values are shown inTable 2. Significant differences among sites were foundfor all soil parameters (po0:05). pH values in the soilsranged between 5.6 and 7.9. The soil around mostspecies was generally neutral to alkaline; only soil of theA. flavum growing place was acidic. While the soils of G.
plicatus and F. bithynica contained the highest organiccarbon (13.4771.26% and 13.0470.54%, respectively),the lowest amount of carbon was in the soils of A.
guttatum (1.6071.01%) and T. sylvestris (1.5571.04%).Total N content was highest in the soil of F. bithynica
(1.4570.09%) and lowest in T. sylvestris soil (0.1570.01%). C/N ratios ranged between 6.173.5 and55.872.9 for all soil samples. It was lowest in the soilof A. guttatum and highest in the soil of C. chrysanthus.The highest actual mineral nitrogen (NH4
+–N andNO3
�–N) contents of all investigated soils was found atthe site of G. plicatus. While the lowest NO3
�–N was inthe soil of both sites of A. coronoria, the lowest NH4
+–Nwas found in the sites of A. flavum, B. sarmatica,L. aestivum, T. sylvestris and A. coronaria.Mean NRA per unit of biomass [mmol NO2
aHighest difference groupsbIntermediate difference groupscLower difference groups
H. Arslan, G. Guleryuz / Flora 200 (2005) 434–443438
compartment [mmol NO2�h�1] and contribution rates of
the respective compartments to total plant nitratereduction are shown in Table 3. Significant differencesbetween compartments in NRA values per gdw areshown for each plant species by minor letters in the sametable. For all species, differences in NRA amongcompartments were significant (po0:05). Mean NRAvalues obtained from the leaves were usually higher thanthose of other compartments of bulbous species exceptfor A. scorodoprasum, C. chrysanthus, F. bithynica andM. neglectum. For example, in B. sarmatica leaf NRA(1.7070.29 mmol NO2
� gdw�1 h�1) was 10-fold higher than
root NRA (0.1770.03 mmol NO2� gdw
�1 h�1). It was 2.6-fold higher in G. plicatus leaves as compared with theroots. Although the highest biomass was not found inthe leaves of A. guttatum, B. sarmatica, G. plicatus, L.
aestivum, O. nutans and T. sylvestris, the contribution ofleaves to total plant NRA was the highest in theseplants. Here, the contribution of leaf NRA to total plantNRA ranged between 40.4% and 76.8%. Mean NRA ofother compartments and their contribution to totalplant NRA varied depending on species. For instance, inA. flavum and B. sarmatica, there was no significantdifference for NRA among plant compartments exceptfor leaves. However, NRA obtained from roots, bulbs,flowering stems and flowers was higher than that oftunics and leaf bases in G. plicatus (Table 3). In contrast
to G. plicatus, NRA of root, bulb and tunics were lowerthan those of flowering stem and flower in O. nutans. Inthis species the contributions of these compartments tototal plant NRA varied between 0.2% and 22.1%.With respect to the distribution of nitrate reduction,
A. scorodoprasum, C. chrysanthus, F. bithynica and M.
neglectum were different from the other examinedbulbous species. In these species, highest NRA valuesper gdw were not detected in the leaves, but in othercompartments. For example, highest NRA per gdw wasobtained from tunics (0.3770.19 mmol NO2
� gdw�1 h�1) in
A. scorodoprasum, from flowers (0.4670.18 mmolNO2
� gdw�1 h�1) in M. neglectum, from bulbs in F.
bithynica (0.5470.19 mmol NO2� gdw
�1 h�1) and from leafbase (1.5470.65 mmol NO2
� gdw�1 h�1) in C. chrysanthus.
The within-plant NRA distribution of tuberousspecies was similar to that observed for most of thebulbous species. Both examined tuberous species, A.
coronaria and C. coum, had the highest NRA in theirleaves (0.4770.54 mmol NO2
� gdw�1 h�1 and 3.247
0.83 mmol NO2� gdw
�1 h�1, respectively). NRA values ofleaves in C. coum were significantly different from thoseof all other compartments. In A. coronaria NRA of thetuber was lowest (0.0970.03 mmol NO2
� gdw�1 h�1). NRA
of other compartments in this species was similar to eachother. In tuberous species, NRA values per gdw of thetuber were generally low, but the contribution of these
ARTICLE IN PRESS
Table 3. Mean NRA (mmol NO2�gdw
�1 h�1), dry weight (g) per compartment and NRA per compartment in geophytes plants from
Mediterranean environment. For mean NRA values, different letters indicate significant differences between the groups according to
Tukey’s HSD test (rejection level 0.05). Means7Standard Deviaiton.
aHighest difference groupsbIntermediate difference groupscLower difference groups
Fig. 1. Correlation between leaf NRA and nitrate content in
soil.
H. Arslan, G. Guleryuz / Flora 200 (2005) 434–443 441
compartments to total plant NRA was high because oftheir high biomass.NRA distribution in I. suaveolens, a rhizomatous
species, was different from that in other geophyte speciesexcept M. neglectum. In the case of I. suaveolens, NRAper gdw of the flowers was higher than the NRAfound for all other compartments (0.3570.13 mmol
NO2� gdw
�1 h�1). Furthermore, the contribution of theflowers to total plant NRA was also the highest (30.3%).NRA per gdw in the flowers of I. suaveolens wassignificantly different from all other compartments(Table 3). High contributions of roots and rhizome(24.2%) to total NRA resulted from the high biomass ofthese compartments (0.282 and 0.626 gdw, respectively).Calculation of the nitrate reduction capacities (NRA
per unit of total plant material) was based on measure-ments of total biomass and of NRA per unit biomass.NRA per unit total plant material was used to evaluatethe nitrate reduction capacities of the geophytes. Thenitrate reduction capacity was the highest in G. plicatus
(0.85 mmol NO2� gdw
�1 h�1). The lowest capacity wasfound in M. neglectum (0.07 mmol NO2 gdw
�1 h�1) and I.
suaveolens (0.08 mmol NO2� gdw
�1 h�1).The correlation between leaf NRA values and actual
NO3�–N content of the respective soils was tested and
a significant positive correlation was found (po0:05)(Fig. 1).
Discussion
High leaf NRA values occurred in most of the bul-bous (A. flavum, A. guttatum, B. sarmatica, G. plicatus,
ARTICLE IN PRESSH. Arslan, G. Guleryuz / Flora 200 (2005) 434–443442
L. aestivum, O. nutans and T. sylvestris) and the twotuberous (A. coronaria and C. coum) species underinvestigation. The high leaf NRA values evidence leavesas the main nitrate reducing compartments of thesespecies. With exception of some species (A. scorodopra-
sum, C. chrysanthus, F. bithynica, M. neglectum and I.
suaveolens), we can propose this as a general NRAdistribution pattern of geophytic plant species in whichNO3
� reduction mainly occurs in leaves. This conclusionis in agreement with the general rule that most herbac-eous plants reduce nitrate preferentially in the leaves(Gebauer et al., 1988; Pate,1980). However, although thehighest NRA was found in the leaves of tuberous andmany bulbous species, other compartments—even sto-rage organs such as bulbs and tubers—have nitratereducing capabilities too. The ability to assimilate NO3
�
of these compartments is important for efficient nitrateutilization of these plants. Due to the contribution ofthese compartments to total plant, nitrate reduction ishigh depending on high biomass rates. This situation wasparticularly clear in the tested tuberous species.Even if nitrate was reduced by preference in leaves of
most of the examined species, this NRA distributionpattern was not observed in all species in our study. Forexample, in F. bithynica the highest NRA was found inthe bulb, an in M. neglectum it was found in the flowers.This can be characterized as a variation of the normalNRA distribution model among plant species. Partition-ing of nitrate reduction has been shown to be affected bygenotype, growth stage and NO3
� supply (Hunter et al.,1982; Lewis et al., 1982; Samuelson et al., 1995).Among the investigated geophytes there was also a
variation in nitrate assimilating capacity. The highestnitrate reduction capacity was calculated for G. plicatus
taken from a nitrate rich habitat. In contrast, it was nothigh in A. guttatum taken from another nitrate richhabitat. This suggests that nitrate reduction capacity isrelated to species rather than to NO3
� supply of soil. Ithas been reported that the ability to assimilate differentsources of nitrogen varies among plant species (Lee andStewart, 1978).There was a positive correlation between leaf NRA
and actual NO3� content of soil. This correlation can be
seen as a result of the substrate-based induction of NRin plants, as exemplified by high leaf NRA in G. plicatus
collected from a nitrate rich habitat. This is consistentwith similar findings of many authors (Solomonson andBarber, 1990; Tischner et al., 1993). Nitrate reductioncapacity is related to species characteristics rather thanto NO3
� supply of soil. On the other hand, depending onthe induction of NR by NO3
� supply of soil, NRA of aplant is assumed to reflect the nitrate supplying power ofthe habitat (Lee and Stewart, 1978). Consequently,Stewart et al. (1972) have successfully showed theactivity of this key enzyme in nitrate assimilation to bean indicator of nitrate supply in ecological studies.
Our findings suggest that leaves are the main nitratereducing compartments in most geophytes. However,other compartments also contribute to total plantnitrate reduction. In addition, our results show thatnitrate reduction capacity was different among geo-phytes species. Even if nitrate reduction capacity isinfluenced by nitrogen supply of habitat, differentresults of species collected from same sites indicate thatthe nitrate reducing capacity is also species-specific.
Acknowledgements
We thank to Dr. G. Gebauer (Bayreuth University,Bayreuth, Germany) for critical comments on an earlydraft of the paper and to M. Adanur (Instructor inUludag University, Bursa, Turkey) for linguistic helpwith the preparation of the manuscript.
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