During the growing session of 1995, the total soluble non-protein nitrogen (TSNN) composition and contents of mycorrhizal fine roots, xylem sap and phloem exudates of roots from a coniferous (Picea abies L.(Karst)) and a deciduous (Fagus sylvatica L.) tree species were analysed at a field site (‘Höglwald’, Germany) exposed to high loads of N. In April, TSNN in fine roots of spruce and beech trees amounted to 16 μmol N g−1 f. wt and 23·3 μmol N g−1 f. wt, respectively. It decreased to 9·2 μmol N g−1 f. wt and 18·1 μmol N g−1 f. wt, respectively, after bud break in June. The seasonal maximum of TSNN in fine roots of spruce was observed in July (32·7 μmol N g−1 f. wt) followed by a decline of c. 30% until the end of the growing season in September. TSNN in fine roots of beech trees showed a further decline between June and July, when its seasonal minimum was determined (15·6 μmol N g−1 f. wt), and increased to c. 29 μmol N g−1 f. wt until September. In spruce roots Gln and Arg were the most abundant TSNN compounds during the entire growing season. In roots of beech Asn played an important role alongside Gln and Arg, especially in April, when it was the most abundant TSNN compound. Other proteinogenic and non-proteinogenic N compounds comprised c. 20–30% of TSNN. Nitrate made up <1%, and ammonium <7% of TSNN in the fine roots of both species.

In April, TSNN in the xylem sap of roots of spruce and beech trees amounted to 3·4 and 8·6 μmol N ml−1, respectively. In roots of spruce trees xylem sap TSNN increased after bud break up to 12·7 μmol N ml−1 in July. At the end of the growing season TSNN had declined again to 3·9 μmol N ml−1. TSNN in the root xylem sap of beech trees decreased after bud break until July (2·4 μmol N ml−1 in July) followed by a slight increase until September (2·9 μmol N ml−1). Arg, Gln and Asp were the most abundant TSNN compounds in the xylem sap of spruce trees contributing together c. 90% to TSNN. The same TSNN compounds prevailed in the root xylem sap of beech trees in April and July, whereas in June and September Asp was replaced by Asn comprising 57% of TSNN in June. In addition to the N compounds mentioned above, a number of other proteinogenic and non-proteinogenic amino compounds were found in root xylem sap of both species. In either species, nitrate and ammonium were present in small amounts, contributing <1% and <4% to TSNN, respectively. Apparently, inorganic N taken up by the mycorrhizal roots is mainly assimilated in root tissues or by the mycorrhiza and N uptake by the roots is largely adapted to the assimilatory capacity of this organ.

In phloem exudates of spruce roots, TSNN amounted to 10·7 μmol N g−1 f. wt in April, increased in June to 23·4 μmol N g−1 f. wt and decreased again until September to a seasonal minimum of 4·8 μmol N g−1 f. wt. In contrast to spruce, TSNN content in phloem exudates of beech roots showed a seasonal maximum (c. 20 μmol N g−1 f. wt) in April with a subsequent decrease in June after bud break (c. 2 μmol N g−1 f. wt). A fourfold increase in July was followed by a decrease in September, when TSNN in phloem exudates of beech roots amounted to 4·3 μmol N g−1 f. wt. Arg was the most abundant N compound in the phloem of roots from spruce trees and made up c. 60–85% of TSNN during the entire growing season. In beech trees the seasonal course of TSNN correlated with the relative abundance of Arg. Arg comprised 69 and 57% of TSNN in April and July, respectively, but contributed <20% in June and September. Besides Arg, other proteinogenic and non-proteinogenic amino compounds could be detected in the phloem of both species. In addition, nitrate and ammonium were present in considerable amounts.

From these results and a previous report on TSNN in above-ground parts of spruce and beech at the same site, a whole-plant model for the cycling of TSNN in both species is proposed. Differences in the location of storage pools are assumed to be responsible for the differences in the seasonal course of TSNN composition and contents observed between the two tree species.

Aqueous extracts of homogenized shoot and root tissue of alfalfa (Medicago sativa L.), white mustard (Sinapis alba L.), and cress (Lepidium sativum L.), with the exudates of sterile roots of these crop plants, were examined spectrophotometrically for the activities of 20 oxidoreductase enzymes by standard procedures. In tissue extracts and root exudates, the reactions of laccase (EC 1.10.3.2), ascorbate oxidase (EC 1.10.3.3), monophenol monooxygenase (EC 1.14.18.1), and phenol 2-monooxygenase (EC 1.14.13.7) were readily detected. Of the aromatic-ring cleavage dioxygenases, those of the meta-cleavage pathway (EC 1.13.11.2 and 1.13.11.8) could also be detected. Guaiacol peroxidase (EC 1.11.1.7) was dominant in all samples. In sterile root exudates of alfalfa, this enzyme was represented by at least seven acidic isoforms. The formation of the ligninolytic Mn3+/malonate and Mn3+/citrate complexes from Mn2+ occurred in all tissue extracts and in root exudates of alfalfa. In root extracts of soybean (Glycinemax L.), the rate of Mn3+ generation correlated (P=0·993) with the activities of endogenous plant guaiacol peroxidase and horseradish peroxidase (HRP) supplements and also with the total phenol content in tissue extracts (P=0·984). Plant guaiacol peroxidase and purified HRP decolorized four aromatic dyes, an activity reported to be involved in ligninolysis. Although no enzymes capable of generating H2O2 as a consequence of the oxidation of simple sugars, amino acids, organic acids, and aldehydes were found, traces of peroxide were detected in tissue extracts and in the root exudate of alfalfa. It is concluded that the oxidoreductases found in plant tissues also occur in root exudates of aseptic whole plants. The significance of interrelations between oxidoreductase enzymes and enzymically generated higher-valency metal ions is discussed in the context of the oxidative conversion of phenolic compounds in soil and plant tissue.

To investigate nitrogen assimilation in Lolium perenne L. colonized by the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum (Thax. sensu Gerd.), nitrate uptake, key enzyme activities, and 15N incorporation into free amino acids were measured. After a 4-h labelling period with [15N]nitrate, 15N content was higher in roots and shoots of AM-plants than in those of control plants. Glutamine synthetase (GS) and nitrate reductase (NR) activities were increased in shoots of AM-plants, but not in roots. More label was incorporated into amino acids in shoots of AM plants. Glutamine, glutamate, alanine and γ-aminobutyric acid were the major sinks for 15N in roots and shoots of control and AM plants. Interactions between mycorrhizal colonization, phosphate and nitrate nutrition and NR activity were investigated in plants which received different amounts of phosphate or nitrate. In shoots of control plants, NR activity was not stimulated by high levels of phosphate nutrition but was stimulated by high levels of nitrate. At 4 mM nitrate in the nutrient solution, NR activity was similar in control and AM plants. We concluded that mycorrhizal effects on nitrate assimilation are not mediated via improved phosphate nutrition, but could be due to improved nitrogen uptake and translocation.

An experiment was carried out in open-top chambers located in eastern Spain. One-yr-old Pinus halepensis Mill. seedlings were exposed during three consecutive summers to the following ozone (O3) treatments: charcoal-filtered air (CFA), non-filtered air (NFA) or non-filtered air plus 40 nl l−1 O3, 9 h d−1, 5 d wk−1 (NFA+40). Seasonal variations in Aleppo pine performance were observed since reductions in chlorophyll and cellular peroxidase levels associated with increases in superoxide dismutase activity, were recorded during the summer. Similarly, a reduction in epoxidation state was found at midday during the summer, derived from an activation of the xanthophyll cycle associated to an increment in radiation and temperature levels.

The first O3-induced effects were recorded in previous-year needles (1991) during the first summer exposure as an increase in extracellular and total peroxidase activities and in zeaxanthin levels in the NFA+40 treatment along with a trend to a higher SOD activity in this treatment. A carry-over effect was detected since a lower winter recovery of chlorophyll levels was found in the NFA+40 seedlings along with a reduction of xanthophyll levels. A reduction in chlorophyll levels was observed in the previous-year needles (1992) from the NFA+40 treatment at the end of the second fumigation period. Realistic ozone exposures induced alterations in plant antioxidative systems and plant pigments as shown in this paper. These observations together with the reductions in stomatal conductance and net photosynthesis recorded in the same experiment, indicate that Aleppo pine is a species sensitive to ozone.

Responses of gas exchange and photosynthesis to changes in CO2 concentration and PPFD were examined in well watered plants of Delosperma tradescantioides Bgr. to establish the relative importance of these environmental changes on the photosynthetic machinery in this CAM-cycling species which grows naturally in both exposed and partly shaded environments. Plants were grown at two PPFDs (220 [LL] and 550 [HL] μmol m−2 s−1). HL plants had larger leaves with higher specific weight, water content and diurnal malic acid fluctuation. Photosynthetic PPFD responses were typically those of sun and shade species for HL and LL plants, both under 21% O2 and non-photorespiratory (2% O2) conditions. The CO2 compensation point in the absence of non-photorespirational CO2 evolution in the light (Γ∗) was c. 30 μmol mol−1. Irradiation reduced mitochondrial respiration by >50%. Comparison of the PPFD responses of linear electron flow rates derived from gas exchange measurements and from fluorescence analysis ([Jscr ]F) indicated effective photosynthetic control. [Jscr ]F was always larger than electron flow rates calculated from gas exchange, indicating that processes other than carboxylation and oxygenation were consistently important in energy consumption under all sampled environmental conditions. Regardless of PPFD during growth, electron flow to carboxylation and [Jscr ]F were linearly correlated, demonstrating that the photosynthetic apparatus was well adapted to PPFD during growth. In HL plants, non-photochemical quenching increased, and photochemical quenching and the quantum yield of linear electron transport through PS II decreased more slowly with increasing PPFD than in LL plants. In plants of both treatments non-photochemical energy dissipation seemed to be exhausted when the proportion of photons not utilizable by photochemistry exceeded 0·7. Results illustrate a pronounced ability of D. tradescantioides to acclimate to a 100% change in the prevailing PPFD and lend support to the hypothesis that CAM cycling might act as a photoprotective process.

Two-yr-old Norway (Picea abies (L.) Karst.), red (P. rubens Sarg.), black (P. mariana (Mill.) B.S.P.) and white (P. glauca (Moench.) Voss) spruce seedlings were exposed from bud break for 35 or 63 d (4·5 h irradiance d−1) to a gradient of biologically effective u.v.-B radiation (λ=280–315 nm) ranging from 0·61 kJ m−2 d−1 to 5·99 kJ m−2 d−1. No visible symptoms of u.v.-B injury were observed. Epicuticular wax production was not affected by needle exposure to increasing u.v.-B irradiance. Seven constituent classes were resolved by GC and confirmed by GC–MS in wax recovered from needles of the four species. Wax composition of Norway, black and red spruce was altered following needle exposure to increasing u.v.-B dose. White spruce wax composition was unaffected. Direction and magnitude of wax composition response was species-dependent. The proportion of nonacosane diols on Norway spruce needles increased (P<0·05) whereas that of alkyl esters decreased with increasing u.v.-B dose. The proportion of fatty acids in black spruce needle wax increased (P<0·05), and that of estolides (GC-identified) in red spruce needle wax increased (P<0·05) with increasing u.v.-B dose. Changes in wax chemical composition reported were induced following to daily, 4-h duration exposures of needles to u.v.-B centred on 1200 hours. Affected variables exhibited a continuum of response. The highest dose applied was within the range of measured or predicted increases in mid-northern latitudes. Such changes in conifer needle epicuticular wax chemical composition might result in increased seedling sensitivity to the changing atmospheric environment, especially from co-exposure to tropospheric ozone in mid-northern latitudes where much of Canada's productive forest is located.

Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992–1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered air+35 nl l−1 ozone for 8 h d−1 (NF+) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF+, 45 nl l−1 1000–1800 hours, seasonal mean) the net CO2 uptake of wheat flag leaves was decreased by c. 40% relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO2 uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF+ treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.

A new experimental system was developed for studying the hormonal mechanisms which control tracheid differentiation. In this system the tracheids redifferentiated from parenchyma cells in the hypocotyl of young Pinus pinea L. seedlings. The experimentally induced tracheids have unique shapes and patterns, and are therefore easily distinguished from the primary and secondary tracheids formed before the experiments. Auxin (0·1–1% NAA) alone sufficed to cause the redifferentiation of short tracheids, usually in discontinuous patterns across the hypocotyl. Gibberellin by itself did not induce redifferentiated tracheids. Combinations of auxin with gibberellin (0·1% NAA+0·1–1% GA3) promoted the differentiation of long tracheids (up to threefold greater than those induced by auxin) in continuous patterns along the stem axis. Gibberellin in the presence of auxin promoted tracheid elongation by stimulating intrusive growth of both the upper and lower ends of the differentiating tracheids. The role of auxin and gibberellin in controlling the evolution of tracheary elements, from tracheids to vessels and fibres, is discussed.

Sclerophylly and synthesis of phenolic compounds are active responses of plants subjected to environmental stress (drought, low nutrient supply, u.v.-B radiation, ozone). Here we describe the morphological and histochemical alterations occurring in field-grown leaves of Fagus sylvatica L. from three sites located along an ecological gradient: from a site in cool and protected conditions to one located on a mountain ridge, where the trees grow on a thin layer of soil and are exposed to the wind and to intense solar radiation in summer. The morphological data show that, as the ecological conditions of the stand worsen, individual leaf surface decreases, while the thickness of the leaves and their specific d. wt (i.e. d. wt per unit leaf area) increases. Histochemical and ultrastructural tests show a marked increase of phenolics during the course of the year. These substances, present primarily in the leaves of trees growing in stress conditions, have been identified mainly as tannins. They accumulate in the vacuoles, especially those of the upper epidermal layer and the palisade mesophyll; at a later stage they appear to be solubilized in the cytoplasm and retranslocated, eventually impregnating the outer wall of the epidermal cells amidst the cellulose fibrils, where they cluster together and form an electron-opaque layer between the wall and the cuticle. Observation of the epidermal cells also reveals that the outer cell wall is thicker. The paper discusses the roles of secondary metabolites in protection and detoxification processes; the possible ecological significance of these alterations in the ecophysiology of beech trees.

Direct pollen interactions, as well as interactions mediated by a recipient, can have a remarkable influence on pollen fertilization ability. Under conditions of pollen competition it could be advantageous if pollen grains interfered with the germination of other pollen. The aim of this study was to find out if there are direct negative or positive pollen–pollen interactions between pollen grains from genetically slightly different donors. The in vitro germinability of the pollen from several Betula pendula Roth clones was investigated. The pollen interactions between the clones were examined pairwise by using equal pollen mixtures. In three of the eight cases the germination percentage of the pollen mixture was significantly higher than the average germination percentage of the separate clones that formed the mixture, which indicates some type of interaction between the pollen populations. We found only positive interactions between the pollen of clones. This study also documented density-dependent germination of pollen grains in vitro (=pollen population effect). Adding an aqueous pollen extract to the incubation medium increased the germination percentages of poorly germinating pollen and small pollen populations. Germination-stimulating effects were found to exist both with fresh and dead pollen. Such direct pollen–pollen interactions could be explained by specific water-soluble substances diffusing from pollen grains.

In the monocarpic perennial Cynoglossum officinale L. the probability of flowering is related to the size of the plant. In previous work it was observed that this relation varies between years. We hypothesized that variable conditions during the winter, the period of vernalization, explain this variation.

We collected plants from the field in autumn and placed these under different simulated winter conditions in a climate room. In contrast to our hypothesis, the probability of initiating flowering at a given size was not affected by: (a) the temperature during the cold period, (b) the duration of the cold period, or (c) the application of a plant hormone (GA3) or an inhibitor of gibberellin synthesis (paclobutazol) during the cold period. Winter cold is not necessary for floral initiation, and is only required for elongation of the inflorescence. It is unlikely that winter temperature affects the fraction of plants flowering.

Subsequent morphological investigation of flower development in material collected in the field showed that large plants had primordial inflorescences well before vernalization, sometimes as early as August. In plants grown from seeds under constant conditions in a climate room, the probability of initiating the inflorescence differed for plants grown at various temperatures (34·1% at 15°C, 100% at 20°C, and 95% at 25°C). Our results suggest that environmental conditions in August and September, up to 10 months before actual flowering, could affect the fraction of flowering plants.

An epidemic of the ergot fungus (Claviceps purpurea (Fr.) Tul), a non-systemic floral pathogen, appeared in populations of Spartina anglica C. E. Hubb in Poole Harbour in the 1980s. Over 70% of inflorescences were infected between 1985 and 1995. Between 1983 and 1995, there was no consistent pattern in the rank order of sites with respect to the proportion of inflorescences infected, and variation in the amount of infection was not related to inflorescence density. Among years, there was significant variation in the total biomass of ergot per inflorescence, but no significant difference in the number of ergots per inflorescence. Log–log regressions of total weight of ergot per inflorescence against the number of ergots per inflorescence showed that in all years resources for each additional ergot were limited.

There was no statistically significant difference between the number of seed set on infected and uninfected inflorescences in 1985 or 1995. Further analysis showed that, compared with uninfected inflorescences, there was higher seed output from inflorescences with fewer than 10% of spikelets infected, which balanced reduced seed output from heavily infected inflorescences. At high levels of ergot infection, a lower proportion of uninfected spikelets set seed, compared with spikelets on uninfected inflorescences. This suggests that conditions which favour ergot growth are detrimental to seed production. Compared with uninfected inflorescences, mean and total seed weights were significantly lower in inflorescences with >10% of spikelets infected.

The fungus Fusarium heterosporum Nees ex Fr. was found in association with the ergots on Spartina. There was a significant positive correlation between the average severity of Fusarium infection and the number of ergots on an inflorescence. There was a non-significant negative correlation between severity of Fusarium and mean ergot weight, when ergot number per inflorescence was held constant. These data suggest that any negative effect of Fusarium on Claviceps is small.

The high, and temporally and spatially uniform levels of ergot infection probably result from genetic uniformity of Spartina and the lack of zonation in the salt marshes of Poole Harbour. This situation is likely to persist for the foreseeable future.

Phytoplankton infections by fungal parasites in the upper, mixed layer of a mesotrophic northern temperate lake were analysed according to the following parameters: host and parasite species, host population density and prevalence of infection, resting spore formation by the parasite, and the lowest host density at which parasites appeared. The phytoplankton taxa recorded included the Cyanobacteria, Dinomastigota, Chrysophyceae, Bacillariophyceae, Chlorophyceae, Cryptophyceae and Haptophyceae, but infection was never found in the last two classes. The parasites belonged almost exclusively to the monocentric Chytridiomycetes. Fungal epidemics occurred at all times of the year. Parasites appeared at population densities as low as about 1 cell ml−1 in some host species, with infection prevalence sometimes exceeding 80%. The proportion of the total phytoplankton biovolume infected by fungi was usually much <1%, but occasionally reached 10%. Parasitism proved to be highly species-specific, with one parasite species usually infecting only one host species. In the case of Zygorhizidium planktonicum, which infected both Asterionella formosa and Synedra acus, there is evidence that two species-specific formae speciales, each infecting only one of these two host species, are present in the lake.

Patterns produced by foraging mycelial systems of decomposer fungi might be the visual evidence of strategies for obtaining differently distributed carbon sources. Mycelial systems might also be responsive to unevenly distributed mineral nutrients in the soil. The nature of the mycelial response might differ between species inhabiting eutrophicated disturbed sites, such as Stropharia caerulea (Kriesel), and inhabitants of more typical woodland soils, such as Phanerochaete velutina DC.: (Pers.) Parmasto. We examined the effect of enrichment of soil by addition of different concentrations of N, P and potassium on inoculum decay rate and on extension rate, biomass production and fractal dimension (determined by image analysis) of extra-resource mycelia of S. caerulea and P. velutina extending from wood blocks across compacted non-sterile soil in trays.

S. caerulea was more responsive than P. velutina to both type and quantity of soil nutrient enrichment. P. velutina grew more rapidly, produced more biomass and decayed inocula more rapidly than S. caerulea under most conditions, rates of growth and resource decay of the former remaining stable over a range of soil nutrient enrichments. In contrast, morphology of S. caerulea altered on enriched soils compared to controls with greater and more pronounced fanning (divergence of separate hyphae) of the foraging mycelial front. These changes were reflected in fractal values. Extension rate and biomass production of S. caerulea was less than controls on N-amended soil, but greater on P-amended soils, with the greatest biomass produced on 0·13–0·27 mg P g−1 air-dried soil. Decay rate of inocula was often positively related with mycelial production rates. These responses indicate extra-resource mycelium of S. caerulea is able to respond rapidly to a range of soil nutrient enrichments, and this is discussed in terms of the different foraging strategies exhibited by the two fungi.

Sequence-based markers were developed to study the genetic structure and reproductive biology of the ectomycorrhizal fungus Suillus pungens Thiers & Smith in a Bishop pine (Pinus muricata D. Don) forest. Six different basidiome genotypes were found in a 1200 m2 area. Five of the six genotypes were represented by single basidiomes. The remaining genotype comprised 13 basidiomes and covered an area of at least 300 m2, with maximum measured dimensions of 40 m and 14 m. This is the largest genet of an ectomycorrhizal fungus described to date, and is likely the result of vegetative growth, because analysis of single spore isolates eliminates the possibility of genetic identity resulting from either apomixis or fortuitously indistinguishable recombinant genotypes. Genetic analysis also shows that although out-crossing appears to predominate in the population, at least a low percentage (1·4%) of spores are secondarily homothallic.

The combination of extensive vegetative growth and abundant fruiting suggests S. pungens utilizes more carbon than might be expected for a species which accounts for <3% of the total ectomycorrhizal abundance at the site. Additional carbon might come from either more efficient host–fungus transfer, pooling of carbon from the roots of different host plants, or saprophytism.

Seedlings of Betula pendula Roth. were grown in the presence of Paxillus involutus (Batsch) Fr., and metabolic changes during mycorrhiza formation were examined by measuring organic acid and amino acid pools and related enzyme activities, following sequential harvests. Glutamine, aspartate and asparagine pools were always lower in infected roots than in non-infected roots, especially during Hartig net initiation and formation. Glutamate concentration was similar in both tissues. Citrate and malate were the two major organic acids detected and their concentrations were equal in infected and non-infected roots. Aspartate aminotransferase, glutamine synthetase, NAD-dependent malate dehydrogenase and glucose-6-phosphate dehydrogenase activities were higher in infected roots than non-infected roots. For all enzymes revealed on polyacrylamide gels, both root and fungal isoforms were present in infected roots. Quantitative changes in enzyme capacities and metabolite pools indicated that mycorrhiza formation caused a re-arrangement of the main metabolic pathways during the very early stages following contact, which might be related to the structural changes.

Variation within the internal transcribed spacer (ITS) of the ribosomal RNA gene of 15 isolates representing seven European Armillaria species, was examined by sequencing of the PCR-amplified products. The analysis of an 744-bp region showed that the 5.8S gene appeared to be highly conserved in the 15 isolates and in other Basidiomycetes and Ascomycetes, whereas ITS1 and especially ITS2 spacers exhibited polymorphisms due to base substitutions, insertions or deletions of up to eight nucleotides. An initial dendrogram for the full sequence was drawn using cluster analysis (UPGMA), and a tree was constructed using the maximum parsimony method. Both methods indicated that the isolates could be divided into four major groups. One group, consisting of A. ectypa, was distinct from all the other species. Examination of the other groups indicated that A. tabescens and A. mellea were in a separated cluster, with a significant variation between the two isolates of the latter species. A. gallica and A. cepistipes constituted another closely related group distinguishable from A. ostoyae and A. borealis, these latter two species exhibiting the highest similarity. The results are consistent with, and discussed in regard to, the relationships estimated previously by pairing tests, morphological and physiological comparisons, as well as by restriction fragment length polymorphism of the rDNA.