MORFOLOGICAL AND ANATOMICAL MODIFICATION OF BRACKEN FERN (PTERIDIUM AQUILINUM (L.) KUHN.) ON SERPENTINITE MABLE

Aleksandra Halarewicz
Department of Botany and Plant Ecology, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

In the 1 year study, selected morpho-anatomical traits of two populations of bracken fern were observed at two sites within Ślęża Massif  (Lower Silesia, Poland). The study demonstrates modifications of at least some of the observed traits in fronds growing on serpentinite mable as compared to another bracken fern population, growing of non-serpentinite soil. The modified traits are the number of primary and secondary divisions of a frond, the leaf blade surface and the length of stomata. These modifications may result from different edaphic conditions of the two studied bracken fern populations.

Key words: Pteridium aquilinum, leaf anatomy, morfological variation, serpentinite.

INTRODUCTION

Allelopatic properties of bracken fern Pteridium aquilinum [2], along with its resistance to infections by pathogenic fungi and with not too many insect species known foraging on it [5] are the key features making the plant a strongly dominant species, not subject to competitive pressure from other components of a particular ecosystem. This cosmopolitan fern species, in some countries considered even as an invasive plant [7], colonizes also serpentinite mable.

Plant species growing on serpentinite sites develop some adaptive traits enabling their survival despite of high soil concentrations of Cr, Ni and Pb [17]. In Pteridium aquilinum, the adaptative mechanisms involve shortening of the plant's green parts vegetation period and, by the same token, extending of the rhizomes' hibernation time [3]. No doubt it is not but one difference between plants of that species growing on serpentinite – derived and non-serpentinite soils [14].

The increased soil concentrations of heavy metals result in many negative changes in organism of a plant. In seed plants populating serpentinous soils dwarfish growth is observed as an effect of the anomalous ratio in soil between Ca and Mg ions [12], as well as of the toxic influence of Cr and Pb [1]. Furthermore, nickel uptaken by the plants in excess results in reduction of the surface and thickness of the leaf blade and brings about disturbance of many physiological processes, in particular of photosynthesis [8,15].

The influence of serpentinous soil containing heavy metals on the bracken fern is so far entirely unknown.
The leaves (fronds) of bracken fern include a stipe (leaf-stalk). The blades of the fronds are divided into pinnae, the bottom pair of which are sometimes large enough to give the impression of a three-part leaf.  Each pinnae is in turn divided into segments called pinnules and the pinnules are then divided into the blade segments of smallest level – ultimate segments [6]. Pteridium aquilinum has bifacial leaves. Although leaves are hypostomatal, stomata there are only on abaxial side (lower leaf surfaces). Mesophylll is differentiated into the palisade and spongy layers.

The aim of this study was to find out whether there are differences in morfological and anatomical structure of bracken fern leaves between the plants in two natural habitats, populating serpentinite and non-serpentinite soil.

OBJECTS AND METHODS

The observations were carried out at two natural sites of Pteridium  aquilinum within  Ślęża Massif  (Lower Silesia, Poland). Site I was spruce monoculture developed in human-transformed habitat of acidophilic mountainous beech forest (Luzulo luzuloidis-fagetum community) of Radunia (a separate crest in Ślęża Massif), growing on proper ranker developed from serpentinite mable. Site II was submontane acidophilous oak forest (Luzulo-Quercetum community) of Ślęża, growing on acid brown soil developed from gabro.

Both these sites were chosen for the study considering their equal altitude (400 m above MSL), southern exposure of the slopes, presence of the windows in the trees' crowns and the absence of other shade-tolerant species in the shrub layer. On the other hand, the most important differences between the studied habitats lie in the chemical properties of the soils, and in the concentration of heavy metals in particular. In the upper layer of serpentinous soil (site I) the total content of all forms of chromium, nickel and lead reached as much as 140 mg·kg^-1, 60 mg·kg^-1 and 130 mg·kg^-1 respectively, thus exceeding levels acceptable in soils. On the contrary, the soil concentrations of chromium and nickel in samples from site II did not exceed 30 mg·kg^-1 and only the lead concentration was higher than standard, reaching the level of 100 mg·kg^-1 [3].

The study included field and laboratory observations. Only the fully developed leaves have been investigated, therefore the field measurements and sampling of 50 composite leaves from each site were accomplished in the second half of July 2006. The field-measured and the laboratory-measured parameters of the leaves are listed in Table 1.

Plant material was sampled in order to make anatomical preparations of the leaf tissue. The sampled plant parts were taken from ultimate segments of the leaf blades from three such same locations on a frond, from both sampling sites. Preparations were made in ten replicates for each location on a frond. Nevertheless, the figures presented in this paper refer only to the parameters measured in the apical part of each frond, in the top-most one of the three sampling locations.

Stomatal density was determined on abaxial leaf sides by making replicas of stomata on silicone rubber. For each replica the total number of stomata and their length in a given area inter veins were recorded. The frequencies of stomata were determined per 1 mm².

For anatomical studies of mesophyll structure samples were always taken from the midblade region. Hand-cut transverse sections were optically bleached, the content of cells was dissolved with chloral hydrate, washed with 1% acetic acid and stained with alcian blue, 8GS, iodine and carmine alum. Observation of mesophyll and thickness of leaf blades from each series were made by light microscope Axioscop 2 plus (Zeiss) with an ocular micrometer.

The obtained data have normal distribution, therefore ANOVA and Tukey's HSD test were used to analyse the differences between means. These procedures were performed in Statistica 7.1. package.

RESULTS AND DISCUSSION

Morfological traits of the leaves
The morphological traits of the bracken fern leaves are different between the sampling sites (Table 1). Fronds are clearly shorter and more slender in all the individuals populating serpentinous soil. Differences in length and width are statistically significant. Number of pinnae, pinnules and ultimate segments of the leaves in sampled populations are significantly different – fronds on serpentinite mable are less branched. The highest test F value (F=187,08, p<=0,05) is observed for the number of pinnules, making the modification of this parameter higly significant. The length of the ultimate segments as well as their surface are significantly lower for the site 1 compared to site 2.

In general, the small surface of the ultimate segments, along with the limited number of primary and secondary divisions in plants on serpentinite mable result in reduction of the total surface of the leaf blade and as such can be considered symptoms of the plants' dwarfish growth.

Selected traits of stomata
Some authors demonstrate the reduction of the total number of stomata as a plant response to the presence of Ni in the soil [9,13]. However, the microscopic analyses of epiderma of bracken fern's leaves in the presented study revealed no statistically significant difference between the number of stomata in the two observed samples. In contrary to that, the length of stomata has been shown different for each one of the studied sites (Table 2). Stomata on leaves of the top as well as at the base of fronds, taken from site II are longer when compared to those from the analogous samples from plants growing at site I.

Mesophyll structure
Anatomical preparations of the leaves' cross sections show anatomical differences between the examined samples. The thickness of the leaf blade is reduced  by 15-17 μm on average in plants growing on serpentinite mable. In these same leaves the number of vascular bundles is reduced from nine to seven, and geminate, small bundles accompanying the main vascular bundle in the midrib of ultimate segments, are missing from the leaves of these plants (Fig. 1-2).

According to other authors, the thinner leaves occurr in many plants as a result of water deficit [10,11]. However, in the presented study, no limitation in water accessibility to the plants was demonstrated at the two sites of bracken fern. Acclimation, defined as the reduced number of veins with the accompanying reduction of the number of mesophyll layers have no influence on water supply to mesophyll [18]. On the other hand, studies in vitro on higher plants grown in Ni contaminated soils show reduction of  the leaf blade thickness [9]. Therefore the effect observed by the author may be a manifestation of a similar plant response to nickel concentraction in the soil.

In all the studied bracken fern's leaves the size of spongy and palisade cells is similar. It was demonstrated that in ultimate segments taken from plants growing on the serpentinous soil, palisade parenchyma is more conspicuous. Long cylindrical cells are regularly, tightly packed in two rows. In these same leaves intercellular spaces in spongy parenchyma are wider (Fig. 3-4).

One may presume that the different development of chlorenchyma is a result of different light conditions in each one of the studied habitats. Plants can adapt the structure of their palisade and spongy parenchyma to light intensity. It has been found by other authors that there are more palisade layers in leaves if they had grown under high light intensities, whereas shaded leaves, or older ones that grow closer to the soil, are usually single-layered [16]. It is difficult to unequivocally assess whether the structural variation of mesophyll observed in this study resulted also from the plants' adaptation to the local light conditions or were exclusively bracken fern's response to the heavy metals concentrations, with nickel in particular.

CONCLUSIONS

Variation in morphology and particularly the dwarfish growth of a whole local population on serpentinite mable might suggest the phenomenon known as autotoxicity. Furthermore, it comes as a first choice explanation for any person familiar with bracken fern. Autotoxicity is usually described as plants' switching into latent vegetation phase at a site where they have been growing for a long time, which results in the degeneration of that site. After some period of time, during which new fronds are absent, the recolonization of the previously populated site takes place [4]. However, the serpentinous site studied in 2006 has been also observed in preceding seasons, namely 2002-2004, but the plants' vigour and the site "status" was not altered over that time.

Hence it is supposed that high heavy metals concentration in serpentinous soil is the reason for the dwarfish growth of the bracken fern`s above-ground parts, expressed by developing shorter fronds of lower number of segments and of their smaller surface area. The interpopulation variation of anatomical traits such as the length of stomata is probably also influenced by the different edaphic conditions of the studied populations.

REFERENCES

Accepted for print: 18.11.2008

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