Annals of Botany 89: 109-114, 2002
© 2002 Annals of Botany Company
An Ecological Interpretation of the Difference in Leaf Anatomy and its Plasticity in Contrasting Tree Species in Orange Kloof, Table Mountain, South Africa
1Department of Botany, University of Transkei, Private Bag X1, Umtata, South Africa
* For correspondence. E-mail: Bhatr{at}getafix.utr.ac.za
Received: 19 July 2001; Returned for revision: 20 August 2001; Accepted: 29 September 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Leaf anatomy and morphology were studied in 11 tree species growing in an undisturbed forest and the adjoining fynbos for over 50 years. Functional anatomical results suggest that the forest and the fynbos are ecologically distinct. Moreover, leaf anatomy suggests that the foliage is primarily adapted for photosynthesis rather than for control of transpirational water loss. Forest precursor tree species and scrub species exhibit xeromorphy in the fynbos whereas they exhibit mesomorphic features inside the forest. The wide-ranging species, such as Olea capensis subsp. capensis, simulated the response of the forest precursors, with the cuticle being phenotypically plastic between the forest and the fynbos but not between the stream and non-stream habitats. Finally, the forest precursors, the scrub species, and the wide-ranging taxa seem to have anatomical characters which can be modified in the fynbos and therefore allow its colonization by a variety of different species.
Key words: Leaf anatomy, phenotypic plasticity, xeromorphy, morphological adaptation, habitat.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Individual plants can respond to the environment in two ways: in the short-term they can respond via morphological, physiological and biochemical changes (Bradshaw, 1965), while in the long-term plant populations respond by changing their genetic composition. Genotypes with a high fitness for a given environment are maintained while the less well-adapted disappear. Genetic differentiation and phenotypic plasticity have been studied in many plants in great detail (Kuiper, 1990). Although mutually interacting, they have been discussed independently; the latter given sole coverage by Fahn (1964), Johnson (1980) and Kuiper (1990).
This paper considers the phenotypic plasticity of leaves of 11 woody species growing at Orange Kloof, Table Mountain, South Africa. According to Fahn (1964), anatomical and morphological characters may serve as reliable indicators in the study and understanding of genetic relationships, physiological processes and ecological adaptations of living organisms. Therefore, in this study, qualitative and quantitative anatomical leaf characters are used to deduce ecological phenomena of the foliage of 11 species. According to Passioura (1976), the control of leaf area and morphology is the most powerful means by which a mesophytic plant can influence its fate when subjected to long-term water stress in the field. The present investigation of leaf anatomy and morphology of 11 tree species growing in the forest and the adjoining fynbos on Table Mountain was carried out with a view to determining the importance of the functional anatomy of these species (see Table 1).
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MATERIALS AND METHODS |
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Sampling
For the purpose of the study, three zones were recognized: forest interior, forest margin and fynbos. The latter zone refers to the sclerophyllous vegetation designated as Macchia (Acocks, 1975). The forest interior zone was subsampled depending upon proximity to a stream (forest interior stream and forest interior non-stream habitats), as was the forest margin. In the fynbos zone the stream habitat is lacking as the forest always extends along the stream in a riparian forest patch, shielding the stream from the fynbos. In each zone (except fynbos), 24 leaves were collected from 12 individual trees of each species; 12 leaves being collected from six trees in the stream habitat and the other 12 from six trees in the non-stream location. In the fynbos which lacked the stream habitat, six trees of the same species sampled in the other two zones provided 12 leaves for the study. Of the two leaves collected from each tree, one was an exposed sun-leaf and the other a shade-leaf taken from deep within the canopy (Warming, 1909; Cutter, 1971). Some species were not present in every zone (see Table 1).
Microtechniques
Formalin-propiono-alcohol (F.P.A.) was used to fix leaves as soon as they were collected (Johansen, 1940). After 24 h in F.P.A., lamina strips from the middle part of the leaf, consisting of the midvein, the intercostal area and the margin, were excised from the leaves, dehydrated in a graded ethanol series, infiltrated with and embedded in paraffin wax, and cut into 16 µm sections. The cross-sections thus obtained were stained with Sudan IV in 70 % alcohol and counterstained with aqueous Astra blue. All the leaves collected were treated, for transverse sections, in this manner, and were permanently mounted in entellan and deposited in the herbarium of the University of the Western Cape. To obtain paradermal sections, leaf strips similar to those described above, and adjacent in position, were boiled in equal volumes of 30 % hydrogen peroxide and concentrated glacial acetic acid (Engelbrecht, 1981). The cuticles thus obtained were stained in 0.5 % safranin (alcoholic) which stains epidermal cells red but leaves the guard cells unstained (Lo Gullo et al., 1986). Paradermal sections were prepared for both sun and shade leaves. On each paradermal section, stomatal frequency was determined at the base, the apex, the leaf margin and in the immediate vicinity of the midvein on each leaf. This was done because stomatal frequency is generally higher at the apex than at the base and higher at the margin than in the vicinity of the midvein (Salisbury, 1927; Knecht and Orton, 1970). All the leaves studied are hypostomatic except those of Euclea which are amphistomatic. In all species studied, stomatal counts were made on the abaxial surface. All slides were examined under a light microscope and measurements and counts were performed using a Kontron Image Analyzer. The number of counts and measurements for any character usually exceeded 30, and the frequency bar graph associated with this sample number always produced a normal curve. Analysis of variance was used to test for the significance of differences, and the modified t-test (Parker, 1979; Sokal and Rholf, 1981) was used to test for differences in means of the characters in different environments.
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RESULTS |
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Comparison of leaf anatomical characters of species between the forest interior, forest margin and fynbos
Cuticle thickness.
The thickest adaxial cuticle was observed on leaves of individuals growing in the fynbos zone. The scrub species, Euclea and Maytenus, had the thickest cuticle (Table 2), and were rivalled only by the wide-ranging Olea (6.41 µm), commonly found in all three environments. Cassine and Kiggelaria had the thinnest cuticles while the forest precursor species, Rapanea melanophloeos and Cunonia, had a cuticle of intermediate thickness. Olinia, which is predominantly restricted to the forest interior, also had a thin (2.21 µm) cuticle in the fynbos. Ilex, Scolopia and Chionanthus, which are confined to the interior and margin of the forest, had the thinnest cuticles.
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In the interior and margin of the forest, Maytenus still had a thick cuticle while that of Olea was even thicker than in the fynbos (Table 2).
Epidermal cell size.
Species which had a thin or medium-thick cuticle tended to have small epidermal cells in all three environments. In this regard, Kiggelaria and Olinia had the smallest epidermal cells in the fynbos environment (Table 3). This phenomenon is also observed in the interior and margin of the forest. Meanwhile, species that had thick cuticles in the fynbos had large epidermal cells in this zone. This was true of Maytenus and Olea (776 and 992 µm2, respectively). In the latter case, the phenomenon also extended into the interior and margin of the forest. In contrast, epidermal cells of Maytenus were substantially smaller (460 and 398 µm2) in the interior and margin of the forest than in the fynbos (776 µm2). The forest species Rapanea and Cunonia, which had cuticles of intermediate thickness in the fynbos, also possessed cells of intermediate size. Significantly, their cell size decreased from the fynbos to the interior of the forest. The forest interior species, C. foveolatus and Olinia, had virtually identical cell sizes in the interior and margin of the forest.
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Stomatal frequency and length.
Different species showed a rather complex relationship with regards stomatal frequency. With the exception of Cassine, all had the highest frequency of stomata in the fynbos (Table 4). There was a vast difference in average stomatal frequency between the fynbos on the one hand, and the forest interior and margin on the other, whilst the difference was only marginal between the latter two zones. This point was clearly illustrated by Maytenus and Olea. Rapanea and Cunonia also conformed to this trend, having 18 and 15 % more stomata per mm2 between the fynbos and the forest margin and only 8 and 9 % more between the margin and the interior of the forest, respectively. Ilex and Chionanthus—species restricted to the forest—had a higher stomatal frequency at the margin (166 and 152) than in the interior (133 and 133) of the forest. Scolopia proved exceptional to the rule. Cassine was also exceptional, with the difference between the forest interior and forest margin (23 %) exceeding that between the fynbos and these two environments (15 %). Olinia had large frequency differences between all three environments.
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The lowest stomatal frequency (147 mm–2) was observed in Euclea. This low frequency was associated with the longest stomata (28 µm). As a rule, the higher the stomatal frequency the shorter the stomata; a trend clearly expressed in leaves of Olinia and Cunonia. Forest precursor species, Rapanea and Cunonia, had somewhat longer stomata in the interior (20 and 15 µm) and forest margin (22 and 14 µm) than in the fynbos (18 and 13 µm, respectively). The same was true of Olinia. In the case of Maytenus, this pattern was reversed; the stomatal length in the interior (18 µm) and forest margin (15 µm) being less than in the fynbos (21 µm). Stomatal length remained unchanged regardless of environment in the case of Olea.
In summary, except for Cassine, stomatal frequency decreased from the fynbos towards the interior of the forest. With few exceptions, the higher the stomatal frequency, the shorter the stomata. The forest precursor species (Rapanea and Cunonia) responded similarly in most cases.
Comparison of characters between species in the stream and non-stream habitats
Cuticle thickness.
Maytenus had a significantly thicker adaxial cuticle (Table 2) further away from the stream. In direct contrast, the forest-interior species, Olinia and Chionanthus, had a significantly thicker cuticle close to the stream compared with further away from it.
The adaxial and abaxial cuticle thickness of the forest-precursor species, Rapanea, Cunonia and Kiggelaria, was not affected by distance from the stream. Olea posessed a significantly thicker adaxial cuticle (5 µm) in the vicinity of the stream than further away (4 µm). However, the opposite was true for the abaxial cuticle.
Epidermal cell size.
The smallest epidermal cells characterized leaves of Olinia and Chionanthus (Table 3). These forest-interior species had equal sized cells. Furthermore, their cell size differed between the stream and non-stream habitats. The large cell size of Olea did not differ between the two habitats. Maytenus had larger cells (520 µm2) in the stream than in the non-stream habitat (338 µm2). The same was true of Cunonia. In contrast, Rapanea had larger cells in the non-stream habitat compared with the stream habitat.
Stomatal frequency and length.
Stomatal frequency was always higher in the non-stream than in the stream habitat. However, unlike the appreciable differences reflected between the fynbos, on the one hand, and forest interior and margin, on the other, that between the stream and non-stream habitats was non-significant except in the case of Cunonia (66 mm–2). With the exception of Maytenus, the difference in stomatal length was also small.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Quantitative assessment of the cuticle shows that this character varies according to the environment. The thickness of the cuticle was maximal in Euclea and Maytenus growing in the fynbos. The latter is the second most dominant species in the fynbos at Orange Kloof after Widdringtonia nodiflora, which is absent inside the forest (Masson, 1990). Maytenus is thus ipso facto the most dominant forest species in the fynbos zone. Euclea, on the other hand, is confined to the fynbos. According to Shields (1950), Johnson (1980) and Christodoulakis (1989), leaf anatomy appears to vary according to environment; our present observations confirm this result.
On the basis of cuticle thickness, Olea is the only rival of Euclea and Maytenus. It is the most dominant species in the scree forests at Orange Kloof (Masson, 1990). Like Cassine, it is found within the forest, the forest margin and the fynbos (McKenzie et al., 1977). A thick cuticle therefore appears characteristic of leaves of species dominant in the fynbos and of those that transcend different environments. These species also exhibited high plasticity in this character. Rapanea and Cunonia also possess plastic cuticles, with cuticles being approximately twice as thick in the fynbos than in either the interior or margin of the forest. This suggests significant ecological differences between the fynbos, on the one hand, and either the margin or interior of the forest on the other. Significantly, there was little difference in cuticle thickness of either Rapanea or Cunonia between the margin and the interior of the forest, suggesting a lack of ecological variability between these environments.
The significantly thicker cuticle of Maytenus further away from the stream may be a mechanism to conserve water in the drier habitat. By forming a hydrophobic barrier, the cuticle restricts water vapour loss (Pallardy, 1981). Thickening of the cuticle is the simplest method by which to conserve water (Fritsch and Salisbury, 1955) and cuticle biosynthesis is energetically inexpensive (Shulze, 1982). Forest-precursor species Rapanea, Cunonia and Kiggelaria, on the other hand, had thin cuticles in both the stream and non-stream habitats.
Leaves of Olinia and Chionanthus had thicker cuticles in the proximity of the stream than further away. These two species are restricted to the forest and its margin, and are amongst those species with an extremely thin cuticle. Their cuticles, which were thin in all environments, are probably determined genetically, as a result of conditioning to the low radiation and favourable humidity within the forest. The lack of any difference in cuticle thickness between the stream and non-stream habitats for forest-precursor species suggests that, within the forest, water availability does not affect the cuticle thickness appreciably. The thicker cuticle of the forest precursors in the fynbos may have been significant with regard to water conservation. Temperature may also be another responsible stimulus or environmental factor. Cuticle-related changes in the spectral properties of leaves have an indirect effect on water relations, being a result of alterations in the leaf temperature (Pallardy, 1981).
Epidermal cell size was used by Myers et al. (1987) as an approximation of mesophyll cell size. Cuttler et al. (1977) reported that a reduction in average cell size, and hence in the relative symplasmic water per cell, may result in a decrease in tissue osmotic potential. This is an energetically inexpensive way of maintaining osmotic homeostasis as osmotic adjustment can be very difficult when performed by plants growing on Mediterranean soils which typically have a low ionic content (Lo Gullo et al., 1986). The fact that the species with a thin cuticle in the fynbos, e.g. Kiggelaria and Olinia, possessed smaller cells than in the forest environment suggests that these species were, in fact, not completely devoid of a mechanism for water conservation. Furthermore, these species are xeromorphic in the fynbos, with leaves insulated by thick hydrophobic cuticles. Maytenus and Olea probably did not require any further osmoregulatory mechanism, which explains their large sized cells across the environments. Osmoregulation by cell size is not as effective as regulation by changes in solute concentration. The latter process accounted for much of the difference in tissue osmotic potential of leaves of Castanospermum australe growing between the forest and adjacent clearing (Myers et al., 1987). Even between the stream and non-stream habitat, Olinia and Chionanthus showed osmoregulation by cell size. In the stream and non-stream habitats, Maytenus and C. capensis may have employed this method merely to supplement their water regulation by a thick cuticle.
The large difference in stomatal frequency (of Maytenus, Olea, Rapanea and Cunonia) between the fynbos, on the one hand, and the interior and the margin of the forest, simulates the response shown by cuticle thickness. The small difference between the interior and margin of the forest also parallels the non-significant difference in cuticle thickness between these environments. It is also interesting that for any given leaf character, Maytenus, Olea, Rapanea and Cunonia often responded in a similar fashion. The latter two species are forest precursors whilst the former is a wide-ranging forest species (see Hlwatika et al. 1999).
High stomatal frequencies are usually characteristic of leaves of xerophytes (Shields, 1950; Fahn, 1964). Plants grown under high light intensities also show a high stomatal frequency (Fetcher et al., 1983). Since the difference in stomatal frequency was always small between the stream and non-stream habitats, as well as between the margin and the interior of the forest, the higher stomatal frequencies in the fynbos may primarily be a reaction to the favourable photosynthetic conditions. Radiation is many times higher under pioneer vegetation than within the forest (Bazzaz, 1979; Messier and Bellefleur, 1988). As carbon dioxide constitutes only 0.03 % of the atmosphere it forms diffusion shells on both sides of the epidermis covering the leaf, thereby maximizing diffusion efficiency. In this way diffusion of carbon dioxide is minimally hindered by the epidermis. Water vapour is near saturation inside the leaf, and thus never forms diffusion shells. In addition, there is always a boundary layer of moist air around the leaf, preventing steepening of the diffusion gradient (Bidwell, 1974). Consequently, the leaf is maximally effective for reducing water evaporation whilst permitting high rates of carbon dioxide transfer. Directly or indirectly, the high stomatal frequency in the fynbos may also be a response to the scarcity of water. The higher light intensity and temperature in addition to exposure to wind in the fynbos compared with the forest would accelerate evapotranspiration.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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The investigation of leaf anatomy and morphology of trees in Orange Kloof reveals ecological differentiation. The present observations suggest that Cunonia, Rapanea and, to some extent, Kiggelaria respond in a similar fashion to the environment. These species exhibited high phenotypic plasticity; their leaf structure changed to suit the particular environment. Their leaves are of a xeromorphic nature in the fynbos but assume a mesomorphic structure within the forest. In the case of Cunonia, Rapanea and Kiggelaria, all exhibited leaf anatomical plasticity. Maytenus belongs to a different class. Unlike the leaves of the forest precursors, its foliage showed no phenotypic plasticity. In the fynbos it was even more xeromorphic than the forest precursors. Its leaf anatomy is apparently a result of genetic influence. This may limit Maytenus to environmental conditions found in the fynbos. Its dominance in the fynbos adjacent to the scree forest, its co-dominance at the margin of the scree, and its characteristic absence within any forest type (Masson, 1990), attest to this. Euclea occurs only in the fynbos and its leaf characters are similar to those of Maytenus. Like Maytenus, its confinement to the fynbos may be linked to a lack of phenotypic plasticity. Species restricted to the forest also exhibited similar leaf characters. Their foliar features occupied the other end of the scale compared with those of Maytenus. In this category, only Olinia exhibited leaf plasticity as it had xeromorphic leaves in the fynbos environment. Transcending all these categories, and found in all environments, were Olea and Cassine.
The slow rate of forest recovery in Orange Kloof (50 years since the last fire) may be explained by the mode of response of these species to the environment. Adaptive capacity is a prerequisite for invasion of the fynbos by forest species. Maytenus is genetically adapted to the most adverse environments: the scree margin and fynbos zone. In these environments it may facilitate colonization by forest species. Imbued with phenotypic plasticity, the forest precursors may set the pace for regeneration by modifying the less adverse fynbos environments. The wide-ranging species, Olea and Cassine, are probably facilitators like the forest precursors. Finally, the forest-precursor species, scrub species and the wide-ranging taxa appear to have anatomical characters that can be modified in the fynbos, thus allowing it to be colonized by a variety of different species.
In conclusion, it does appear to be possible to identify plastic behaviour in anatomy which can be related to the ecology of a species. However, to understand this relationship more fully would require examination of a larger number of species than was undertaken in the present investigation, perhaps covering a wider breadth of environments.
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ACKNOWLEDGEMENTS |
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We thank Professors B. McKenzie and C. T. Johnson, University of the Western Cape, South Africa, for their encouragement.
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LITERATURE CITED |
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