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Annals of Botany 89: 605-612, 2002
© 2002 Annals of Botany Company

The Anatomy and Chemistry of the Colour Bands of Grasstree Stems (Xanthorrhoea preissii) used for Plant Age and Fire History Determination

WENDY I. COLANGELO0, BYRON B. LAMONT*,0, ANTHEA S. JONES0, DAVID J. WARD0 and SANDRO BOMBARDIERI0

0School of Chemical and Biological Sciences, Curtin University of Technology, PO Box U1987, Perth WA 6845, Australia

* For correspondence. Fax 0061 8 9266 2495, e-mail rlamontb{at}cc.curtin.edu.au

Received: 5 September 2001; Returned for revision: 30 November 2001; Accepted: 7 January 2002.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 

A new method of ageing and determining the fire history of grasstrees, based on colour bands running along the stem, has been developed. As part of our evaluation of the technique, we examined the structural and chemical basis of the colour differences. Exposed ends of the leaf bases are cream, brown and black, with the inner cortex, especially in the black leaf bases, being darker than the outer cortex. There was no structural difference between the three leaf base types. Tannin concentration increased from cream to brown to black leaf bases, and from the inner to outer cortex, and remained quite stable over many years. Both water-soluble and insoluble pigments contribute to the darkness of the black leaf bases. A hydrophobic naphthoquinone was present in the conducting tissues of the vascular bundles, and related naphthalene-derivatives were present in the surrounding tissues. We conclude that the colour differences between the leaf bases have a chemical basis that can be linked to environmental changes: tannin cells to phenological effects, and naphthalene-derivatives in the vascular core to the passage of fire.

Key words: Age determination, fire history, grasstrees, growth rings, leaf anatomy, leaf bases, monocotyledon, naphthoquinone, resin, tannin, vascular bundles, Xanthorrhoea.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Grasstree is the common name used for several plant genera in Australia, viz. Dasypogon, Kingia and Xanthorrhoea (Staff and Waterhouse, 1981). Grasstrees consist of a stem up to 5 m long that is surrounded by dead leaves and that supports a dense crown of long spiny leaves. The dead leaves form a skirt around the stem until they are burnt back to the leaf bases by occasional fires to form a sheath around the true stem. Grasstrees are widespread in southern and eastern Australia, occurring in wet and dry sclerophyll forest to open heath and swamp margins (Staff and Waterhouse, 1981). In Xanthorrhoea, the leaf bases are embedded in resin that melts during fire and flows to lower parts of the stem. Lamont and Downes (1979) harvested grasstrees, removed their persistent leaf bases and identified annual undulations running along the stem and occasional flower remnants. Since flowering is usually associated with fire, they used the presence of flower remnants as an indicator of fire and hence determined the fire history of the site. These authors estimated ages up to 350 years for tall Xanthorrhoea preissii Endl. and 600 years for Kingia australis R. Br., with an average growth rate of about 1·5 cm per year.

Recent research by Ward et al. (2001) has confirmed that X. preissii may be useful in reconstructing the fire history of various sites but using a more convenient method. When the blackened outer surface of the sheath of leaf bases is ground off, alternating cream and brown bands with intermittent black bands are revealed. Ward et al. (2001) showed that these bands may be used to determine plant age and fire history, with each pair of cream and brown bands depicting 1 year’s growth, and the black bands denoting the occurrence of fire. Cream bands represent the spring/summer flush of growth, and brown bands the slower autumn/winter production of leaves. Black bands are caused by the burning of the oldest green leaves. Since this new technique may provide a detailed fire history of a site over the past 350 years, as well as a non-destructive method of ageing old plants, confirmation that the colour differences have a biological origin is important (Ward et al., 2001). Thus, we undertook an anatomical, histological and chemical analysis of the three colour bands to determine whether they had a structural/chemical basis.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Histology
Whole cream, brown and black leaf bases were removed from two stems harvested from two contrasting sites within the northern jarrah (Eucalyptus marginata) forest of south-western Australia (Ward et al., 2001, Figs 1 and 2A). One stem was collected from Gyngoorda Block (32°35'S, 116°28'E, burnt at 5–15 year intervals) and another from Poison Gully (32°11'S, 116°15'E, unknown fire history), both administered by the Department of Conservation and Land Management. From each stem, leaf bases in the three colour bands were collected at two positions about 30 cm apart.



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  		Fig. 1. Grinding off charcoal from leafbases of Xanthorrhoea preissii to reveal colour bands used for growth rate and fire history determination. Dead leaves removed from unburnt (upper) part of stem before grinding. Trunk diameter = 20 cm.

 
Xanthorrhoea preissii leaf bases have a hard woody texture. To enable sectioning, the leaf bases were boiled for 30 min in deionized water (O’Brien and McCully, 1981). Unboiled leaf bases could not be sectioned and embedding the leaf bases in wax and glycomethylacrylate was also unsuccessful. Leaf bases were sectioned (50–75 µm) using a sliding wood microtome (Model 860; American Optical Corp., New York, USA). Unstained sections were mounted on slides with Gürr’s aquamount. Photographs were taken of the slides using a Olympus Vanox-S AHBS-513 photomicroscope. Ten sections were stained with safranin/fast green, toluidine blue or ferrous sulfate for each of the two stems and two ages. Using 1 % aqueous safranin and 0·5 % fast green in 95 % ethanol, lignified walls, tannin deposits and chromatin are stained red, while all other constituents are stained green (O’Brien and McCully, 1981). Cell walls impregnated with lignin or other phenolics and tannin-rich vacuoles stain green or blue with 0·05 % toluidine blue in acetate buffer, pH 4·4 (O’Brien and McCully, 1981; Gahan, 1984). Ferrous sulfate, which stains tannins specifically, was prepared according to Schneider (1981). Sections were placed in 2 % ferrous sulfate for 2 h. Tannins stain orange to black.

Tannin assay
Four stems were collected from four sites within SW Australia: Gyngoorda (32°35'S, 116°28'E), Amphion (32°48'S, 116°12'E), Yarra Road (32°14'S, 116°26'E) and Mundaring Weir Road (31°59'S, 116°9'E). Leaf bases from three consecutive bands were collected from two positions on the one stem, termed ‘upper’ and ‘lower’. The Gyngoorda leaf bases were taken from about the years 1905 and 1960, the Amphion leafbases from 1960 and 1985, Yarra Road leaf bases from 1940 and 1970, and Mundaring Weir Road leaf bases from 1964 and 1970. About 40 leaf bases were used per colour band analysis. The first centimetre that maintained the external colour of the leaf base was collected. The hard outer layer was removed with a razor blade and then the inner (with vascular bundles) and outer cortex were separated. The inner cortex could be distinguished by colour and texture from the outer cortex. The inner and outer cortex were ground separately using a mill (Arthur Thomas, Philadelphia, USA) and were passed through an Endecott laboratory sieve with aperture 1·4 mm.

When whole leaf bases were boiled in deionized water for 30 min prior to sectioning, the water changed to a brown colour, increasing in intensity from the cream, to brown to black leaf bases. Based on this observation, we used a colorimetric approach to determine the relative concentration of this water-soluble brown substance (tannin?). To 50 mg of milled leaf bases, 6 ml of deionized water was added to a test tube, sealed with non-absorbent cottonwool and boiled for 30 min in a water bath (C. Joll, pers. comm.). Twelve samples of 40 leaf bases were treated per stem, viz. the three colour bands, inner and outer cortex, and two ages (upper/lower). All the black leaf base extracts went cloudy, as did that from Amphion cream leaf bases. To enable absorbance readings to be performed, 0·6 ml 100 % acetone was added to each cream, brown and black extract to ensure a clear solution. The resulting supernatants were dispensed into spectrophotometer cuvettes. An LKB-Biochrom 4060 UV-visible spectrophotometer (Pharmacia, Uppsala, Sweden) was used. Absorbances were read at 400 nm. Results were subjected to ANOVA to examine the (null) hypotheses that there were no differences between the four stems, the three band colours, the position on the stem (age effect) or the inner and outer cortex. Equality of variances was checked by Bartlett’s test, and corrected where required by taking square roots of the raw data. Selected results were then subjected to paired t-tests with sequential Bonferroni corrections applied for non-independent data to identify where the differences lay (Rice, 1989).

The Price and Butler method (Waterman and Mole, 1994) was used to determine total tannin (phenolic) concentration of the cream, brown and black Amphion, Gyngoorda and Mundaring Weir Road leaf base extracts. The two reagents were prepared according to Price and Butler: ferric chloride reagent (0·1 mol l–1 solution of FeCl3 in 0·1 mol l–1 HCl), which was filtered until the final solution was clear yellow, and potassium ferricyanide reagent (8 mmol l–1 K3Fe(CN)6 in deionized water). Twenty-five millilitres of deionized water was mixed with 250 µl of sample in a 50 ml container. The 6·6 ml cream, brown and black, inner and outer Amphion leaf base extracts from the initial analysis (plus blanks) were used again. To the diluted sample, 3 ml FeCl3 reagent was added. After 3 min, 3 ml K3Fe(CN)6 reagent was added, and mixed. After 15 min, the absorbance was read at 720 nm. To convert absorbance readings to quantities of phenolics, the assay was performed on tannic acid (C76H52O46; Lab Reagent; BDH Chemicals, Poole, UK) as the standard.

Pyrolysis gas chromatography and mass spectrometry (GC-MS)
Vascular bundles were excised from 75 µm sections of the Gyngoorda leaf bases under a dissecting microscope. About 15 to 20 vascular bundle cross-sections from each of the cream and black bands on the leaf bases, a resin sample from the Amphion stem and a lapachol (C15H14O3; Lab Reagent; Sigma-Aldrich, NSW, Australia) sample were placed in quartz pellets. Lapachol was tentatively identified as a unique component of the inner cortex of black leaf bases in preliminary pyrolysis GC-MS (J. Challinor, pers. comm.). The inner cortex of Amphion 1960 black leaf base sections was placed in 70 % acetone, the supernatant collected and evaporated and the residue placed in a quartz pellet. Pyrolysis GC-MS was performed using a SGE Pyrojector 2 (SGE International, Victoria, Australia) operating at 550 °C. This was fitted to an HP 5890 series II gas chromatograph (Hewlett Packard, Pennsylvania, USA) with a 30 m x 0·2 mm fused-silica column with a 0·33 µm film (HP-5MS). An oven temperature programme of 0 to 280 °C at a rate of 8° min–1 was used, with initial and final hold times of 2 and 10 min, respectively. The carrier gas was helium at a linear velocity of 35 cm s–1. Mass spectra were recorded using an HP 5971 series MSD (Hewlett Packard, Pennsylvania, USA) operating in scan mode with typical operating conditions of EM voltage 2100 V, 70 eV electron energy and source temperature of 190 °C. Wiley library spectra were used for compound identification. Compounds common to the resin, the black inner cortex and vascular bundles, and/or lapachol, and which differed from the cream leaf bases, were noted.

The occurrence of black bands on the Gyngoorda stem were used, in conjunction with the Department of Conservation and Land Management fire records, to establish the age and fire history of leaf bases for the period 1894–1903. There were three external dark bands for the decade, implying three fires. Leaf bases were removed using a heat gun, washed in alcohol to remove resin and cut into 1-cm pieces measured from the proximal end. Seven cream, nine brown and four black leaf bases were cut into five 1-cm long sections, giving a total of 100 samples. The inner cortex of the leaf base samples was analysed by pyrolysis GC-MS as before to determine the presence of the unique pigments within the vascular core.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Transverse sections of unstained leaf bases from the three colour bands (Fig. 2A) are shown in Fig. 2B–D. Their anatomy is identical, with a characteristic inner and outer cortex, scattered vascular bundles in the inner cortex and a reaction fibre group present on the abaxial side. Cells with yellow-brown contents were scattered in both parts of the cortex. At a higher magnification, there was no apparent difference between the cream and brown leaf base sections but the inner cortex of the black leaf bases was markedly different (Fig. 2E and F). Black leaf base sections usually showed the V-shaped xylem and the band of phloem, forming a triangle with the xylem, filled with an orange-brown substance. The three fibre groups comprising the rest of the vascular bundle were a darker yellow-brown than those in the other two bands. The inner cortical cells appeared brown rather than yellow, while some cells surrounding the vascular bundles appeared to have blackened walls with a brown to black substance filling some cells and their intercellular spaces.



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  		Fig. 2. A, Portion of ground grasstree stem showing alternating cream and brown bands and one black band. Transverse sections of unstained cream (B), brown (C) and black (D) leaf bases (Poison Gully, 1974; 75 µm thick). Brown (E), black (F) (Poison Gully, 1943; 50 µm) and cream (G; Poison Gully, 1958; 50 µm) leaf bases stained with safranin and fast green; H, black leaf base stained with safranin and fast green (Poison Gully, 1958; 50 µm); I, brown leaf base stained with toluidine blue (Gyngoorda, 1972; 75 µm); J, black leaf base stained with toluidine blue (Poison Gully, 1974; 75 µm); brown (K; Gyngoorda, 1972; 75 µm) and black (L; Gyngoorda, 1972; 50 µm) leaf bases stained with ferrous sulfate.

 
The cortical cells of the cream and brown leaf bases stained blue-green with safranin/fast green, while those of the black leaf bases stained red-brown (Fig. 2G and H). The scattered yellow-brown cells in the cortex stained dark red throughout. The fibre bands stained green and red and the xylem stained red, but these tissues did not stain in the black leaf bases. The brown substance in the xylem and surrounding cortical cells stained bright red using safranin/fast green and blue-black with toluidine blue (Fig. 2J).

The fibre groups stained pale green and the xylem blue with toluidine blue in the cream and brown leaf bases, while these tissues stained bright green in the black leaf bases (Fig. 2I and J). The scattered brown cortical cells stained dark blue with toluidine blue and brown-black with FeSO4 throughout (Fig. 2I–L). The contents of some narrower vessels of the xylem stained brown in the cream and brown leaf bases, while the cortical cells of the black leaf bases turned brown-black, the fibre group walls brown, their contents black and the brown substance surrounding the bundles black. The orange substance in the vessels and phloem did not stain.

Absorbance readings for the four sets of leaf base extracts are given in Table 1. Three-way ANOVA performed on the complete data set showed that there was a highly significant difference between the overall colour of the four stems, between the three leaf base colours and between the inner and outer cortex (Table 2). The only significant interaction term showed the intensity of the colour pattern varied between stems. Repetition of this analysis, but omitting the black leaf bases, produced a highly significant difference between stems, cream and brown leaf bases, and the inner and outer cortex. There was a significant interaction between stems, colour and cortex. Paired t-tests were then performed on the inner vs. outer cortex and on the different colour bands to identify where the differences lay (Table 3). t-Tests showed that the inner cortex of the black leaf bases was much darker than that of the cream or brown leaf bases, while the brown leaf base cortex was slightly darker than that of the cream leaf base. The black leaf base outer cortex was much darker than that of the corresponding cream or brown leaf bases, while the brown leaf base outer cortex was 25 % darker than that of the cream leaf base. The inner cortex was significantly darker than the outer cortex for the three colours of leaf bases.


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  		Table 1. Individual absorbance readings at 400 nm for water extracts from 40 pooled leaf bases from each of four X. preissii stems, three colour bands, and inner and outer cortex
 

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  		Table 2. A, Three-way ANOVA for four stems, three colours, and cortex (inner/outer). B, Three-way ANOVA for four stems, two colours (cream and brown), and cortex
 

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  		Table 3. One-tailed, paired t-tests (with sequential Bonferroni correction) comparing inner and outer cortex leaf base extracts from the three colour bands
 
The ferricyanide test confirmed that the brown colouration of the water extracts was closely associated with tannins (Fig. 3). Tannins, as tannic acid equivalents, were notably higher in the black leaf bases, with tannins in the brown leaf bases being slightly higher than in the cream leaf bases. The curvilinear relationship (slope < 1) indicated that as the brown colouration increased, the proportional contribution of tannins decreased. In particular, much of the dark colour of the leaf bases must be due to compounds other than tannins.



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  		Fig. 3. Mean ± s.e (n = 4) of absorbance values (water extracts at 400 nm) per colour band per tree (from Table 1) vs. mean ± s.e. tannic acid equivalents of the same extracts. Cream bands (open circles); brown bands (black circles), black bands (black squares); blank (open squares) (not included in equation).

 
Compared with the cream leaf bases, four compounds unique to the black leaf base vascular bundles were detected by pyrolysis GC-MS in the 30·9–34·1 min range (Table 4). The most abundant was a phenolic related to 1-hydroxynaphthalene-2,8-dicarboxaldehyde that was also abundant in the vascular inner cortex of the other plant. Two other water-insoluble compounds with naphthalene skeletons were present in both samples. The compound at 32·7 min was by far the most abundant in the inner cortex. The naphthoquinone, demethoxyisoeleutherin, is a rearrangement product of lapachol and is a strongly coloured pigment. Unique to the vascular bundles, but not as abundant, was a pyrido-indole alkaloid. A closely related compound was detected at low levels in the resin. Also in the resin was a compound closely related to that in the vascular tissues at 34·1 min. This, or a related compound, was detected in small quantities in the lapachol sample, but otherwise was unmatched. The three naphthalene-derived compounds, especially those related to naphthoquinone, identified above in the vascular cores of the black leaf bases were detected towards the point of attachment of most leaf bases (Fig. 4). Only in the black leaf bases did they reach the surfaces exposed by grinding off the charcoal.


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  		Table 4. Pyrolysis GC-MS peaks for black vascular bundles which were not found in cream vascular bundles, but were found in black inner cortex in 70 % acetone, grasstree resin and lapachol samples, including GC peak height, most likely identity (Wiley library) and closeness of match
 


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  		Fig. 4. Presence of naphthalene derivatives detected by pyrolysis GC-MS in vascular cores of leaf bases cut into 1-cm length sections from sequential cream (c), brown (br) and black (bl) bands over an estimated 10-year period for the Gyngoorda grasstree.

 


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Histological examination failed to show consistent differences between the cream and brown leaf bases. The occasional reddish-brown cortical cells observed in all three colour bands appear to contain tannin, based on their appearance and responses to safranin, toluidine blue and FeSO4. The lack of a difference between leaf base types appears to be due to boiling the tissue before sectioning, causing most of the tannins to be leached out. However, the histology of the three bands showed a basic difference between the black leaf bases on the one hand, and the cream and brown leaf bases on the other, in terms of the chemistry of the cell walls and the deposits associated with the vascular bundles. Cell walls of the black leaf bases did not stain with fast green but stained strongly with safranin, toluidine blue and FeSO4. This indicates that they may contain tannin-like substances that mask the expected response to cellulose only. In addition, the brown substance spreading through the fibre groups and tissues surrounding the vascular bundles of the black leaf bases stained strongly with FeSO4, specific for tannins. However, the substance filling the xylem and phloem of the black leaf bases did not appear to stain for tannins using FeSO4. This indicates that the unusual dark brown deposits pervading the cortical cells of the black leaf bases are not the same substance as in the vascular tissues. Results of the three stains tested indicate that the substance in the vascular tissues is different from, but possibly related to, the substance apparently oozing from the vascular bundles into the cortex or tannin cells in the cortex. Unfortunately, there are no stains for particular types of tannins.

Colorimetric results agreed with visual impressions: the brown leaf base is darker than the cream, the black leaf base is darker than the brown and the inner cortex is darker than the outer (Fig. 2A; Table 2). The greater abundance of typical tannin cells, especially in the inner cortex, appears to be the basis of the colour difference between the cream and the brown leaf bases (Fig. 3). At least some of the colour difference distinguishing the black from the cream and brown leaf bases appears to be due to additional tannin-like substances that leak from the vascular tissues into the intercellular spaces, walls and lumens of the cortex and fibre groups. This substance differs from that in the xylem and phloem, which is insoluble in boiling water and does not stain with FeSO4. However, its staining response with safranin and toluidine blue, and close proximity to the greatest concentrations of cortical tannin, indicate that it is structurally related to, and probably the source of, this extra tannin. This vascular substance also appears to contribute to the darkening of the inner cortex following fire.

Pyrolysis GC-MS showed that the vascular core of the black leaf bases contains unique compounds, mostly naphthalene derivatives, with varying affinities to tannins, intensity of colour and solubility in water (Table 4). The hydroxynaphthalene and pyrido-indole related compounds probably contributed to pigmentation outside the conducting tissues. The hydrophobic pentamethylnaphthalene and naphthoquinone probably contributed to the insoluble pigment in the conducting tissues. These may have some affinity with resin secreted by the stem supporting the leaf bases, but xanthorrhoeol, the major component of X. preissii resin (Birch and Dahl, 1974), was not detected. The rearrangement of lapachol during pyrolysis highlights the problem of identifying any of the components with certainty, and this part of our work must be considered preliminary.

We interpret the clogging of the vascular tissue with a hydrophobic substance and its leakage, in modified form, across the cortex as a general means of limiting further loss of water and metabolites from heat-killed leaves. Younger leaves recover from loss of their outer portions by intercalary growth (Staff and Waterhouse, 1981) and consequently show no vascular clogging. Vascular clogging by naphthoquinone-related compounds is a normal process in dying leaves, but is both hastened and heightened by fire (Fig. 4).

The reason why the cream leaf bases have less cortical tannin than brown leaf bases is less clear. Leaves produced in autumn/winter elongate at a time of year when conditions are not conducive to rapid growth. Their bases are thinner (J. Eldridge, unpubl. res.), confirming that they are not as vigorous as leaves produced in spring/summer. Tannins accumulate in response to less favourable conditions, and this may explain their presence in autumn/winter-formed leaves. Tannin accumulation is also time-dependent. Since all leaves die in summer, leaves initiated in autumn/winter are 5 months older than spring/summer-initiated leaves when they die (D. Korczynskyj, pers. comm.) and this could also explain the differences.

We conclude that colour differences between the bands of Xanthorrhoea preissii leaf bases have a chemical origin. Since this is associated with seasonal changes or fire events experienced by the plant, the results provide biochemical support for use of the colour bands in determining the age and fire history of grasstrees (Ward et al., 2001).


   ACKNOWLEDGEMENTS
 
We thank the Australian Research Council (Linkage), Department of Conservation and Land Management and Curtin Infrastructure Program for financial support and Dr John Challinor (WA Chemistry Centre) for assistance with the initial chemical analyses.


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 

    Birch AJ, Dahl CJ. 1974. Some constituents of the resins of Xanthorrhoea preissii, australis and hastile. Australian Journal of Chemistry 27: 331–344.

    Gahan PB. 1984. Carbohydrates: Tannins. In: Sutcliffe JF, Cronshaw J. eds. Plant histochemistry and cytochemistry – introduction. Sydney: Academic Press.

    Lamont BB, Downes, S. 1979. The longevity, flowering and fire history of the grasstrees, Xanthorrhoea preissii and Kingia australis. Journal of Applied Ecology 16: 893–899.

    O’Brien TP, McCully ME. 1981. The study of plant structure principles and selected methods. Melbourne: Termarcarphi.

    Rice WR. 1989. Analyzing tables of statistical tests. Evolution 43: 223–225.[CrossRef][Web of Science]

    Schneider H. 1981. FeSO4 for tannins. In: Clark G, ed. Staining procedures. 4th edn. London: Williams and Wilkins, 338–339.

    Staff IA, Waterhouse JT. 1981. The biology of arborescent monocotyledons, with special reference to Australian species. In: Pate JS, McComb AJ. eds. The biology of Australian plants. Nedlands: University of Western Australia Press, 216–257.

    Ward DJ, Lamont BB, Burrows CL. 2001. Grasstrees reveal contrasting fire regimes in eucalypt forest before and after European settlement of southwestern Australia. Forest Ecology and Management 150: 323–329.[CrossRef]

    Waterman PG, Mole S. 1994. Analysis of phenolic plant metabolites. Oxford: Blackwell Scientific.


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