The Wood of Pinus Canariensis and Its Resinous Heartwood

IAWA Journal, Vol. 26 (1), 2005: 69–77

THE WOOD OF PINUS CANARIENSIS AND ITS RESINOUS HEARTWOOD

Luis García Esteban1, Peter Gasson2, José María Climent3, Paloma de Palacios1 & Antonio Guindeo1

SUMMARY

Pinus canariensis (Canary Island Pine or Pitch Pine) forms natural for-ests on the islands of Tenerife and La Palma. The heartwood has an extra-ordinarily high resin content, and this paper provides an anatomical description of the wood as well as an interpretation of the factors relating to this resinification.Pinus canariensis possesses many subsidiary parenchyma cells surround-ing the axial resin canals. Similarly, the percentage of rays is high, which means there are many parenchyma cells capable of accumulating large amounts of starch, which in turn can be used for the synthesis of the pitch extractives, primarily terpenoids and polyphenols. The presence of sub-sidiary parenchyma cells and the high percentage of rays are a major contributor to the heartwood of Pinus canariensis being rich in extrac-tives.

Key words: Pinus canariensis, Canary Islands, pine forest, resinous heart-wood.

INTRODUCTION

Pinus canariensis C.Smith occurs naturally on the highest islands of the archipelago, Gran Canaria, Tenerife, La Gomera, El Hierro and La Palma, but it is not found in Lanzarote or Fuerteventura. The species occupies a total area of 66,700 ha, with 30,000 ha in Tenerife, 23,000 ha in La Palma, 11,000 ha in Gran Canaria and 2,700 ha in El Hierro. The species is found from sites at sea level to nearly 2400 m. The present-day condi-tions of the eastern islands do not allow for the establishment of pine forests, although archaeological sites have been located in Fuerteventura (Machado Yanes 1995) and many of the place names on the island of Lanzarote bear witness to the former presence of the Canary Island pine in these islands.

1) Escuela Técnica Superior de Ingenieros de Montes, Universidad Politécnica de Madrid, De-partamento de Ingeniería Forestal, Cátedra de Tecnología de la Madera, Ciudad Universitaria, E-28040 Madrid, Spain [E-mail: [email protected]].

2) Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, United Kingdom.3) Escuela Técnica Superior de Ingenieros de Montes, Universidad Politécnica de Madrid, De-

partamento de Silvopascicultura, Cátedra de Anatomía y Fisiología Vegetal, Ciudad Universi-taria, E-28040 Madrid, Spain.

IAWA Journal, Vol. 26 (1), 200570 71García Esteban et al. — Wood of Pinus canariensis

Although it occupies a limited and very specific natural area in comparison with other species, Pinus canariensis has a broad genetic base due to its adaptation to the diverse ecological conditions of the islands (Climent 1995). Four areas of provenance and five sub-regions have been established for this species (Climent et al. 1996). There is a wide variety of opinion on the absence of true pine forests on the island of La Gomera. For some (Ceballos & Ortuño 1951; Rivas-Martínez 1987), the altitude of the island is not sufficient to allow a dry montane forest. For others (Ferreras & Arozena 1987), the presence of relic examples in el Garabato and in the Imada and Agando rock formations is significant. However, the possible existence of a former pine forest at some stage in the past should not be discounted. In many woods of the genus Pinus the resin content of the heartwood is much greater than that of the sapwood. Species such as Pinus rigida, P. merkusii, P. ponderosa and P. caribaea have heartwood with high resin content, for which reason they are given the name of pitch pine in commerce. The Canary Island pine is a clear example of this group of pines, although the small amount of wood placed on the market means that it is virtually unknown outside the Islands. The first complete description of the wood of Pinus canariensis was made by Nájera and was included in Ceballos & Ortuño (1951). Greguss (1955) includes a brief description of the xylem of the Canary Island pine, and Peraza (1964) provides an ex-haustive description of this wood. In 1967 he enlarged this description. García Esteban and Guindeo (1988) revised these works and expanded the description. Climent (1995) did an exhaustive study of the genetic and environmental aspects of the resinification of the Canary Island pine, including a full anatomical description of the axial paren-chyma sheath that surrounds the resin canals, which Wiedenhoeft and Miller (2002) term subsidiary parenchyma cells. Lastly, García Esteban et al. (2002) presented a new description with features that had not been considered previously.

MATERIAL AND METHODS

The material used for this study was collected in the natural forests of Tenerife and La Palma. In order to locate the natural forests the Mapa Forestal de España (Ceballos 1966) and the Atlas Cartográficos de los Pinares Canarios (Del Arco et al. 1992; Pérez de Paz et al. 1994) were used. The sample sites were chosen by using the Estudio Ecológico del Pino Canario (Blanco et al. 1989). The specimens collected in Tenerife covered the three provenance subregions of the island, and in La Palma specimens could only be taken from the northern subregion. Eleven trees were cut, six in Tenerife and five in La Palma. The microscopic preparations were made following the usual methods of softening, sectioning, staining, and mounting. For the observation of starch content in the parenchyma cells Lugolʼs solution was used, prepared following the method of Johansen (1940). The lipids and resins contained in the lumen of the tracheids were identified with Sudan IV dye using the method of Jensen (1962). The tannins were identified by being immersed in an aqueous solution of potassium dichromate for 15 minutes (Locquin & Langeron 1985).


WOOD ANATOMICAL DESCRIPTION

Growth rings clearly defined, with a relatively gradual change from earlywood to late-wood. Axial tracheids without spiral thickenings. The axial tracheids of the early-wood are of irregular shape, with rounded edges and intercellular spaces (Fig. 1a). These intercellular spaces also occur in non-compression wood. The axial tracheids of the latewood are hexagonal or square in cross section. These intercellular spaces also occur in non-compression wood. In the resinous heartwood, all the axial tracheids have the lumen occupied by translucent substances of a caramel colour and of terpenic nature, which react positively with Sudan IV and cupric acetate. These deposits appear abruptly either in the sapwood ring closest to the heartwood or in the two closest rings. The average radial diameter of the axial tracheids of the earlywood is 51 μm (37–58 μm) (12.1)*) and in the latewood it is 37 μm (24–43 μm) (6.4). The average tangential diameter of the axial tracheids is 49 μm (45–51 μm) (4.1). The axial resin canals appear in the transition wood or latewood and very rarely in the earlywood. Their average tangential diameter is 221 μm (168–293 μm) (8.3) (Method A in IAWA Committee 2004), with a density of 0.25 to 0.60 canals per mm2. The latewood was impregnated with resin earlier and with greater intensity than the earlywood. The epithelial cells that surround the canals are thin-walled and are surrounded by a very extensive group of subsidiary parenchyma cells which in some cases number as many as 65 cells (normally around 35, Fig. 2). The appearance of the subsidiary tissue is on occasion aliform, and when two contiguous canals are in proximity it may be confluent. The further away we are from the epithelial cells, the greater the degree of lignification of the secondary walls of the subsidiary parenchyma cells. Groups of subsidiary cells in transverse section can frequently be found, and even diffuse groups of parenchyma cells between the axial tracheids, which can be confused with ʻnormal ̓diffuse paren-chyma, although after making contiguous transverse cuts above or below these cells it appeared that all of the parenchyma cells are continuous with the subsidiary tissue of the axial resin canals (Fig. 1b). The subsidiary parenchyma cells communicate with the adja-cent axial tracheids through semi-bordered pits, which are smaller than those of the cross fields in the ray-tracheid contacts. The epithelial cells in the initial stages are turgid, completely covering the canal internally, with starch grains being very evident in the subsidiary parenchyma cells and less so in the epithelial cells. Lipids, detected by Sudan IV staining, are also present in the epithelial cells. Once the tree has reached the age of ten years, the epithelial cells in the newly-formed wood become disorganised and appear squashed against the walls of the subsidiary tissue of the resin canal, apparently due to the excess of resin in the interior of the canal (Climent 1995). In the subsidiary parenchyma cells there is a pro-cess of substitution of the grains of starch by substances of a more intense colour, usually reddish-brown, with a base of stilbenes and flavonoids, sometimes with tannic properties. At the end of the maturation process of the resin canals in the pitch wood, the canals are totally blocked by solid resin. All the rings near the resinous heartwood show this condition.

*) 51 μm (37–58 μm) (12.1) = average value (minimum value – maximum value) (standard devi-ation).


Fig. 1–4. Pinus canariensis. – 1: Transverse section showing intercellular spaces throughout the wood (a) and subsidiary cells (b) of an axial resin canal not visible in this section. – 2: Sub-sidiary cells associated with the axial resin canal. – 3: Starch grains in ray parenchyma cells. – 4: Cross-field pits pinoid. — Scale bars for 1 = 150 μm; for 2 = 200 μm; for 3 & 4 = 25 μm.


The height of the uniseriate rays varies from 2 to 32 cells and their average value is 9 cells (5.46). The number of rays per mm2 is 17 (16.2–19.5) (1.2); the number of ray tracheids per mm2 is 35 (27.2–54.1) (4.3); and the number of ray parenchyma cells per mm2 is 76 (61.3–97.1) (4.3). The presence of radial resin canals gives rise to the typical appearance of fusiform multiseriate rays, with an average diameter of 36 μm (24–48 μm) (6.3). Bordered pits on the tangential wall of the axial tracheids were not observed either in early- or latewood. The rays are heterogeneous with an average percentage of parenchyma cells in rela-tion to the number of cells of the ray of 70% (58–81%) and an average percentage of ray tracheids of 30% (19–41%). The cells of the ray parenchyma are generally thick-walled, the horizontal walls are pitted and the end walls are smooth. The ray cells have a high content of starch grains, which can have a diameter of up to 20 μm (Fig. 3). On the boundary of the sapwood and the heartwood the starch grains transform into reddish-brown substances, in particular in the latewood, of the same nature as those of the subsidiary parenchyma cells of the resin canals, but in a much more abrupt manner than in the latter. In the pitch wood, although some isolated cells can be observed where the transformation has not been completed, starch grains have virtually disappeared.

Fig. 5 & 6. Pinus canariensis. – 5: Dentate ray tracheids. – 6: Biseriate and opposite bordered pits with crassulae on the radial wall of the axial tracheids. — Scale bars for 5 = 25 μm; for 6 = 150 μm.


The cross-field pits are of the pinoid type, generally 1 or 2 per field, although there can be as many as 4 (Fig. 4). Their greatest diagonal diameter has an average value of 18 μm (13–22 μm) (2.5). The ray tracheids are located both in the marginal position as well as in the interior of the ray. Their walls show irregular thickenings without marked dentations, although some dentations can be found sporadically (Fig. 5). The bordered pits of these tracheids have an average diameter of 11 μm (10–11 μm) (0.8). The bordered pits on the radial wall of the axial tracheids are normally uniseriate, and occasionally biseriate, and in both cases crassulae are present, although not in all the tracheids (Fig. 6). The average pit diameter is 24 μm (21–26 μm) (1.5).

DISCUSSION

The lack of references to the presence of the aliform subsidiary parenchyma cells found around the axial resin canals could be attributed to the loss of this subsidiary tissue during the sectioning process, and accounts for the fact that the measurements shown in the previous descriptions give a larger diameter to the canals (350 μm) than those obtained in the present biometric study (221 μm). Only the photographs in Nájera (1951) clearly show the subsidiary parenchyma tissue of the resin canals, but the author does not establish any differences between the subsidiary cells and the epithelial cells. La Pasha and Wheeler (1990) note a similar structure in the resin canals of Pinus taeda, but no references have been found to indicate that the heartwood of this species is particularly resinous (Mirov 1967; Farjon 1984; Vidakovic 1991). Baas et al. (1986) describe subsidiary parenchyma cells around the resin canals in Pinus longaeva and note an abundant resin content in the axial tracheids and in the ray cells of the heartwood. Although the abundance of resin canals (from 0.25 to 0.60 canals per mm2) is greater than in other pines, this is not sufficient to explain the large amount of resin in the resin-ous heartwood of this species. In fact, when heartwood formation occurs in this wood, the epithelial cells have already lost their nuclei and became disorganised several years previously. In Pinus pinaster, Pardos (1976) showed that only the epithelial cells of the first rings of the sapwood were able to proliferate. Therefore, the resin of the resin canals is preformed inside the lumen of the canal and is liberated towards the tracheids when the tracheids have lost their water conducting function. However, it is possible that the radial resin canals, considered to have little to do with resinification, help to increase the volume of resin due to their connection with the living cells of the sapwood. The fact that the radial resin canals are interconnected with the axial resin canals, forming a three-dimensional network (La Pasha & Wheeler 1990), does not account for the resinification of this wood either. Only a metabolic route such as that described by Hillis (1987) for the impregnation by resin of the axial tracheids of Pinus radiata after the application of Paraquat, through the cross fields from the ray parenchyma cells, can explain resinification of this type. Yamada (1992) also highlights this circumstance, attributing to the resin canals the first stage in response to a fungus attack, as the resin is preformed, but at a later stage it is the ray parenchyma cells which are responsible for completing the impregnation.


The most significant episodes in the formation of the heartwood are: aspiration of the bordered pits (Liese & Bauch 1967); death of the parenchyma cells of the rays, which corresponds to the most important cytological change, after a process of increasing lignification (Balatinez & Kennedy 1967; Fengel 1970; Nobuchi & Harada 1985); and the transformation of the starch (Yamamoto 1982) and impregnation of substances. The microscopic observation of the samples of Pinus canariensis analysed has shown that the starch present in the transition zone disappears completely in the heartwood in order to give rise to different impregnation substances. In all of the descriptions, with the exception of the one made by Greguss (1955), the extraordinary abundance of starch is shown. Its presence is not only limited to the rays, but also to the subsidiary parenchyma cells that surround the axial resin canals. This abundance is obvious in the transition zone between the sapwood and the heart-wood, and affects two or three rings. In the transition zone many authors have defined an area with a high level of biological activity responsible for the principal processes of heartwood formation (Frey-Wyssling & Bosshard 1959; Bamber & Fukazawa 1985). Climent (1995) shows the larger percentage of rays in Pinus canariensis in relation to other conifers, which occupy up to 10% of the volume, as opposed to 6% in Pinus strobus (Panshin & DeZeeuw 1980), 5% in Larix (Côté et al. 1966), 5–7% in Coniferae (Wilson & White 1986) and 9% in Pinus spp. (Koch 1972). This high ray volume implies a higher number of metabolically active cells capable of synthesising a larger amount of products, thereby providing some explanation for the high content of extractives in the Canary Island pine in relation to other species. In the same way, other characteristics of this species such as its ability to grow stump shoots and epicormics, and even to survive after trunk girdling, are probably related to the ability to store and synthesise the compounds in the xylem, as shown in the results on other species by Kramer and Kozlowski (1979). Finally, the fact that the latewood is impregnated earlier and more intensely was also seen in the wood of Pinus radiata (Campbell et al. 1965; Lloyd 1978) and is ex-plained both by the greater amount of resin canals in the latewood and by the fact that the bordered pits remain unaspirated in this area (Hillis 1987).

ACKNOWLEDGEMENTS

We are grateful to Prof. Pieter Baas and an anonymous referee for critical reading of the manuscript and valuable suggestions; and to all the staff of the Servicio de Medio Ambiente of the islands of Tenerife and La Palma.

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The Wood of Pinus Canariensis and Its Resinous Heartwood

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