Señal de CD86
IX. DISCUSION
Northern temperate zone trees belonging to the rosid groups in the APG sys-tem (e.g., Salicaceae and Betulaceae), produce resins in epidermal cells and bud trichomes of leaves and young stems (Table 3-1). Tropical trees, rare among asterids, also produce bud trichomes (Rubiaceae). The ecological roles of these kinds of resin-secreting structures have been considered.
In contrast to secretion from glandular trichomes, resin exudation in Pop-ulus (Salicaceae) occurs from epidermal cells, seen as secretory spots on the
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surface of stipules, and from teeth of young leaves (Curtis and Lersten 1974).
The secretory tissue of P. pyramidalis and P. alba buds consists of palisade-like epidermal cells called prismatic cells. The secreted material is eliminated into a space formed between the outer walls of the prismatic cells and the cuticle covering them, forming a blister or spot. Later, the cuticle bursts and the resin collects between leaves and stipules of the bud (Figure 3-10). Char-rière-Ladreix (1973) deduced from optical fluorescence microscopy of P.
nigra that the resin is not only eliminated to the outside but is secreted intra-cellularly in the epidermis and extraintra-cellularly into the parenchyma below the prismatic epidermis.
The timing of resin secretion in Populus is important ecologically. In P.
deltoides, for example, buds accumulate resin in late summer and secrete it over leaves the following spring, thus enhancing their resistance to leaf beetles
Figure 3-10. Elimination of resin from specialized epidermal cells of Populus pyramidalis. Cross section of a stipule, about ×140. Lower left, a portion of a stipule with the cuticle beginning to detach from the epidermal (prismatic) cells as resin collects in the subcuticular space. Lower right, as more resin collects in the space, the cuticle ruptures and the resin flows onto the leaf surface. Redrawn from Fahn (1979).
stipule
cuticle resin
epidermal cells
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(Curtis and Lersten 1974) and gypsy moths (G. Meyer and Montgomery 1987). Resistance to gypsy moth declines rapidly as the leaves expand and the resin is diluted, volatilized, or weathered. Furthermore, the timing of the resin’s availability to honeybees is important for their nest construction (Chapter 5) as well as to beekeepers who collect it commercially as propolis (Chapter 10).
Bud trichomes of Betulaceae are more similar to epidermal cells than to trichomes, so they are discussed here. Various classes of triterpenoids are major constituents of Alnus, Betula, and Ostrya resins. Species of these gen-era, along with some of Aesculus (Sapindaceae), also contain methylated fla-vonoid aglycones (flavones and abundant flavonols). Sometimes mixed with mucilage (Charrière-Ladreix 1973, 1975; Wollenweber and Dietz 1981; Palo 1984), the resins are generally secreted via multicellular glandular trichomes (often called colleters), which may not have stalk cells.
A comparison of bud trichomes and their chemical content on juvenile shoots of six Betula species from different geographic areas showed that mor-phological differences in the secretory structures are often related to chemical differences in the triterpenoid-dominated resins (Taipale et al. 1994). Apical shoots of B. ermanii, B. pendula, B. platyphylla, and B. resinifera are covered with resin droplets secreted by multicellular peltate glands (Figure 3-11)
Figure 3-11. Light micrograph of cross section of an apical leaf bud of Betula pendula, showing peltate trichomes in which resin is secreted. Scale, 100 µm.
From the archives of S. P. Lapinjoki.
138 | CHAPTER 3 How Plants Secrete and Store Resin
whereas those of B. papyrifera and B. pubescens are covered with long non-glandular hairs that form a pubescence. Interestingly, both resinous and pubescent hairs can occur within a single species, such as B. pendula. Resin gland morphology and chemistry are similar in B. pendula, B. platyphylla, and B. resinifera but are different from those in B. ermanii.
Detailed study of the resin secretory tissue in European birch (Betula pen-dula) exemplifies the importance of understanding secretory activity to solve ecological problems arising in commercial use of the plant. Lapinjoki et al.
(1991) analyzed resin secretory activity in B. pendula because juvenile plants of this species, important as a source of short fiber pulp and for reforestation in Scandinavia, are susceptible to many herbivores. Although resin in birch had been shown to deter hares (Tahvanainen et al. 1991) and other animals that browse on the juvenile plant (Chapter 5), there was marked intraplant variation in resistance (Roussi et al. 1989). Little was known, however, about the relationship between development of resin-secreting structures, changes in resin chemistry, and ontogeny of young organs of European birch.
Lapinjoki et al. (1991) reported that two phases of resin production in Betula pendula are related to different stages of growth of juvenile shoots and twigs as well as to defense of these tissues. They found that the resin trichomes, composed of a peltate gland of differentiated columnar epithelial cells (appear-ing as a palisade-like epidermis) radially surround(appear-ing a core of cells, are sim-ilar in the young leaf and young stem (Figures 3-11 and 3-12). They suggested that material stored from the preceding season is used for trichome forma-tion and early secreforma-tion from densely arranged glands resulting from primary growth. The sticky resin in the buds and along the young stems may serve as an antidesiccant, protecting developing tissues from the harsh conditions of late winter and early spring, and also deter insects and birds when less food is available. Secondary growth in later seasons moves glands farther apart, and resin production depends on large trichomes that are gradually worn off.
Large resin droplets on the stem (Figure 3-13), as well as particular triterpenes that lead to winter protection (Chapter 5), are produced only after the most intensive stage of primary growth. These observations helped clarify ecologi-cal confusion about differences in palatability for vertebrate herbivores between first-year shoots and twigs of older saplings (Roussi et al. 1989).
The peltate glandular trichomes on young leaves or buds of alder (Alnus) possess a short stalk of four or more cells, several intermediate cells, and a
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tened head composed of a layer of relatively large secretory cells (Figure 3-7).
In ultrastructural studies of the development of the glandular trichomes in Alnus, Wollenweber et al. (1971) discovered that some of the trichomes are active in fall, others in spring, a situation perhaps similar to what is found in Betula. At the stage of secretion in Alnus, the cuticle together with the
cutic-Figure 3-13. Close-up of resin droplets from trichomes along an adventitious shoot of Betula pen-dula. Scale, 1 mm. From the archives of S. P. Lapinjoki.
Figure 3-12. Light micrograph of cross section of newly grown internode of Betula pendula with the same peltate resin-secreting trichomes as in the leaf buds. The columnar epithelial cells that surround the core of the gland radially are more obvious in this section than in Figure 3-11. Scale, 100 µm. From the archives of S. P. Lapinjoki.
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ular layer splits from the cell wall to form subcuticular spaces that become filled with resin. During the course of this exudation, subcuticular spaces in-crease and the resin passes through openings in the cuticle and fills the spaces between young leaves and bud scales.
Certain genera of Rubiaceae produce resin in multicellular colleters (Fahn 1979). In some cases the exudate is primarily phenolic resin whereas in others it may be mucilage or may contain both substances. Solereder (1908) described colleters in several species of Burchellia, Carphalea (syn. Dirichletia), and Gardenia, and Mora-Osejo (1977) found that this type of trichome in Elaea-gia has a structure somewhat similar to that in other rubiaceous species. The resin exudes from this type of trichome at the base or edge of stipules and accumulates at the ends of shoots, forming a spherical cap that encloses the entire bud. The resin thus impregnates leaves and inflorescences, in fact, prac-tically the entire aerial portion of the plant. Krause (1909) thought that the stipular resin decreases transpiration in species of Gardenia, observing that some species in moist habitats do not possess colleters. On the other hand, Gardenia species in Indian and eastern Asian moist forests produce resin in colleters, with the species producing the most resin growing in drier habitats.
On the contrary, Mora-Osejo pointed out that Colombian species of Elaea-gia in humid montane tropical forests produce the greatest amount of resin.
Therefore, he proposed that resin in E. pastoensis protects against the insects and microorganisms abundant in these moist tropical habitats. The use of Elaeagia resin, as barniz de Pasto, is discussed in Chapter 9.
Resin is produced in thin-walled epidermal cells, rather than colleters, in young stems of Elaeagia (Mora-Osejo 1977). It also occurs in the interior regions of the secondary cortex, found as well in Gardenia (Solereder 1908).
Scattered sclerified cells in this region in Elaeagia had resin, which Mora-Osejo reported as probably similar to that observed by Solereder in the stems of Cinchona and Coffea. Apparently, rubiaceous plants contain diverse secre-tory tissues that also may contain different kinds of chemicals.