DEVELOPMENT, TAXONOMIC SIGNIFICANCE and ECOLOGICAL ROLE of the CUTICULAR EPITHELIUM in the SANTALALES Carol A. Wilson & Clyd

Home , Antidaphne, Arceuthobium americanum, Bark (botany), Dendrophthora, Eremolepidaceae, Eubrachion

IAWA Journal, Vol. 24 (2), 2003: 129–138

DEVELOPMENT, TAXONOMIC SIGNIFICANCE AND ECOLOGICAL ROLE OF THE CUTICULAR EPITHELIUM IN THE SANTALALES by Carol A. Wilson & Clyde L. Calvin Department of Biology, Portland State University, Portland OR 97207, U.S.A.

SUMMARY All genera in the mistletoe family Viscaceae develop a secondary protective covering, the cuticular epithelium, that replaces the epidermis. The cuticular epithelium also occurs in the Eremolepidaceae and some genera within the related family Santalaceae. This secondary covering, unlike the periderm, lacks lenticels or their functional equivalent. We suggest that the cuticular epithelium provides a greater degree of control over transpirational water loss in older plant parts. The cuticular epithelium may arise in the epidermis, subepidermal layer, or in deeper tissues of the stem. Bark strands of the endophytic system, where they are in contact with either nonliving host tissues or the external environment, also form a cuticular epithelium. The epidermal feature, stomatal orientation, was also studied. All genera in the Viscaceae and Eremolepidaceae have stomata with a transverse orientation. The presence or absence of a cuticular epithelium and stomatal orientation are vegetative characters with potential taxonomic value. Key words: Cuticular epithelium, parasitic angiosperm, periderm, San- talales, stomatal orientation, transpiration, Viscaceae.

INTRODUCTION

In the vast majority of plants having secondary growth, the epidermis is replaced by a covering of secondary origin, the periderm. The first periderm may arise in the epidermis, the subepidermal layer, or in deeper tissues of the stem (Esau 1977). The roots of woody plants also form a periderm, although its origin is typically from pericyclic derivatives rather than from cortex or epidermis. The formation of a periderm in woody plants is not universal, however. In one plant family, the Viscaceae, a strikingly different secondary protective covering – termed the cuticular epithelium – replaces the epidermis (Damm 1902; Calvin 1970; Wilson & Calvin 1996). The cuticular epithelium is a relatively thick tissue produced by living epidermal and subepidermal cell layers and is high in cutin. Older cuticular epithelia typically contain necrotic epidermal and subepidermal cells. The cuticular epithelium differs from the periderm in several ways. First, the periderm contains a discrete tissue-initiating layer, the phellogen, whereas tissue formation within the cuticular epithelium is more diffuse. Second, cells within the periderm are radially seriate when viewed in transverse sections of stem. This radial alignment of cell files

Downloaded from Brill.com09/23/2021 11:08:58AM via free access 130 IAWA Journal, Vol. 24 (2), 2003 Wilson & Calvin — Cuticular epithelium in Santalales 131 is either poorly defined or absent in stems with a cuticular epithelium. Third, a prominent feature of most periderms is the presence of lenticels that allow for exchange of gases between the tissues of the stem and the external environment. Lenticels, or their functional equivalent, are absent in the cuticular epithelium (Damm 1902; Calvin 1970; Wilson & Calvin 1996). Fourth, the periderm has a high suberin content (Roelofsen 1959). In the cuticular epithelium suberin is absent or present as a minor constituent. Instead, successive cuticular layers assume the functional roles attributed to suberin in the periderm. A cuticular epithelium was first described for stems of the European mistletoe,Vis- cum L. (Damm 1902). Damm also noted the presence of a cuticular epithelium in stems of two additional genera of Viscaceae, Notothixos Oliver and Dendrophthora Eichler. Later, Calvin (1970) provided an account of the cuticular epithelium in stems of Phora- dendron Nutt. Wilson and Calvin (1996) confirmed the presence of a cuticular epithelium in stems of several species of the dwarf mistletoe, Arceuthobium M. Bieb., also in the Viscaceae. Korthalsella Tieghem and Ginalloa Korth., the remaining genera of Viscaceae, have not been examined previously for this character. Further, while a cutic- lar epithelium is known to occur in stems, its possible presence in the highly modified roots making up the endophytic system has not been determined previously. One additional morphological vegetative character, stomatal orientation with respect to the plant axis may have taxonomic potential within the Santalales. Past work has shown that a transverse orientation of stomata on stems is uncommon in angiosperms (Butterfass 1987). Butterfass concludes that a transverse orientation of stomata is more common in plant groups with succulent members. We have slide preparations of many species of Viscaceae. Because host /parasite tissue relationships were our main interest, we focused mainly on study of the endophytic system. However, we usually also prepared stem and leaf material for microscopic study. This report utilizes these slide preparations, as well as materials prepared from herbarium specimens, to elaborate on the occurrence and functional role of the cuticular epithelium in the plant family Viscaceae. Where possible, stomatal orientation was also determined. Selected genera in four other families of the Santalales – Santalaceae, Eremolepidaceae, Loranthaceae, and Misodendraceae – were also examined for com- parative purposes. We are interested in identifying vegetative characters having potential taxonomic value for mistletoes because few characters are known for reduced members of this group.

MATERIALS AND METHODS

Field collected aerial shoot and endophytic system tissues of Arceuthobium and Pho- radendron were used in this study (Table 1). Materials were embedded in paraffin, sec- tioned at 8–12 µm using a rotary microtome and stained with either safranin-fast green or tannic acid-ferric chloride-lacmoid. These prepared specimens were augmented by small samples (Table 1) obtained from herbarium sheets at the University of California, Berkeley, Herbarium (UC) and Portland State University Herbarium (HPSU). These samples were softened (Schmid & Turner 1977) and free-hand sections were prepared.

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Table 1. Species analyzed for the presence (+) or absence (–) of a cuticular epithelium (CE) and stomatal orientation (St-Or). Stomata orientation is either transverse (tv) or vertical (vt).

Family / genus CE St-Or Material studied or reference Viscaceae Arceuthobium americanum Nutt. ex Engelm. + Tv Calvin US01-03 (HPSU) globosum Hawksworth & Wiens + Tv Authorʼs prepared slides tsugense (Rosendahl) G.N. Jones + Tv Authorʼs prepared slides Dendrophthora clavata (Benth.) Urb. + Tv Damm (1902); Butterfass (1987) Korthalsella complanata (v. Tiegh.) Engl. + Tv Forbes 187 (UC) Ginalloa arnottiana Korth. + Tv Clemens 18028 (UC) Notothixos subaureus Oliver + Tv Blakely s.n. July 1921 (UC) Phoradendron macrophyllum (Engelm.) Cockerell + Tv Calvin US00-08 (HPSU) californicum Nutt. + Tv Calvin US00-09 (HPSU) Viscum album L. + Tv Damm (1902); Butterfass (1987) Santalaceae* Exocarpus bidwellii Hook. f. + Tv Calvin & Wilson NZ98-05 (HPSU) Eremolepidaceae Antidaphne viscoidea Poepp. & Endl. + Tv Weston 4213 (UC) Eubrachion ambiguum (Hook. & Arn.) Engl. + Tv Herter 82379 (UC) Lepidoceras chilense (Molina) Kuijt + Tv Eyerdam 10609 (UC) Loranthaceae** Tupeia antarctica (Cham. & Schldl.) Korth. – Vt Oeuci s.n. 6 May 1930 (UC) Misodendraceae Misodendrum brachystachyum DC. – Vt S. Carlquist (pers. comm.)

* Butterfass (1987) records 10 additional genera of Santalaceae with transverse stomata on stems. ** Butterfass (1987) records two genera of Loranthaceae with transverse stomata on stems.

Some sections were stained with Sudan IV, safranin, and/or phloroglucinol-HCl to en- hance cellular and extracellular materials. The prepared slides and tissue samples from herbarium sheets allowed us to study the stem anatomy of all seven genera comprising the Viscaceae, as well as selected members of related families. Sherwin Carlquist provided the information on Misodendrum G.Don f. in the Misodendraceae (Table 1). Specimens were also examined for stomatal orientation. In most angiosperm families stomata are vertically oriented or parallel with the longitudinal axis of the stem. A few families have transverse stomata, where the long axis of stomata is perpendicular to the longitudinal axis of the stem (Butterfass 1987). The herbarium sheets from which samples were taken and the genera examined are given in Table 1.

RESULTS We found that a cuticular epithelium is present in two genera of the Viscaceae – Kort- halsella and Ginalloa – that had not previously been examined for this character. These findings confirmed that stems of all genera within the Viscaceae develop a thick cuticular layer covering epidermal cells (Fig. 1). Prominent pegs of cuticular material also form

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Fig. 1–5. Transverse and longitudinal sections through older stems of Viscaceae genera. – 1–4: Phoradendron macrophyllum. – 1: Break, (b) in continuity of thick cuticular layer (cl), × 100. – 2: Repair of break in cuticular layer by formation of new layer beneath, × 100. – 3: Detail of Figure 2 showing presence of new cuticular layer in space between anticlinal wall (da) and inner periclinal wall, × 250. – 4: Formation of cuticular layer by cortical cells surrounding necrotic tissue (nt) beneath stoma (s), × 100. — 5: Arceuthobium globosum, cuticular epithelium (ce); note thickness of cuticular layer and presence of embedded necrotic cells, × 100. between cells. As stems continue to increase in circumference, more or less vertical breaks appear in the epidermis, often directly above the cuticular pegs. At first these breaks are shallow, extending only partially through the thick cuticular layer. Later, larger breaks appear and these are repaired by the production of additional cuticular material, either by existing epidermal cells (Fig. 1), or by cells in the subepidermal layer. Continued growth results in even larger breaks in epidermal continuity (Fig. 2).

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These breaks are repaired by the formation of new cuticular layers, connected on their flanks with the original cuticular material (Fig. 2). An unusual feature of these new cuticular layers is that they may be formed by daughter cells that are within the con- fines of mother cells with thickened walls (Fig. 3). In the cell to the left of the arrow, a daughter cell with a protruding outer periclinal wall has formed a thick cuticular layer. Initially this cuticular layer was separated from cuticular layers of cells at its flanks by the intervening anticlinal walls of the mother cell. In the course of cuticular deposition discontinuities developed in the anticlinal walls of flanking cells (Fig. 3), allowing for the formation of continuous cuticular layers beneath surface breaks. It is often difficult to determine the nature of the cells that produce additional cuticular layers. In Figure 3, for example, production appears to be from epidermal cells or their derivatives. In Figure 4, in contrast, the cuticular layer surrounding the necrotic substomatal region is clearly derived from cortical cells. Deeper-lying cells may also form cuticular layers. In an older stem of Notothixos an extensive cuticular layer is present in the secondary phloem, as evidenced by the presence of bundles of primary phloem fibers external to this layer. Thus, in stems, additional cuticular layers may be formed by epidermal cells or their derivatives, cortical cells, or in the parenchyma of the secondary phloem. In older stems the cuticular epithelium varies markedly in thickness and may appear somewhat disorganized (Fig. 5). In stems, thick cuticular layers with embedded necrotic cells characterize older cuticular epithelia. Analysis indicates that embedded cells include guard and subsidiary cells, the bases of trichomes, and ordinary epidermal cells that were present during development of the stem. Ultimately the cuticular epithelium may become thick, approaching, or exceeding 100 µm. Notably absent in the cuticular epithelium are lenticels or their functional equivalent, a feature also noted by Damm (1902). The highly modified root system of mistletoes, when embedded within host branches, is termed the endophytic system. The system has two components: bark strands and sinkers. The bark strands grow more or less longitudinally within host branches (Fig. 6–10). At intervals elongating bark strands turn inward, coming into contact with the host cambial zone (Fig. 6 & 7). The strands then displace host cambial cells, thus establish- ing a position within the cambial cylinder. Thereafter they grow in unison with the vascular cambium of the host (Salle 1983), producing new cells both centripetally and centrifugally. These derivatives form the elongate, radially oriented structures termed sinkers (Fig. 8). The xylary portion of the sinker seen in Figure 8 is embedded in five years of host wood. Proximally it merges with the bark strand, identified by its centrally located vascular tissue. The bark strand shown in Figure 8 has formed a cuticular epithelium even though it is completely embedded within the host bark. The cuticular epithelium occupies the region of the bark strand that is in contact with nonliving tissue of the host (Fig. 8). A cuticular epithelium is also present in the bark strand seen in Figure 9. Although this bark strand is exposed at the branch surface, the cuticular epithelium occurs not only at the exposed surface but also around the flanks of the strand. The inward progress of the cuticular epithelium stops at the juncture with living host tissues. The formation

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of cuticular epithelia by bark strands is somewhat independent of their age. The bark strands seen in Figures 8 and 10 are more than five years old, based on sinker age, and both have cuticular epithelia. In contrast, the bark strand in Figure 9 appears to be younger, yet it also has a well-developed cuticular epithelium. Selected members of four additional families of the Santalales – Santalaceae, Eremo- lepidaceae, Loranthaceae, and Misodendraceae – were examined for the presence of a cuticular epithelium (Table 1). The presence of a cuticular epithelium is confirmed for older stems of Exocarpus Labill. in the Santalaceae, and Antidaphne Poeppig & Endl., Eubrachion Hook.f. and Lepidoceras Hook.f. in the Eremolepidaceae, but is not present in Tupeia Cham. & Schldl. in the Loranthaceae, or Misodendrum (S. Carlquist, pers. comm.) in the Misodendraceae. It was determined that all seven genera within the Viscaceae have a transverse orientation of stomata on their stems (Table 1). The transverse orientation of stomata is illustrated for Arceuthobium in Wilson & Calvin (1966; Fig. 10.3a-c and 10.9a-c) and Phoradendron in Calvin (1970; Fig. 39– 42). The stomata on stems of Antidaphne, Eubrachion, and Lepidoceras (Eremolepidaceae) are also oriented transversely, whereas those of Tupeia (Loranthaceae) and Misodendrum (Misodendraceae) are oriented vertically (Table 1). DISCUSSION

This study establishes for the first time that a cuticular epithelium is present in all seven genera comprising the family Viscaceae. We illustrate that a cuticular epithelium occurs not only on older stems but also on bark strands of the endophytic system. A cuticular epithelium has also been reported for plants not belonging to the Santalales. Damm (1902) illustrates the presence of a cuticular epithelium in older stems of Menispermum L. (Menispermaceae). Johnson (1992) describes and illustrates an outer protective layer in Bitelaria (Istchenko & Istchenko) Johnson & Gensel, an early Devonian vascular plant of uncertain affinity from New Brunswick, that appears strikingly similar to the cuticular epithelium present in the Viscaceae. These reports indicate that the cuticular epithelium is of ancient origin, and that this unusual protective covering has arisen independently on several occasions. It is assumed that the Viscaceae evolved from a woody stock in which a periderm was present. Therefore, in the Viscaceae the periderm has been replaced by a novel secondary protective covering, the cuticular epithelium. If this hypothesis is correct, what adaptive advantage does the cuticular epithelium confer that is not provided by a

Fig. 6–10. Bark strands and sinkers of Phoradendron juniperinum as seen in transverse sections through a branch of a Juniperus host. – 6. Young bark strand (bs) initiating sinker. – 7: Larger bark strand surrounded by living host tissue (lht); note absence of cuticular layer. – 8: Bark strand with elongate sinker (s) and prominent cuticular epithelium. – 9: Bark strand; note presence of cuticular layer (cl) at both the interface with necrotic host tissue (nht) and external environment. – 10: Older bark strand with sinker xylary portion embedded in six years of host wood, prominent cuticular epithelium (ce) present where bark strand is in contact with necrotic host tissue (nht). — Magnification of 6–10 = × 40.

Downloaded from Brill.com09/23/2021 11:08:58AM via free access 136 IAWA Journal, Vol. 24 (2), 2003 Wilson & Calvin — Cuticular epithelium in Santalales 137 periderm? We suggest that the primary advantage of the cuticular epithelium is that it provides plants a greater degree of control over water loss. This concept can be sum- marized as follows: Dermal type Control over water loss Epidermis (with stomata) High Epidermis + periderm (& lenticels) Moderate Epidermis + cuticular epithelium High

Evidence suggests that the cuticular epithelium is efficient in the control of water loss. Richard Tocher (pers. comm.) measured transpiration in stem segments of the mistletoe, Arceuthobium. Results indicated that transpiration was limited almost exclusively to segments near the stem tip. Older stem segments of Arceuthobium have been shown to have a well-developed cuticular epithelium (Fig. 5; Wilson & Calvin 1996). Further, bark strands like stems form a cuticular epithelium when they are in contact with either the external environment or nonliving tissues of the host (Fig. 8–10). This suggests that bark strands are also highly resistant to water loss. It has long been known that parasitic angiosperms transpire at a greater rate than their hosts, thus creating a gradient of decreasing water potential (Ehleringer & Marshall 1995). While both host and parasite respond to evaporative demands, the parasite is less responsive than its host (Ehleringer & Marshall 1995). The maintenance of this gradient keeps water and minerals moving from host to parasite, and is critical to the parasitic mode of life. It is important to note that gradient regulation occurs at the stomatal level, and, in the family Viscaceae, is not confounded by the presence of leaky lenticels. Remarkably, the mistletoes within the Viscaceae transpire at a greater rate than their hosts in spite of several morphological adaptations that reduce water loss such as squamate habit, sunken stomata, and small substomatal chambers. If our assumptions are correct, the cuticular epithelium also retards or prevents the exchange of gases between older portions of the plant and the ambient environment. It is of interest to note that in other plant groups the periderm may serve in this capacity (Nedoff et al. 1985). In Ocotillo (Fouquieria splendens Engelm.) a transparent cork forms in the furrows between the sclerified leaf bases during secondary growth. Beneath the cork and leaf bases is a well-developed, competent chlorenchyma. Although light is transmitted through the cork to the chlorenchyma beneath, no exogenous uptake of CO2 occurs. The transparent cork layers appear to lack lenticels, and the authors conclude that no CO2 uptake occurs because the cork is impermeable to CO2, and pres- umably to water. The Viscaceae is regarded as sister to the Santalaceae (Nickrent & Duff 1996). In- deed, some authors suggest merger of the Viscaceae into the Santalaceae (APG 1998). The circumscription of the Santalaceae is further complicated by a molecular analysis of the Eremolepidaceae by Nickrent (2000) that places all three genera (Antidaphne, Eubrachion, and Lepidoceras) in a clade with members of the tribe Antoboleae in the Santalaceae (Exocarpus and Omphacomeria (Endl.) A. DC.). Nickrent concludes that

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further work is needed to arrive at an evolutionarily supported circumscription for the family Santalaceae. While some genera within Santalaceae are reported to form a periderm (Metcalfe & Chalk 1950), we found a cuticular epithelium to be present in Exo- carpus. We also found a cuticular epithelium to be present in all genera of the Eremo- lepidaceae (Antidaphne, Eubrachion and Lepidoceras). The presence of a cuticular epithelium in all genera within the Viscaceae, all genera within the Eremolepidaceae, and at least some members of the Santalaceae provides further support for the close rela- tionship between the three families. Support is further strengthened by the occurrence of stomata with a transverse orientation on stems in the Viscaceae, Eremolepidaceae and Santalaceae. A cuticular epithelium was reported to be absent in the related mistletoe families, Loranthaceae and Misodendraceae (Damm 1902; Metcalfe & Chalk 1950). This study of Tupeia (Loranthaceae) and the monotypic genus Misodendrum confirms this finding. The genus Misodendrum provides a classic example of a plant with a well-developed periderm (S. Carlquist, pers. comm.). Thus, within the Santalales a cuticular epithelium occurs in the Viscaceae, all members of the Eremolepidaceae, and some members of the Santalaceae, and is absent in the Misodendraceae and at least some members of the Loranthaceae. The family Opiliaceae has not been examined for this character.

In summary, the cuticular epithelium is a novel protective covering that replaces the epidermis in Viscaceae, Eremolepidaceae and some members of the Santalaceae. To- gether with the orientation of stomata the cuticular epithelium may be taxonomically informative in determining relationships between these closely related families. In addi- tion, the cuticular epithelium may provide information on the divergent strategies utiliz- ed by mistletoes in the regulation of water loss and uptake of water and nutrients from their hosts. We suggest that a cuticular epithelium restricts water loss from older stem and root (bark strand) surfaces more effectively than a periderm with lenticels. As a consequence, transpiration is regulated almost entirely by the stomata, giving plants a greater degree of control over transpirational water loss. The precise control of transpiration rates is critical to mistletoes because it is through water potential gradients that they obtain water and solutes from their hosts. If the role that we assign to the cuticular epithelium proves to be correct it raises other questions. In the large, primarily southern mistletoe family, Loranthaceae, a periderm with lenticels is present. Indeed, in the many genera of Loranthaceae with epicortical roots, lenticels are often especially abundant near where secondary haustorial attachments to the host occur. This would suggest that the mechanisms employed for water and nutrient uptake are somewhat different in the two families. Resolving these questions will give us greater insight into the complex ecoanatomy and ecophysiology of the mistletoes.

ACKNOWLEDGEMENTS

We thank Sherwin Carlquist for graciously providing information on the stem anatomy of Misoden- drum and Rudi Schmid for obtaining some collection numbers from the UC herbarium that we were missing.

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