IAWA Journal, Vol. 18 (3),1997: 229-245 WOOD ANATOMY OF FOUR CALIFORNIAN MISTLETOE SPECIES (PHORADENDRON, VISCACEAE) by Vanessa E.T.M. Ashworth & Gracielza Dos Santos Rancho Santa Ana Botanic Garden, Claremont, CA 917ll, U. S. A. SUMMARY Secondary xylem characteristics were compared in four species of Phor­ adendron Nutt. (Viscaceae) native to Califomia. All have extremely short, thick-walled vessel elements with simple perforation plates. They also share high vessel density, radial vessel arrangement, thick-walled fibres, and multiseriate, heterocellular rays. The fibres show considerable in­ trusive growth. Features of the vessel elements (i.e. vessel dimensions, arrangement, type of wall sculpturing) and calcium oxalate crystals in the ray parenchyma cells are useful diagnostic traits to separate species. Grooved vessel walls are shared by the morphologically similar P. villosum and P. macrophyllum. Differences between these two spe­ cies may reflect contrasting drought response strategies pursued by respective hosts. Vulnerability and mesomorphy ratios of the wood of P. caZifomicum are higher than those of P. pauciflorum and P. macrophyl­ Zum. Phoradendron pauciflorum has the most xeromorphic wood of the four species studied. Key words: Phoradendron, Viscaceae, wood anatomy, parasitic plants, vessel wall sculpture, spiral thickenings, mesomorphy ratio, vulnerability ratio. INTRODUCTION Phoradendron Nutt. is a New World mistletoe in Viscaceae, a santalalean family of epiparasites that used to be treated as a subfamily of Loranthaceae s.l. Viscaceae was formally segregated from Loranthaceae by Barlow (1964) on the basis of embryology and is today generally seen as being a highly derived family, whereas Loranthaceae s.s. appears to be descended from Olacaceae, the basal family within the order (Kuijt 1968; Nickrent & Duff 1996). Phoradendron has a wide distribution, extending from a centre of diversity in Meso­ america into temperate latitudes to the north and south. Its members parasitize mainly arborescent angiosperms and occasionally conifers by forming connections with the host xylem. Phoradendron species occur in a multitude of disparate environments, ranging from tropical to desert and lowland to montane habitats. In general, the wood of epiparasites is xeromorphic (Carlquist 1975, 1977; Carlquist & Hoekman 1985), but has relatively high vulnerability ratios (vessel diameter divided by vessel density Downloaded from Brill.com09/28/2021 02:36:10PM via free access 230 IAWA Journal, Vol. 18 (3), 1997 per mm2; Carlquist 1977) eompared with non-parasitie xerophytes, The ability of wood attributes to refleet eeological eonditions is abundantly doeumented (e. g. Baas et al. 1983; Carlquist 1975, 1977). However, selective pressures are exerted not only by cli­ mate but also arise from the parasitic lifestyle and a need to adapt to a partieular host (Atsatt 1983). Many anatomical studies of epiparasites are devoted to the host parasite interface (e.g.,Alosi & Calvin 1985; Calvin 1967; Calvin et al. 1991; Calvin & Wilson 1995; Salle 1979; Sehmid & Lindeman 1979; Srivastava & Esau 1961; Thoday 1956, 1957, 1960). Evidenee of a direct influence of the host on the wood anatomy of the parasite has been presented for two members of Loranthaceae (Patel 1991). The systematic utility of wood anatomie al features for Phoradendron is the focus of our work. Only a few studies of epiparasites (e. g. Beaman 1971; Carlquist 1985; Patel 1974,1991; Smith & Gledhilll983) have used wood anatomical traits for diagnostic/ systematic purposes. Phoradendron has a troubled taxonomie history, in part a eonse­ quence of the highly reduced floral organs that offer littIe help in interspeeific differen­ tiation. Trelease (1916), the monographer of the genus, eireumseribed many species and higher taxonomie ranks on the basis of variable taxonomie eharacters, such as leaf size and shape. As we now know, however, leaves in Phoradendron are particularly subject to phenotypic plasticity. Today many ofTrelease's speeies (some 240) are plaeed in synonymy and his supraspecifie ranks are largely dismissed as invalid or unnatural. Regional revisions of Phoradendron (e.g. Wiens 1964) have reexamined some of Trelease's sections. Our study covers four speeies representing three ofWiens' (1964) sections. We have not sampled a suffieient number of species to permit validation or refutation of his sections, but our work does enable us to look for attributes of the wood that follow sectional boundaries. Moreover, we diseuss anatomical features of the wood in terms of ecology and influences exerted by the host. Given the eomplex nature of wood we believe that any interpretation of its anatomy needs to take aceount of all contributing factors (Baas 1982; Carlquist 1977). The four species included in this study are: Phoradendron californicum Nutt. (desert mistletoe); P. macrophyllum (Engelrn.) Coekerell (big leafmistIetoe); P. villosum (Nutt.) Nutt. (oak mistIetoe); and P. pauciflorum Torrey (fir mistIetoe). These taxa exhibit a range of adaptive strategies, especially reflecting differences in host range (broad ver­ sus narrow), host phenology (deciduous versus evergreen; broadleaved versus conif­ erous) and climatic adaptation (desert versus riparian, low versus high altitude) (Table 1). Phoradendron pauciflorum is a parasite of Abies concolor, a conifer oecur­ ring mainly at altitudes of 1700-3300 metres (the speeimens used in this study origi­ nated at an altitude of 2100-2300 metres). The leaves in P. pauciflorum are 0.5-2.5 cm long and 0.5-0.8 em wide. Phoradendron californicum, by contrast, parasitizes a range of desert shrubs and trees, especially leguminous taxa, such as Prosopis, Acacia, Cercidium, Olneya, and oeeasionally Larrea tridentata. It is eharaeterized by seale­ like leaves. The other two speeies, P. macrophyllum and P. villosum, are morphologi­ eally similar, both having large obovate leaves (3.0-4.2 cm long, 1.5-2.3 cm wide; 1.5-4.7 cm long, 1.0-2.5 em wide, respectively), comparable intemode lengths and similar inflorescence and fruit eharacteristics. Their distinctness derives from minor differences in degree and type of leaf and stern pubescenee, contrasting phenology Downloaded from Brill.com09/28/2021 02:36:10PM via free access Ashworth & Dos Santos - Wood anatomy of Phoradendron 231 Table 1. Host ecology. Phoradendron Host Host Host alti- Host phenology habitat tude (m) P. californicum Prosopis glandulosa (Fabaceae) deciduous desert 1100 Acacia greggii (Fabaceae) deciduous desert 600-800 Acacia sp. deciduous desert 600 P. macrophyllum Cinnamomum camphora (Lauraceae) deciduous irrigated 200 Fraxinus velutina (Oleaceae) deciduous riparian 400 Fraxinus sp. deciduous riparian 1100 Platanus racemosa (Platanaceae) deciduous riparian 400 Robinia pseudoacacia (Fabaceae) deciduous riparian 300 P. paucijlorum Abies concolor (Pinaceae) evergreen subalpine 2100-2300 P. villosum Quercus lobata (Fagaceae) deciduous woodland 270 Quercus wislizenii var.frutescens evergreen woodland 400 Quercus spp. evergreen or woodland 1500 deciduous (flowering period in December-March and July-September, respectively) and host preference. Phoradendron macrophyllum is an avowed generalist parasitizing broad­ leaved trees belonging to distantly related families, such as the Platanaceae (Platanus), Oleaceae (Fraxinus) and Lauraceae (Cinnamomum camphora ). Phoradendron villosum is rarely found on hosts other than Quercus. The nomenclature we use for Phoradendron follows Hawksworth and Wiens (1993). Nomenclatural variants in common use today exist for all taxa but P. californicum. Phoradendron macrophyllum and P. villosum, in particular, are sometimes cited as P. flavescens (Pursh) Nutt. var. macrophyllum Enge1m. and P. flavescens (Pursh) Nutt. var. villosum (Nutt.) Engelrn., respectively (Munz 1959). Wiens (1964) made the new combinationP. tomentosum (DC.) Enge1m. subsp. macrophyllum (Engelrn.) Wiens but retained P. villosum (Nutt.) Nutt. MATERIALS AND METHODS Collection data and voucher specimens are cited at the end of each species description. Following the collector's number is the collection date and the stern diameter (in pa­ rentheses). Voucher specimens and wood sampies are deposited at Rancho Santa Ana Botanic Garden. Downloaded from Brill.com09/28/2021 02:36:10PM via free access 232 IAWA Journal, Vol. 18 (3),1997 Fig. 1-4. Phoradendron califomicum. - 1: Columbus 2464, transverse seetion, vessels in radial multiples, two or more radial ehains arranged side by side, distinet growth ring; seale bar = 100 J1Ill. - 2: Columbus 2464, tangential seetion, storied vessel elements and axial parenehyma; sea1e bar = 100 J1Ill. - 3: Ashworth 103, radial seetion, heterocellu1ar rays composed ofprocum­ bent, square, and upright eeHs mixed; seale bar = 250 J1Ill. - 4: Ashworth 81, SEM mierograph, eoarse spiral thiekenings in vessel element; seale bar = 10 J1Ill. - In Fig. 1: g = growth ring. Downloaded from Brill.com09/28/2021 02:36:10PM via free access Ashworth & Dos Santos - Wood anatomy of Phoradendron 233 Fig. 5-8. Phoradendron macrophyllum. - 5: Ashworth 64, transverse section, patches of very thick-walled fibres; scale bar = 250 Iffil. - 6: Ashworth 76, tangential seetion, thick-walled and somewhat contorted fibres, wide rays, and storied to irregularly storied vessel elements and axial parenchyma; scale bar =100 Iffil. - 7: Ashworth 76, druses in pith cells; scale bar = 40 Iffil. - 8: Ashworth 65, SEM micrograph, grooves forrned by interconnecting pit apertures in vessel element;