A Taxonomic Revision of Rhododendron L. Section Pentanthera G
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A TAXONOMIC REVISION OF RHODODENDRON L. SECTION PENTANTHERA G. DON (ERICACEAE) BY KATHLEEN ANNE KRON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1987 , ACKNOWLEDGMENTS I gratefully acknowledge the supervision and encouragement given to me by Dr. Walter S. Judd. I thoroughly enjoyed my work under his direction. I would also like to thank the members of my advisory committee, Dr. Bijan Dehgan, Dr. Dana G. Griffin, III, Dr. James W. Kimbrough, Dr. Jonathon Reiskind, Dr. William Louis Stern, and Dr. Norris H. Williams for their critical comments and suggestions. The National Science Foundation generously supported this project in the form of a Doctoral Dissertation Improvement Grant;* field work in 1985 was supported by a grant from the Highlands Biological Station, Highlands, North Carolina. I thank the curators of the following herbaria for the loan of their material: A, AUA, BHA, DUKE, E, FSU, GA, GH, ISTE, JEPS , KW, KY, LAF, LE NCSC, NCU, NLU NO, OSC, PE, PH, LSU , M, MAK, MOAR, NA, , RSA/POM, SMU, SZ, TENN, TEX, TI, UARK, UC, UNA, USF, VDB, VPI, W, WA, WVA. My appreciation also is offered to the illustrators, Gerald Masters, Elizabeth Hall, Rosa Lee, Lisa Modola, and Virginia Tomat. I thank Dr. R. Howard * BSR-8601236 ii Berg for the scanning electron micrographs. Mr. Bart Schutzman graciously made available his computer program to plot the results of the principal components analyses. The herbarium staff, especially Mr. Kent D. Perkins, was always helpful and their service is greatly appreciated. Finally, I thank my husband, Paul, for his unending support and patience during the completion of this project. iii TABLE OF CONTENTS Page ACKNOWLEDGMENTS ii ABSTRACT vi INTRODUCTION 1 GENERIC AND SECTIONAL RELATIONSHIPS 5 Discussion 5 Phylogenetic Analysis 8 SPECIES RELATIONSHIPS WITHIN RHODODENDRON SECTION PENTANTHERA 27 Discussion 27 Phylogenetic Analysis 34 PHENETIC ANALYSIS 56 Methods 56 The R. prinophvllum-periclvmenoides - canescens Complex 60 The R. viscosum Complex 78 Variation and Delimitation of Rhododendron viscosum and R. atlanticum 83 The Rhododendron calendulaceum- cumberlandense Complex 93 Variation in Rhododendron occidentale 106 Variation in Rhododendron molle 116 DISTRIBUTION AND ECOLOGY 13 0 TAXONOMIC CRITERIA 135 Taxonomic Philosophy 135 Habit 135 Indumentum 13 7 Leaves 138 Bud Scales 139 Flowers 143 Fruits and Seeds 147 Vegetative Anatomy 169 Phenology 166 Cytogenetics 170 iv HYBRIDIZATION. .172 ALLOPOLYPLOID SPECIATION 17 6 TAXONOMIC TREATMENT 179 Measurements and Terminology 179 Specimens Examined ... 179 Key to the Subsections of Rhododendron section Pentanthera 184 Key to the Species of Rhododendron sect. Pentanthera Using Floral, Fruit and Vegetative Characters 185 Key to the Species of Rhododendron sect. Pentanthera Using Fruit and Vegetative Characters 192 Hybrid Names 402 Nomina Nuda et Ambigua 404 LITERATURE CITED 4 05 BIOGRAPHICAL SKETCH 416 V Abstract of a Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy A TAXONOMIC REVISION OF RHODODENDRON L. SECTION PENTANTHERA G. DON (ERICACEAE) By Kathleen Anne Kron December 1987 Chairman: Dr. Walter S. Judd Major Department: Botany Rhododendron sect. Pentanthera comprises a group of closely related, highly ornamental plants which are commonly called "azaleas." Thirteen of the fifteen species recognized in this section are indigenous to North America. and One species ( Rhododendron mol_le) is native to Japan China, and one species is indigenous to the Caucasus region is (R. luteum) . Taxonomic treatment of these plants difficult due to their great inter- and infraspecific morphological variation. Hybrids may occur when habitats or flowering times overlap, causing further complexity. Cladistic analysis of the species within the section indicates that the section is monophyletic. Rhododendron molle is the basal member of the section and is the sole member of Rhododendron subsect. Sinensia . The remaining species form a monophyletic group recognized as R. subsect. presence of a Pentanthera . Within this subsection the blotch on the upper corolla lobe defines two primarily red- flowered monophyletic groups. The first group has a vi Tertiary Period disjunct distribution and comprises R. luteum . R. austrinum . and R. occidentale . The second group is indigenous to eastern North America and comprises R. calendulaceum , R. cumberlandense . R. f lammeum , R. prunifolium . and R. alabamense In both groups the cladistically basal species has white flowers with a yellow blotch on the upper corolla lobe (R. occidentale , R. flowering alabamense . respectively) . The pink-white early species, R. canescens . R. periclvmenoides , and R. prinophvllum are paraphyletic. Rhododendron atlanticum , R. group that arborescens . and R. viscosum form a monophyletic is basal in the subsection. Phenetic analysis indicated that the eastern Asian with two taxon, R. molle . is best recognized as one species geographical subspecies. Rhododendron prinophvllum is quite distinct from R. canescens and R. periclvmenoides . The latter two species are very similar morphologically, but their similarities are due to the retention of plesiomorphic characters and they should be recognized as distinct species. Rhododendron calendulaceum can be distinguished from R. cumberlandense using a combination of morphological and phenological characters. The various taxa previously segregated out of R. viscosum are merely extreme forms of a widespread and variable species and are not given any formal rank. No subspecific taxa are species and recognized in R. occidentale . Keys to the species descriptions are included. vii INTRODUCTION Rhododendron section Pentanthera is a remarkable group of closely related, highly ornamental plants. Although the group is very "well-known" to the layperson and horticulturist, it presents a paradox to amateur and professional taxonomists alike because historically species delimitation and recognition have been a "mystery." These "native azaleas" are one of the most widely and copiously collected groups of plants in eastern North America. The plants are notorious for their ability to hybridize in addition to exhibiting a broad range of natural morphological variation. Although plethora of articles has been written about the perplexing variation of the species in R. sect. Pentanthera . the last monograph was published in 1921 by E. H. Wilson and A. Rehder. The "mystery"' surrounding species identification has been exacerbated by the complex taxonomic and nomenclatural history which has involved garden hybrids, mixed collections and superfluous names. While Wilson and Rehder thoroughly addressed the nomenclatural and taxonomic history of R. sect. by the Pentanthera , their treatment is highly influenced typological philosophy and taxonomic methods of their era. Since then, several attempts have been made to sort out the taxonomic problems within Rhododendron sect. Pentanthera 1 2 (Skinner, 1952, 1955, 1959, 1961, 1971; Galle, 1968, 1985; Solymosy, 1974, 1976; Willingham, 1973, 1974, 1975, 1976; King, 1977a, b, 1980) . One conclusion has been consistent: the section is monophyletic and comprises highly variable, interfertile, closely related species. However, many questions still remained concerning species delimitations, evolutionary relationships, and the extent of hybridization within the section. These questions have been considered from various perspectives, ranging from classical, morphological approaches (Skinner, 1961) to phenetic methods (King, 1977a, 1980) . However, the group still lacked a comprehensive treatment which incorporated modern philosophical and methodological approaches in species delimitation and in construction of phylogenetic relationships. In addition, since 1921, information from many new sources has become available. Flavonoid chemistry (Harborne and Williams, 1971; King, 1977a, b, 1980, Evans et al., 1980; Spethmann, 1980), indumentum development and differentiation (Siethe, 1960, 1980), anatomical and cotyledonary characters (Philipson and Philipson, 1968, Philipson, M. N., 1980), and chromosomal information (Sax, 1930; Nakamura, 1931; Janaki-Ammal , et al., 1950; Li, 1957) have all been studied since the last monograph of the section. This study was undertaken in order to clarify the species limits, the evolutionary relationships, and the extent of hybridization within Rhododendron sect. 3 Pentanthera . This requires that the typological view of a species be discarded because it does not adequately deal with variation within and among natural populations. It also requires the employment of phenetic approaches in species delimitation, and cladistic approaches in the development of hypothetical evolutionary relationships. With these new approaches in mind it has been possible to take full advantage of the vast amount of information available from herbarium material and from the literature. These sources were supplemented by extensive field work which was concentrated in areas where species problems were particularly difficult. Populational variation was considered important, especially in taxa where hybridization had been frequently cited as common (e.g., R. calendulaceum . R. cumberlandense . R. canescens, R. flammeum) While the status of all the taxa within the section was reevaluated, particular emphasis