Ultrastructure of Sieve-Element Plastids of Myrtales and Allied Groups

Abstract

The core families of Myrtales (69 species investigated) contain S-type sieve-element plastids. In these, the presence of several medium-sized spherular starch grains is probably a common trend in the order. Among those families usually closely associated or more or less distantly related, the great majority also have S-type plastids, some with the same characteristics. However, Rhizophoraceae, in contrast to the close allies and Connaraceae, Gunneraceae, and Rhabdodendraceae of the distantly related families, have developed P-type plastids. That subtype PV-plastids, which exclude all starch, are found in all tribes of Rhizophoraceae except Anisophylleae supports family recognition of the latter. Sieve-element characters investigated with the transmission electron microscope have repeatedly contributed to the circumscription and classification of higher taxa, e.g., Leguminosae (Behnke & Pop, 1981), Monocotyledoneae (Behnke, 198 lb). Of these, the most intensively studied feature is the ultrastructure of sieve-element plastids. The great number of investigated angiosperms (some 2,000 species from nearly 400 families-as of 1983-12-31) allow a subdivision of the plastids into Pand S-type, a number of subtypes, and a great many of characteristic forms by using both chemical and morphological composition at the ultrastructural level (see Behnke, 198 la, for more detail). The successful delimitation of the order Caryophyllales to the PIll plastid families (Behnke, 1976; Mabry, 1977) and the use of sieve-element plastid data in separating Vitidales from Rhamnales (Behnke, 1974; Dahlgren, 1980) and Buxaceae from Simmondsiaceae and Euphorbiaceae (Behnke, 1982a) stimulated, among others, the screening of all the core families of Myrtales and of those closely or more distantly related to the order, with the results to be included in this symposium report. MATERIALS AND METHODS Preferentially young stem pieces of the plant species listed in Table 1 were cut into longitudinal sections fixed in a formaldehyde-glutaraldehyde mixture followed by 1% buffered OsO4 and dehydrated in acetone. Small pieces containing phloem tissue were embedded and polymerized in epoxy resins, cut with an ultramicrotome, and the final ultrathin sections viewed and photographed with a transmission electron microscope (for exact procedures see Behnke, 1 982a). RESULTS AND DISCUSSION Sieve-elements of many of the myrtalean families are extremely difficult to fixate in a closeto-natural condition. There is less plasmatic content which is more labile and, during preparation, sieve-element plastids burst more often than in the average dicotyledon family. Nevertheless, all of the 69 investigated species of the core families of Myrtales (cf. Dahlgren & Thorne, 1984) could be shown to contain S-type plastids (cf. Table 1). Number, size, and shape of the starch grains in these plastids certainly vary (Figs. 1-14), but some common trends may be recognized. There are always several starch grains in one plastid-often six or more (Figs. 1-4, 6, 81 1), their size being variable (sometimes all of the same size) but never very large and their shape almost exclusively is a spherule. Considerably smaller starch grains are present in adI For permitting collection of living material used in this study the author wishes to thank the directors and curators of the Botanical Gardens in Berlin-Dahlem, Bogor, Bonn, Copenhagen, Edinburgh, Heidelberg, Kew, Leiden, Mainz, and Utrecht. I am particularly indebted to A. Assi (Abidjan), P. Berry (Caracas), D. Clark (Kirstenbosch), I. Dorr (Kiel), D. M. C. Fourie (Pretoria), I. S. Gottsberger (Botucatu), A. Irvine (Atherton, Qld), A. Juncosa (Petersham), K. Kubitzki (Hamburg), Lee Ying Fah (Sandakan, Sabah), G. Merz (Heidelberg), H. B. Nicholson (St. Michaels-on-Sea, RSA), H. T&h6 (ORSTROM), R. Tracey (Indooropilly) for collecting and shipping living plant material, A. P. Bennell (Edinburgh) and L. B. Jorgensen (Copenhagen) for helping with fixations, and A. R. A. Noel (Pietermaritzburg) and P. H. Raven (St. Louis) for making contact to collectors. I gratefully acknowledge the expert technical help of Mrs. L. Pop and Mrs. D. Laupp (both Heidelberg). This work was supported by grants from the Deutsche Forschungsgemeinschaft. 2 Zellenlehre, Universitat Heidelberg, Im Neuenheimer Feld 230, D-6900 Heidelberg, Federal Republic of Germany. ANN. MISSOURI BOT. GARD. 71: 824-831, 1984. This content downloaded from 157.55.39.35 on Thu, 01 Sep 2016 05:46:04 UTC All use subject to http://about.jstor.org/terms 1984] BEHNKE-SIEVE-ELEMENT PLASTIDS 825