Senin, 21 Januari 2008

Introduction - Developmental Anatomy and Branching of Roots of Four Zeylanidium Species (Podostemaceae), with Implications for Evolution of Foliose Ro

Members of Podostemaceae are aquatic angiosperms that grow on water-worn rocks in waterfalls and rapids which are subject to seasonal fluctuations in water level. The plants grow submerged during the rainy season and are usually exposed to the air during the dry season. They flower shortly after exposure to air and set fruits whilst drying. The family has evolved specialized morphologies that appear to have adaptive significance. Within the family, considerable discontinuous morphological variation is reflected by the current classification in which approx. 270 species are classified into about 47 genera, mostly monotypic or oligospecific (Cook, 1996).

The extensively modified plant body of the Podostemaceae is difficult to interpret using the ordinary root–shoot concept. A seedling of subfamily Podostemoi deae either has a rudimentary primary shoot (plumule) or lacks one, and may or may not have a relatively short-lived primary root (radicle) (Mohan Ram and Sehgal, 1997; Suzuki et al., 2002). The mature plant usually consists of an adventitious (secondary) root, which arises from various parts of the seedling with the exception of the hypocotyl tip, and root-borne adventitious shoots. The morphological nature of the diversely structured roots is controversial, and they have been variously termed a thallus, root–thallus or crust, as well as a root (Rutishauser and Huber, 1991). In Asian Podostemoideae species, roots are flattened subcylindrical, ribbon-shaped or foliose (thalloid) (Willis, 1902; Engler, 1930; Troll, 1943; Cusset, 1992; Cook, 1996; Schnell, 1998; Jäger-Zürn, 2000a; Uniyal and Mohan Ram, 2001). In Z. subulatum (Gardn.) C. Cusset and Z. lichenoides (Kurz) Engl., as in many other Podo stemoideae, the root is subcylindrical or ribbon-shaped, respectively, and alternately branched. Unusually, there is a shoot at every point of root branching (Warming, 1888; Willis, 1902; Mathew and Satheesh, 1997; Rutishauser, 1997; Jäger-Zürn, 2000a). The pattern is as regular as that commonly seen in shoots of most angiosperms where shoots occur in the axil of each subtending leaf. The shoot emerges on the dorsal surface of the root near the lateral edge between the main axis and lateral root. This association between root branching and shoots is also seen in Cladopus, a species of Polypleurum and Asian members of Podostemoideae, whereas there is no such association in other related species of Polypleurum (S. Koi et al., unpubl. res.).

The foliose root is chlorophyllous, with extremely reduced vegetative and reproductive adventitious shoots scattered on the dorsal surface, and root hairs (adhesive hairs) on the ventral surface. Such roots are seemingly multifunctional. A molecular phylogeny shows that Asian Podostemoideae are monophyletic and divided into two clades, one consisting of Zeylanidium and Polypleurum, and the other consisting of Cladopus, Torrenticola, Hydrobryum and Synstylis (Cook and Rutishauser, 2001; Kita and Kato, 2001). This suggests that the foliose roots of Z. olivaceum (Gardn.) Engl. and Z. maheshwarii C. J. Mathew & Satheesh, and of Hydrobryum and Synstylis are derived independently from subcylindrical or ribbon-like roots in the two clades. This proposed parallel evolution is also supported by the developmental morphology of seedlings (Suzuki et al., 2002). However, no attention has been paid to a possible association between root lobing and shoot formation in Z. olivaceum and Z. maheshwarii in which the shoots are scattered irregularly over the dorsal surface of the root (Willis, 1902; Mathew and Satheesh, 1997; Jäger-Zürn, 2000b), although they are phylogenetically close to Z. subulatum and Z. lichenoides.

This paper describes the developmental anatomy of roots of Z. subulatum and Z. lichenoides, and Z. olivaceum and Z. maheshwarii, with emphasis on the development of the root meristem, which is associated with shoot initiation, during the course of root branching. Anatomical data are compared for the four species to clarify the evolution from subcylindrical to foliose roots and the evolution of the developmental relationships of root branching or lobing with such a regular association with shoots.


Tidak ada komentar: