Leaf anatomy and ultrastructure of the North American marine angiosperm Phyllospadix (Zosteraceae)

Abstract

The leaf anatomy and ultrastructure of the North American Phyllospadix species P. serrulatus Rupt. ex Aschers., P. scouleri Hook, and P. torreyi Watson are described. The unique anatomical and ultrastructural features of these species are compared with those of other seagrasses and their possible functional significance is discussed. All three species have ultrastructures similar to those in other members of the family Zosteracae. Subcuticular cavities, wall ingrowths, and numerous mitochondria and chloroplasts with well-developed grana are present in the blade epidermal cells and the adaxial sheath epidermal cells, indicating that these cells may play a major role in photosynthesis, osmoregulation, and absorption. Plasmodesmata are present occasionally between adjacent epidermal cells, and also between epidermal and mesophyll cells, suggesting that solutes can be transferred symplastically between these tissues. The vascular bundle sheath cells are not easy to recognize, as cell walls are thin and not suberized. The phloem contains both normal and nacreous-walled sieve tubes that may be functional. The walls of the phloem parenchyma cells facing nacreous-walled sieve tubes possess weak wall ingrowths, leading to speculation that these parenchyma cells may play an important role in solute translocation. The absence of suberin lamella in bundle sheath cells and the presence of a small xylem element in each vascular bundle suggest that the water flow in xylem elements in these seagrasses may be limited and that water is taken directly from the water column by leaf epidermal cells and is transported apoplastically along cell walls. The three North American Phyllospadix species can be separated by anatomical characters such as number of vascular bundles, the shape of epidermal cells in both transverse sectional and surface views, and the distribution of fibre bundles. It is proposed that P. serrulatus is taxonomically more closely related to the Japanese P. iwatensis Makino than to P. scouleri and P. torreyi and that there is no detectable hybrid species occurring between P. scouleri and P. torreyi. Key words: anatomy, ultrastructure, seagrasses, Phyllospadix, North America.