Functional leaf anatomy and ultrastructure in a marine angiosperm, Syringodium isoetifolium (Aschers.) Dandy (Cymodoceaceae)

Abstract

The terete leaf blade of Syringodium isoetifolium (Aschers.) Dandy from south-western Australia has uniformly small epidermal cells and a few large secretory cells. Mesophyll tissues contain several air lacunae, a central longitudinal vascular bundle, and eight to twelve peripheral longitudinal vascular bundles, but no fibre bundles. The total volume of air lacunae is about 10% that of the leaf blades, but the total surface area of air lacunae is similar to that of leaf blades. The leaf cuticle appears as a thin, electron-transparent layer. Leaf-blade epidermal cells have a large central vacuole and peripheral cytoplasm containing many chloroplasts that lack starch grains. Wall ingrowths are absent. However, the small region between the walls and the plasmalemma could play an important role in nutrient absorption. Plasmodesmata appear to be absent between adjacent epidermal cells, and also between epidermal and mesophyll cells, suggesting that there is only an apoplastic pathway for the transport of photosynthate to the vascular tissues. Each vascular bundle is surrounded by a layer of sheath cells, which are characterized by the presence of suberized lamellae in their walls. These may act to reduce the apoplastic exchange of solutes and water between the mesophyll and vascular tissues. Xylem elements, represented by large lumens and intensely hydrolysed walls, may not be present in all peripheral vascular bundles. Two types of sieve elements occur in S. isoetifolium leaf blades: normal thin-walled ones with large lumens, and thick-walled ones with reduced lumens, representing the photophloem and the metaphloem, respectively.