Structural features of the leaf epidermis of Halodule uninervis

Leaf blade epidermal cells of the marine angiosperm Halodule uninervis (Forssk.) Aschers resemble transfer cells since they possess a highly invaginated plasmalemma with which numerous mitochondria are often associated. However, wall ingrowths found in transfer cells of terrestrial plants are lacking. The plasmalemma is invaginated around fibrillar material which is not continuous with the cell walls and which appears to be structurally different from cell wall material. Epidermal cells possess an organelle-rich cytoplasm in which chloroplasts and electron-dense deposits feature prominently. The outer cell wall is about twice as thick as the other walls and seems to have a porous texture. A thin but distinct cuticle covers the leaf surface. Often minute cavities occur within the cuticle adjacent to the wall. Epiphytic algae and bacteria are frequently found in close association with the epidermis. Based on structural features, it would appear that leaf blade epidermal cells of H. uninervis play an important role in the leaf’s activities such as synthesis of carbohydrates, absorption and secretion of solutes and osmoregulation

Ultrastructure and cytochemistry of squamulae intravaginales of the marine angiosperm, Halophila ovalis

Small delicate structures found in the leaf bases in vegetative buds of the marine angiosperm Halophila ovalis (R. Br.) Hook. f. showed a fine structure of the component cells typical of secretory cells. They are rich in organelles particularly ER, dictyosomes, ribosomes and mitochondria. Endoplasmic reticulum occurs at two localities in each cell, one surrounding the central nucleus and the other in the peripheral regions close to the plasmalemma. Groups of dictyosomes occur between the two layers of ER and each group is separated from another by interconnecting strands of ER. A mucilaginous secretion accumulates on the cell surface and between the outer and inner layers of cell wall, particularly at the corners. The involvement of ER in the secretion of mucilage is discussed. Staining of ultrathin sections with silver proteinate after thiocarbohydrazide showed the presence of insoluble carbohydrates and histochemical tests with alcian dyes showed the secretion to contain non-sulphated acid polysaccarides. Staining of epoxy resin sections with Coomassie blue R250 indicated that the mucilage secretion on the surface of and within the cell walls also contained proteins. Cellulase–gold complex marked the cellulose wall layers as distinct from the included secretion

Structural changes and associated microbial activity accompanying decomposition of mangrove leaves in Mgeni Estuary

Leaves of Avicennia marina (Forssk.) Vierh. and Bruguiera gymnorrhiza (L.) Lam. were placed in litter bags and allowed to decompose in situ before collection at 7, 14, 21, 42, 64 and 85days. Material from each collection was examined for structural changes using light microscopy and SEM. Samples of each collection were also cultured for fungi and bacteria. Examination of senescent leaves still attached to the trees revealed the presence of phylloplane micro-organisms. Decomposition in both species followed a similar pattern although the leaves of A. marina decomposed more rapidly than those of B. gymnorrhiza. By day 7 there were no obvious changes in leaf structure, initial decomposition of the leaf base was the only apparent change in structure on day 14; by day 21 portions of the cuticular layer had separated from the leaf tissue; on day 42 most of the leaves were split longitudinally, separating the adaxial and abaxial surfaces; in the day-63 material, fragmentation of the leaves was evident; further fragmentation and ‘skeletonization’ of leaves was observed after 85days of decomposition. Both bacteria and fungi were important in the decomposition process. Leaves of both mangroves were found to harbour 20–25 species of fungi. This work appeared to confirm the significance of micro-organisms in the estuarine food web

Abnormal concentrations of Cu-Co in Haumaniastrum katangense, Haumaniastrum robertii and Aeolanthus biformifolius: contamination or hyperaccumulation?

The Central African Copperbelt of the DR Congo and Zambia hosts more than 30 known Cu-Co hyperaccumulator plant species. These plants can accumulate extraordinarily high concentrations of Cu and Co in their living tissues without showing any signs of toxicity. Haumaniastrum robertii is the most extreme Co hyperaccumulator (able to accumulate up to 1 wt% Co), whereas Aeolanthus biformifolius is the most extreme Cu hyperaccumulator (with up to 1 wt% Cu). The phenomenon of Cu-Co hyperaccumulator plants was studied intensively in the 1970s through to the 1990s, but doubts arose regarding earlier observations due to surficial contamination of plant material with mineral particles. This study set out to determine whether such extraneous contamination could be observed on herbarium specimens of Haumaniastrum robertii and Aeolanthus biformifolius using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Further, synchrotron X-ray absorption spectroscopy (XAS) was used to identify the chemical forms of Cu and Co in newly collected Haumaniastrum katangense plant material from the DR Congo. The results show that surficial contamination is not the cause for abnormal Cu-Co concentrations in the plant material, but rather that Cu-Co enrichment is endogenous. The chemical form of Cu and Co (complexation with carboxylic acids) provides additional evidence that genuine hyperaccumulation, and not soil mineral contamination, is responsible for extreme tissue concentrations of Cu and Co in Haumaniastrum katangense.Antony van der Ent, François Malaisse, Peter D. Erskine, Jolanta Mesjasz-Przybyłowicz, Wojciech J. Przybyłowicz, Alban D. Barnabas, Marta Sośnicka and Hugh H. Harri