Antheridial dehiscence in ferns

We investigated the mechanism of antheridial dehiscence in ferns for the first time using fluorescence microscopy as well as scanning and transmission electron microscopy. The mechanism leading to antheridial dehiscence in Polystichum setiferum, Asplenium trichomanes and A. onopteris was found to depend on the different cellulose contents of the inner and outer walls of the ring cells detected with calcofluor white stain and the Thiéry test. The extremely low cellulose content of the ring cell walls facing spermatozoids made them less mechanically resilient than external wall cells. When the ring cells absorbed water they expanded only into the antheridial cavity, pushing the gametes against the cap cell, which detached from the ring cell below and enabled spermatozoid release. The newly released spermatozoids were spherical bodies covered in cellulose fibrils. The significance of cellulose fibrils could be to isolate the gametes from each other, to reinforce the electron transparent material and to protect the gamete from pressure created by the ring cells during release.     

An electron microscope study of gametangia in the green seaweed Halimeda tuna

Abstract

The gametangia of the green seaweed Halimeda tuna are spherical bodies of diameter up to 250–300 μm. They are clustered in groups of 8–10 on hundreds of threads sprouting from all pale white segments of the fertile individuals. In addition to gametes, starch-containing chloroplasts, naked starch grains and two types of spherical bodies different in size and ultrastructure are the main corpuscular components. A layer of amorphous material of irregular thickness underlies the walls which are finely and evenly rough, structureless, electron translucent and 1–1.5 μm thick. Gametangia with superficial wall warts were found also.     

Ultrastructural features of spermatocytes and spermatozoids in the fern Phyllitis scolopendrium (L.) Newm. subsp. scolopendrium

Phyllitis scolopendrium Newm. subsp. scolo-pendrium spermatozoids are cells 10 μm long in the form of spirals with about four turns. Their chromatin is partly honeycomb-shaped and partly highly condensed. The nuclear envelope over the latter has a regular, thin intermembrane space crossed by fibers that are probably involved in connecting the chromatin with elements of the microtubular ribbon. The cytoplasm is traversed by long cistern-shaped folds of the plasma membrane, believed to be involved in a late process of cell simplification through segregation and detachment of parts of the cytoplasm. The spermatozoids are embedded in 1–1.5 μm thick amorphous electron-transparent material containing cellulose fibrils. These fibrils are considered a network connected to the original spermatocyte wall and elements of elastic support for the amorphous material. The different polysaccharide composition of the inner and outer parts of the walls causes changes in the size and shape of the ring cells, so that the spermatozoids are pushed against and past the cap cell. The gametes are released through limited laceration of the cap cell. The laceration is due to the generally weak substructure of the cell wall. A light microscope sequence of spermatozoid release and scanning electron microscope features of newly released spermatozoids are shown. Received: 24 November 1999 / Revision accepted: 29 December 1999     

Response to Salinity in Legume Species: An Insight on the Effects of Salt Stress during Seed Germination and Seedling Growth

The process of soil salinization and the preponderance of saline water sources all over the world represent one of the most harmful abiotic stress to plant growth. This pointed to the importance of obtaining plants which are tolerant or resistant to salt, considering that projection of climate change for the coming years indicate an increase in temperature and rain scarcity. In the current study, the effect of NaCl was investigated on germinating seeds of Lathyrus sativus L., Vicia sativa L., Vigna radiata L. R.Wilczek and Vigna unguiculata L. Walp., by combining physiological, biochemical, biostatistical and ultrastructural analyses. Our results revealed that germination was not influenced by high NaCl concentrations, while seedling growth was affected even at low NaCl concentrations, probably due to an alteration in water uptake and in organic matter biosynthesis. Nevertheless, the synthesis of antioxidant enzymes, phenolic acids and flavonoids was registered in all species, which tended to cope with the increasing salt stress, allowing a response mechanism such as cytoplasm detoxification and cellular turgor maintenance. Besides, the ultrastructural analysis evidenced plasmolyzed cells close to cells with a normal ultrastructure with no appreciable differences among the species. This research deeply investigates the mechanism of salt-stress response focusing on species never tested before for their possible tolerance to salinity.     

Structural and histochemical changes during seed development of Brassica macrocarpa Guss.

During seed formation of Brassica macrocarpa the development of the embryo precedes that of the integuments; structural changes and histochemical changes are associated. Esterases, acid phosphatases, phenols and starch follow a sigmoid pattern, increasing during embryogenesis and decreasing during seed maturation. In the mature seed, esterase activity is localized in the embryo and in the cells of the mucilaginous, aleuronic and hyaline layers. Acid phosphatases are present in the mucilaginous cells, mainly in the column, the cell walls delimiting intercellular spaces of the cortical cylinder and the adhesion areas of the cotyledons. Phenols are scanty in the root apex, mucilaginous cells and the palisade layer, and abundant in the pigmented layer. Starch is absent in ripe seeds which have lipid and protein reserves. The major classes of storage proteins have molecular weights of 21, 22, 27 and 30 KD and accumulate in the late stages prior to complete drying. Esterases and acid phosphatases in mucilaginous cells of the seed integument suggest that these enzymes are involved in hydrolytic processes occurring prior to germination and that mucilages have a metabolic function in seed-soil interactions.     

An ultrastructural study of the mature spermatozoid of the fern Asplenium trichomanes L. subsp. trichomanes

 Asplenium trichomanes L. subsp. trichomanes spermatozoids are spirals of about five turns. Keels link the elements of the microtubular ribbon with the plates of the lamellar layer (LL) which are uninterrupted, parallel and curved with an inner angle of about 150°. Electron-opaque filaments connect the microtubules of the multilayered structure (MLS) and the osmiophilic crest, the LL and the MLS-associated mitochondrion and the latter and the plasmalemma. The nucleus occupies the 2.5–3 posterior turns and has an inner honeycomb-shaped chromatin mass and an outer highly condensed chromatin mass with randomly scattered electron-transparent areas. The basal bodies of the ca. 50 flagella are bounded by a reticulum of granular material which forms a plug inside their proximal region; the proximal region of the flagellum has a 9 + 0 pattern. The axoneme has a 9 + 2 pattern. Received: 15 January 1997 / Revision accepted: 1 April 1997