Reversing cavitation in tracheids of Pinus sylvestris L. under negative water potentials

Xylem cavitation is a frequent event, but since resistance to flow does not generally increase in vivo, reversal must also occur even under negative potentials. We demonstrated that this can occur in excised wood. Our results suggest that refilling of cavitated tracheids at negative water potentials may result from a change in equilibrium between gas concentrations, water potential and surface tension at the embolism interface. Excised branch-wood specimens from small trees of Pinus sylvestris were dried on the bench to a range of relative water contents and then rehydrated in a permeability apparatus using ultra-filtered, de-aerated water as permeant. Water inflow and outflow were measured gravimetrically by recording the gain or loss from two reservoirs held on balances. Flow was induced through the specimen by holding the balances at different levels, while an overall negative water potential could be imposed by raising the specimen above the inflow/outflow reservoirs. Changes in water content of the specimen were calculated as the difference between inflow and outflow. The time-course data for both relative water content and permeability were fitted to an exponential function to give initial and final estimates and a time constant. Rehydration occurred at all imposed water potentials, but the speed of recovery was affected at lower potentials. Where drying of the specimen was more protracted, permeability was initially lower but also recovered during permeation. Both flow and de-aeration were necessary for complete rehydration. A model requiring new information on gas concentrations and transport coefficients is suggested.     

SIALIC ACID AND THE SOCIAL BEEIAVIOUR OF CELLS

1. It is suggested that specific carbohydrate side-chains of membrane glycoproteins are the sites for cell recognition or adhesion when the terminal sugar, sialic acid, is absent. 2. It is suggested that sialic acid plays a ‘protective’ or ‘blocking’ role in cell interactions so that addition of sialic acid to asialo side-chains converts them to forms inactive for recognition. This principle of ‘blocking’ by sialic acid has been observed in other situations as in covering tumour antigens and in protecting glycoproteins from uptake by the liver. It is here extended to cell-cell adhesions. 3. It is to be expected that specific ‘protective’ actions of sialic acid in membrane-bound glycoproteins will be difficult to detect. As a charged residue, sialic acid is likely to have a strong influence both on the glycoproteins on which it is borne and on their interactions with each other at the cell surface. Removal of sialic acid by enzymes could therefore perturb the structure of the cell surface in several ways and so obscure the ‘protective’ effects of sialic acid. Sialic acid is therefore suggested to have a structural role also. 4. Evidence is assembled in favour of a model in which sialysation of specific adhesive receptors affects the social behaviour of cells. This may be an effect associated with growing cells since the contact properties of mitotic cells (and populations rich in dividing cells) are decreased by the increased sialysation of receptors. One of the factors associated with malignant behaviour could be that adhesive receptors are permanently blocked by sialic acid. 5. A schematic representation of some of the points is given in Fig. 4.     

Modelling the benefits of American Mink Mustela vison management options for terns in west Scotland

American Mink Mustela vison is a semi-aquatic predator that has invaded the west coast of Scotland and many of its associated islands. We developed a GIS model of their potential range based on their dispersal abilities and habitat use, which revealed that most islands in west Scotland are accessible to Mink, and that these host a large proportion of the region's Common Sterna hirundo and Arctic Terns S. paradisaea . Mink are predators on tern eggs and chicks, and statistical modelling of long-term productivity data demonstrated that unprotected sites within their range have an average productivity of 0.33 chicks per pair, whereas that at sites where Mink were trapped was 253% higher. We assessed the benefits of current Mink control projects for terns in the Western Isles and the remainder of west Scotland using a population modelling approach. This showed that both projects delivered considerable benefits for Common Terns, because a large proportion of their numbers were within the area of the control programmes and in sites that would be accessible to Mink if no control were in operation. For Arctic Terns, the benefits were less clear, as a larger proportion of their numbers were outside the control areas, and many of these were in sites isolated from, or unsuitable for, Mink. We discuss the implications of these findings for future strategic planning of Mink management in west Scotland.     

Molecular phylogeny of the grass genus Brachypodium P. Beauv. based on RFLP and RAPD analysis

Nuclear genome analysis using RFLPs and RAPDs has been assessed within different species of the genus Brachypodium P. Beauv. and representatives of other grasses in order to determine the characteristics of the Brachypodium genome and to establish its evolutionary position in relation to other Pooideae. Distinctive features of the Brachypodium genome are its small size, the low amount of repetitive DNA, the lack of restriction fragment length polymorphisms within the genus for the assayed probe/enzyme combinations, and the genomic variability demonstrated at species level by random DNA amplification. These molecular studies confirm Brachypodium as an isolated ancient genus best placed in its own tribe (Brachypodieae). Its relationships to other tribes Bromeae, Triticeae, Poeae are resolved, Brachypodieae being the earliest tribe to diverge from this core of pooids. Within the genus two major Old World clades are distinguishable: an annual clade, represented only by B. distachyon; and a perennial clade, represented by all the other species studied (except B. mexicanum). The perennial American species B. mexicanum appears equally attached to these two clades. RFLP data were found to be useful in obtaining phylogenies at generic and higher rank levels, whereas the highly variable RAPD data were more suitable for resolving interspecific and intraspecific evolutionary pathways.     

Systematics of the Genus Eubothrium Nybelin (Cestoda, Pseudophyllidea), with Partial Re-description of the Species

Material from as many species of the cestode genus Eubothrium as could be obtained (six out of nine existing species), from a range of hosts and from both marine and freshwater habitats, were examined both by light and scanning electron microscopy. The scolex shape as seen with SEM turned out to be a very valuable specific character. Particular attention was also paid to the possibility that intra-specific races, distinguishable on biological criteria, might in addition exhibit characteristic morphological differences.