Low-temperature-SEM study of dihedral angles in the ice-I/sulfuric acid partially molten system.

The transport and mechanical properties of partially molten materials are influenced by the wetting behaviour of the melt with respect to the crystalline solid. The equilibrium microstructure of an ice + melt system was examined using low-temperature scanning electron microscopy. The samples were prepared by spraying a liquid solution of H(2) O-H(2) SO(4) into liquid nitrogen and packing the frozen particulates into aluminium capsules. Samples were then sintered at -35°C or -55°C (above the eutectic temperature, T(E) =-62°C) for various durations and were quenched in liquid nitrogen to capture the equilibrium microstructure. This paper reports the first quantitative measurements of dihedral angle in this system. The measured median dihedral angle between the solid and vitrified melt is approximately 26 ± 2° at -35°C and increases slightly as temperature decreases and approaches the solidus (32 ± 3° at -55°C).

Paludification and forest retreat in northern oceanic environments.

Examination of temperature variations over the past century for Europe and the Arctic from northern Norway to Siberia suggests that variations in the North Atlantic Oscillation are associated with an increase in oceanicity in certain maritime regions. A southward depression of the tree line in favour of wet heaths, bogs and wetland tundra communities is also observed in northern oceanic environments. The physiological basis for this change in ecological succession from forest to bog is discussed in relation to the long-term effects of flooding on tree survival. The heightened values currently detected in the North Atlantic Oscillation Index, together with rising winter temperatures, and increased rainfall in many areas in northern Europe, presents an increasing risk of paludification with adverse consequences for forest regeneration, particularly in areas with oceanic climates. Climatic warming in oceanic areas may increase the area covered by bogs and, contrary to general expectations, lead to a retreat rather than an advance in the northern limit of the boreal forest. High water-table levels are not automatically detrimental to forest survival as can be seen in swamp, bottom land and mangrove forests. Consequently, the inhibitory effects of flooding on tree survival and regeneration in northern regions should not be uncritically accepted as merely due to high water levels. Evidence is discussed which suggests that physiological and ecological factors may interact to inhibit forest regeneration in habitats where there is a risk of prolonged winter-flooding combined with warmer winters and cool moist summers.

An examination of the relationship between flowering times and temperature at the national scale using long-term phenological records from the UK.

This paper examines the mean flowering times of 11 plant species in the British Isles over a 58-year period, and the flowering times of a further 13 (and leafing time of an additional 1) for a reduced period of 20 years. Timings were compared to Central England temperatures and all 25 phenological events were significantly related (P<0.001 in all but 1 case) to temperature. These findings are discussed in relation to other published work. The conclusions drawn from this work are that timings of spring and summer species will get progressively earlier as the climate warms, but that the lower limit for a flowering date is probably best determined by examining species phenology at the southern limit of their distribution.

Plant regeneration from encapsulated embryoids and an embryogenic mass of pistachio, Pistacia vera L.

Pieces of an embryogenic mass (EMS) induced in culture from immature fruits of pistachio, Pistacia vera L., were encapsulated into calcium alginate beads. Somatic embryos were also encapsulated individually into calcium alginate beads to produce synthetic seeds. The viability of the encapsulated EMS and somatic embryos was investigated immediately following encapsulation, and after storage for 60 days at 4°C. The encapsulated-stored EMS fragments recovered their original proliferative capacity after two months storage following two sub-cultures, but non-encapsulated-stored EMS failed to recover. The conversion frequency of synthetic seeds to seedling plants was 14% after storage for 60 days at 4°C, from which it may be concluded that encapsulation is a practical procedure for short-term storage of embryogenic pistachio tissue, and may be applicable to the preservation of desirable elite genotypes.

Somatic embryogenesis in cultured immature kernels of Pistachio, Pistacia vera L.

Embryogenic tissue was produced from kernels of immature fruits of Pistachio (Pistacia vera L.) cultured in liquid Murashige and Skoog media, supplemented with 200 mgl(-1) casein hydrolysate, 114 µM 1-ascorbic acid, and benzylaminopurine. Compact embryogenic masses differentiated directly from the fruit explants after culture for 2 weeks in liquid medium with 8.9 µM benzylaminopurine. After transfer of the embryogenic masses into the same medium, but with 4.4 µM benzylaminopurine, somatic embryos appeared. Several stages of embryogenesis were present in the cultures. Adventive embryos were readily separated from the friable embryogenic masses by shaking. Separated somatic embryos, germinated on solidified Murashige & Skoog medium without growth regulators, developed into plantlets.

Low-temperature scanning electron microscopy in biology.

A review of low-temperature scanning electron microscopy (LTSEM) with regard to preparation protocols, specimen preservation, experimental approaches, and high-resolution studies, is provided. Preparative procedures are described and recent developments in methodologies highlighted. It is now well established that LTSEM, for most biological specimens, provides superior specimen preservation than does ambient-temperature SEM. This is because frozen-hydrated samples retain most or all of their water, are rapidly immobilized and stabilized by cryofixation, and are not exposed to chemical modification or solvent extraction. Nevertheless, artefacts in LTSEM are common and most arise because frozen-hydrated specimens contain water. LTSEM can be used as a powerful experimental tool. Advantages of employing LTSEM for this purpose and ways in which it can be used for novel experimentation are discussed. The most exciting development in recent years has been high-resolution LTSEM. The advantages, problems and requirements for this approach are defined.

Water droplets and ice deposits in leaf intercellular spaces: redistribution of water during cryofixation for scanning electron microscopy.

An experimental study is described of the formation of extracellular deposits on the surfaces of cells in freeze-fractured, frozen-hydrated primary leaves of Phaseolus vulgaris examined by low-temperature scanning electron microscopy. The deposits, observed under a range of experimental conditions, consisted of (a) droplets with diameters of 1.5 to 3.0 µm, (b) droplets with diameters of 10 to 30 µm, (c) crystals with diameters of 1.0 to 6.0 µm, and (d) granules with diameters up to 0.15 µm. The types of deposit were influenced by specimen cooling rate, and their distribution was influenced by the direction of the thermal gradient during cooling. All deposits were predominantly water ice. The quantities of deposited water (up to 4.0% of the leaf water content) increased as the cooling rate was reduced. It is concluded that the ice deposits were primarily artefacts of cryofixation and do not represent the location of water in vivo, as recently suggested. We propose that the deposits arose in four main ways: (1) displacement of water from underlying cells by a pressure wave resulting from the volume increase of intracellular water as it freezes, (2) evaporation of water from warmer cells and its condensation onto colder cells, (3) withdrawal of water from underlying cells by extracellular ice crystallization, (4) condensation of pre-existing water vapour in the intercellular spaces onto cells. The significance of the findings is discussed in relation to the use of lowtemperature scanning electron microscopy in studies of plant morphology and for localizing water and soluble ions within plant cells and tissues.

A comparison of tomato (Lycopersicon esculentum) lectin with its deglycosylated derivative.

A deglycosylated derivative of tomato (Lycopersicon esculentum) lectin was prepared with the use of trifluoromethanesulphonic acid. Its properties were generally similar to those of the native lectin, but differences were evident in terms of relative agglutinating activity towards sheep, (untreated) human and trypsin-treated human erythrocytes. The two forms of tomato lectin were used in conjunction with a battery of specific antisera to investigate structural relatedness among solanaceous lectins. Immunological cross-reactivity between tomato, potato and Datura lectins depends on the integrity of the glycosylated region of those lectins; that between Datura lectin and other seed lectins, however, has a separate structural basis.

Epicuticular wax in the stomatal antechamber of sitka spruce and its effects on the diffusion of water vapour and carbon dioxide.

The distribution of wax tubes on the leaf surfaces is described, especially the presence of wax tubes in the antechambers of the stomata. The extra resistances which the wax-filled antechambers add to the other resistances in the pathway for diffusion of water vapour and of carbon dioxide are calculated. We conclude that the wax-filled stomatal antechambers reduce the rate of transpiration by about two thirds but reduce the rate of photosynthesis by only about one third. Thus wax-filled stomatal antechambers are excellent antitranspirants.

Negative stain in wax tubes from the surface of Sitka spruce leaves.

Carbon replicas and scanning electron micrographs of wax outgrowths on some leaf surfaces have not shown conclusively whether the outgrowths are tubes or solid rods. Some workers have suggested that they are solid rods. We find that negative stains will penetrate into a cavity in the centre of wax outgrowths on leaves of Sitka spruce (Picea sitchensis Bong. Carr) and Tulipa kaufmanniana Elliot. Thus these outgrowths appear to be tubes.