No abstract

Xylem cavitation in winter and recovery from cavitation in the spring were visualized in two species of diffuse-porous trees, Betula platyphylla var. japonica Hara and Salix sachalinensis Fr. Schm., by cryo-scanning electron microscopy after freeze-fixation of living twigs. Water in the vessel lumina of the outer three annual rings of twigs of B. platyphylla var. japonica and of S. sachalinensis gradually disappeared during the period from January to March, an indication that cavitation occurs gradually in these species during the winter. In April, when no leaves had yet expanded, the lumina of most of the vessels of both species were filled with water. Many vessel lumina in twigs of both species were filled with water during the period from the subsequent growth season to the beginning of the next winter. These observations indicate that recovery in spring occurs before the onset of transpiration and that water transport through twigs occurs during the subsequent growing season. We found, moreover, that vessels repeat an annual cycle of winter cavitation and spring recovery from cavitation for several years until irreversible cavitation occurs.In plants "cavitation" refers to the formation of a cavity within a body of water, usually in the xylem watertransport system (Milburn, 1991). All conduits for the transport of water in plants are potentially vulnerable to cavitation. Cavitation disrupts water transport and longterm consequences can include reduced growth and dieback of the plant. Cavitation occurs mainly in response to water stress or freezing (Tyree and Sperry, 1989), and cavitation caused by water stress has been explained in terms of air-seeding, as follows. When the tension inside xylem conduits exceeds a certain critical value, air is sucked into the lumina of water-filled conduits from adjacent gas-filled conduits through the pores of intervascular pit membranes (Zimmermann, 1983;Sperry and Tyree, 1988). By contrast, the cavitation that is caused by freezing seems to occur as a result of the low solubility of gases in ice. When frozen sap containing air bubbles thaws under tension, air bubbles expand in the xylem conduits and cavitation occurs (Zimmermann, 1983; Sperry, 1993).In dicotyledonous trees the region of the xylem that functions in water transport varies among species. In ringporous trees it is the earlywood vessels of the current-year xylem that are mainly involved in water transport (Chaney and Kozlowski, 1977; Ellmore and Ewers, 1986). The earlywood vessels lose the capacity to function in the transport of water by the end of each winter, and no refilling with water occurs in these vessels during the following spring (Zimmermann, 1983). Consequently, earlywood vessels play a role in water transport during a single growth season. The cavitation that occurs in earlywood vessels of ring-porous trees is believed to be irreversible.In diffuse-porous trees, by contrast to ring-porous trees, water transport generally occurs in a larger part of the sapwood (Greenidge, 1958; Chaney and Kozlowsk...

Bryophyte species growing in areas in which temperatures fall below zero in winter are likely to have tolerance to freezing stress. It is well established in higher plants that freezing tolerance is acquired by exposure to non-freezing low temperatures, accompanied by expression of various genes and increases in levels of the stress hormone abscisic acid (ABA). However, little is known about the physiological changes induced by cold acclimation in non-vascular plants such as bryophytes. We examined the effects of low temperatures on protonema cells of the moss Physcomitrella patens (Hedw.) Bruch and Schimp. The freezing tolerance of protonema cells was clearly increased by incubation at low temperatures ranging from 10 degrees C to 0 degrees C, with maximum tolerance achieved by incubation at 0 degrees C for several days. The enhancement of freezing tolerance by low temperatures occurred in both light and dark conditions and was accompanied by accumulation of several transcripts for late-embryogenesis-abundant (LEA) proteins and boiling-soluble proteins. By de-acclimation, low-temperature-induced expression of these transcripts and proteins, as well as the freezing tolerance, was reduced. Interestingly, endogenous levels of ABA in tissues or that secreted into the culture medium were not specifically increased by low-temperature treatment. Furthermore, removal of ABA from the medium by addition of activated charcoal did not affect low-temperature-induced freezing tolerance of the protonema cells. Our results provide evidence that bryophytes have an ABA-independent cold-signaling pathway leading to expression of stress-related genes and resultant acquisition of freezing tolerance.

Thermal damage to the outer membrane of Escherichia coli W3110 was studied. When E. coli cells were heated at 55°C in 50 mM Tris-hydrochloride buffer at pH 8.0, surface blebs were formed on the cell envelope, mainly at the septa of dividing cells. Membrane lipids were released from the cells during the heating period, and part of the released lipids formed vesicle-like structures from the membrane. This vesicle fraction had a lipopolysaccharide to phospholipid ratio similar to that of the outer membrane of intact cells, whereas it had a lower content of protein than the isolated outer membrane. After heating bacterial cells at 55°C for 30 min, the resulting leakage from the cells of a periplasmic enzyme, alkaline phosphatase, amounted to 52% of the total activity, whereas no release of a cytoplasmic enzyme, glucose-6-phosphate dehydrogenase, was detected. The results obtained suggest that surface blebs formed by heat treatment almost completely consist of the outer membrane and that the blebs may be gradually released from the cell surface into the heating menstruum to partially form vesicles.

No abstract