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Biomedical subjects

M J Canny

Publications and source records attributed to M J Canny.

10 recordsLinked to original sources
PubMed2006

Freeze/thaw-induced embolism depends on nadir temperature: the heterogeneous hydration hypothesis.

Freeze/thaw-induced embolism was studied in leaves of field-grown snow gum (Eucalyptus pauciflora) subject to frequent morning frosts. Juvenile trees were grown in buried pots, brought to the laboratory at different stages of acclimation and subjected to simulated frost-freezes (at 2 degrees C h(-1)) to nadir temperatures of -3 or -6 degrees C, which snow gums commonly experience. Frost-frozen and subsequently thawed leaves were cryo-fixed to preserve the distribution of water and were then examined by cryo-scanning electron microscopy. No embolisms were found in leaves frozen to -3 degrees C and thawed. In contrast, 34% of vessels were embolized in thawed leaves that had been frozen to -6 degrees C. This difference was seen also in the extent of extracellular ice blocks in the mid-vein expansion zones in leaves frozen to -3 and -6 degrees C, which occupied 3 and 14% of the mid-vein area, respectively. While the proportion of embolism depended on nadir temperature, it was independent of season (and hence of acclimation state). From the observation that increased embolism at lower nadir temperature was related to the freeze-induced redistribution of water, we hypothesize that the dehydration of cell walls and cells caused by the redistribution exerts sufficient tension on xylem water to induce cavitation on thawing.

Adaptation, Physiological↗
PubMed2006

Leaf water content and palisade cell size.

The palisade cell sizes in leaves of Eucalyptus pauciflora were estimated in paradermal sections of cryo-fixed leaves imaged in the cryo-scanning electron microscope, as a quantity called the cell area fraction (CAF). Cell sizes were measured in detached leaves as a function of leaf water content, in intact leaves in the field during a day"s transpiration as a function of balance pressure of adjacent leaves, and on leaf disks equilibrated with air of relative humidities from 100 to 58%. Values of CAF ranged from 0.82 at saturation to approx. 0.3 in leaves dried to a relative water content (RWC) of 0.5, and in the field to approx. 0.58 at 15 bar (1.5 MPa) balance pressure. At a CAF of 0.58, the moisture content of the cell walls is in equilibrium with air at 90% relative humidity, which is the estimated relative humidity in the intercellular spaces. It is shown that at this moisture content, the cell walls could be exerting a pressure of approx. 50 bar on the cell contents.

Cell Size↗
PubMed2004

The management of extracellular ice by petioles of frost-resistant herbaceous plants.

BACKGROUND AND AIMS: Some frost-tolerant herbaceous plants droop and wilt during frost events and recover turgor and posture on thawing. It has long been known that when plant tissues freeze, extracellular ice forms. Distributions of ice and water in frost-frozen and recovered petioles of Trifolium repens and Escholschzia californica were visualized. METHODS: Petioles of intact plants were cryo-fixed, planed to smooth transverse faces, and examined in a cryo-SEM. KEY RESULTS: With frost-freezing, parenchyma tissues shrank to approx. one-third of their natural volume with marked cytorrhysis of the cells, and massive blocks of extracellular icicles grew under the epidermis (poppy) or epidermis and subepidermis (clover), leaving these layers intact but widely separated from the parenchyma except at specially structured anchorages overlying vascular bundles. On thawing, the extracellular ice was reabsorbed by the expanding parenchyma, and surface tissues again contacted the internal tissues at weak junctions (termed faults). These movements of water into and from the fault zones occurred repeatedly at each frost/thaw event, and are interpreted to explain the turgor changes that led to wilting and recovery. Ice accumulations at tri-cellular junctions with intercellular spaces distended these spaces into large cylinders, especially large in clover. Xylem vessels of frozen petioles were nearly all free of gas; in thawed petioles up to 20 % of vessels were gas-filled. CONCLUSIONS: The occurrence of faults and anchorages may be expected to be widespread in frost-tolerant herbaceous plants, as a strategy accommodating extracellular ice deposits which prevent intracellular freezing and consequent membrane disruption, as well as preventing gross structural damage to the organs. The developmental processes that lead to this differentiation of separation of sheets of cells firmly cemented at determined regions at their edges, and their physiological consequences, will repay detailed investigation.

Cryoelectron Microscopy↗
PubMed2002

Evidence for protection of nitrogenase from O(2) by colony structure in the aerobic diazotroph Gluconacetobacter diazotrophicus.

Gluconacetobacter diazotrophicus is an endophytic diazotroph of sugarcane which exhibits nitrogenase activity when growing in colonies on solid media. Nitrogenase activity of G. diazotrophicus colonies can adapt to changes in atmospheric partial pressure of oxygen (pO(2)). This paper investigates whether colony structure and the position of G. diazotrophicus cells in the colonies are components of the bacterium's ability to maintain nitrogenase activity at a variety of atmospheric pO(2) values. Colonies of G. diazotrophicus were grown on solid medium at atmospheric pO(2) of 2 and 20 kPa. Imaging of live, intact colonies by confocal laser scanning microscopy and of fixed, sectioned colonies by light microscopy revealed that at 2 kPa O(2) the uppermost bacteria in the colony were very near the upper surface of the colony, while the uppermost bacteria of colonies cultured at 20 kPa O(2) were positioned deeper in the mucilaginous matrix of the colony. Disruption of colony structure by physical manipulation or due to 'slumping' associated with colony development resulted in significant declines in nitrogenase activity. These results support the hypothesis that G. diazotrophicus utilizes the path-length of colony mucilage between the atmosphere and the bacteria to achieve a flux of O(2) that maintains aerobic respiration while not inhibiting nitrogenase activity.

Acetobacteraceae↗
PubMed2000

The reliability of cryoSEM for the observation and quantification of xylem embolisms and quantitative analysis of xylem sap in situ.

The reliability of cryoSEM for visualizing gas embolisms in xylem vessels of intact, functioning roots is examined and discussed. The possibility that these embolisms form as a result of freezing water columns under tension is discounted by a double-freeze experiment. Two regions of the same root, one frozen under tension, the other isolated from the tension by the first freeze, had the same percentage of embolisms, as did also long pieces of root frozen simultaneously along their length. The reliability of energy-dispersive X-ray analysis to measure xylem sap concentration in situ in frozen tissue was established by measurement of KCl standard solution frozen on stubs, and within xylem vessels. Solute heterogeneity within the vessels varied with freezing procedure; deep-freeze > LN2 > cryopliers > liquid ethane, but only the deep-freeze method gave unsatisfactory estimates of concentration for the standard solution. It is concluded that cryoanalytical SEM is useful for direct observation of gas and liquid-filled compartments, and for solute analyses at depth within intact plant organs.

Cryoelectron Microscopy↗
PubMed1997

Retention in situ and spectral analysis of fluorescent vacuole components in sections of plant tissues.

The contents of plant vacuoles vary in different organs and with the health of the plant, but little is known of the cell-to-cell distribution of soluble organic compounds within plant tissues. Soluble fluorescent phenolic compounds can be immobilized in plant tissues using an anhydrous freeze-substitution and resin embedment process. The vacuolar fluorescence can be characterized in fluorescence photomicrographs for variations in color and intensity, or more quantitatively with spectra obtained using a microspectrofluorometer. This is demonstrated here in freeze-substituted roots and leaves of soybean. Excitation and emission spectra of individual vacuoles can be compared with spectra of pure compounds to form profiles of the varied phenolic contents of plant vacuoles. Such analyses will add an important anatomical dimension to the study of plant defense and stress responses.

Cryoultramicrotomy↗
PubMed1994

Planing frozen hydrated plant specimens for SEM observation and EDX microanalysis.

A procedure is described for forming a flat face on a frozen piece of plant tissue, which may then be observed fully-hydrated or lightly etched, and coated or uncoated with a metal film, in scanning electron microscopy (SEM). The frozen sample was planed with a glass knife at -80 degrees C in a cryo-ultramicrotome. The sections were discarded, and the planed block face placed on the cold stage in the microscope column, either for observation uncoated at low kV, or for light etching (-90 degrees C) to reveal the cell outlines. If a higher accelerating voltage was needed, the face was given an evaporative coating of Al in the cryo-preparation chamber and returned to the column. The advantages of the planed face over the usual fracture face are illustrated: imaging at a chosen rather than a chance position; clearer cellular and subcellular detail; preservation of hydrated gels like mucilage and swollen cell walls; the possibility of making serial parallel sections through the same piece of tissue; opportunities for accurate morphometric analyses on the planed face; capacity to produce longitudinal sections; preservation of very delicate structures that are destroyed by fixation and drying. A major advantage of the Al-coated planed face is the increased accuracy of energy-dispersive X-ray (EDX) microanalyses on a smooth rather than a rough surface. Tests are included which show that neither the light etching employed, nor successive planing, interferes with the analyses of elements in the frozen face.

Cryopreservation↗
PubMed1989

Development of water conducting capacity in the root systems of young plants of corn and some other c4 grasses.

Development of the primary and early nodal roots was studied in Zea mays L., Zea mexicana (Schrad.) Reeves & Mangelsd., Sorghum bicolor (L.) Moench., and Sorghum sudanese (Piper) Stapf. in relation to shoot development. In all the types studied all roots reached lengths of about 30 centimeters before the late metaxylem (LMX) was open, and young plants with total root lengths of around 100 centimeters had almost no open LMX. On average, corn seedlings with up to 36 square centimeters of leaf had no open LMX. The name "immature apices" is suggested for such long but not fully functional roots. In plants up to 50 days old a fairly constant proportion of less than half the total root length had open LMX. A pilot study of stomatal resistance on days of high evaporative demand suggested that young seedlings may show higher resistance than older plants in the afternoon. Estimates of longitudinal permeability of corn roots with only early metaxylem vessels open indicate very steep gradients of water potential would develop under such conditions.

Journal Article↗
PubMed1986

Transport in living plants.

This paper provides a general over-view to introduce the subsequent papers on particular topics of plant transport. A revised statement is given of the transport processes reviewed in Ann. Rev. Fluid Mech., 9, 275 (1977), outlining the tissue and cell structures of the plant body which carry out the long distance movements of water, mineral nutrients, and organic material. Some of the questions posed in that review are now better understood, e.g. the breaking of xylem water columns under tension, the loading of sugars into phloem in leaves, and the dissemination of water in leaf veins. Intractable questions remain to which there are no agreed answers, especially the organisation within phloem sieve tubes and the relative roles of the apoplast and symplast in the uptake of water and mineral ions by roots. New techniques are available for tracing water-soluble substances at high resolution in microscopic preparations which may lead to the resolving of some of these questions.

Biological Transport, Active↗