A model of bubble growth leading to xylem conduit embolism

The dynamics of a gas bubble inside a water conduit after a cavitation event was modeled. A distinction was made between a typical angiosperm conduit with a homogeneous pit membrane and a typical gymnosperm conduit with a torus-margo pit membrane structure. For conduits with torus-margo type pits pit membrane deflection was also modeled and pit aspiration, the displacement of the pit membrane to the low pressure side of the pit chamber, was found to be possible while the emboli was still small. Concurrent with pit aspiration, the high resistance to water flow out of the conduit through the cell walls or aspirated pits will make the embolism process slow. In case of no pit aspiration and always for conduits with homogeneous pit membranes, embolism growth is more rapid but still much slower than bubble growth in bulk water under similar water tension. The time needed for the embolism to fill a whole conduit was found to be dependent on pit and cell wall conductance, conduit radius, xylem water tension, pressure rise in adjacent conduits due to water freed from the embolising conduit, and the rigidity and structure of the pits in the case of margo-torus type pit membrane. The water pressure in the conduit hosting the bubble was found to occur almost immediately after bubble induction inside a conduit, creating a sudden tension release in the conduit, which can be detected by acoustic and ultra-acoustic monitoring of xylem cavitation.     

The effectiveness of a bubble curtain for preventing physical damage to antarctic krill in captivity

Krill in captivity endure spatial restrictions, regardless of the size or shape of tank used. Visual inspection of freshly caught and captive animals suggests that their appendages, particularly the antennae and antennules, are often subject to injury. We have developed a novel method for reducing physical damage to captive krill based on their reported reaction to feeding whales. This method involves creating a bubble curtain that keeps the krill in the centre of the tank and away from potentially damaging hard surfaces. There was no difference in the length of the antennae between any of the captive animals. Trends in the data suggest that the antennules of krill maintained in a 200?L bubble curtain tank and a1000?L holding tank, were longer and less variable in length, than for krill in a 200?L tank with no bubble curtain. Freshly caught krill from the field were found to have damage to both antennae and antennules. The implications of these findings for husbandry of krill and for behavioural studies in the laboratory are discussed.     

The effectiveness of a bubble curtain for preventing physical damage to antarctic krill in captivity

Krill in captivity endure spatial restrictions, regardless of the size or shape of tank used. Visual inspection of freshly caught and captive animals suggests that their appendages, particularly the antennae and antennules, are often subject to injury. We have developed a novel method for reducing physical damage to captive krill based on their reported reaction to feeding whales. This method involves creating a bubble curtain that keeps the krill in the centre of the tank and away from potentially damaging hard surfaces. There was no difference in the length of the antennae between any of the captive animals. Trends in the data suggest that the antennules of krill maintained in a 200 L bubble curtain tank and a1000 L holding tank, were longer and less variable in length, than for krill in a 200 L tank with no bubble curtain. Freshly caught krill from the field were found to have damage to both antennae and antennules. The implications of these findings for husbandry of krill and for behavioural studies in the laboratory are discussed.     

A parallel bubble column system for the cultivation of phototrophic microorganisms

An incubator with up to 16 parallel bubble columns was equipped with artificial light sources assuring a light supply with a homogenous light spectrum directly above the bioreactors. Cylindrical light reflecting tubes were positioned around every single bubble column to avoid light scattering effects and to redirect the light from the top onto the cylindrical outer glass surface of each bubble column. The light reflecting tubes were equipped with light intensity filters to control the total light intensity for every single photo-bioreactor. Parallel cultivations of the unicellular obligate phototrophic cyanobacterium, Synechococcus PCC7942, were studied under different constant light intensities ranging from 20 to 102 microE m(-2)s(-1) at a constant humidified air flow rate supplemented with CO(2).     

Thermodynamic analysis of the interaction of the xylem water and phloem sugar solution and its significance for the cohesion theory

The cohesion theory explains water transport in trees by the evaporation of water in the leaves (transpiration), which in turn generates the tension required for sap ascent, i.e. the flow of pure water from the soil through the root system and the non-living cells of the tree (xylem tracheids) up to the leaves. Only a small part of this water flow entering the leaves is used in photosynthesis to produce sugar solution, which is transported from the leaves through the living cells (phloem) to everywhere in the tree where it is needed and used. The phloem sieves are connected to the xylem tracheids by water transparent membranes, which means that the upflow of pure water and downflow of sugar solution interact with each other, causing the osmotic pressure in the sugar solution (Münch model). In this paper we analyse this interaction with a thermodynamic approach and we show that some open questions in the cohesion theory can then perhaps be better understood. For example, why under a quite high tension the water can flow in the xylem mostly without any notable cavitation, and how the suction force itself depends on the cavitation. Minimizing Gibbs energy of the system of xylem and phloem, we derive extended vapor pressure and osmotic pressure equations, which include gas bubbles in the xylem conduits as well as the cellulose-air-water interface term. With the aid of the vapor pressure equation derived here, we estimate the suction force that the cavitation controlled by the phloem sugar solution can generate at high moisture contents. We also estimate the suction force that the transpiration can generate by moisture gradient at low moisture contents. From the general osmotic pressure equation we derive an equation for calculating the degree of cavitation with different sugar solution concentrations and we show the conditions under which the cavitation in the xylem is totally avoided. Using recent field measurement results for a Scotch pine, the theory is demonstrated by showing its predictions for possible amounts of cavitation or embolism from morning hours to late afternoon.     

Impact of long-term drought on xylem embolism and growth in Pinus halepensis Mill.

 The present study was carried out to elucidate the response mechanisms of 50-year-old Pinus halepensis Mill. trees to a long-term and severe drought. The amount of water available to trees was artificially restricted for 12 months by covering the soil with a plastic roof. Over the short term a direct and rapid impact of drought was evident on the water relations and gas exchanges of trees: as the soil dried out in the Spring, there was a concurrent decrease of predawn water potential; transpiration was strongly reduced by stomatal closure. Seasonal changes in the water volume fractions of twig and stem xylem were observed and interpreted as the result of cavitation and refilling in the xylem. When droughted trees recovered to a more favourable water status, refilling of embolized xylem was observed; twig predawn water potentials were still negative in the period when the embolism was reversed in the twig xylem. A few months after the removal of the covering, no differences in whole plant hydraulic resistance were observed between droughted and control trees. Needle and shoot elongation and stem radial growth were considerably reduced in droughted trees; no strategy of trees to allocate carbon preferentially to the stem conducting tissues was apparent throughout the experiment. An after-effect of the drought on growth was observed. Received: 4 August 1997 / Accepted: 1 October 1997     

Native root xylem embolism and stomatal closure in stands of Douglas-fir and ponderosa pine: mitigation by hydraulic redistribution

Hydraulic redistribution (HR), the passive movement of water via roots from moist to drier portions of the soil, occurs in many ecosystems, influencing both plant and ecosystem-water use. We examined the effects of HR on root hydraulic functioning during drought in young and old-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] and ponderosa pine (Pinus ponderosa Dougl. Ex Laws) trees growing in four sites. During the 2002 growing season, in situ xylem embolism, water deficit and xylem vulnerability to embolism were measured on medium roots (2–4-mm diameter) collected at 20–30 cm depth. Soil water content and water potentials were monitored concurrently to determine the extent of HR. Additionally, the water potential and stomatal conductance (g_s) of upper canopy leaves were measured throughout the growing season. In the site with young Douglas-fir trees, root embolism increased from 20 to 55 percent loss of conductivity (PLC) as the dry season progressed. In young ponderosa pine, root embolism increased from 45 to 75 PLC. In contrast, roots of old-growth Douglas-fir and ponderosa pine trees never experienced more than 30 and 40 PLC, respectively. HR kept soil water potential at 20–30 cm depth above –0.5 MPa in the old-growth Douglas-fir site and –1.8 MPa in the old-growth ponderosa pine site, which significantly reduced loss of shallow root function. In the young ponderosa pine stand, where little HR occurred, the water potential in the upper soil layers fell to about –2.8 MPa, which severely impaired root functioning and limited recovery when the fall rains returned. In both species, daily maximum g_s decreased linearly with increasing root PLC, suggesting that root xylem embolism acted in concert with stomata to limit water loss, thereby maintaining minimum leaf water potential above critical values. HR appears to be an important mechanism for maintaining shallow root function during drought and preventing total stomatal closure.     

Plasmodesmatal pores in the torus of bordered pit membranes affect cavitation resistance of conifer xylem

The pit membrane in bordered pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plasmodesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which correspond relatively well with the pressure inducing cavitation. Predictions based on earlier correlations between pit structure and cavitation resistance were only weakly supported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavitation than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gymnosperms and unlikely to represent the only air-seeding mechanism.     

A method for inducing xylem emboli in situ: experiments with a field-grown tree

A pressure collar, assembled around 25cm sections of 4-year-old willow twigs, was used to examine cavitation events under field conditions. When the air pressure inside the collar was raised to between 1–8 and 2–8MPa, ultrasound acoustic emission signals were triggered which indicated the breaking of water columns in the xylem. The hydraulic conductivity of the twig portion inside the chamber decreased markedly. As a result, water potentials and conductances in leaves at the end of the twig decreased. Similar changes were induced at comparable pressures in detached twigs. The equipment used is described in detail, and evidence is presented that the mechanism of this artificial production of emboli follows the air-seeding principle hypothesized for natural cavitation events.     

Acoustic emission analysis and experiments with physical model systems reveal a peculiar nature of the xylem tension

Advanced acoustic emission analysis, special microscopic examinations and experiments with physical model systems give reasons for the assumption that the tension in the water conducting system of vascular plants is caused by countless minute gas bubbles strongly adhering to the hydrophobic lignin domains of the xylem vessel walls. We ascertained these bubbles for several species of temperate deciduous trees and conifers. It is our hypothesis that the coherent bubble system of the xylem conduits operates as a force-transmitting medium that is capable of transporting water in traveling peristaltic waves. By virtue of the high elasticity of the gas bubbles, the hydro-pneumatic bubble system is capable of cyclic storing and releasing of energy. We consider the abrupt regrouping of the wall adherent bubble system to be the origin of acoustic emissions from plants. For Ulmus glabra, we recorded violent acoustic activity during both transpiration and re-hydration. The frequency spectrum and the waveforms of the detected acoustic emissions contradict traditional assumptions according to which acoustic emissions are caused by cavitation disruption of the stressed water column. We consider negative pressure in terms of the cohesion theory to be mimicked by the tension of the wall adherent bubble system.     

A resource budget model to explain intraspecific variation in mast reproductive dynamics

Rowan, Sorbus aucuparia L., exhibits conspicuous geographic variation in its inter-annual patterns of reproduction across southern Norway. Along the southern west coast, trees flower and produce clusters of succulent berries every other year, while in the east, trees mast every 3 years. We investigate two hypotheses for this variation: local adaptation to environmental conditions that favors different reproductive schedules at different locations or variation arising from single stereotyped reproductive strategy in the face of a geographic variation in productivity. To assess the theoretical plausibility of each of these hypotheses, we develop a refined resource budget model for mast reproduction. The refined model assumes that there is a lower bound on the amount of resource that may be allocated to reproduction, which allows the prediction of the relation between reproductive schedules and productivity. From the analysis of the model, we find that the observed geographic transition in rowan mast is caused if either productivity in the east is lower than that in the west or if plants in the eastern populations have evolved to pause reproduction until energy reserves are replenished to higher levels. The prediction by the productivity gradient hypothesis match with the empirical finding that the productivity is, in fact, likely to be lower in the east than the west, although we lack empirical data to test the likelihood of the local adaptation hypothesis. There is a need for experimental studies to clarify the validity of the local adaptation hypothesis. Akiko Satake is the recipient of the 11th Denzaburo Miyadi Award.     

Overall assessment of Monodus subterraneus cultivation and EPA production in outdoor helical and bubble column reactors

The outdoor production of Monodus subterraneus wasstudied in bubble column and helical reactors, mainly analysing the influenceofdilution rate, air flow rate and solar irradiance on growth rate andbiochemicalcomposition. Photoinhibition and photo-oxidation phenomena were also analysed.The cultures were stressed at high solar irradiance and dissolved oxygenconcentrations. A clear relationship between stress of the cultures and thefluorescence from PSII measurements was observed, the Fv/Fm ratio being lowerinthe helical reactor than in the bubble column. Growth rate and biomassproductivity were both a function of the average irradiance and the Fv/Fmratio;maximum values of 0.040 h^–1 and 0.54 gL^–1 d^–1 were measured. The influenceofphotoinhibition and average irradiance was modelled, the model also fitting theexperimental data reported by another author. The chlorophyll contenthyperbolically decreased, whereas the carotenoid content decreased linearlywiththe average irradiance. The higher the dilution rate the higher the protein andcarbohydrate content of the biomass, and the lower the lipid content. Theeicosapentaenoic acid (EPA) content ranged from 2.3 to 3.2% d.wt, the higherthe dilution rate, the lower EPA content, although the higher the EPAproportion. Maximum EPA productivity was only 9 mg L^–1d^–1, due to the stress to which the cultures wereexposed.     

Daily dynamics in xylem cell radial growth of Scots pine (Pinus sylvestris L.)

Daily dynamics of radial cell expansion during wood formation within the stems of 25-year-old Scots pine trees (Pinus sylvestris L.), growing in field conditions, were studied. The samples of forming wood layers were extracted 4 times per day for 3 days. Possible variations in the growth on different sides of the stem, duration of cell development in radial cell expansion phase and dynamics of cell growth in this phase were taken into account. The perimeters of tracheid cross-sections as a reflection of primary cell wall growth were the criterion of growth in a radial direction. For the evaluation of growing cell perimeters a special system for digital processing and image analysis of tracheid cross-sections of the forming wood was used. Growth rate for certain time intervals was estimated by the change in the relation of the perimeter of each observed cell in each of ten tracheid rows in each of 12 trees to the perimeter of the xylem cell of the same row before the expansion. Temporal differences in average values of the relations were estimated by Analyses of Variance. The existence of daily dynamics of Scots pine xylem cell radial growth has been proved. Intensive growth of pine tracheids has been shown to occur at any time of the day and to depend on the temperature regime of the day and the night as well as water supply of stem tissues. Moreover, reliable differences (P = 0.95) in the increment of cell walls during tracheid radial expansion have been found. Pulsing changes of the water potentials both of the cell and the apoplast, as the reason for the fluctuations of radial cell growth rate, were discussed.     

Interactions between animal cells and gas bubbles: The influence of serum and pluronic F68 on the physical properties of the bubble surface

We describe a method by which the degree of bubble saturation can be determined by measuring the velocity of single bubbles at different heights from the bubble source in pure water containing increasing concentrations of surfactants. The highest rising velocities were measured in pure water. Addition of surfactants caused a concentration-dependent and height-dependent decrease in bubble velocity; thus, bubbles are covered with surfactants as they rise, and the distance traveled until saturation is reached decreases with increased concentration of surfactant. Pluronic F68 is a potent effector of bubble saturation, 500 times more active than serum. At Pluronic F68 concentrations of 0.1% (w/v), bubbles are saturated essentially at their source. The effect of bubble saturation on the interactions between animal cells and gas bubbles was investigated by using light microscopy and a micromanipulator. In the absence of surfactants, bubbles had a killing effect on cells; hybridoma cells and Chinese hamster ovary (CHO) cells were ruptured when coming into contact with a bubble. Bubbles only partially covered by surfactants adsorbed the cells. The adsorbed cells were not damaged and they also could survive subsequent detachment. Saturated bubbles, on the other hand, did not show any interactions with cells. It is concluded that the protective effect of serum and Pluronic F68 in sparged cultivation systems is based on covering the medium-bubble interface with surfaceactive components and that cell death occurs either after contact of cells with an uncovered bubble or by adsorption of cells through partially saturated bubbles and subsequent transport of cells into the foam region. (c) 1994 John Wiley & Sons, Inc.     

Microbiological air analysis in dental surgeries: A comparison between two methods

The microbiological quality of indoor air is creating an increasing interest especially as far as places at risk such as hospitals, clinics, medical and odontological surgeries are concerned. Working with the odontologists of our province we have been carrying out a research aimed at preventing cross-infection in odontology. Data obtained from the microbiological analysis of the air in 36 surgeries using S.A.S were discussed during the V National Congress of Aerobiology. During that congress the need of setting a standardized technique of air sampling in indoor environments emerged and two routes have been identified: (1) the gravimetric technique on open plate exposed for an hour close to the dental unit and (2) the use of the volumetric sampler which gives qualitative data expressed as colonies forming units per cubic metre of air. However, both of these techniques present some problems: using the first a loss of micro-organisms has been noticed due to the variability of the air fluxes and the different weight of the biological particles; using the second one the bacterial charge is also undervalued, because of the stress suffered by the bacteria with the use of the volumetric sampler. In the light of these statements we decided to use both in dental surgeries to be able to compare the results obtained. Our project is expected to carry out at least one inspection and the relative sampling (indoor air, water of the dental unit, air of the syringe, disinfectant solution, surface tampons, biological test of sterility) in each dental surgery in the territory of our health Unit, located in Ferrara, Northern Italy.