Water vapor dissolves easily as the added molecules have essentially no effect on the concentration of the solvent; refilling of cavitated conduits containing only water vapor should occur almost instantaneously once the vapor pressure of water is exceeded. In contrast, forcing air into solution is more difficult as the dissolving gas molecules locally increase their concentration in the liquid surrounding the embolus. The movement of more gas into solution is held in check by the rate at which the newly dissolved gases diffuse away from the gas—liquid boundary.
Thus, the time needed to force an air-embolus into solution depends upon both the magnitude of the applied pressure and the diffusional limitations imposed by the surrounding environment Yang and Tyree In considering the generation of positive pressures needed to effect refilling, it is important to take into account both the hydrostatic pressures in the liquid phase and the forces arising from the surface tension of curved gas-liquid interfaces.
While the hydrostatic pressure can be either positive or negative, the surface tension of water always produces a force directed towards the inward concave side of the meniscus see Figures 3. This means that the hydrostatic pressures in the xylem needed for refilling depends upon the size of the conduit being refilled, because the curvature of the gas:water interface, and therefore the force produced by surface tension, will be set by the dimensions of the conduit.
Thus, a small conduit containing only water-vapor i. However, for larger conduits and particularly those that become air-filled, positive pressures in the liquid phase are needed to refill embolized conduits.
Plants may generate the positive pressures necessary for refilling within the roots, throughout the stem, or locally, within the cavitated conduit. The mechanisms by which these processes occur range from fairly well understood in the case of root pressure to poorly understood for the phenomenon of local pressurization. Root pressure : Many species are able to raise the hydrostatic pressure throughout their vascular system by actively loading solutes into their root stele.
Most evidence suggests that this process can only occur when the soil is saturated and transpiration is low, but the critical thresholds at which root pressure is inhibited have not been established experimentally. While the phenomenon is well documented in herbs and vines Tyree et al.
Diurnal refilling via root pressure has been reported in some crop species Tyree et al. Stem pressure : Several temperate species draw water into their stems when temperatures drop near or below freezing. Upon thawing, the positive pressure built up by this influx of water refills embolism induced over the winter by both freezing and dehydration Sperry et al. It is this process that causes sugar maple trees to bleed in the spring, which is the raw material for maple syrup.
The pressures produced by these species are generated throughout the stem, but can co-occur with some degree of root pressure Ewers et al. While the precise mechanism for this is still not fully understood, it is clear that this process is limited to the spring before leaf flush when the soil is saturated and transpiration is minimal.
Local pressurization : Local refilling, where pressurization occurs solely within an embolized conduit or a few adjacent cells, is the most ecologically interesting mechanism, as it could allow plants to recover conductivity during periods of active transpiration and therefore adjust to changing conditions throughout the day.
However, it is also the most controversial. The remainder of this Web Essay explores the limitations and possible mechanisms by which plants could generate pressure within individual conduits while the remainder of the xylem remains under tension.
Evidence for local refilling derives from a suite of studies employing a variety of experimental and measurement techniques: artificially inducing embolism by injecting pressurized air into the xylem Salleo et al. The strongest evidence to date for the occurrence of local refilling is provided by studies in which treatments designed to alter physiological parameters such as phloem transport girdling , starch reserves shading , or membrane activities addition of sodium orthovanadate or fusicoccin reduce the degree of refilling compared with control plants.
If local refilling occurs it requires a metabolic source of energy, most likely sugars either stored in the xylem parenchyma as starch or supplied by the phloem. Refilling also requires a source of water, which is generally thought to be the phloem. However, how local refilling actually occurs remains a matter of active discussion. The basic questions surrounding local refilling center on the development of a sufficient driving force to move water into the embolized vessel as well as a way to prevent water that has entered the cavitated conduit from being pulled away by tensions present in the apoplast.
Several mechanisms have been suggested to overcome these problems, but each either lacks experimental evidence or has failed to withstand rigorous theoretical analysis. Below is a brief outline of the mechanisms currently proposed; a more thorough treatment is available in a review by Clearwater and Goldstein Hacke and Sperry proposed that refilling occurs osmotically, with xylem parenchyma cells secreting solutes of sufficient size that they cannot pass through the inter-vessel pit membranes.
The key element of this hypothesis is that the refilling vessel remains hydraulically connected to adjacent conduits due to the proposed semi-permeable action of the pit membranes. Thus adjacent conduits could serve as a source of water for refilling. While the simplicity of this hypothesis is appealing, the presence of large solutes has not been detected. All of the other proposed mechanisms for local refilling require the embolized conduit to be isolated from the transpiration stream during the refilling process and then reconnected once refilling has occurred.
A hypothesis for how such hydraulic compartmentalization might occur was first proposed by Holbrook and Zwieniecki The authors suggest that the shape of the pit chamber and its hydrophobic walls may lead to the formation of a meniscus in the neck of the pit chamber that traps a small gas barrier preventing contact between the water in the refilling vessel and the transpiration stream.
Based on measurements of the pit chambers of six species they conclude the maximum pressure this meniscus could sustain ranges from 70 to kPa Zwieniecki and Holbrook Figure 1 Hydraulic compartmentalization of vessel refilling. Left: Living cells adjacent to the embolized vessel create a driving gradient that draws water into the vessel lumen blue arrows. Low permeability of the secondary wall prevents tension in adjacent vessels from being transmitted.
Influx of water into the lumen compresses the gas phase black arrows , forcing it into solution yellow arrows. The dissolved gas then diffuses away from the refilling vessel, where it may be carried off by the transpiration stream. The upper conduit is actively refilling and the water is under positive pressure; the lower vessel is under tension. Figure reprinted from Holbrook and Zwieniecki Although this mechanism could, at least in theory, isolate a refilling xylem conduit, difficulty arises when the refilled conduit must be reconnected to the transpiration stream.
If the pit chambers do not all fill simultaneously, the gas that isolated the vessel from the transpiration stream may induce cavitation and lead to a futile cycle of cavitation and refilling. Mathematical models based on the geometry and surface chemistry of bordered pits have been used to argue how such coordination might be achieved Konrad and Roth-Nebelsick , but the details of how this might occur in nature remain unexplored.
Two proposals exist to describe the driving force for water movement into hydraulically isolated conduits. The first is that refilling occurs osmotically Holbrook and Zwieniecki , Tyree et al. An alternative hypothesis, first postulated by Canny and later expanded by Bucci et al. This hypothesis requires a build-up in cortical turgor pressure due to an accumulation of solutes, followed by the net inward movement of water due to cortical cells being physically constrained by surrounding rigid tissues.
Both of these proposed mechanisms remain, to a large degree, unsubstantiated. To date no one has demonstrated sufficient osmotic concentrations in xylem sap to induce refilling. This may be due to the difficulty in accurately determining the osmotic concentration of nanoliter to picoliter samples extracted from single vessels or dilution of the contents of refilling vessels with water from full vessels when larger volumes are extracted.
The length of the conduit formed by xylem vessels or tracheids, diameter of the conduit, and size of the pits or bordered pits play important role in cavitation and embolism.
An effective method of repairing embolism in herbaceous plants occurs at night when transpiration is low or absent and root pressure is high. Under such situation, root pressure generates positive xylem pressure which reduces tension in xylem water and allows air to re-dissolve in the xylem solution. But, how embolism might be reversed in tall trees is not so clear.
However, positive xylem pressures have been observed in trees such as sugar maple and woody vines in spring and those plants are known to recover from freezing- induced embolisms in spring. Another effective mechanism to restore hydraulic conductivity in xylem after cavitation is to produce new xylem conduits in those plants which possess capacity for secondary growth. New xylem vessels and tracheids produced each spring in such plants shrubs or trees replace the older cavitated and non-functional xylem conduits which may fulfill the hydraulic conductivity needs of these plants.
Top Menu BiologyDiscussion. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes.
Answer Now and help others. Answer Now. Here's how it works: Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Do not sell my personal information. Cookie Settings Accept. Manage consent. Close Privacy Overview This website uses cookies to improve your experience while you navigate through the website.
Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. We also use third-party cookies that help us analyze and understand how you use this website. These cookies will be stored in your browser only with your consent.
You also have the option to opt-out of these cookies. But opting out of some of these cookies may affect your browsing experience. Necessary Necessary. Necessary cookies are absolutely essential for the website to function properly. These cookies ensure basic functionalities and security features of the website, anonymously. The cookie is used to store the user consent for the cookies in the category "Analytics". The cookies is used to store the user consent for the cookies in the category "Necessary".
The cookie is used to store the user consent for the cookies in the category "Other. The cookie is used to store the user consent for the cookies in the category "Performance".
0コメント