Biomedical subjects

M A Zwieniecki

Publications and source records attributed to M A Zwieniecki.

5 recordsLinked to original sources

PubMed2001

Hydrogel control of xylem hydraulic resistance in plants.

Increasing concentrations of ions flowing through the xylem of plants produce rapid, substantial, and reversible decreases in hydraulic resistance. Changes in hydraulic resistance in response to solution ion concentration, pH, and nonpolar solvents are consistent with this process being mediated by hydrogels. The effect is localized to intervessel bordered pits, suggesting that microchannels in the pit membranes are altered by the swelling and deswelling of pectins, which are known hydrogels. The existence of an ion-mediated response breaks the long-held paradigm of the xylem as a system of inert pipes and suggests a mechanism by which plants may regulate their internal flow regime.

Hydrogels↗

PubMed2001

In vivo observation of cavitation and embolism repair using magnetic resonance imaging.

Magnetic resonance imaging (MRI) was used to noninvasively monitor the status of individual xylem vessels in the stem of an intact, transpiring grape (Vitis vinifera) plant over a period of approximately 40 h. Proton density-weighted MRI was used to visualize the distribution of mobile water in the stem and individual xylem vessels were scored as either water or gas filled (i.e. embolized). The number of water-filled vessels decreased during the first 24 h of the experiment, indicating that approximately 10 vessels had cavitated during this time. Leaf water potentials decreased from -1.25 to -2.1 MPa during the same period. Watering increased leaf water potentials to -0.25 MPa and prevented any further cavitation. Refilling of xylem vessels occurred as soon as the lights were switched off, with the majority of vessels becoming refilled with water during the first 2 to 3 h in darkness. These measurements demonstrate that MRI can be used to monitor the functional status of individual xylem vessels, providing the first method to study the process of cavitation and embolism repair in intact plants.

Magnetic Resonance Imaging↗

PubMed2001

Hydraulic properties of individual xylem vessels of Fraxinus americana.

Studies of the hydraulic properties of xylem vessels have been limited to measurements of whole plant or whole stem segments. This approach allows the longitudinal transport properties of the ensemble of vessels within a stem to be determined, but provides little information on radial transport. Here the xylem of Fraxinus americana L. has been examined using a new method that allows the transport properties of individual vessels to be examined. One goal of this study was to quantify transport parameters relevant to embolism repair. The longitudinal conductivity of vessel segments open at both ends (i.e. no end walls) agreed with values predicted by the Poiseuille equation. Radial specific conductance (conductance per unit area) was approximately six orders of magnitude lower than the longitudinal conductance of the vessel segment normalized by the cross-sectional area of the vessel lumen. There was a step increase in the radial specific conductance of previously gas-filled vessels when the delivery pressure exceeded 0.4 MPa. This is consistent with the idea that positive pressure, required for embolism repair, can be compartmentalized within a vessel if the bordered pit chambers are gas-filled. The diffusion coefficient for the movement of gas from a pressurized air-filled vessel was of the same order of magnitude as that for air diffusing through water (1.95 e(-9) m(2) s(-1)). Estimates of the time needed to displace all of the gas from an air-filled vessel were in the order of 20 min, suggesting that gas removal may not be a major limitation in embolism repair.

Biological Transport↗

PubMed2000

Bordered pit structure and vessel wall surface properties. Implications for embolism repair.

The idea that embolized xylem vessels can be refilled while adjacent vessels remain under tension is difficult to accept if the cavitated vessels remain hydraulically connected to vessels under tension. A mechanism by which embolized conduits could be hydraulically isolated from adjacent conduits requires the existence of a non-zero contact angle and a flared opening into the bordered pit chamber such that a convex air-water interface forms at the entrance into the pit chamber. We measured the contact angle and pit chamber geometry for six species. The contact angle measured in the vessel lumen ranged between 42 degrees to 55 degrees, whereas the opening into the pit chamber ranged between 144 degrees and 157 degrees. If the surface properties within the pit chamber are similar to those in the lumen, a convex meniscus will form at the flared opening into the pit chamber. The maximum pressure difference between water in the lumen and gas in the pit chamber that could be stabilized by this interface was calculated to be within the range of 0.07 to 0.30 MPa.

Biological Transport↗

PubMed1998

Stomatal plugs of Drimys winteri (Winteraceae) protect leaves from mist but not drought.

Two outstanding features of the flowering plant family Winteraceae are the occlusion of their stomatal pores by cutin plugs and the absence of water-conducting xylem vessels. An adaptive relationship between these two unusual features has been suggested whereby stomatal plugs restrict gas exchange to compensate for the presumed poor conductivity of their vesselless wood. This hypothesized connection fueled evolutionary arguments that the vesselless condition is ancestral in angiosperms. Here we show that in Drimys winteri, a tree common to wet forests, these stomatal occlusions pose only a small fixed resistance to water loss. In addition, they modify the humidity response of guard cells such that under high evaporative demand, leaves with plugs lose water at a faster rate than leaves from which the plugs have been experimentally removed. Instead of being adaptations for drought, we present evidence that these cuticular structures function to maintain photosynthetic activity under conditions of excess water on the leaf surface. Stomatal plugs decrease leaf wettability by preventing the formation of a continuous water film that would impede diffusion of CO2 into the leaf. Misting of leaves had no effect on photosynthetic rate of leaves with plugs, but resulted in a marked decrease ( approximately 40%) in leaves from which the plugs had been removed. These findings do not support a functional association between stomatal plugs and hydraulic competence and provide a new perspective on debates surrounding the evolution of vessels in angiosperms.

Journal Article↗