Abstract
Ion-mediated changes in xylem hydraulic resistance are hypothesized to result from hydrogel like properties of pectins located in the bordered pit membranes separating adjacent xylem vessels. Although the kinetics of the ion-mediated changes in hydraulic resistance are consistent with the swelling/deswelling behavior of pectins, there is no direct evidence of this activity. In this report we use atomic force microscopy (AFM) to investigate structural changes in bordered pit membranes associated with changes in the ionic concentration of the surrounding solution. When submerged in de-ionized water, AFM revealed bordered pit membranes as relatively smooth, soft, and lacking any sharp edges surface, in contrast to pictures from scanning electron microscope (SEM) or AFM performed on air-dry material. Exposure of the bordered pit membranes to 50 mM KCl solution resulted in significant changes in both surface physical properties and elevation features. Specifically, bordered pit membranes became harder and the fiber edges were clearly visible. In addition, the membrane contracted and appeared much rougher due to exposed microfibers. In neither solution was there any evidence of discrete pores through the membrane whose dimensions were altered in response to the ionic composition of the surrounding solution. Instead the variable hydraulic resistance appears to involve changes in the both the permeability and the thickness of the pit membrane.
Highlights
Self-actuating hybrid materials are used in a wide range of microdevices ranging from flux control in microfluidic systems using gels (Beebe et al, 2000) to shape memory actuators (Vyawahare et al, 2008)
Bordered pit membranes create a porous boundary between adjacent micro-channels, that prevents the spread of embolism that could halt water transport from soil to leaves and cause plant death
Most often the entire process resulted in sections that were unsuitable for atomic force microscopy (AFM) scans due to tilt, unexposed or torn bordered pits, problems impossible to detect in light microscope
Summary
Self-actuating hybrid materials are used in a wide range of microdevices ranging from flux control in microfluidic systems using gels (Beebe et al, 2000) to shape memory actuators (Vyawahare et al, 2008). The growing interest in the use of “smart materials” results from the fact that such materials do not require external triggers, rather they respond to changes in their surroundings Such autonomously acting materials occur within the plant vascular network. The movement of water through the plant vascular system occurs via specialized micro-channels composed of dead cells arranged in series The ability of these cells to respond to the ionic concentration in the transpiration stream allows the seemingly dead xylem micro-channels to modulate their hydraulic properties (Zwieniecki et al, 2001). Pit membranes are a composite material that includes cellulose, lignin, hemicelluloses, and pectin hydrogels (Zwieniecki and Holbrook, 2000; Liu et al, 2004) It provides a significant hydraulic resistance to xylem sap flow that was shown to be variable in response to sap ion concentration
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