Water relations

cohesion

attraction between water molecules due to Hydrogen bonding

adhesion

attraction of water to a solid phase such as glass slide or a plant cell wall

surface tension

property of the surface of a liquid that allows it to resist an external force

Hydrostatic Pressure

force inside a standing fluid that pushes equally in all directions

Tensile strength

max force a column of water can withstand before “breaking”

negative hydrostatic pressure may lead to

cavitation

diffusion

solute molecules move from high to low concentration

osmosis

solvent molecules move from low to high solute concentration

hypertonic

higher concentration of solutes outside compared to inside the cell

Isotonic

same concentration of solutes outside and inside the cell

Hypotonic

lower concentration of solutes outside compared to inside the cell

soil hydraulic conductivity

the ease in which water moves through soil

root hairs

filamentous outgrowths of root epidermal cells that greatly increase surface area for absorption

Mature regions of the root absorb water less due to

the Exodermis which contains suberin

suberin

bipolymer composed of long-chain fatty acids, glycerol and phenolic compounds

suberin functions

prevents water loss, regulates nutrient absorption, and defends plants against pathogens, pests, and env stress

Apoplastic

movement through cell walls, intercellular spaces, lumens of nonliving cells

Symplastic

movement through the network of cell cytoplasm interconnected by plasmodesmata

Transmembrane transport

enters and exits cell via plasma membrane channels

Cohesion-Tension Theory of Sap Ascent

As water evaporates from cells in the leaf the remaining water is drawn into the interstices of the cell wall. This causes a decrease in bulk flow similar to that seen in soil

Capillarity

upward movement of water within a narrow tube driven by adhesion, cohesion, and surface tension

driving force for water movement across a membrane

difference in water potential

rate of water movement depends on

magnitude of the water potential gradient and membrane permeability

water moves down energy gradient from _____ to ______ water potential

higher to lower

causes of plasmolysis

hypertonic environment

primary site of water absorption in roots

root tip / apical region

Guttation

exudation of liquid water droplets from specialized pores called hydathodes located on leaf margins or tips

cause of guttation

positive root pressure that builds up in the xylem when roots actively pump ions into the xylem under low transpiration conditions

when is guttation most commonly observed

at night or early in the morning when soil moisture and atmospheric humidity is high, and stomata are closed

Exodermis

Outer layer of root cortex

Endodermis

Innermost layer of the root cortex surrounding the vascular cylinder;
contains Casparian strip

Casparian strip (suberin barrier)

forces water/solutes out of apoplast into symplast to regulate xylem entry

exodermis water/solute control

Develops suberized walls in mature root regions to restrict outer water loss and pathogen entry

elasticity of a cell wall is determined by

composition and structural arrangement of wall polymers

Soil to Leaf movement driven by

gradient in hydrostatic pressure potential

leaf to atmosphere movement driven by

water vapor concentration gradient

gymnosperms xylem

tracheids only, torus-margo pit valves, lower water transport efficiency and slower bulk flow rates

angiosperms xylem

both tracheids and vessel elements, simple/bordered pit membranes, higher flow rates but risk of cavitation

cell water potential

sum of solute potential and pressure potential

in response to drought, plant cells accumulate solutes to

lower solute and cellular water potential which allows plant to maintain positive turgor pressure

cells adjust to high salt content by osmotic means

accumulate solutes in cytosol, put toxic inorganic ions into vacuoles to maintain lower internal water potential

cells adjust to high salt content through ion transport

active membrane transporters and ion pumps to exclude Na+ at the plasma membrane or pump it in vacuole

xylem prevent cavitation by

pits which contain cell wall membranes/torus-margo structures to allow water pass