BOT3015 - Water Movement

CO2, H20, and Light

Plants require to bring together these items within photosynthetic organs (leaves and stems)

Water Potential (Ψ)

Difference in potential energy between pure water and water in a system. Water moves from areas with higher to lower Ψ - a difference in water potential (ΔΨ) must exist

between two systems

Osmotic Potential (Ψπ)

From dissolved solutes

Pressure Potential (ΨP)

From hydrostatic and pneumatic pressures - turgor and transpiration

Matric Potential (Ψm)

Cohesive and adhesive forces of water

Gravitational Potential (Ψg)

Gravity

Gradient in Ψ

Provides the driving force for water to move from soil, through the plant, into the atmosphere.

Transpiration

Evaporation of water from the leaves of a plant. Water vapor diffuses from the leaf to the atmosphere via the open stomata. To prevent excessive diffusion, leaves form a boundary layer: a relatively undisturbed gradient of water vapor. Larger the boundary layer → gentler gradient → slower rate of diffusion

Trichomes

Tiny, spikelike projections on some leaves for protection

Stomatal crypts

Sunken Stomata

Guard Cells

The two cells that flank the stomatal pore and regulate the opening and closing of the pore. The concentration of solutes in the vacuoles of these cells increases: starch is converted to malic acid, proton pumps expel H+, creating a gradient across the plasma membrane: this leads to K+ and Cl- to move into the cell → osmosis brings water in → turgor

Diffusion

Molecules in fluids move from areas of higher to areas of lower concentrations.

Osmosis

Diffusion of water across a differentially permeable membrane. In cells, it creates hydrostatic pressure

(turgor)

Capillary Forces

Cohesion and adhesion

Cohesion

Water molecules stick to each

Adhesion

Water molecules stick to hydrophilic molecules (e.g. cellulose)

Negative Hydrostatic Pressure

Pressure created from large tension between cohesion and adhesion

Loss of Water Vapor

This loss reduces the water potential of intercellular spaces. The increase in ΔΨ leads to evaporation of water from

cell walls which leads to drier cell walls have increased tension, bringing water from surrounding areas (including inside and outside cells)

Vascular Tissue (The Stem) - Cohesion

Transfers the tension as water water moves from xylem to mesophyll in the leaves, hydrogen bonds pull in the rest of the water column

Vascular Tissue (The Stem) - Adhesion

Also transfer the tension to the walls of the xylem - but walls are too rigid to move. The amount of tension that can be induced is inversely proportional to the cross-sectional area

Tracheids

Can support greater tension than vessel elements

Water Loss from the Xylem

In the leaves, this reduces the water potential of xylem in the roots. The xylem is directly connected to the apoplast of

the stele (central region inside the endodermis)

Water Loss from the Stele

The lose in this area is replaced with water from the cortex, which in turn is replaced with water from the soil.

Casparian Strip

A water-impermeable ring of wax in the endodermal cells of plants that blocks the passive flow of water and solutes into the stele by way of cell walls.

Permanent Wilting

This happens if soil is too dry, the hydrostatic pressure is too

large for plants to be able to obtain water

Water in Soil

The hydrostatic tension in soil around the root increases, drawing in water from soil particles further away.

Soil Water Potential

The plant cannot have a xylem pressure that is above the water potential of its active rooting zone.

Gravitational Force

Generally the smallest contributor to the pressure

gradient generated within the xylem

Stomata

Closes partially as conditions get drier. In extreme soil drought, stomata are likely to close completely.

Xylem Vulnerability Curves (VCs)

Quantify an organ's resistance to xylem cavitation.

Cavitation

Formation of a cavity or hole