Lecture 12 - Water Relations

Tallest fossil tree?

120 m

Tallest living tree?

116 m Redwood

112 m Eucalyptus

Modes of water movement

1. bulk flow

2. osmosis

3. diffusion

Bulk flow

overall movement of water driven by potential energy

driven by pressure gradient

mechanism of long-distance travel via xylem

Diffusion

Net movement from more to less concentrated zone, towards equilibrium

Rapid over short distances, slow over long distances

Osmosis

Movement of water across selectively permeable membrane

Osmotic properties

Osmotic forces on cells

Turgor pressure

Water pressure in plant cells keeps plant tissue rigid

Plasmolysis

Separation of protoplast from cell wall because of water movement by osmosis

occurs with any high contrast solution

Plasmodesmata

Cytoplasmic strands that interconnect protoplast of adjoining plant cells

Cell-to-cell communication

Modes of solute transport across a membrane

1. simple diffusion

2. facilitated diffusion

3. active transport

Carrier protein types

Uniporter

Symporter

Antiporter

Uniporter

One solute moved directly across in one direction

Symporter

Two different solutes moved in same direction

Antiporter

Two different solutes simultaneously moved in different directions

Primary active transport

Directly energy requiring, against gradient

Secondary active transport

Driven indirectly by primary pumps, using H+ protons and sucrose

Transpiration

Loss of water vapor in plants

~99% of water taken up by plant is transpired, up to 100 gallons from a single tree per day

Water movement up a tree

1. Root pressure

2. Capillary pressure

3. Adhesion/cohesion

4. Transpiration

Capillary pressure

Water climbs up a small tube

Smaller tube, higher water climb

Cohesion

water/water attraction

Adhesion

water/other substance attraction

Transpiration stream

Flow of water through the xylem through the roots to the leaves where water vapor is evaporated through the stomata

Open stomata

due to increased turgor pressure

Closed stomata

Decreased turgor pressure

Factors that influence stomata

• CO2 concentration

• light, open in light, closed in dark

• blue/red light

• high temps close stomata

• circadian rhythm

Continuous water network

Continuous network of water in xylem that forms columns from root, through stem, out to stomata in leaves

Cohesion-Tension theory

Water is pulled to treetop via water tension and cohesion, as water molecules evaporate out of the stomata, tension in the cohesive water column network pulls water up from roots

Why can’t trees get any taller?

above 110m, the force of gravity is greater than that of cohesion

How water gets to the top of tallest tree

Combo of:

Cohesion-tension (mostly)

Capillary pressure - 1m

Root pressure - variable, small

Vacuum pump - 11m

Cavitation

Rupture of water column or embolism, breaking water connections due to vessels filling with air

Net flow in whole plant

• Transpiration moving water in xylem from roots to leaves

• Assimilate transport moving sugars in the phloem from sources to sinks

Source

organ such as mature leaf or starch storing root that produces more sugars than it requires

Sink

organ that consumes sugar

Examples of sinks

• root

• flower

• developing fruit

Source and sink development

Sources and sinks change during development

Assimilates

Sugars

Flow of water/assimilates

Assimilates are transported through the phloem from the source/production site, to the sink

Aided by interaction of xylem transpiration stream

Assimilate transport in phloem

1. Requires loading of sucrose into phloem at source

2. Bulk flow

3. Unloading at sink

Assimilate Transport - Bulk Flow

• high sugar at source

• sugar diluted with water from xylem, creating pressure

• sugar unloaded at sink, then metabolized to starch

• excess water flows to xylem back to source

What type of transport does assimilate movement use?

Active - loading/unloading of sucrose into phloem

Passive - bulk flow

Also, osmosis

Net flow - whole plant

Transpiration - water movement in xylem from roots to leaves

Assimilate transport - moving sugars in the phloem from sources to sinks