Transcript The Importance of Water

Biology 751- Advanced Plant
Physiology
Plant Biology
Fall 2006
Prof. Tamimi
Spring 2010
• Reading
Reading material (Taiz & Zeiger):
Chapter 3, Water and Plant Cells
material
(Taiz & Zeiger):
Chapter 4, Water Balance of Plants
• Chapter 3, Water and Plant Cells
• Chapter 4, Water Balance of Plants
2- Plant-water relationship
Water is essential for plants because of its important functions in plants:These include
I. Water is a Component of plant cells
II. Reactant
III. Solvent
in addition
1. Water transports minerals and nutrients that are absorbed by a plant’s root
system.
2. Evaporation between intercellular spaces provides a cooling mechanism that
allows plants to maintain favorable temperatures necessary for metabolic
processes.
3. water also provides physical support for plants. (Tugor Pressure):-
Well-watered plants maintain
their shape
due to this internal
pressure in plant cells
Loss of this pressure
due to insufficient water
supply can be noticed
as plant wilting.
Plant Water
relations
Water movement
There are three major ways to move
water molecules:
Bulk flow
(mass flow )
Diffusion
Osmosis
Movement of water in plants
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• Bulk flow:
– Concerted movement of groups of molecules en masse, most
often in response to a pressure gradient
– water flowing in a pipe
•
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•
•
Affected by:
Radius of pipe (r)
Viscosity of liquid (h)
Pressure gradient dyp/dx
• Dependant on the radius of the system water is traveling
in.
– Double radius – flow rate increases 16 times!!!!!!!!!!
• This is the main method for water movement in Xylem,
Phloem and in the soil.
Diffusion
• Molecular diffusion
– Water moves from an area of high free energy to an area of
low free energy
• Diffusion works down a chemical potential gradient.
Leads to the gradual mixing of molecules & eventual
dissipation of conc. Differences.
• It is rapid over short distances, but extremely slow over
long distances
Movement of water into a plant cell
occurs by osmosis
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Diffusion across the
membrane
aquaporins (water filled
pores
The water permeability of
plasma membrane from
Arabidopsis suspension cells
or root cells was reduced in
the presence of free Ca2+
and/or low pH (Gerbeau et
al., 2002
Movement of water into a plant cell
occurs by osmosis
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• Water uptake is driven by a free energy gradient
– Free energy gradient for water movement is referred to as a
Water Potential Gradient
Yw and water status of plants
• Water potential has two main uses
– 1: Governs water transport across
membranes.
– 2: uses as a measure of the water status
of plant.
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The Importance of Water Potential
• Physiological aspects
Rate of Osmosis
Osmosis is controlled
Measuring Yw
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Scholander’s pressure bomb
A leaf or shoot is excised and placed
in the chamber
• Cutting the leaf breaks the tension in
the xylem causing water to retreat into
the surrounding cells
Pressurizing the leaf chamber
returns water to the cut surface of
the petiole
• The amount of pressure to return water
to the cut surface equals the tension (Yp)
present in the xylem (but is opposite in
sign) before excision
From Plant Physiology on-line (http://4e.plantphys.net/)
Values obtained approximate the
tension in the xylem and are used as
a measure of Yw
• Strictly speaking to know the actual Yw
some xylem sap should be collected to
Cell wall water
Measuring Yw
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Relative water content
Assesses the water content of plant tissues as a fraction of the fully turgid water
content
• relevant when considering metabolic / physiological aspects of water deficit stress
Considered to be a better indicator of water status and physiological activity
Captures effects of osmotic adjustment
• Osmotic adjustment lowers the Yw at which a given RWC is reached
Simple technique:
• Leaf disks are excised, weighed (W) then allowed to reach full turgidity and re-weighed (TW).
disks are dried to obtain their dry weight (DW) .
RWC (%) = [(W – DW) / (TW – DW)] X 100
Leaf