Plant Biology Fall 2006Biology 751- Advanced Plant

PhysiologyProf. TamimiSpring 2010

• Reading material (Taiz & Zeiger):• Chapter 3, Water and Plant Cells• Chapter 4, Water Balance of Plants

Reading material (Taiz & Zeiger): 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. ReactantIII. 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):-

Loss of this pressuredue to insufficient water supply can be noticed

as plant wilting.

Well-watered plants maintain their shape

due to this internal pressure in plant cells

Plant Water relations

Water movement

There are three major ways to move water molecules:

Bulk flow(mass flow)

Diffusion Osmosis

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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• Affected by:

• Radius of pipe (r)

• Viscosity of liquid ()

• Pressure gradient p/x

• 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.

Movement of water in plants

• 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

Diffusion

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Movement of water into a plant cell occurs by osmosis

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

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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

Movement of water into a plant cell occurs by osmosis

w 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

BISC 367Measuring w

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 (p) present in the xylem (but is opposite in sign) before excision

Values obtained approximate the tension in the xylem and are used as a measure of w

• Strictly speaking to know the actual w some xylem sap should be collected to measure s

Scholander’s pressure bomb

From Plant Physiology on-line (http://4e.plantphys.net/)

Cell wall water

BISC 367Measuring w

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 w 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). Leaf

disks are dried to obtain their dry weight (DW).

RWC (%) = [(W – DW) / (TW – DW)] X 100

Relative water content