Water Relations Principles and Theory. John S. Boyer University of Delaware Lewes, DE 19958

Transcription

1 Water Relations Principles and Theory John S. Boyer University of Delaware Lewes, DE 19958

2 Water Use Is Large Compared to Dry Matter Gain Water Use per Gm Dry Weight Water Use by Crops in Akron, CO Alfalfa Oats Barley Wheat Maize Millet Sorghum C 3 Photosynthesis C 4 Photosynthesis Miller Plant Physiology. 2nd Edition

3 Water Yield from a Watershed with Natural Woody Vegetation in N. Carolina Process Deciduous Forest Clear Cut Precipitation Interception Runoff Soil storage Evaporation Transpiration Hoover Trans. Am. Geophys. Union 25,

4 Chemical Potential and Its Components Control Water Movement in Plants and Soils µ µ Ψ = Water Potential = = w V w w o RT V w ln a w µ w = chemical potential of water in cell (energy per mol) µ o = chemical potential of pure, free water (energy per mol) V w = partial molal volume of water (volume per mol) R = gas constant (energy mol -1 K -1 ) T = Kelvin (K) a w = water activity controlled by solute, pressure, matrix, gravity (no units) Ψ w = water potential (energy per volume = force per area = pressure) Gibbs Trans. Conn. Acad. Sci. III, Slatyer and Taylor Nature 187, Kramer and Boyer Water Relations of Plants and Soils. Pg. 35

5 Water Potential Is the Sum of All Forces Acting on the Water Ψ w =Ψ s +Ψ p +Ψ m +Ψg Solute (-) Pressure (+ or -) Matrix (-) Gravity (+ or -) 0 MPa Ψ w Ψ s(p) Ψ p(p) -1.0 MPa

6 There Are Two Compartments for Water: Protoplast and Apoplast Water diameter is about 0.3 nm Kramer and Boyer Water Relations of Plants and Soils. Pg. 46

7 Large Polysaccharides Are Trapped Inside While Water Moves Through Isolated Wall Turgor = 0.5 MPa Turgor = 5.0 MPa Dextran, 580 kd, 27 nm diameter Proseus and Boyer Ann. Bot. in press

8 Component Potentials In Cells Kramer/Boyer Water Relations of Plants and Soils. Pg. 52

9 Water Potentials in a Maize Plant during the Day and Night Tang and Boyer J. Exp. Bot. 53:

10 Mechanism of Water Movement Across a Membrane: Osmosis (a numbers game!) Solute is reflected by membrane Number of water molecules is greater at L than at R Therefore, net movement is L to R Ray Plant Physiol. 35, Mauro Science 149, Kramer/Boyer Water Relations of Plants and Soils. Pg

11 Carefully Remove Membrane Immediately after Later Mixing occurs, but no osmosis

12 Enlarging Pores of Membrane Has the Same Effect As Removing the Membrane No solute is reflected by membrane Mixing occurs but no osmosis

13 Measuring Reflection Coefficient (σ) for a Cell (Tradescantia virginiana epidermis) dψ σ = d Ψ p s Reflection of membrane can be measured! Tyerman and Steudle Aust. J. Plant Physiol. 9,

14 How Does Water Potential Change When Water Content Changes? dψ w = dψ s + dψ p dψ = Ψ s s dv V dv dψ p =ε V dψ w = Ψ dv dv s + ε = ( ε Ψs) dv V V V dψ = (10 + 1)( 0.1) = 1.1 w MPa Notice: Change in water potential is controlled mostly by elastic modulus!! Kramer/Boyer Water Relations of Plants and Soils. Pg. 61

15 Elasticity of Wall Affects Change in Leaf Water Potential Kramer/Boyer Water Relations of Plants and Soils. Pg. 63

16 Water Moves through Membrane According to 1) Potential Difference 2) Conductance (Lp) and Reflection (σ) Properties of Membrane J = Lp( Ψ Ψ + σ ( Ψ Ψ )) v m( a) p( p) s( a) s( p) For fully reflective membrane, J = Lp( Ψ Ψ ) = Lp( Ψ ) v w( a) w( p) w and flow across a membrane is controlled simply by the difference in water potential and the conductivity Lp is x 10-6 m s -1 MPa -1, depending on cell Kramer/Boyer Water Relations of Plants and Soils. Pg

17 Lp for Single Cell is Less Than for Tissue Westgate and Steudle Plant Physiology 78,

18 Summary Large amounts of water move through land plants Plant water exists in two compartments: apoplast and protoplast Water potential and its components move water from soil into the apoplast and protoplast Solute in cells is the ultimate source of the forces (potentials) that move water Osmosis depends on reflection of solute by membrane, causing less water to move from solution side than from water side of membrane Dehydration initially lowers the water potential mostly by decreasing turgor

19 Some Consequences of Osmosis and Water Relations Theory Small plants use less water than large plants. Water potentials of small plants tend to be higher than in large plants. Drought tolerance??? Drought tolerance defined as ability to leave progeny in water limited conditions Two types: Desiccation avoidance (deep roots, osmotic adjustment, water conservation) Desiccation tolerance (viability retained in seeds, spores, resurrection plants)

20 Some Consequences of Osmosis and Water Relations Theory Water potential is not a plant character like a gene. It is dynamic and reacts to environment as well as plant characters. Water Light Sugar CO 2 Modifying plant characters may modify water potential and components, but only detectable if environment is rigorously controlled Environment control may be achieved in field by comparing in common soil and atmosphere at same time, or in climate chambers