Chapter 2 Water Metabolism Plant Water Relations For
Chapter 2 Water Metabolism
Plant Water Relations Ø For every g of organic matter made by the plant, approximately 500 g of water is transpired by the plant. Ø Leaves loose up to ~ 100% of their water/hr.
Amounts of water in plants ØLiving plants =~ 80 to 95% water. ØVegetables =~ 85 to 95% water Ø Sapwood =~ 35 to 75% water Ø Seeds =~ 5 to 15% water
Roles of Water in Plant cells Ø Medium for biochemical reactions. Ø Physical support. • Turgor pressure. Ø Thermal regulation. Ø Transport of water, nutrients and other molecules.
Status of water in plants Ø bound water Ø free water ü Cytoplasm Protein = 60% • Hydrophilic • Hydrophobic
How Does Water Move? – Passive Processes Ø Diffusion - down a concentration gradient. Ø Bulk Flow - down a pressure-driven gradient. Ø Osmosis - down both a concentration & pressure driven gradient (= water potential). • Across a selectively semi-permeable membrane.
Diffusion Ø Diffusion - directed movement - high to low concentration (higher to lower free energy). Ø Random thermal motion. Ø Significant role in: • short-distance movement
Bulk Flow = Mass Flow Ø The concerted movement of groups of molecules by mass, most often in response to a pressure gradient. Ø Examples: • Water moving through the xylem. • A river flowing, rain falling. Ø Controled by : • Aquaporin (水孔蛋白)
A A. Diffusion B B.Bulk flow
Osmosis Ø Movement of water across a selectively semi-permeable membrane in response to chemical and pressure gradients (together = water potential). • Selectively semi-permeable membrane prevents exchange of solutes, but allows passage of water.
Water Potential t p e c on C Ø Water potential = the free energy associated with water = potential for doing work. Ø Water moves down a water potential gradient (from higher to lower potentials) = gives up energy as it moves.
Water Potential Ø Water potential is not an absolute value. It is relative to a reference state. • Reference state = pure water at ambient temperature and pressure. Ø The water potential within a cell is usually less than the reference state.
Plant cell—osmotic system Ø The Plant Cell • Cell wall –permeable • Membrane and protoplasm – semi-permeable.
If a cell is placed in a: Ø Hypotonic Solution (lower solute concentration) – water rushes in and generates turgor pressure. Ø Hypertonic Solution (higher solute concentration) - water flows out of the cell. Ø Isotonic Solution (equal solute concentration) - equilibrium.
Concepts Ø Plasmolysis (质壁分离)- protoplast shrinks away from the cell wall. • Turgor pressure = 0
Water Potential of the plant cell
Osmotic Potential Ψs = - RTCs Ψs = osmotic potential. ØR = the gas constant (0. 00832). ØT = temperature. ØCs = solute concentration (mol L-1). dissolved solutes Ψs
Pressure Potential Ø Cell expands with water and pushes the cell membrane against the rigid cell wall => pressure. • Turgor pressure = positive hydrostatic pressure inside cells pressing against the cell wall. Ø Water continues to enter the cell until pressure counteracts the negative osmotic pressure.
Water Movements in Cells Ø Water moves in and out of cells because of differences in water potential between the cell and the surrounding solution. Ø Water moves from cell to cell in the same manner.
Water movement from cell to cell
Root Water Absorption Root Hairs Ø Thin-walled outgrowths of epidermal cells. • Provide a large root surface area and close contact between root & soil. Ø Greatly increase contact of root with soil H 2 O. • Small diameter permits them to penetrate capillary spaces.
Vertical section of root tip
Transversal section
Root Water Absorption Ø Pathways § Apoplast § Transmembrane § Symplast Ø Driving force § Root pressure § Transpirational pull Ø Soil factors affecting root water absorption § § Available water Aeration conditions Temperature Solute concentration
3 Pathways: Water Movement Inside the Root Ø Via the Apoplast – e. g. through cell walls and intercellular spaces. Ø Via the Symplast – e. g. passing from cell to cell via the plasmodesma. Ø Via Transmembrane Pathway – e. g. through membranes (cell membrane and vacuole membrane).
Apoplast Symplast Apoplast and Symplast pathway
Transmembrane Pathway
Driving force for root water absorption Root Pressure Active absorption Ø Roots actively absorb and transport ions into the xylem. Ø As solutes build up, water potential decreases and water flows into the cells => increases pressure = Root pressure.
Bleeding because of root pressure
Driving force for root water absorption Water potential gradient Transpirational pull : Passive absorption Ø Transpiration: Evaporated Water moves from the leaf to the outside. • Primarily through the stomata. Ø Water potential gradient from leaf to root
Effects of Soil on Root Water Absorption: Available water Ø Sand: low • Large Diameter = 20 -2000 µm. • Low Surface Area = <1 -10 m 2. • Large spaces between particles =water drains. • Lower Field Capacity. Ø Clay: high • Small Diameter =<2 µm. • Large Surface Area =100 -1000 m 2. • Small spaces between particles =holds water. • Greater Field Capacity.
Effects of Soil on Root Water Absorption: Aeration conditions Ø CO 2 • anaerobic respiration • alcoholism • water absorption inhibited
Effects of Soil on Root Water Absorption: Temperature Ø Low T Ø High T • Viscidity • Aging • Respiration • Enzyme • Growth • Enzyme
Effects of Soil on Root Water Absorption: Solute concentration Ø Low solute • Water potential Ø High solute • Water potential water absorption inhibited
Transpiration
Absorbed water by plant Metabolism 1%— 5% Loss 95%— 99% Through: 1) Liquid state (e. g. bleeding, guttation) 2) Evaporated state (transpiration) Major way
Transpiration (蒸腾作用) Ø The loss of evaporated water from the plant to the outside air via diffusion. • Most is lost through the stomata (>90%). • But also through the cuticle (表皮), lenticels (皮孔), etc (5 -10%).
Significance of transpiration Ø Responsible for the movement of water and nutrients through the plant. Ø Cooling of the leaves. Ø Improving gas exchange.
Dicot Monocot
Guard Cells Ø Take up water (cells swell) = opens pore. Ø Lose water – closes pore.
Mechanisms for stomatal movement Ø Starch–sugar interconversion Ø Potassium ion uptake Ø Malate production
Starch–sugar interconversion Ø Day time: photosynthesis (CO 2 used) p. H Starch phosphorylase: starch sugar (soluble) Water potential Take up water opens pore
Starch–sugar interconversion Ø Night time: respiration (CO 2 produced) p. H Starch phosphorylase: starch sugar (soluble) Water potential lose water closes pore
Potassium ion uptake Ø ATP proton pump: H+ out, K+ in water potential pores open H+ K+, Cl-
Malate production Photosynthesis CO 2 used, p. H PEP Carboxylase PEP + HCO 3 - → OAA→ Malate Water potential Take up water Pores open
Factors Affecting Stomatal Movement Light light——open dark——close Temperature —— opening • To a point… (~35 C) CO 2 Low ——open High ——close ABA close 返回
Rate of Transpiration Depends On Ø Driving force: Water vapor pressure gradient from inside to outside the leaf. Ø Diffusional resistance: • High resistance through the cuticle and boundary layer. • Variable resistance through the stomata.
Factors Affecting Transpiration Ø Factors affecting stomatal movement: Light, temperature, CO 2, ABA Ø relative humidity/wind/temperature. Ø Leaf surfaces, stomatal frequency, etc.
Plant Water Transport
The water transport pathway from soil to outside of the leaves.
Plant Water Transport Ø Dead cell: xylem § Low resistance § Fast § Long distance § Major pathway Ø Alive cell: § High resistance § Slow § Short distance
Evolution of the Plant Kingdom v Algae v Bacteria v Lichen v Moss v Fern v Gymnosperm v Angiosperm Vascular plant
Xylem 。 Vessel Tracheids
Water Transport Through Xylem q For long distance transport in plants, >99. 5% of the water travels via the xylem. Ø Relatively simple cells – dead at maturity Ø No membranes or other cellular constituents to slow water transport = low resistance.
Long Distance Water Transport Ø Approximately 3 MPa of pressure is needed to transport water to the top of a 100 m tree. Ø How do plants generate this large of a water potential gradient?
Driving force for root water absorption Root pressure : active absorption • Root pressure may only generate ~ 0. 1 MPa. • Capillary rise = ~ 0. 6 m for xylem. Transpirational pull : Passive absorption • The major driving force
Long Distance Water Transport q Transpiration-cohesion-tension Theory. Ø Evaporation from the leaves generates the large negative water potential (Transpirational pull). Ø This pulls the water up the xylem. Ø Water’s adhesive and cohesive properties support this tension.
Transpiration-cohesion-tension Theory pull Ø H.H.Dixon t p e c con Ø Cohesive force:adhesion Cohesive force between similar moleculara,> 20 MPa。 Ø Tension force: gravity. 0. 5~ 3 MPa in xylem。 Ø Cohesive force >> Tension force Continuous water column G
The Problem Ø How can plants: • Replenish lost water • Regulate water loss. • But still allow gas exchange for metabolic processes? Ø Agriculturist’s Challenge: • Achieve maximum crop yield. • Use minimum irrigation.
Crop-water relations depends on Ø Different crops: § C 4<C 3; Wheat<Rice Ø Different developing stages: § Critical period of water
Guideline for suitable irrigation Ø Soil Environment Ø Morphology § Wilting; § leaf color turned dark green or red, etc. Ø Physiology § Osmotic potential, water potential; § Stomatal opening
Irrigation Methods Ø Wild flooding irrigation Ø Spray irrigation Ø Drip irrigation
Quiz v Explanation (select one): § Osmotic potential § Transpiration-cohesion-tension theory v Factors affecting transpiration include: _______, __________, etc. v. What are the driving forces for root water absorption?
Home work v Explanation: § Diffusion, bulk flow, osmosis § Starch–sugar interconversion § Apoplast, symplast § Water potential v Mechanisms for stomatal movement include: _____, ____. v How does water move? What is the difference among the water move processes?
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