Water Balance of Plants PDF

WATER BALANCE OF PLANTS CHAPTER 4 Water balance of plants Why do plants need & loose so much water? • Earth’s atmosphere presents problems to plants – The atmosphere is a source of CO2 • Required for photosynthesis – Atmosphere is relatively dry • Can dehydrate the plant • Plants have evolved w...

WATER BALANCE OF PLANTS CHAPTER 4 Water balance of plants Why do plants need & loose so much water? • Earth’s atmosphere presents problems to plants – The atmosphere is a source of CO2 • Required for photosynthesis – Atmosphere is relatively dry • Can dehydrate the plant • Plants have evolved ways to control water loss from leaves and to replace water loss to atmosphere • Involves – A gradient in water vapor concentration (leaves) – Pressure gradients in xylem and soil Water in the soil The water content and the rate of water movement in soils depend to a large extent on soil type and soil structure. Sand •Desert •Too much soil porosity Silt •Under water bodies (canals) Clay •Traditional houses •Poor soil porosity Very porous soil Too large Less porous soil Ideal Too small An ideal soil has a mix of particle sizes Water flow in the soil • The rate of water flow in soil depends on: – The size of pressure gradient through the soil – Soil hydraulic conductivity: how easily the water moves through the soil • Sandy > silt > clay soil Capacity of a soil to hold water (saturation). Excess water: drains away Plants can not regain turgor pressure; even after the transpiration stops Ψw < Ψs Cell water potential - Ψw • The equation Ψw = Ψs + Ψp + Ψg Affected by three factors: • Ψs : Solute potential or osmotic potential – The effect of dissolved solutes on water and the cell • Ψp : Hydrostatic pressure of the solution. Pressure is known as Turgor pressure – This is important in moving water long distances in plants • Ψg : Gravity - causes water to move downwards unless opposed by an equal and opposite force Why is Ψp negative in most soils? Soil dried out or water absorbed by plants • Air space expand • Because of adhesive force, water tends to cling to the surface of soil particles • Root hairs make intimate contact with soil particles: amplify the surface area for water absorption by the plant. Water in the Soil Plants absorb water from the soil, and they deplete water near the surface of the roots The depletion reduces Ψp in the water near the root surface and establishes a pressure gradient with respect to neighboring region of soil that have higher Ψp Ψp decrease Water moves through soil by bulk 5low Bulk flow: mass movement, responding to pressure gradient • Dependent on the radius of the tube that water is traveling in. Double radius – flow rate increases 16x!!!! • Main method for water movement in Xylem, Cell Walls and in the soil. • Independent of solute concentration gradients – to a point* – VERY different from diffusion • The rate of water flow depends on: – Size of the pressure gradient – Soil hydraulic conductivity (SHC): measures the ease in which water moves through soil • SHC varies with water content and type of soil – Sandy soil: high SHC; Clay soil: low SHC Water absorption by roots • Moves from soil, through plant, and to atmosphere by a variety of mediums – Cell wall – Cytoplasm – Plasma membranes – Air spaces Three pathways that water moves through the roots 1. Apoplast 2. Symplast 3. Transmembrane pathway How water moves depends on what it is passing through Water absorption by roots 1. Apoplast: • Water moves through cell walls without crossing any membranes – Continuous system of cell walls and intercellular air spaces 2. Symplast: • Water travels from one cell to the next via plasmodesmata – Network of cell cytoplasm interconnected by plasmodesmata 3. Transmembrane pathway: • Water sequentially enters a cell on one side, exits the cell on the other side, enters the next cell, and so on. Water absorption by roots At the endodermis: • Water movement through the apoplast is stopped by the Casparian Strip • The Casparian strip breaks continuity of the apoplast, forcing water and solutes to cross the endodermis through the membrane Band of radial cell walls containing suberin (a wax-like water-resistant material) or lignin – All water movement across the endodermis occurs through the symplast “Plants are taking up microplastics” Casparian strip is not filtering these plastics… https://phys.org/news/2020-07-cropmicroplastics.html?fbclid=IwAR3SVYvtfohoJa7PxFWIASm Q-Pg4Sqjma9efn0J6B-Ifvzov_foX3RZ1q-k Nature Sustainability 2020 Suberin exists mainly in roots and contains very long chain (C20-C26) fatty acids and fatty alcohols and high phenol* contents Plant phenolics: generally involved in defense against ultraviolet radiation or aggression by pathogens, parasites and predators, as well as contributing to plants' colors. Water across plant membranes • Diffusion of water directly across the bilipid membrane. Aquaporins: Integral membrane proteins that form water selective channels à faster water diffusion • Alters the rate of water Slow but NOT direction • Permeability: regulated in response to intracellular pH – Decreased rate of respiration: increases in intracellular pH – High cytoplasmic pH: roots are less permeable to water Root pressure A positive hydrostatic pressure (Ψp) builds in the xylem of roots. For example: • The cut stump will exude sap from the cut xylem • Guttation, liquid droplets on the edges of their leaves Most of the time transpiration rates are high. Water is taken up so rapidly into the leaves and lost to the atmosphere that a positive pressure never develops in the xylem • When would it? Root pressure At night: water enters the roots through osmosis. Since stomata are closed and evaporation is low, water pressure builds up à plants to re-hydrate at night It also occurs when soil water potentials are high and transpiration rates are low Root absorb ions from the dilute soil solution Transport ions into the xylem Build up the solute in the xylem sap Decrease the xylem osmotic potential (ψS) ψS = -RTCS CS increase => ψS decrease Decrease the xylem water potential (ψW) The lowering of xylem ψW provides a driving force for water absorption. This leads to a positive hydrostatic pressure in the xylem. Guttation is important to prevent flooding of the air spaces in the leaf due to root pressure. Typical values for Ψw • Ψw = -0.2 to -0.6 MPa – Plants are never fully hydrated due to transpiration • Ψs = -0.5 to -1.5 MPa – Plants living in saline or arid environments can have lower values • Ψp = 0.1 to 1.0 MPa – Positive values needed to drive growth and provide mechanical rigidity: cellular level** Water transport Xylem • Xylem is a simple pathway of low resistance • Dead cells: no organelles or membranes – Water can move with very little resistance • Two types of tracheary elements: tracheids & vessel elements (only in angiosperms, some gymnosperms and some ferns) • For trees with wide vessels (r = 100 ~ 200 m), velocity = 16 – 45 m/h (=4 – 13 mm/s) • Trees with smaller vessels (r = 25 ~ 75 m), velocity = 1 – 6 m/h (=0.3 – 1.7 mm/s) High pressure is needed in tall trees! How can we explain the movement of water from the roots of a tree up to 100 meters above ground? • A pressure difference of roughly 2-3 MPa from the base to the top branches, is needed to carry water to the tallest trees of 100 m. • How is the water lifted to a treetop? – Relies on the fact that water is a polar molecule – Water is constantly lost by transpiration in the leaf. When one water molecule is lost another is pulled along • Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. Cavitation (or embolism) As the tension in water increase à tendency for air to be pulled through pores in the xylem cell walls and finally formed an air bubble. Also due to freeze-thaw of xylem water. Impact minimized by several means: • Gas bubbles can’t easily pass through the small pores of the pit membranes • Xylem are interconnected so one gas bubble does not completely stop water flow: water detours by moving through neighboring, connected vessels • Eliminated Elimination: • Create positive root pressure – Gases can dissolve back into the solution in the xylem or be pushed out. • Secondary growth: new xylem forms each year. – As a back up to finding a way around gas bubbles. Water evaporation in the leaf affects the xylem Transpiration pull: causes water to move up the xylem. In the leaves: • Water adheres to cellulose and other hydrophilic wall components • Mesophyll cells are in direct contact with atmosphere à air spaces • Negative pressure exists à cause surface tension on the water • As more water is lost to the atmosphere à the remaining water is drawn into the cell wall • As more water is removed from the wall, pressure of the water becomes more negative This induces a motive force to pull water up the xylem h"ps://www.youtube.com/watch?v=d60lqIfGeQw Water movement from leaf to atmosphere • After water has evaporated from the cell surface of the intercellular air space diffusion takes over The path of water: Xylem à Cell wall of mesophyll cells à Evaporated into air spaces of leaf à Diffusion occurs & water vapor then leaves via stomatal pore Water goes down a concentration gradient Water vapor diffuses quickly in air • Diffusion à primary means of any further movement of the water out of the leaf. Water movement is controlled by the concentration gradient of water vapor • Diffusion of water out of the leaf is very fast – Diffusion is much more rapid in a gas than in a liquid Transpiration from the leaf depends on two factors: 1. Difference in water vapor concentration between leaf air spaces and the atmosphere • High surface area to volume ratio; allows rapid vapor equilibrium inside the leaf 2. The diffusional resistance of the pathway from leaf to atmosphere • Leaf stomatal resistance & boundary layer resistance A thin film of still air on the surface of leaf and its resistance to water vapor diffusion is proportional to its thickness. The thickness is determined primarily by wind speed Boundary layer resistance • Thickness of the layer is determined by wind speed • Still air – layer may be so thick that water is effectively stopped from leaving the leaf • Windy conditions – moving air reduces the thickness of the boundary layer at the leaf surface. Stomatal resistance has the largest amount of control over water loss. • Size and shape of leaves inHluence air Hlow, but stomata play the most critical role • When air surrounding the leaf is still à increases in stomatal aperture have little effect on transpiration rate • Thickness of the boundary layer is the primary deterrent to water vapor loss from the leaf. Transpiration E = Cwv (leaf) – Cwv (air) rs + rb • E : transpiration rate (mol/m2/s) • rs : resistance at the stomatal pore (s/m) • rb : resistance due to the boundary layer boundary layer : the layer of unstirred air at the leaf surface • However, it is difficult to measure the Cwv (leaf). Sometimes vapor pressure are used instead of concentrations, and the difference is called “water vapor pressure deficit”. • Water vapor pressure deficit = Pwv (leaf) - Pwv (air) RELATIVE HUMIDITY RH = Cwv / Cwv (sat.) Cwv (sat.) is strongly dependent on temp Stomatal control • The concentration gradient for CO2 uptake is smaller than the concentration gradient driving water loss • How can plant prevent water loss without simultaneously excluding CO2 uptake? • Almost all leaf transpiration results from diffusion of water vapor through the stomatal pore – Remember the cuticle? • Provide a low resistance pathway for diffusion of gases across the epidermis and cuticle • Regulates water loss in plants and the rate of CO2 uptake – Needed for sustained CO2 fixation during photosynthesis Ion and organic molecules Guard cell Ψs decrease (Ψs = -RTCs) Ψw decrease Water moves into the guard cell Ψp increase Stoma open Relationship between water loss and CO2 gain • Effectiveness of controlling water loss and allowing CO2 uptake for photosynthesis à transpiration ratio • There is a large ratio of water efflux and CO2 influx – Concentration ratio driving water loss is 50 larger than that driving CO2 influx – CO2 longer diffusion path: membrane à cytoplasm à chloroplast membrane à chloroplast All add resistance!! Summary • Land plants faced with dehydration by water loss to the atmosphere • There is a conflict between the need for water conservation and the need for CO2 assimilation – This determines much of the structure of land plants: 1. Extensive root system – to get water from soil 2. Low resistance pathway to get water to leaves – xylem 3. Leaf cuticle – reduces evaporation 4. Stomata – controls water loss and CO2 uptake 5. Guard cells – control stomata Leaves that “eat” insects Figure 11.8 (1) • Some plants obtain N from digesting animals (mostly insects). • Pitcher plants: “passive trap”. Insects fall in and can’t get out • Pitcher plants have specialized vascular network to tame the amino acids from the digested insects to the rest of the plant Nepenthes sp. (Borneo, Sabah). Pitfall trap with slippery surfaces and fluid pool at bottom containing digestive enzymes Sarracenia sp. (US, Green Swamp, NC). Leaves that “eat” insects Figure 11.12 (2) • The Venus fly trap (Dionaea muscipula) has an “active trap” • Good control over turgor pressure in each plant cell. • When the trap is sprung, ion channels open and water moves rapidly out of the cells. • Turgor drops and the leaves slam shut • Digestive enzymes take over EJERCICIO # 1 • Calcula el potencial hídrico en un árbol de 30 m. Es mediodía y el día está caliente. La planta ha perdido mucha agua y ha excedido la recarga por parte de las raíces. Completa los valores de ψ con valores realistas. – Cuáles pueden ser las respuestas fisiológicas de este árbol? EJERCICIO # 1 • Respuesta EJERCICIO # 2 • En un laboratorio con temperatura de 20 °C, un investigador sumerge una célula flácida con una concentración interna de soluto de 0.3M, en un vaso con una solución de azúcar de 0.1M. – Calcula el potencial hídrico (ψw =ψs+ψp+ψg) de la célula – Calcula el potencial hídrico de la solución en el vaso – Concluye que va a pasar con la célula. EJERCICIO # 2 • Respuesta: Célula: Ψp= 0 Ψs= - 0.0083 x 293 x 0.3M = -0.73MPa Ψw= -0.73 + 0 = -0.73MPa Vaso Ψp= 0 Ψs= -0.0083 x 293 x 0.1M=-0.24Mpa Ψw= -0.24MPa + 0 = -0.24Mpa El agua entra a la célula