Water in Plants Progetto cofinanziato dal programma LIFE+ Stefania De Pascale Stefania De Pascale Department of Agricultural Engineering and Agronomy - University of Naples Federico II
Why do plants need water? Water is essential for: Chemical reactions in the plant Translocation of nutrients Cooling Turgor pressure Turgor pressure helps to keep the plant erect It is accomplished when the plasma membrane pushes against the cell wall
Water in plants 80-90% of a growing plant cell is water This varies between types of plant cells Carrot has 85-95% water Wood has 35-75% water Seeds have 5-15% water Plant continuously absorb and lose water Lost through the leaves Transpirationi i
Transpiration The evaporation of water into the atmosphere from the leaves and stems of plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO 2 and O 2 during photosynthesis. Transpiration is not simply a hazard of plant life. It is the "engine" that pulls water up from the roots to: supply photosynthesis (1%-2% of the total) bring minerals from the roots for biosynthesis within leaf cool the leaf
A single oak tree can transpire 400 liters of water in a day
Water from transpiration accounts for 50% of the Water from transpiration accounts for 50% of the rainfall in some tropical rain forests
Properties of Liquid Water that are Important to Plants Excellent solvent of other polar molecules High tensile strength due to cohesion (force to pull water apart) Capillary effects (attraction between water and hydrophilic surfaces) High heat capacity (this means that the temperature won t change very much when you add or remove energy) High heat of vaporization (this means that it takes a lot of energy to vaporize water, which h results in evaporative cooling) It is abundant!
Water is a polar molecule. It sticks to other water molecules through hydrogen bonding. This results in cohesion which gives water high tensile strength.
Due to its partially positive H and partial negative O atoms, water readily dl dissolves hydrophilic chemicals like sugar, salt (solutes) [Water repels uncharged hydrophobic molecules like lipids or wax, excluding them from solution.]
Cohesion (water/water) Adhesion (water/solid) Shell of hydration
Capillarity
Water Potential Concept Water potential is a measure of the free energy content of water. The potential of a particular sample of water is defined relative to energy status of pure free water (which by definition has zero potential)! Water potential is the energy that would be required to move water from where it is to the pure free state.
The status of water in plants is described by: Water potential, w Technically, theunits of the chemical potential ti of water are Joules/mole. But in plant physiology it is much more common to describe water potential in units of pressure (derived from the chemical potential divided by the volume of a mole of water)
Standard Unit for w is the MegaPascal (a unit of pressure): MPa 1 atmosphere = 1.013 bar = 0.1013 MPa = 1.013 x10 5 Pa
Water Potential Components The components of water potential ( w) are: matric gravity osmotic pressure
Chemical potential of water Gravity (0.01 MPa m -1 ) -- usually f d t b zero at the soil surface and increases in the up direction w = s + p + m + g referenced to be Matric potential (very important in soils; often ignored in plants, although it is very important in cell walls) Pressure potential solutes (osmotic potential)
Water Potentials The matric potential is always negative and is caused by attraction ti of water molecules l tosolid surfaces. It is most important within soils, but also in xylem tubes, where water is held on cell walls. The osmotic (solute) potential is also negative and is caused by the dissolution of solutes in water. Solute potential is most important in soil solution and insideid livingi plant cells, where water almost always has solutes dissolved in it. Thepressurepotentialcanbeeitherpositiveornegative and is dependent on the pressure acting on (surrounding) the water molecules. This is usually positive inside plant cells (turgor pressure) and negative in xylem (tension).
Cell water potential - w The equation: w = s + p + g Affected by three factors: s : Solute potential or osmotic potential The effect of dissolved solutes on water and the cell p: Hydrostatic pressure of the solution. A positive pressure is known as Turgor pressure Can be negative, as in the xylem and cell wall this is important in moving water long distances in plants G it t t d d g: Gravity -causeswater to movedownwards unless opposed by an equal and opposite force
Osmosissis and Tonicity it Osmosis is the diffusion of water across a plasma membrane. Osmosis occurs when there is an unequal concentration of water on either side of the selectively permeable plasma membrane. Remember, H 2 O CAN cross the plasma membrane. Tonicity is the osmolarity of a solution--the amount of solute in a solution. The cell has a specific amount of sugar and salt. Solute--dissolved substances like sugars and salts. Tonicity it is always in comparison to a cell.
Tonic Solutions A Hypertonic solution has more solute than the cell. A cell placed in this solution will give up water (osmosis) and shrink. A Hypotonic solution on has less solute than the cell. AA cell placed in this solution will take up water (osmosis) and blow up. An Isotonic solution has just the right amount of solute for the cell. A cell placed in this solution will stay the A cell placed in this solution will stay the same.
Plant cell in hypotonic solution Flaccid cell in 0.1M sucrose solution. Water moves from sucrose solution to cell swells up becomes turgid This is a Hypotonic solution - has less solute than the cell. So higherh water concentration Pressure increases on the cell wall as cell expands to equilibrium ium
Plant cell in hypertonic solution Turgid cell in 0.3M sucrose solution Water moves from cell to sucrose solution A Hypertonic solution has more solute than the cell. So lower water concentration Turgor pressure reduced and protoplast pulls away from the cell wall
Water relations in plant cells
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. Because of water loss to the atmosphere plants are seldom fully hydrated. They suffer from water deficits Leads to inhibition of Plant growth most likely to be affected Photosynthesis
Water availability affects photosynthesis and growth in plants Relationships between leaf water potential, leaf elongation and net photosynthesis in corn
w and water status of plants Cell diii division slows down Reduction of synthesis of: Cell wall Proteins Closure of stomata Due to accumulation of the plant hormone Abscisic acid This hs hormone induces closure of stomata during water stress Naturally more of this hormone in desert plants
Summary w always a negative number (pure water at standard temperature is a reference, with zero water potential s (solute potential) - zero for pure water, negative number when there are solutes ( s = -RTcs) p (pressure potential) - positive in healthy, living cells negative in xylem g (gravitational potential) - zero at ground level increases with height 0.01 MPa per meter
Some Important points Measurements of w give a measure of water status leaves of well-watered plants: w =-0.2/-0.6MPa leaves of plants in arid climates w = -2.0/-5.0 MPa Plants can change the s of the cell there is an upper limit ~-0.5 MPa; typically s =-0.8/- 1.2 MPa can increase osmotic concentration (make s more negative) in order to lower w to allow the plant to extract water in response to drought stress or in halophytes plants often synthesize higher levels of proline as a stable osmoticum Values for p are rarely fully equal (but opposite) to values for s therefore ww is always aways negative (or zero) for living cells & tissues
How is water transported t into and up through a plant? There are three ways that water (and other materials) moves in plants
Water transport processes Moves from soil, through plant, and to atmosphere by a variety of mediums Cell wall Cytoplasm Plasma membranes Air spaces How water moves dependsd onwhat it is passing through
1. Diffusion Dff Diffusion is driven by a concentration gradient (usually( we think of this as a difference in concentration of the solute, not water molecules, which make up the solvent, although you can consider it from either perspective) technically, s, where s = solute potential = -RTc s ; R is the gas constant, T is Kelvin temperature and c s is solute concentration)
The law of diffusion From high water content to low Fick s s Law of Diffusion Js = -Ds ( Cs/ x) Js=flux [mol/(s*m m 2 )] Ds = diffusion coefficient (how easily substance moves) [cm 2 /s] /] Cs = concentration gradient (C int -C ext ) [e.g. mol/cm 3 ] x = distance between two points (m) C internal C external
Diffusion Diffusion works down a concentration gradient. Leads to the gradual mixing of molecules & eventual dissipation of concentration differences. It is rapid over short distances, but extremely slow over long distances It would take about 32 years for a sugar molecule l to diffuse throughh a stem 1 meter long!
Diffusion throughg the lipid bilayer (simple diffusion) throughg water channels called aquaporins (channel diffusion) with single transport protein carriers (facilitated diffusion)
Water across plant membranes There is some diffusion of water directly across the bilipid membrane. Auqaporins: Integral membrane proteins that form water selective channels allows water to diffuse faster Facilitates water movement in plants Alters the rate of water flow across the plant cell membrane NOT direction
Osmosissis and Diffusion The plasma membrane is selectively permeable. This means that only some molecules can cross. Small uncharged Diffusion is the movement of molecules l from an area of high concentration to an area of low concentration. molecules like O 2, CO 2 and H 2 O pass. 10 20 Large or charged molecules like proteins or ions cannot pass. 15 15
Osmosis Cell wall Cell membrane water water Vacuole Membrane (tonoplast) Osmosis is driven by a water potential difference across a membrane in other words, both pressure and concentration ti are important t
For osmosis we talk about the potential water molecules have to move the OSMOTIC POTENTIAL. Distilled water has the highest potential (zero). When water has another substance dissolved in it, the water molecules have less potential to move. The osmotic potential is NEGATIVE!
Osmosis Movement of a solvent across a biological membrane (selectively permeable) Movement is driven by the sum of a concentration gradient and pressure gradient Water can move across the lipid bilayer portion of a membrane or many membranes have waterspecific channel proteins that facilitate osmosis (called aquaporins) Osmotic pressure Flow rate = L x w = water potential gradient L = conductance = (1/resistance to flow)
Distilled water separated by a partiallypermeable membrane: Pure Water w = 0 Water molecules are moving from one side of the membrane to the other but there is no net osmosis
If a substance is dissolved in water, the kinetic energy of the water molecules is lowered. This is because some water molecules aggregate on the surfaces of the other molecules Add sucrose to 0.1 M (0.1 mol to 1 L): s =-02MPa 0.2 MPa, p =0 MPa w = - 0.2 MPa + 0 = -0.2 MPa
Water molecules always move from less negative to more negative water potential. Net osmosis = 0 MPa -0.2 MPa
Examples Here are some examples of cell-level water relations with no change in gravitational potential. On a whole plant level, the gravitational component can be important, especially in large trees!
EXAMPLE 1: lets suppose we drop a plant cell into an hypotonic solution Living Plant Cell Water can move by osmosis s across the cell wall and cell membrane but most solutes cannot Pure water + 0.1 M sucrose
Plant Cell: before equilibrating with water s = -0.7 MPa p = 0 What is the total w =? water potential of the plant cell? Hypotonic Solution s = -0.2 MPa p = 0 w = s + p = -0.2 MPa What will happen to the total water potential of the plant cell when it is dropped in the water solution?
Hypotonic Solution s =-0.2 MPa p = 0 w = s + p = 0.2 MPa s = -0.7 MPa p = +0.5 MPa w = -0.2 MPa This is what produces turgor, or positive pressure, in p g, p p, plant cells!
EXAMPLE 2: Putting the plant cell into an hypertonic solution Plant Cell Water can move by osmosis across the cell wall and cell membrane but most solutes cannot 0.3 M solution of sucrose s = -0.7 MPa
Plant Cell: before equilibrating with the hypertonic solution s = -0.7 MPa p = +0.5 MPa What is the total ti w = -0.2 MPa water potential of the hypertonic solution? Hypertonic solution s = -0.7 MPa p = 0 w = s + p =?? What will happen to the total water potential of the plant cell when it is dropped in the solution?
Hypertonic solution s = -0.7 s p = 0 w = s + p = -0.7 s = -0.7 MPa p = -0 MPa w = -0.7 When turgor falls to zero, the cell plasmolyzes! s p in a living cell cannot fall below zero!! If the solute potential of the solution is lower than the solute potential of the cell, the membrane ruptures and the cell contents spill
If the solution is hypotonic, net endosmosis occurs and dthe cell becomes fully turgid.
If the solution is hypertonic, net exosmosis occurs and causes plasmolysis (the cell membrane pulls away from the cell wall. The cell wall stays intact).
If the solution is isotonic, i no net osmosis occurs. The cell is not plasmolysed, l but it is not fully turgid either.
Important rules : mass flow is driven by p not s Differences in solute concentration affect w but, solute concentration affects mass flow of water only if there is a semipermeable membrane! Adding salts or sugars here will not affect mass flow -0.1 01MPa Mass flow -0.5 05MPa Unless there is a semipermeable membrane here
How do solutes move into plant cells? Water molecules are small enough to pass throughh the bilayer membrane. Sugar molecules are both polar and too large to easily pass through the membrane.
Transcellular Vascular
Lateral transport of minerals and water in roots
Compartments of plant cells and tissues and routes for lateral transport
Tapping phloem sap with the help of an aphid
A chemiosmotic model of solute transport in plant cells
Loading of sucrose into phloem
Pressure flow in a sieve tube
Pressure-driven bulk flow drives long-distance water transport Bulk flow: Concerted movement of groups of molecules en masse, most often in response to a pressure gradient. Dependant on the radius of the tube that water istraveling in. Double radius flow rate increases 16 times!!!!!!!!!! This is the main method for water movementm in Xylem, Cell Walls and in the soil. Independent of solute concentration gradients toapoint So different from diffusion
The mass flow (amount/time) of water movement can always be expressed as the product of a driving force, which is the potential difference, and conductance or conductivity of the material or space that the water is moving through. In a general sense: Flux = k*
Water movement through Plants
The soil-plant-atmosphere p continuum Plants return water to the atmosphere through transpiration. More than 90% of the water entering a plant passes into leaf air spaces and then evaporates throughh the stomata into the atmosphere. Usually less than 5% of water escapes through the cuticle.
In this process, water evaporates from pores in the plant s leaves, after being drawn, along with nutrients, from the root system through the plant. On a warm, dry, sunny day a leaf will exchange up to 100% of its water in an hour.
Summary of forces moving water from soil through plant + solute
Water Movements Water is absorbed by roots through root hairs (epidermis) through the cortex and endodermis, and then to the xylem, which conducts to leaves as a liquid. Water can pass from leaves to atmosphere as a gas in a flow called transpiration. This movement is considered to be almost entirely passive with water flow following a water potential ti gradient. Water potential is the difference in free energy of water in soils, cells or atmosphere and that of pure water. Water potential of pure water is used for comparison and is given a value of zero. When there are differences in water potential, water will ALWAYS move passively from higher to lower water potential.
Water potential and water movement: a mechanical model water moves from high to low (from high water potential to low water potential) p=pressure s=solute
Water moving between soil and plants flows down a water concentration gradient
Transpiration takes place so long as the following is true: soil > root cells > stem cells > leaf cells > atmosphere
Estimated water potentials in a soil-plant-air system soil = m + s while plant = m + s + p
Ascent of water in a tree
The generation of transpirational pull in a leaf
The generation of transpirational pull in a The generation of transpirational pull in a leaf
Root Pressure and Guttation Guttation occurs when the soil and atmosphere are saturated t with water. Water secretion occurs through modified Stoma called Hydathodes. Root Pressure provides the motive force for this process.
Root Pressure and Guttation ti Root pressure is not sufficient to move water very far up a stem. Transpirational pull!
An overview of transport in whole plants
Summary Water is important to plants Makes up the media in which all biochemical processes occur that are essential to plant life. Influences the structure and function of proteins, cell membranes, nucleic acids, & carbohydrates Water movement driven by free energy. Moves by Osmosis, bulk flow, diffusion or a combination HelpH l moves water from soil through h plant to atmosphere Water potential is a measure of water Water potential is a measure of water status of a plant