Transport of Water Across the Root Water is absorbed from the soil by osmosis Water moves down the  gradient Water only enters the root near the root tip Here there are root hairs which increase the surface area for osmosis Water potential is higher in the epidermal cells than in the central cells Water moves across the cortex down the  gradient to xylem vessels, Water can move via the symplast or apoplast routes

Transverse section of a root endodermis Casparian strip in the walls of the endodermal cells xylem vessels Subject to copyright clearance a suitable image showing a transverse section of a root could be inserted here. e.g. one similar to that found at: www.uri.edu/artsci/bio/plant_anatomy/31.html stele

Diagram of transverse root section epidermis with root hairs cortex endodermis pericycle xylem phloem

Water is transported across the root by two routes Apoplast route Symplast route between the cells via the cell walls cell cytoplasm to cell cytoplasm

The Symplast Route Through the cytoplasm Water enters the root hair cells across the partially permeable membrane by osmosis Water moves from higher  in the soil to the lower  in the cell Water moves across the root from cytoplasm to cytoplasm down the  gradient It passes from one cell to the other via plasmadesmata Water moves into the xylem by osmosis The only way across the endodermis Normally the most important pathway

The Apoplast Route Water moves through the cellulose cell wall and intercellular spaces The permeable fibres of cellulose do no resist water flow Water cannot pass the endodermis by this route Because the Casparian strip in the endodermis cell wall is impermeable to water Due to the waterproof band of suberin So all water must pass the endodermis via the cytoplasm Therefore it is under cellular control Apoplast route is important when transpiration rates are high as it is faster and requires no energy

The Casparian strip acts as an apoplast block The Casparian strip is made of suberin, which is impermeable to water Water is unable to pass through the endodermis by the apoplast route Subject to copyright clearance a suitable image showing the casparian strip could be inserted here. e.g. one similar to that found at: www.botany.uwc.ac.za/ecotree/root/rootA.htm The endodermis actively transports salts into the root xylem Lowering the  in the xylem, so water moves in down the  gradient by osmosis Water moves up the stem in the xylem vessels

Transverse Section of a Stem (Dicot) Vascular bundles Subject to copyright clearance a suitable image showing a transverse section of a stem could be inserted here. e.g. one similar to that found at: http://www.skidmore.edu/academics/biology/plant_bio/

Vascular bundle from a stem Epidermis Subject to copyright clearance a suitable image showing a vascular bundle could be inserted here. e.g. one similar to that found at: http://www.skidmore.edu/academics/biology/plant_bio/ Phloem Xylem vessels

no cell contents (dead) lignin fibres strengthen the cell walls Xylem vessels with different types of lignin strengthening the cell walls Subject to copyright clearance a suitable image showing xylem vessels with different strengthening could be inserted here. e.g. one similar to that found at: http://www.skidmore.edu/academics/biology/plant_bio/ Xylem vessels no cell contents (dead) lignin fibres strengthen the cell walls form continuous tubes so do not collapse when pressure inside falls

Mechanisms for the Transport of Water up the Xylem Capillarity Root Pressure Cohesion-Tension

Capillarity Water rises up narrow tubes due to the adhesive forces between the water molecules and the wall of the tube Water rises higher in narrower tubes Xylem vessels are very narrow Limitations 1.Water will only rise 50mm 2.The flow rate is slower than the rate observed in xylem

Root Pressure Root pressure causes the mercury to rise in the manometer Water Cut stump of a well watered plant Mercury Manometer

Root Pressure Water is pushed up the xylem by hydrostatic pressure Mineral salts are pumped into the xylem vessels in the root by the endodermal cells Lowering the  in the xylem Water moves in from the surrounding cells by osmosis Raising the hydrostatic pressure so pushing water up the xylem What would happen if the roots were deprived of O2? The ‘pumping’ of the ions would stop as it requires ATP produced in aerobic respiration. O2 required for aerobic respiration

Root Pressure: Evidence Cut stumps of plants exude water from their cut ends In certain conditions some leaves exude water from their leaves = guttation Pressures recorded by mercury manometers attached to the cut stumps could push water in the xylem up to 30m

Guttation Water droplets exude from the leaves Subject to copyright clearance a suitable image showing guttation could be inserted here. e.g. one similar to that found at: http://grapes.msu.edu/guttation.htm

Limitations of the Root Pressure Hypothesis The pressure measured is not enough to get water to the top of trees Only find root pressure in spring Relies on the use of the plant’s energy (ATP) for active transport

Cohesion - Tension + + - - Water is pulled up the xylem by the water lost in transpiration The sun provides the energy to ‘pull’ the water up by providing the energy for evaporation Water moves up the xylem by mass flow from the higher pressure in roots to the lower pressure in the leaves The column of water does not break because of the cohesive forces between the water molecules Hydrogen bonds between individual water molecules is the force of attraction + - + -

Evidence for the Cohesion Tension Hypothesis Cut stems attached to a tube containing water over mercury can pull the mercury up almost 1m Dendrographs record that tree trunks have a narrower diameter during the day when transpiration rate is high i.e. when most tension is created. Puncturing the xylem of the stem of a transpiring shoot under water containing a dye causes the dye to move into the xylem both ways. The dye must be pulled in so the xylem is under tension.

Variation in trunk diameter and transpiration rate over 24 hours 2400 Transpiration rate Branch diameter 1200 The diameter of the trunk decreases as transpiration rate increases Evaporation from the leaves draws water from the xylem by osmosis, water is pulled up the xylem creating a tension. The tension pulls the xylem vessel walls in, so the trunk diameter gets smaller The trunk has a larger diameter when there is less transpiration This supports the cohesion tension hypothesis but not root pressure.

Water movement across the leaf cuticle upper epidermis palisade mesophyll xylem water is pulled along the xylem spongy mesophyll water moves into cells down  gradient by osmosis lower epidermis water evaporates from the spongy mesophyll cell surface lowering cell  cuticle stoma lowest  in the air water vapour diffuses into the air down  gradient

The Cohesion Tension Hypothesis for Movement of Water up the Xylem Vessels Transpiration Lower pressure/tension at top of xylem Water evaporates from the spongy mesophyll cells and diffuses into the atmosphere Lower  in the leaf cells Water is pulled up xylem vessels Water moves from down the  gradient Water moves across root from soil down  gradient Via the apoplast and symplast paths Cohesive forces between water molecules prevent water column breaking

Click on the marks above to check Questions Explain, in terms of water potential how water moves from the soil to the endodermis in a root (5marks) Explain why, in summer, the diameter of a branch is smaller at noon than at midnight. (4 marks) Explain the root pressure hypothesis for water movement in the xylem. (3 marks) Give two limitations of this hypothesis, (2marks) click here to end Click on the marks above to check your answers

Water is absorbed from the soil by the root hairs Answer Q1 Water is absorbed from the soil by the root hairs By osmosis down the water potential gradient The water potential is higher in the epidermal cells than in the xylem in the root centre Water moves from cell to cell through the cytoplasm down the water potential gradient Water also moves through the fibres of the cell wall and intercellular spaces But must go through the endodermal cells due to the Casparian strip Any 5 points Back to question

Temperature higher at noon so transpiration rate higher Answer Q2 Temperature higher at noon so transpiration rate higher More water evaporates from the surface of the mesophyll cells Reducing the the water potential Water moves from the xylem in the leaves into the cells Creating a tension pulling the water up the xylem This pulls the xylem vessels in so reducing the diameter of the trunk Any four points Back to question

Root pressure is a hydrostatic pressure pushing water up the xylem Answer Q3 Root pressure is a hydrostatic pressure pushing water up the xylem Mineral ions are actively transported out of the endodermal cells into the xylem vessels Lowering the water potential in the xylem So water moves in from the surrounding cells by osmosis / down the water potential gradient Raising the hydrostatic pressure Any three points Back to question

The pressure measured is not enough to get water to the top of trees Answer Q4 The pressure measured is not enough to get water to the top of trees Only find root pressure in spring Relies on the use of the plant’s energy (ATP) for active transport Any two Back to question

Now think of some synoptic links and make a list. Try out this web site to review transport of water in plants www.biologymad.com