Ascent of Sap

I.  The Problem
  A.  How do you raise the water column in a plant without
       having the column of water cavitate?
  B.  Root Pressure: occurs when there is: (1) ample moisture is present
       in the soil, (2) humidity is high, (3) transpiration is
       low.  Droplets of water exude from hydathodes creating the
       phenomenon of guttation.  Root pressure is not found in
  C.  Capillarity is the interaction between surface contact
       of a liquid and a solid.  This interaction distorts the liquid
surface from a planar shape to a concave shape (meniscus).
This distortion caused the rise of liquids in small tubes.
The liquid is able to wet the side of the tube and pull itself up. Liquids can rise higher in tubes of small diameter (like a cell!). II. Ascent of Sap: Cohesion Mechanism A. Three basic elements of the cohesion theory for the ascent of sap: the driving force, hydration (adhesion), and the cohesion of water. 1. Driving Force: the gradient of water potentials from the
soil through the plant to the atmosphere. These water potentials
decrease (become more negative) going from the soil to the
a. The xylem cell's small diameter and thick walls prevent them
from collapseing
b. The xylem cell's of the leaf and stem have low osmotic potentials c. The xylem cell walls have a great hydration property, especially
in leaves
d. WATER POTENTIAL: Y = YP Ys Y = water potential (negative) YP = pressure potential (positive) Ys = osmotic or solute potential (negative) 2. Adhesion: hydrogen bonding between water and cell walls. 3. Cohesion: hydrogen bonding between the water molecules. Cohesive forces of water are so great that when it is pulled, by osmotic and evaporative forces at the top of a tall tree,
the pull extends through the trunk and the roots
into the soil. Thus the water column in a whole tree is a
vertical pipe that would normally cavitate. But cavitation does
not hinder the flow of the sap because of the highly specialized
anatomy of the xylem cells.
III. Xylem Anatomy A. Vascular Tissues in the Stem 1. Vascular bundles are "open" in dicots and "closed" in monocots. "Open" refers to the fact that dicot stems can
increase in girth because they contain a layer
of cambial cells. Monocots lack this cambial layer and
thus contain "closed" vascular bundles that are surrounde
in a layer of cells called the bundle sheath.
Monocot vascular bundles are scattered in the stem while Dicot vascular bundles are found in a ring. In woody plants the xylem makes up the wood and the phloem makes up the bark as well as the cortex and cork. 2. Xylem: tracheids, vessel elements, fibers, xylem parenchyma a. Secondary wall: cellulose, lignin, hemicelluloses that cover the primary wall b. Pits: round thin places in the secondary wall (1) Simple pits: small round hole in the secondary wall. (2) Bordered pits: secondary walls extend over the center of the pit and the primary walls are swollen in the center of the pit to form a torus. The primary wall around the torus appears porous. The torus acts as a valve, closing when pressure on one side is greater than pressure on the other side. 3. Tracheid cells have tapered ends which overlap to form long files of cells. These tracheid cells have numerous pits.
Tracheid diameters are often in the range of 10-25 mm.
4. Vessel elements: have perforation plates at the end walls. Allow rapid movement of water. Vessel elements are aligned so that they form long tubes called vessels. Vessel elements are typically 40-80 mm. 5. The rate of sap flow is reduced by friction (adhesion) between the fluid and the sides of the capillary tube. The sap molecules touching the sides of the xylem wall do not move, while the molecules in the center of the tube move the farthest in a given time. 6. Measurements have shown that flow through large vessels is much more rapid, for a given pressure gradient, than it is through tracheids and small vessels. 7. There is a much greater chance that cavitation will occur in a large vessel than in a small one or in a tracheid, producing an embolism. 8. Some vessel elements with spiral thickenings can elongate and grow while conducing water under tension. They grow as the surrounding cells (with contents under pressure) grow and pull them along, their spiral thickenings expanding like coiled springs. Such growth occurs a the base of most grass leaves. 9. Apical Meristems: Primary xylem, cambium. Secondary xylem: spring wood summer wood B. Vascular Tissues: Root Anatomy 1. Pericycle: layer of living cells 2. Stele: vascular tissues 3. Endodermis: radial and transverse cells walls have Casparian Strips or bands that are impregnated with suberin. Impermeable to water. Tangential walls (the inside and outside walls parallel to the surface of the root) are not impregnated with these substances. Suberin lamella sometimes found on inner tangential wall 4. Cortex, epidermis, root hairs. 5. Water with its dissolved substances enters through the epidermis and then moves freely through the cortical cells, both protoplast (symplast) and cells walls (apoplast) but that it cannot pass through the Casparian strips around the endodermal cells. 6. Just inside the epidermis, of a cell layer, an exodermis also has cells with Casparian strips and that typically has a suberin lamella. 7. Hypodermis: a layer of cells just below the epidermis with no wall modifications. Exodermis: hypodermis with Casparian bands. 8. Most ion uptake and probably water uptake occurs in the epidermis, even in roots without an exodermis. Movement of ions through the cortex may be mostly from protoplast to protoplast (symplastic), although water can move freely through the membranes and thus may move in the cell walls (apoplast) as well. 9. Root Cap and Root Hairs: much water enters through the root hairs and their associated epidermal cells: the region of a young root where xylem vessels are mature and resistance is low. C. Apoplast-Symplast Concept 1. Apoplast: interconnecting walls and water-filled xylem elements. "Dead" part of the plant. Endodermal and exodermal walls with Casparian strips are apoplast. 2. Symplast: "living" part of the plant, cytoplasm of all the cells. Protoplast of adjoining cells are connected through the plasmodesmata. D. Anatomical Basis of Root Pressure 1. Roots in contact with soil solution. Ions diffuse into root via apoplast of epidermis. If root have an exodermis, ions must move into the symplast of the epidermis, otherwise ions remain in the apoplast of the cortex until they reach the endodermis. Ions pass across the cell membranes from the apoplast into the symplast in an active process that requires cellular reparation. Increase in the concentration of ions inside the cells to levels higher than those outside. Ions move freely into the pericycle and other living cells within the stele. Plasmodesmata, velocity of movement inward could be increased by cytoplasmic streaming. 2. Once inside the stele, ions are actively pumped out of the symplast and into the apoplast. Buildup in concentration of solutes within the apoplast tin the stele to a level higher than that of the soil solution. Osmotic potential in the stele is more negative than the osmotic potential of the soil. Water passes through protoplasts of the endodermal and exodermal layers, then layers act as differentially permeable membranes, and the root becomes as osmotic system. IV. Driving Force: A Water-Potential Gradient A. Atmospheric Water Potential 1. Great capacity that dry air has for water vapor 2. Rapid drop in the water potential of increasingly dry air 3. temperature has a marked effect n relative humidity B. Role of Osmosis into Living Cells 1. Living cells obtain water from the apoplast. Water potential in protoplast must be slightly more negative than that in the surrounding walls. 2. Cohesion theory says that water in the apoplast, particularly in the xylem, is under tension or negative pressure. 3. Osmotic potential of water in the apoplast must be less negative (higher) then that of water in the living cells 4. Driving force would be the negative water potentials within living cells, established by the very negative osmotic potentials and the relatively low positive pressures in the cells. C. Role of Cell-Wall Hydration 1. When the water potential in a drying plant drops to some critical level, cavitation and the resulting embolisms occur in the transporting elements 2. Plants are constructed in such a way that cavitation occurs when water potential reaches a critical low level in a manner that allows refilling of the tracheids or vessels if conditions are right. when the pressure in the air spaces on the outside of the pit membrane exceeds the tension inside by that amount, the meniscus will be pulled through the pore allowing a minute quantity of air to enter. The air would lead to vacuum boiling, and the air and water vapor would immediately and explosively expand to fill the tracheid or vessel. 3. The vessel would be filled with water vapor at its vapor pressure for the prevailing temperature. The pressure in the cell would be positive but quite low. Such pressures would immediately seal all the pores, because a water meniscus could never be pulled through such a pore by such a small pressure difference across the cell wall. The torus in bordered pits might also contribute to the sealing. When conditions allowed the vapor pressure in the xylem t increase above that of the vapor pressure of water for the particular temperature, the cells might refill by condensation of vapor and solution of the minute amount of air. D. Soil Water 1. Gravitational water: soil saturated with water, the excess water moves down through the pore spaces. 2. Soil that contains all the capillary water it can retain against gravity is said to be wet to field capacity. 3. When soils are not able to hold enough water to overcome wilting in a plant, the amount of soil water is said to be at the permanent wilting percentage. The minimum plant water potential is apparently determined by the osmotic potential of plant cells and the designed leakage in the xylem. This minimum plant water potential determines the permanent wilting point. 4. The amount of water in the soil at field capacity and at the permanent wilting percentage depends strongly upon soil texture, soil structure, and the amount and kind of soil organic matter. a. Soil Texture: size of the primary particles that make up the soil Clay: extremely small particles in the colloidal size range with much hydrophilic surfaces. Silt: intermediate in size between the two (best) Sand: large, little holding capacity b. Organic materials improve water holding capacity Humus: soil organic material c. Soil Structure depends on Particle Size: All soils are mixtures of sand, silt, and clay. Clay: extremely small particles in the colloidal size range with much hydrophilic surfaces. Can hold more water that is available to plants Loam: intermediate in-between the two (best) - high organic content. 10-25% clay and the rest about equal parts of sand and silt Sand: large, little holding capacity V. Tension in the Xylem: Cohesion A. Does Water Have a High Enough Tensile Strength? water can sustain tensions of up to -3.0 MPa without cavitating B. What are the flow velocities? Are the water columns really continuous? Water columns in the xylem are continuous, and observed flow velocities in stems strongly agree. When velocities are measured at different points along the trunk, increase fist at higher points along the trunk in the morning as transpiration begins. Peak velocities (at noon) range from 1- 6 mh-1 for narrow-vessel trees to 16- 45 m h-1 for trees with wide vessels. Slowest velocities occur in conifers. C. Are the columns really under tension? Tensions always observed varied from a few tenths of a megapascal below zero to more negative than -8.0 MPa Forest and freshwater species have tensions with the least negative values, and desert and seashore plants, growing in soils that are likely to be salty, have more negative values. Water in the xylem of land plants in summer must nearly always be under tension. Tensions normally exist in their conductive xylem tissues, but their turgid leaf cells have osmotic potentials even more negative than those of the xylem sap with its negative pressure. Without the differentially permeable membranes around the living cells and the highly negative osmotic potentials of the sap and cytosol inside, the high tensions in the xylem system would lead to collapse (wilting) of the living tissue. Without osmosis, plants would collapse. D. What if the columns cavitate? When dye is inserted at a point in a trunk it spreads circumferentially around the trunk as it moves up and down, mostly within a single growth ring. The spread amounts to about 1-20, which equals about 17-35 mm per meter of trunk. Water entering the xylem of any root is spread throughout the entire trunk until it reaches virtually all the branches that form the crown. If a given root is damaged, no part of the crown suffers from lack of water. E. What about air excluded from solution in the stem by freezing? 1. Air blockage occurs when some trees are frozen. Blocked pathways are replaced or restored in such trees. 2. "Throw-away" method. Large and efficient vessels that a single growth increment of the trunk is sufficient to provide the crown with water. Vessels are formed before leaves emerge in the spring, the vessels of previous years are not used. Root pressure in the spring, which is quite clearly seen in grapevines. 3. Imagine a northern tree thawing in the spring. As the ice melts, the tracheids become filled with liquid containing the many bubbles of air that had been forced out by freezing. Melting continues and transpiration begins, tension begins to develop in the xylem. A large bubble reaches a critical point at which it expands explosively under tension as water turns to vapor in a fraction of a second. this is confined to the tracheid in which it occurs because of its anatomy. but it sends a shock wave to surrounding tracheids, driving their small air bubbles back into solution. The tracheid in which bubble expansion occurred would be vapor-locked and forever lost to sap movement. About 10% of the tracheids were indeed filled with vapor, but the remaining 90% are ample to handle sap movement. VI. Xylem Anatomy: A fail-safe system A. Plants, especially tall trees, are apparently designed to allow sufficient flow in response to the pressure gradients to prevent cavitation in the transport elements to bypass cells that do become vapor-locked, and sometimes even to restore such vapor-locked cells. B. Wall sculpturing in vessel elements and the various kinds of performation plates might keep bubbles that do form from coalescing; small bubbles are much easier to dissolve than large ones. Highest resistance to water flow occurs at the leaf trace or in the base of the petiole. water stress builds during drought, cavitation occurs first in the leaves, so they may wilt and die; but the water system in the trunk remains relatively intact. C. Tyloses seal against water loss and pathogen invasion. Gums or resins are secreted into nonfunctional xylem cells. D. Measure the circumference of the tree; the circumference contracts slightly during periods of high transpiration and expands at night or during rain.