1. Water Absorption by Plant Roots and Movement through Plants
HORT 301 – Plant Physiology
September 5, 2008
Taiz and Zeiger, Chapter 4 (p ), Web Topics and Web Essays
Water Uptake by Roots – movement in soil, symplastic and apoplastic transport pathways to the xylem
Water Movement through Plants – transport from root to shoot through the xylem and transpiration

2. Water Uptake by Roots
Water availability to plants in soil – composition, particle type and size, and structure affect water movement in soils and availability to plants
Water adheres to soil particles and occupies spaces between particles
Sand (0.2 to 2 mm particle diameter), low surface area per gram relative to clay (<.002 mm particle diameter)
Clay - greater water holding capacity than sand due to large surface area/g and small spaces between particles (resists evaporation)
Organic material increases the moisture holding capacity of the soil but aeration is reduced
Field capacity - water content of soil after saturation, maximum moisture holding capacity
Clay retains up to 40% of soil moisture content while sand retains about 3% a few days after moisture content was at field capacity

3. Water moves through soil by pressure-driven bulk flow
Soil water potential (Ψw) – comprised of solute/osmotic potential (Ψs) and hydrostatic pressure (Ψp): Ψw = Ψs + Ψp
Soil solution Ψs – usually negligible because of low solute concentration (-0.02 MPa), an exception is saline soils (-2.0 MPa)
Ψp contributes the most to soil solution Ψw
Hydrostatic pressure/pressure potential (Ψp) – Ψp is near 0 in soils at field capacity but becomes very negative as a result of drying

4. Permanent wilting point - plant cannot regain turgor after irrigation
Water between soil particles evaporates (accessed by roots)first; water adheres to soil particles and is less accessible
Continued dehydration (drought) causes evaporation creating air spaces with large water-air surface areas, i.e., surface tension (negative hydrostatic pressure)
Ψp = -1 to -2 MPa for clay, -2 MPa is permanent wilting point for plants
Permanent wilting point - plant cannot regain turgor after irrigation
→ = surface
tension

5. Water movement (transport) in soils – dependent on soil hydraulic conductivity and Ψw (water potential) gradient (Ψp, pressure potential gradient)
Hydraulic conductivity - soil structure, sand has high conductance relative to clay, i.e., greater adhesion by water to clay
Pressure potential gradients in soils – difference between less (0 or less negative) and more negative hydrostatic pressures/pressure potentials (Ψp)
Differences in –Ψp generates a positive pressure gradient at one position relative to the other

6. Position A - Ψp = -1 MPa
Position B - Ψp = -2 MPa
then ΨpA - ΨpB = -1 MPa –( -2 MPa) = +1 MPa gradient, water moves from A to B B A ↓

7. Water transport from soil to roots - root soil water contact is maximized by primary root growth, and secondary root and root hair development
Root hairs may constitute more than 50% of the root surface area
Water uptake occurs primarily in the root hair region (fully elongated cells but no secondary growth)

8. Root hairs are microscopic and can enter small soil cavities to access water located in the soil between particles

9. Water uptake into roots facilitates soil water movement in soil - water uptake by plant roots creates a more negative hydrostatic pressure/pressure potential (Ψp) near the root surface, barrier to water uptake
Water moves to these regions because of the pressure gradient that is created but root hairs must grow in order to access water
Plant roots sense water (less negative water potential) and growth is directed towards water (hydrotropism)
Water absorption by roots requires root and root hair growth to soil areas where water is more available

10. Water uptake by roots and root hairs - involves apoplastic and/or symplastic pathways
Apoplastic – water moves along the cell wall, i.e., intercellular spaces
Symplastic – water transport through cells across the membranes or through plasmodesmata (pores that link protoplasm of cells)
Water uptake into cells may occur at the epidermis or cortex but symplastic transport must occur at the endodermis. Casparian strip contains suberin (hydrophobic lipid polymers) that is impermeable to water

11. Water movement (transport) into cells from the soil solution is driven by the Ψw gradient, between apoplast and symplast (osmosis)
Aquaporins – facilitate in symplastic water uptake into roots

12. Water transport through the xylem – xylem, conduit for water movement from roots to leaves by capillarity
Tracheary elements – water-conducting cells of the xylem
Low resistance and high tensile strength (cell wall structure) to withstand negative hydrostatic pressures
Tensile strength - dense and highly lignified secondary walls

13. Tracheary/xylem element types – tracheids (angiosperms and gymnosperms) and vessel elements (angiosperms)
Programmed cell death at maturation - hollow cells devoid of protoplasm and membranes (low resistances), capillary tube-like

14. Tracheids (gymnosperms and angiosperms) – arranged in overlapping vertical files with pits that facilitate water flow from tracheid to tracheid
Vessel element interconnection (angiosperms) – shorter and wider than tracheids, joined end to end to interconnect water movement, pits facilitate lateral water movement between elements
Pits - microscopic holes created by the absence of a secondary wall, thin porous primary wall separating inside from outside, connect xylem

15. Very efficient water conductance through the tracheary elements, resistances caused by pits and irregular inner wall surfaces
1010 more pressure is required to move water through living cells relative to the xylem (Web Topic 4.3)

16. Surface tension facilitate water movement (transport) from roots to leaves – surface tension (air-water interface) in the sub-stomatal cavity of the leaf creates large negative pressure potential (p) that “pulls” water up the xylem
Note, large surface
area to volume ratio in the cavity

17. Cohesion-tension theory for water movement in the xylem – surface tension and cohesive strength of intermolecular interaction between water molecules facilitate development of large water columns in the xylem (capillarity)
The tensile strength of degassed water is greater than -30 MPa (tension)
Only a 3 MPa pressure difference is sufficient to move water from roots to needles 100 meters away in the canopy of redwood trees
Adhesive capacity of water molecules also facilitates capillarity
Pressure potential (Ψp) gradient (more negative at the liquid water-air interface) is the force that drives water movement from the roots to the leaves

18. Water Absorption by Plant Roots and Movement through Plants
HORT 301 – Plant Physiology
September 5, 2008
Taiz and Zeiger, Chapter 4 (p ), Web Topics and Web Essays
Water Uptake by Roots – movement in soil, symplastic and apoplastic transport pathways to the xylem
Water Movement through Plants – transport from root to shoot through the xylem and transpiration
Cohesion-tension theory for water movement through the xylem – surface tension in the substomatal cavity causes capillarity, root to shoot water movement facilitated by cohesion of water molecules

19. Surface tension increases with evaporation of water (transpiration) at the leaf surface - xylem is an interconnected network conduit linking the root to the leaf vascular system (veins)
All leaf cells are within 0.5 mm of a minor vein

20. Evaporation of water to the atmosphere increases the air-water surface area in the sub-stomatal cavity, i.e. surface tension (negative hydrostatic pressure (Ψp) of water in the xylem increases

21. Water movement (transport) from the leaf cells to the atmosphere (transpiration) – water (liquid) evaporates (vaporizes) into the intercellular spaces in the sub-stomatal cavity)
Water vapor diffuses from the cavity to the atmosphere (high to low concentration) through stomatal pores, w gradient between the sub-stomatal cavity vapor and air outside of the leaf, no p

22. Water vapor rapidly diffuses quickly into ambient air, ~1 mm in 0
Water vapor rapidly diffuses quickly into ambient air, ~1 mm in 0.04 s compared to glucose 50 s
~95% of water loss occurs through stomata (transpiration), cuticle and wax restricts water loss through the epidermis

23. Vapor concentration in the sub-stomatal cavity is near saturation (100% relative humidity (RH) even for a transpiring leaf
RH - vapor concentration/vapor concentration at saturation
RH rapidly decreases along the transpiration pathway creating a large vapor gradient, w gradient due mainly to water concentration difference

24. Relationship between water potential (Ψw), and relative humidity (RH) and temperature
Ψw = RT/Vw ln(RH)
R – gas constant, T – temperature (Kelvin), Vw – partial molal volume of liquid water, RH – relative humidity
Saturation water vapor concentration increases with temperature, water potential becomes more negative (Ψw = RT/Vw ln(RH)), i.e. RH decreases

25. Resistances to water movement from leaf to air - boundary layer (unstirred and on the leaf surface) through which water vapor must diffuse and stomatal pore opening and closing
Boundary layer –creates a “microenvironment” of higher RH that reduces water loss from leaves, 20% relative to moving air
Stomatal control of water loss – guard cells at the opening of the stomata control the aperture and regulate water vapor loss

26. Transpiration - drives water (solute) movement up the xylem and cools the plant leaf surface because of the energy required to vaporize water
One gram of dry matter (CO2 fixed) for 500 g of transpired water

27. Primary forces that drive water transport:
1. Soil - p gradient that drives bulk flow
2. Uptake by plant roots - w gradient that facilitates osmosis due mainly to the symplastic s
3. Root to shoot - p gradient resulting from surface tension in the sub-stomatal cavity
4. Sub-stomatal cavity to atmosphere – water vapor concentration gradient 4 3 2 1

29. Water movement (transport) through the xylem – root to shoot, capillarity (adhesion, cohesion and surface tension)
Water movement (transport) from the leaf to the atmosphere –sub-stomatal cavity to the atmosphere, driven by the vapor concentration difference (Δcwv, c – concentration in the leaf and air)