Water Absorption by Plant Roots HORT 301 – Plant Physiology September 26, 2007 Taiz and Zeiger, Chapter 4 (p ), Web Topics 4.1 and 4.2

Water movement (transport) in soil, absorption by roots and transport to the xylem

Water movement (transport) in the soil – pressure-driven bulk flow, transport affected by soil properties Soil composition - sand (particle size ≥1 mm), low surface area per gram relative to clay (<2 µm) Water adheres to soil particles, including in spaces between particles, adhesion Clay has a much greater water holding capacity than sand due to large surface area/g and small particle spaces Organic material substantially increases the moisture holding capacity of the soil but aeration is reduced Field capacity - water content of soil after saturation and draining of excess water Clay retains up to 40% of the water while soil moisture content of sand may be 3% (of field capacity) after three days

Water moves through soil by pressure-driven bulk flow – soil water potential is comprised of solute/osmotic potential and hydrostatic pressure/pressure potential Soil water potential (Ψ w ) – solute/osmotic potential (Ψ s ) and hydrostatic pressure (Ψ p ): Ψ w = Ψ s + Ψ p Solute/osmotic potential (Ψ s ) – usually negligible because of low solute concentration (-0.02 MPa), an exception is saline soils (-2.0 MPa) Hydrostatic pressure/pressure potential (Ψ p ) – Ψ p is near 0 in soils at field capacity but becomes very negative as a result of drying

Water adheres to soil particles forming air spaces creating large water-air surface areas, i.e., surface tension (negative hydrostatic pressure), Ψ p = - 1 to -2 MPa Arrows indicate surface tension regions Soil dehydration first evaporates the most accessible water

Water movement (transport) in soils - dependent on the pressure gradient size and soil hydraulic conductivity Resistance to water transport – soil structure, sand has high conductance relative to clay, i.e., greater adhesion by water to clay Reduced soil moisture Pressure gradient in soils – normally the difference between less and more negative hydrostatic pressure/pressure potential (Ψ p ) Differences in – Ψ p generates a positive pressure gradient at one position relative to the other Position A Ψ p = -1 MPa, position B Ψ p = -2 MPa then Ψ pA - Ψ pB = -1 MPa –( -2 MPa) = +1 MPa, water moves from A to B

Permanent wilting point – negative soil Ψ w at which the plant cannot regain turgor even when transpiration ceases, i.e. extreme negative hydrostatic pressure/pressure potential

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

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

Water uptake into roots facilitates soil water movement in soil - water uptake by plant roots creates a more negative hydrostatic pressure/pressure potential (Ψ p ) near the root surface Water moves to these regions because of the pressure gradient that is created Plant roots sense water (less negative water potential) and growth is directed towards water (hydrotropism) Water absorption by roots is dependent on root and root hair growth to soil areas where water is more available

Water Absorption by Plant Roots HORT 301 – Plant Physiology September 26, 2007 Taiz and Zeiger, Chapter 4 (p ), Web Topics 4.1 and 4.2

Water transport in soil, absorption by roots and transport to the xylem

Water absorption by roots and transport to the xylem

Water uptake by roots and root hairs - movement through the epidermis and cortex involves apoplastic or symplastic pathways Apoplastic – water moves along the cell wall, i.e., intercellular spaces Symplastic – water movement into and out of cells across the membranes or through the plasmodesmata (interconnecting pores that link protoplasm between cells)

Water uptake into cells may occur at the epidermis or cortex but symplastic transport must occur at the endodermis Endodermal cells contain suberin (hydrophobic lipid polymers) in the radial cell walls (Casparian strip), prevents water (and solute) movement through the apoplast

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

Root pressure – develops in plants with high soil moisture content and with little or no transpiration Solute/osmotic potential (Ψ s ) of cells in the root xylem generates a small positive hydrostatic pressure/pressure potential (Ψ p ) gradient with the shoot xylem e.g., 0.05 to 0.5 MPa Contributes little to water transport in the xylem except at night, guttation