Water Uptake and Movement Water Uptake Plants absorb water from the soil with their extensive root system that explore large volumes of soil. In many species the root system comprises more than 50% of the total body weight. The permeability of roots to water depends on age, physiological condition and the water status of the plant. The region of most active water uptake lies near the root tip. The first 0,5 cm of the root tip, including the root cap, is relatively impermeable to water. The next 10 cm of the root is the region where water is taken up most rapidly zone of cell maturation where vascular tissue begins to differentiate and root hairs develop actively. Root hairs are thin-walled extensions of epidermal cells and can increase the absorbing surface of the root by 1.5 to 20 times. Root hairs are about 10 m in diameter and up to 10 mm long and explore the capillary spaces between soil particles. Soil is comprised of inorganic rock particles, organic material, a soil solution containing dissolved solutes and a gas phase in equilibrium with the atmosphere. Soil particles are classified according to their size. Paricle class Particle size (mm) Water retention Aeration Coarse sand Sand Silt Clay 2.0-0.2 0.2-0.02 0.02-0.002 Less than 0.002 Poor Good Exellent Excellent Good Poor Soil texture is determined by the mix of particles of different sizes. Soil porosity is determined by the texture. Two kinds of pores recognized large pores and capillary pores. When soil is freshly watered the water percolates through the pore space and displaces the all the air saturated. Water then drains freely from the large pores due to gravity. The water that remains after drainage is held in the capillary pores field capacity. The smaller the soil particles the higher the field capacity. Sandy soils with coarse particles drain rapidly and has a low field capacity. Clay soils consist largely of capillary pores and has a high field capacity. Due to surface tension, water in soil at or below field capacity is under tension (negative pressure) negative pressure potential. Concentration of solute in soil water is relatively low s is negligible and = p. As soil dries p and thus becomes more negative (smaller). As the soil dries out it reaches a point where the plant cannot extract water and will wilt permanently if water is not added to the soil permanent wilting percentage. 1-2 % for sand and 20-30% for clay. for any soil type at this point is about -1.5 MPa. Available water for the plant is between field capacity and the permanent wilting percentage. Water Movement Water is absorbed by the root as a result of the difference in between the soil and the root. Water can enter the root hair cells from where it moves symplastically to the xylem.Water can also move apoplastically through the cell walls to the endodermis where the Casparian strip (suberinized cell walls) blocks its movement into the root pericycle.It then has to enter the endodermis cells from where it can move symplastically to the root xylem. Water in the apoplast (the free diffusional space outside the plasmalemma) is in equilibrium with water in the symplast (the inner side of the plasmalemma in which water can freely diffuse). Water flow through the root cortex thus involves both pathways. Atmospheric pressure (0,1 MPa at sea level) will support a water column of 10 meters in a closed tube. Some trees are more than 100 m high would require a pressure of 1-1.5 MPa to push a water column up the tree. In the tree water would encounter additional resistance against movement: Gravity Irregular wall surfaces Perforation plates etc. Force required to overcome all resistance ca. 2-3 MPa. The Ascent of Sap What mechanism can generate the force? 1. Root Pressure: Sap exudes from the cut surface of a herbaceous plant if cut just above the soil surface. root is lower than soil water moves into root. xylem is lower than cortex- water moves into xylem Casparian band prevents free return of water positive hydrostatic pressure develops in xylem. Water rises in the xylem. Pressure ranges from 0,1-0,5 MPa not enough. Root pressure not detected in all species. During periods of active transpiration xylem under tension, not pressure. 2. Capillary Rise/Capillarity: Rise of water in capillary tubes due to: Adhesion attraction between water molecules and polar groups along the surface of the tube water moves upward along the wall. Surface tension due to cohesive forces between water molecules pulls the bulk water up the tube lumen. Capillary rise of water is inversely proportional to the radius of the tube. Radius of xylem vessels range from 50 200 m water will rise between 0,6 and 0,08 m in the xylem due to capillarity. 3. Cohesion theory: There is a continuous column of water from the root tips through the stem into the mesophyll cells of the leaves. Surface of mesophyll cells covered with thin water layer which adheres to cellulose and other hydrophilic surfaces. Water evaporates into the air spaces increases the surface tension at the air-water interface. Generates increasing negative pressure, which draws more water to the surface. Due to the cohesion between water molecules in the continuous column, the negative pressure is transmitted all the way to the roots and water is pulled up through the plant. How is the integrity of the water column retained? Water has a high tensile strength as a result of cohesion forces high resistance to breakage of the column. Tensile strength of pure water deplete of gas big enough to withstand tensions of -25 to -30 MPa. Tension in a 100 m tree = -2 to -3 MPa, sometimes as low as -10 MPa. Under tension there is a tendency for dissolve gasses in the water to come out of solution and rapidly form air bubbles in the xylem that relieves the tension cavitation. Large gas bubble formed known as an embolism. Embolisms obstruct water movement in the xylem element, which can no longer conduct water. Water bypasses the blockage by flowing laterally into adjacent elements. = -0,3 MPa -0,5 MPa -0,6 MPa -0,8 MPa -0,8 MPa Atm = -95.2 MPa by 50% RH CO2 -0,8 MPa H2O vapour -95.2 MPa Driving force of transpiration Water potential difference between the leaf and thesurrounding atmosphere. Environmental Factors Affecting the Rate of Transpiration 1. Humidity Relative humidity (RH) - ratio of the actual water content of air to the maximum amount of water that air can hold at that temperature. % Relative humidity RH x 100 Air at 50% RH contains 50% of the maximum amount of water that air can hold at that temperature. The maximum amount of water that air can hold increases with temperature. Air at 20C holds more water at 50% RH than air at 10C. Air in the sub-stomatal spaces close to 100% RH. of air at 100% RH is 0. decreases with a decrease in RH. Relationship between RH and : = 1.06 T log (RH/100) The lower the RH the faster the rate of transpiration. % RH at 20C ( MPa) at 20C 100 95 90 50 20 0 -6.9 -14.2 -93.5 -217.1 2. Temperature The higher the temperature the bigger the amount of water that the air can hold. At a fixed RH, the higher the temperature the lower the . %RH Temperature (C) 05101520 20 30 40 50 60 70 80 85 90 95 -202.85 -151.75 -115.49 -87.36 -64.38 -44.95 -28.13 -20.48 -13.28 -6.47 -206.56 -154.52 -117.60 -88.96 -65.56 -45.78 -28.64 -20.86 -13.52 -6.58 -210.27 -157.30 -119.71 -90.56 -66.74 -46.60 -29.15 -21.23 -13.77 -6.70 -213.99 -160.08 -121.83 -92.16 -67.92 -47.42 -29.67 -21.61 -14.00 -6.82 -217.71 -162.86 -123.95 -93.76 -69.10 -48.25 -30.18 -21.98 -14.25 -6.94 3. Wind On the surface of the leaf is a layer of undisturbed air boundary layer. RH of the boundary layer is higher than the rest of the surrounding atmosphere slows down transpiration. The thickness of the boundary layer ads to the length of the length of the diffusion path and resistance to diffusion. Wind decreases the thickness of the boundary layer the higher the wind speed the thinner the boundary layer and the higher the rate of transpiration up to the point where the stomata close as a result of cooling or desiccation. Boundary layer thickness is also determined by the presence of hair on the leaf and by leaf size and shape.