Water transport across the entire soil-plant-atmosphere continuum Water Acquisition and Transport - Whole Plants 3 possible pathways for water movement across the soil-plant-atmosphere continuum Apoplast - through cell walls and xylem Transmembrane - across biological membranes - acts as a control on ion movement - Aquaporins - channels for bulk flow of water Symplast - through one cell to another via plasmodesmata - small pores allow water through Figure 3.13 Water can cross plant membranes by diffusion of individual water molecules Figure 4.4 Pathways for water uptake by the root 1
Role of the Casparian Strip & Exodermis Waxy coating on root cell (endodermis) forces water to cross a biological membrane before entering xylem - allows control of ion concentration Waxy outer surface coating on the older sections of root tissue allows uptake of water to extend further throughout the distal portions of the root Figure 4.3 Water uptake along roots: role of the exodermis Xylem cells are the high conductivity pathway Represent 95% of the pathway for water movement Xylem cells are dead at functional maturity Two basic types of xylem cell: - Tracheids: short & small diameter tapered cells - Vessel Elements: wider diameter cells that combine to form long vessels - a string of connected vessel elements Figure 4.6 XYLEM: Comparison of tracheids and vessel elements 2
Mechanisms for Water Transport Through Plants 1. Cohesion-Tension Theory (most important) 2. Root Pressure (minor role) - only occurs under some limited conditions Cohesion-Tension Theory 3 main components: solar radiation input provides the energy for the phase change and evaporation of water vapour (energy needed to break the H-bonds) water vapour diffuses out of leaf down a concentration gradient tension is created in cell walls where initial evaporation occurs - the tension pulls water up through plants Cohesion-Tension Theory the tension (negative pressure) is described by: p 2T r r = radius of curvature at the air-water interface (m) T = surface tension of water (7.28 x 10-8 MPa m) as tension is created adjacent water molecules are pulled along because of cohesion (due to H-bonds) the water column is pulled up to replace molecules that diffuse out of a leaf 3
Figure 4.8 The driving force for water movement through plants Cohesion-Tension Theory evidence for negative pressure (tension) in xylem comes from measurements with a pressure chamber Water Potential in Soils w = p + s as with plant cells total soil water potential is primarily influenced by two factors s depends on the concentration of solutes in the water - this is why soil salinity causes physiological drought p depends on water being held in small pores that occur between soil mineral particles 4
Figure 4.2 Root hairs make intimate contact with soil particles Root Pressure can be a force moving water into xylem water is normally pulled up through xylem - NOT pushed by pressure but under some special conditions: - soil water potential is high - transpiration is very low or stopped roots can generate positive pressure in xylem Root Pressure roots can generate a positive xylem pressure ( p ) by absorbing ions from the dilute soil solution and actively transporting them into the xylem Figure 4.5 Guttation in leaves from strawberry (Fragaria grandiflora) the build-up of solutes in xylem sap decreases solute ( s ) and total potential ( w ) which then draws additional water into the root xylem since water is NOT being lost from the leaves - pressure can build up inside the root xylem - this can be important to re-fill cavitated xylem 5
Representative Values of Water Potential across the soil-plant-atmosphere continuum - water moves from high to low water potential - adjacent plant cells can have the same w but very different component values ( p & s ) - the major change in water potential occurs from the leaf intercellular air spaces to the outside atmosphere 6