Lecture 9 Evapotranspiration (2) Transpiration Processes • General Comments • Factors Controlling Transpiration • AET and PET • Water Potential • Stomatal and Root Controls

Transpiration (General Comments)

Vaporization and loss of water from plants through the cuticle or stomatal openings in the leaves Vaporization occurs in the energized leaf and leaf vapor escapes by molecular diffusion Stomata open in sunshine to allow entry of CO 2 sensitive for photosynthesis – extremely light Water loss is usually “passive”; driven by the same factors that govern evaporation although plants may further modulate water flux under certain physiological stresses The rate of transpiration also depends on the rate of uptake from the soil and passage through the plant Root hairs in contact with soil particles and water molecules absorb water through osmosis Water is pulled up through vascular tissue by capillary forces

Photosynthesis

An essential processes by which plants form carbohydrates – fundamental for life

6 CO 2  12 H 2 O  C 6 H 12 O 6  6 O 2  6 H 2 O • The photosynthetic tissue is protected by an outer epidermis • The epidermis contains numerous small pores called

stomata

allow exchange of CO 2 and O water vapor (  1% of leaf area) 2 (singular

stoma

) to with the atmosphere – they also allow the loss of • Vapor loss is a side-effect of assimilating CO 2 from the atmosphere • Vapor loss is an effective way to regulate plant temperature (loss due to latent heat of vaporization) and to distribute nutrients

Factors Controlling Transpiration (1)

1. Meteorological Conditions • Energy input Net available radiation (Q*) Latitude • Drying power of the atmosphere Vapor pressure deficit Wind speed • Type of precipitation Duration Intensity Return interval • Length of growing season Sunlight hours Temperature profiles

Factors Controlling Transpiration (2)

2. Biological Characteristics • • Cell water potential Water movement by water potential gradient and osmosis Turgor pressure (internal cell wall pressure) Wilting (dehydration ) • Height Surface roughness Reflectivity (albedo) Wind characteristics (turbulent transfer) Leaf behaviour varies throughout canopy • Resistances (soil–root, root–leaf, leaf–atmosphere etc.) Similar to Ohm’s law (sum of resistances) Stomatal resistance (diurnal pattern) Roots Soil –plant water supply Extent, depth, and efficiency of root system Potential differences between roots and soil

Factors Controlling Transpiration (3)

3. Soil Characteristics • Available soil water (Field Capacity to Wilting Point) • Texture (porosity, permeability) • Surface albedo • Structure

AET and PET

• In practice it is not possible to distinguish pure evaporation from pure transpiration, so they are usually considered together: i.e., Evapotranspiration (ET) • Theoretically, meteorological factors determine the maximum rate at which ET can occur: hence: Potential Evapotranspiration • Without renewal, ET drains the soil of moisture: (PET) • This rate will not be attained if water supply is limited or other factors restrict the passage of water through the plant thus: actual evapotranspiration

AET



PET

(AET)

AET estimation A simple way to estimate AET:

AET



PET

 f   

AW AWC

   where: AW = (SWC – PWP)  AWC = (FC – PWP)  rooting depth rooting depth AW = Available water AWC = Available water capacity SWC = Soil water content PWP = Permanent wilting point FC = Field capacity

Cell Water Potential

 Cells are the basic structural elements in plants; they control the fundamental plant response to water as it moves through the plants  The water potential gradient alone could not replace water lost due to transpiration  Need to consider osmosis (movement of water across a porous membrane separating two different concentrations  Therefore, the true water potential in a cell is:    

P

  P = = = Water potential Osmotic potential Internal cell wall pressure (turgor)

Stomatal Control

• Stomata generally occur on the undersides of leaves • Each stomate consists of two elongated guard cells and an opening • Cells in the walls of the stoma usually contain water, as CO 2 place in solution and O 2 exchange can only take • Stomata allow for an efficient means of gas exchange necessary for photosynthesis and control the amount of water loss • Stomatal aperture (opening) can be varied by changes in the turgor of the guard cells

Diurnal pattern of stomatal movements Extremely sensitive to light intensity Sensitive to CO 2 concentrations

• Availability of water in soil and plant (conservation of water)

Root Control

• Water availability at roots can be influenced the concentration of various chemicals and acids • Signals sent to leaves may modify stomatal operation • Removal of water by roots and the transpiration process will reduce soil conductivity and therefore increase resistance to liquid flow • Transpiration does not seem to be linked directly with root density, but rather the difference in potential within the system