中文课件：

3 水分的吸收、运输、蒸腾.PPT

soil water properties and plant water absorption

driving force for water absorption and movement

Water balance

Soil – Plant – Atmosphere

   

5% of water absorbed by plant  isretained for growth and biochemistry, others is lost into atmosphere viatranspiration

Water in the soil

The water content and the rate of water movement in soils depend to a large extenton soil type and soil structure.

water in the soil consists of 3parts: Gravitational water: water filled in the big spaces/interstices of soil particles and is readily drained from them bygravitation. Bound water: water tightly adhered to the soilparticles. Capillary water: Water filled in the smallspaces/interstices of particles, easily get to the surface of water by theforce of capillarity.

Field capacity

Soil saturation capacity is the water content of a soil after it has been saturated with water and excesswater has been allowed to drain away.

Field capacity is  the soil saturation capacity minus gravitational water.Field capacity for different types of soil: clay soils (41-47%%) > silt soils(22-27%) and sand soils (14-18%).

Water absorption by the root

Water moves through soils predominantlyby bulk flow drivenby a pressure gradient,although diffusion also accounts for some water movement.

As a plant absorbs water from the soil,it depletes the soil of water near the surface of the roots.

The main absoption area is the root tip.

Water transport from epidermis to and through cortex 

apoplast pathway: water moves exclusively through the cell wall without crossing any membranes.(The apoplast is the continuous system of cell walls and intercellular air spaces in plant tissues.)

Symplast pathway: water moves through the symplast, traveling from one cell to the next viathe plasmodesmata (The symplast consistsof the entire network of cell cytoplasm interconnected by plasmodesmata.)

transmembrane pathway: watersequentially enters a cell on one side, exits the cell on the other side. Inthis pathway, water crosses at least two membranes for each cell in its path.  

Symplast pathway and transmembranepath way are two components of cellular pathway

Driving Forces of Water absorption and movement: Root pressure and Transpiration pull

Solute Accumulation in the Xylem Can Generate “Root Pressure”

The root absorbs ions from the dilutesoil solution and transports them into the xylem. The buildup of solutes in thexylem sap leads to a decrease in the xylem osmotic potential (Ψs) and thus a decrease in the xylem water potential (Ψw). This lowering of the xylem Ψw provides a driving force for waterabsorption.

Root pressure is also sometimes visible on leaves. Under conditions  of high humidity, cool temperature, and low light exposure root pressure can  push xylem fluids through leaf mesophyll and out some larger pores in the leaves  called hydathodes. Thus on a cool morning as you walk across the grass  you notice a drop of liquid on the tip of each blade. You may have thought this  was dew, but because it is on the upward pointing tip, you realize that this  cannot be so. A test of solutes would demonstrate that this is xylem sap, not  condensed humidity! The process by which this exudes is called guttation and it is driven by root pressure. Parking your car under certain species of  trees can leave some nasty "water spots" on your wax job. Again, if this were  dew, the pure condensation would not leave a mineral spot behind; this is xylem  sap that dries, leaving a mineral deposit.

Capillarity allows water to climb up the xylem

We have already discussed this in a previous lecture. Again the  amount of climb that is possible in a tracheid of normal diameter is perhaps a  meter up the plant. This capillarity is a function of adhesion of the liquid to  the cell wall of the xylem, and cohesion of the water molecules to each other.  

Evaporation from the leaves pulls water up the xylem

Evaporation from the intercellular spaces in the leaf into the  atmosphere is a strong pull that removes water from the top of the column of  water in the xylem. This process generates sufficient force to lift the column  of water up against the gravity vector in tall trees.

The negative pressure that causes water to move up through the xylem develops at the surface of the cell walls in the leaf.  The cell wall acts like a very fine capillary wick soaked with water. Water adheres to the cellulose microfibrils and other hydrophilic components of the wall. The mesophyll cells within the leaf are in direct contact with the atmosphere through an extensive system of intercellular air spaces. Initially water evaporates from a thin film lining these air spaces.

As water is lost to the air, the surface of the remaining water is drawn into the interstices of the cell wall, where it forms curved air–water interfaces. Because of the high surface tension of water, the curvature of these interfaces induces a tension, or negative pressure, in the water. As more water is removed from the wall, the radius of curvature of the air–water interfaces decreases and the pressure of the water becomes more negative.