Plant Physiology

Water balance of plants

Water in the soilThe water content and the rate of water

movement in soils depend to a large extent on soil type and soil structure.

Sand

Silt

Clay

•Desert

•Under water bodies (canals)

•Traditional houses

Water in the soil consists of 3 parts:1- Gravitational water: water filled in the big spaces/interstices of soil particles and

is readily drained from them by gravitation.• Gravitational water is found in the macropores. It moves rapidly out of well

drained soil and is not considered to be available to plants.• It can cause upland plants to wilt and die because gravitational water occupies

air space, which is necessary to supply oxygen to the roots.• Drains out of the soil in 2-3 days2- Bound water: water tightly adhered to the soil particles.• This water forms very thin films around soil particles and is not available to the

plant. The water is held so tightly by the soil that it can not be taken up by roots. • not held in the pores, but on the particle surface. This means clay will contain

much more of this type of water than sands because of surface area differences. 

• Gravity is always acting to pull water down through the soil. However, the force of gravity is counteracted by forces of attraction between water molecules and soil particles and by the attraction of water molecules to each other.

3- Capillary water: Water filled in the small spaces/interstices of particles, easily get to the surface of water by the force of capillarity.

• Most, but not all, of this water is available for plant growth• Capillary water is held in the soil against the pull of gravity 

Forces Acting on Capillary Water

• Capillary water is held by cohesion (attraction of water molecules to each other) and adhesion (attraction of water molecule to the soil particle). 

• The amount of water held is a function of the pore size (cross-sectional diameter) and pore space (total volume of all pores) 

Field capacity:• Field capacity is the water content of a

soil after it has been saturated with water and excess water has been allowed to drain away due to the force of gravity.

• Field capacity is large (40%) for clay soils and soils that have a high humus content and much lower (3%) for sandy

Water Moves through the Soil by Bulk Flow

• Water moves through soils predominantly by bulk flow driven by 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.

Water absorption by the root

Root tip—the water absorption zone

The overall scheme of water movement through the plant

1- From soil to root epidermis– Diffusion to the intercellular space• Capillary movement of soil water to plant

roots. Plant root removes water. Tension in the soil right around the root increases gradient flow of water from low tension to high. This keeps a source of capillary water flowing to the plant root.

– Osmosis to the epidermis cells

1- 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.)

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

3- Transmembrane pathway: water sequentially enters a cell on one side, exits the cell on the other side. In this pathway, water crosses at least two membranes for each cell in its path.

Symplast pathway and transmembrane pathway are two components of cellular pathway,

2- From epidermis to and through cortex

Transversing endodermis

• Casparian strip?– Casparian strip is a band

of cell wall material deposited on the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele of a plant.

– To transverse casparian strip, apoplast pathway does not work (blocked), only cellular pathway works

Stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis.

3- From endodermis to root vessel apoplast pathway and cellular pathway

(diffusion or osmosis)4- From root vessel to stem vessel to leaf

vessel apoplast pathway (mass flow)5- From leaf vessel → leaf mesophylls and

intercellular space→stomatal cavity→stomata →air (diffusion or osmosis)

Driving Forces of Water absorption and movement

1- Root Pressure2- Transpiration pull

1- Root Pressure

• Solute Accumulation in the Xylem Generates “Root Pressure”

• The root absorbs ions from the dilute soil solution and transports them into the xylem. The buildup of solutes in the xylem 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 water absorption.

GuttationAppearance of xylem sap drops on the tips or edges of leaves e.g. grasses•Sugars, mineral nutrients and potassium

Dew?

•Transpiration stops at night time due to stomata closing• High soil moisture level• Lower root water potential•Accumulation of water in plants•Plants will start bleeding through leaf tips and edges

2 Transpiration Pull

　 Transpiration-cohesion theory

Transpiration is the loss of water through the stomata in leaves. This loss of water causes an area of low pressure within the plant and water moves from where it is at high pressure to low pressure. The cohesion part is what allows water to do this against gravity.

How do we genetically manipulate plant water relations?

Arabidopsis as example!!!

Mutation in MRH2 Kinesin (ARM domain-containing kinesin-like protein) Enhances the Root Hair Tip Growth Defect

Knockout of PARG-1 gene causes Arabidopsis plants to wilt earlier than the wild type under drought stress

Stomata fail to close under scarce water conditions

Arabidopsis PARG1 mutants