1. Water Balance of Plants
Syed Abdullah Gilani

2. Water Absorption by the Roots
Absorption of water by plant roots requires:
an intimate contact between the root surface and the soil
the growth of the root and root hairs maximizing the surface area in contact with the soil (i.e. maximizing ions and water uptake by the root).
Water enters the root from the portions of the root system near the root tips.
Most mature portions of the root system often have protective tissues (exodermis or hypodermis) containing hydrophobic materials (relatively impermeable to water).
In disturbed soils, new root growth is needed to reestablish soil-root contact.

4. Water Movement in the Root
Water movement in the soil is by bulk flow. Water movement in the root is more complex.
Water can flow from the epidermis to the endodermis of the root through three pathways: the apoplast, symplast, and transmembrane pathways.
The apoplast pathway
Water travels across the root cortex without crossing any membranes.
Water moves through cell walls, any water-filled extracellular spaces, and lumen of cells that have lost their cytoplasm.

5. The symplast pathway
The entire network of the cell cytoplasm interconnected by plasmodesmata.
Water travels across the root cortex by passing from cell to the next via the plasmodesmata, without crossing any semi-permeable membranes.
The transmembrane pathway
Water crosses at least two membranes for each cell in its path (the plasma membrane on entering and on exiting).
The Casparian strip (made from suberin) in endodermis breaks the continuity of the apoplast pathway, forcing water and solutes to cross the endodermis by passing through the plasma membrane.

6. Connection between Root Respiration and Water Uptake
Water uptake decreases when roots are subjected to low temperature or anaerobic conditions, or treated with respiratory inhibitors. 
“Guttation” and “root pressure”
Accumulation of solutes “ions” in the xylem generates a positive hydrostatic pressure (root pressure).
Lowering of the water potential of xylem leads to more water uptake by the root, which in turn leads to a positive hydrostatic pressure in the xylem
High water potential of the soil and low transpiration rate of the plant leads to more root pressure.
Developing of positive root “xylem” pressure results in guttation, exudation of xylem sap through hydathodes.

7. Guttation in leaves from strawberry

8. Water Transport through the Xylem
Xylem forms the longest (99.5% or more in tall trees) of the pathway of water transport through the plant.
Compared to layers of living cells, xylem is a simple pathway of low resistivity for water transport.
How the structure of the xylem contributes to the movement of water from the roots to the leaves?
How negative hydrostatic pressure generated by transpiration pulls water through the xylem?

10. Structure of xylem
Xylem consists of two types of tracheary elements: tracheids and vessel elements.
Tracheids and vessel elements are functional water-conducting cells; they have no membranes and no organelles. (i.e. hollow cells” tubes” with thick, lignified cell walls).
Tracheary elements are pitted (have pits) on their lateral sides “walls”.
Pits are microscopic regions have no secondary walls and their primary walls are thin and porous. Pits occur in pit pairs.

11. The pit membrane is the porous layer between pit pairs
The pit membrane is the porous layer between pit pairs. The pit membrane between pit pairs consists of two primary walls and a middle lamella.
Pit membranes in conifers tracheids have a central thickening (a torus), acts like a valve to close the pit (i.e. prevent cavitation and embolism within the xylem).
Tori are not found in pit membranes, whether in tracheids or vessel elements, in all other plants.
1- Tracheids:
Present in angiosperms, gymnosperms, and ferns.
Elongated, spindle-shaped cells with tapered ends, arranged in overlapping vertical files.

12. 2- Vessel elements:
Present in angiosperms and some gymnosperms and ferns.
Shorter and wider than tracheids.
Have perforated end walls (i.e. a perforation plate exists at each end of the cell).
Vessel members are stacked end to end (through the perforation plates) to form a larger tube (vessel).
The vessel members at the extreme ends of a vessel lack perforations in their end walls and communicate with neighboring vessels via pit pairs.

14. Water Movement through the Xylem
The driving force required to move water through cell-to-cell pathway at a given velocity is ten orders of magnitude greater than the driving force required to move water through the xylem at the same velocity,
i.e. water flow through the xylem is vastly more efficient than water flow across living cells.
Thus, xylem provides a pathway of low resistivity for water movements
In other words, xylem reduces the pressure gradients needed to transport water from the soil to the leaves.

15. The Cohesion-Tension Theory of Sap Ascent
The pressure needed to move water through the xylem could result from:
positive pressures generated at the base of the plant
negative pressures generated at the top of the plant.
Some roots can develop positive hydrostatic pressure in their xylem.

16. For some reasons, root pressure is inadequate to move water up to a tall tree:
it is less than 0.1 MPa (at least 3MPa is needed)
high transpiration rate
dry soils
a mechanism is required by the plant to deal with accumulation of solutes once the water evaporates from the leaves.
Water at the top of a tree develops a large tension force (a negative hydrostatic pressure) (tension pulls water through the xylem).
The cohesive forces (properties) of water is needed to sustain the large tension forces of the water columns in xylem

17. The driving force for water movement through plants originates in the leaves.
What is the source of the negative pressure of water in leaves? (Fig. 4.9)
How does the negative pressure of water in leaves serve to pull water from the soil?
The negative pressure that causes water to move up through the xylem develops at the surface of cell walls in the leaf.
The cellulose microfibrils of the cell wall act as a very fine capillary network.

19. As more water is removed from the walls:
Ψp = -2T/r
T = surface tension of water (7.28 x 10-8 Mpa
r = radius
As more water is removed from the walls:
The air-water interface curvature increases,
the smaller radius results,
more negative water potential develops

20. Water adheres to the cellulose microfibrils and other hydrophilic components of the wall.
The mesophyll cells within the leaf are in direct contact with atmosphere through intercellular air spaces.
Air-water interfaces are developed within the leaf tissues (Interface means thin boundary or layer between water and air).
As water is lost to the air, the surface of the remaining water is drawn into the interstices of the cell wall; resulted in formation of curved air-water interfaces (Interstice means opening or space between objects).

21. As more water is removed from the wall, the curvature of the air-water interfaces increases (becomes of smaller radius, r), resulted in more negative water pressure in the leaves.
The driving force for water movement through plants originates in the leaves.
The movement of water through plants can occur without direct expenditure of metabolic energy.
The energy input that powers the movement of water through plants comes from the sun.