TRANSPIRATION
$\displaystyle \small \bullet$ Transpiration is the loss of water in the form of water vapor from aerial parts of plants.
$\displaystyle \small \bullet$ Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. The remaining is lost by transpiration.
$\displaystyle \small \bullet$ During transpiration, exchange of $\displaystyle O_{2}$ & $\displaystyle CO_{2}$ in the leaf also occurs through stomata.

$\displaystyle \small \bullet$ Opening or closing of the stomata is due to change in the turgidity of the guard cells.
$\displaystyle \small \bullet$ The inner wall of guard cell lining stomatal aperture is  thick and elastic, and the outer wall is thin.  
$\displaystyle \small \bullet$ When turgidity of guard cells increases, the outer walls bulge out and pull the inner walls into a crescent shape.  
$\displaystyle \small \bullet$ Cellulose microfibrils in the guard cells are oriented radially rather than longitudinally making it easier for the stoma to open.
$\displaystyle \small \bullet$ The guard cells lose turgidity due to water loss (or water stress) and the inner walls regain their original shape. As a result, the stoma closes.
$\displaystyle \small \bullet$ Usually the lower surface of a dicot leaf has a greater number of stomata. In monocot leaf, they are about equal on both surfaces.

Factors Affecting Transpiration
$\displaystyle \small \bullet$ External factors: Temperature, light, humidity, wind etc.
$\displaystyle \small \bullet$ Plant factors:  Number and distribution of stomata, Number of stomata open, Water status of the plant, Canopy structure etc.
$\displaystyle \small \bullet$ The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
 $\displaystyle \small \circ$ Cohesion: mutual attraction between water molecules.  
 $\displaystyle \small \circ$ Adhesion: attraction of water molecules to polar surfaces (such as the surface of tracheary elements).  
 $\displaystyle \small \circ$ Surface Tension: water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
$\displaystyle \small \bullet$ In plants capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.
$\displaystyle \small \bullet$ The forces generated by transpiration can create pressures sufficient to lift a xylem sized column of water over 130 metres high.

Transpiration & Photosynthesis – a Compromise
$\displaystyle \small \bullet$ Creates transpiration pull for absorption and transport of plants
$\displaystyle \small \bullet$ Supplies water for photosynthesis
$\displaystyle \small \bullet$ Transports minerals from the soil to all parts of the plant
$\displaystyle \small \bullet$ Cools leaf surfaces, sometimes 10 to 15 degrees, by evaporative cooling
$\displaystyle \small \bullet$ Maintains the shape and structure of the plants by keeping cells turgid

UPTAKE AND TRANSPORT OF MINERAL NUTRIENTS
$\displaystyle \small \bullet$ Most of the minerals enter the roots by active absorption into the cytoplasm of epidermal cells because
(i) Minerals occur in the soil as charged particles (ions) which cannot move across cell membranes.
(ii)  The concentration of minerals in the soil is usually lower than the concentration of minerals in the root.
$\displaystyle \small \bullet$ The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis.
$\displaystyle \small \bullet$ Some ions are absorbed passively.
$\displaystyle \small \bullet$ The specific membrane proteins of root hair cells actively pump ions from the soil into the epidermal cells.
$\displaystyle \small \bullet$ Endodermal cell membrane also has transport proteins.  They allow some solutes cross the membrane, but not others. These proteins are control points, where a plant adjusts quantity and types of solutes that reach the xylem.
$\displaystyle \small \bullet$ The suberin in the root endodermis allows the active transport of ions in one direction only.

Translocation of Mineral Ions
$\displaystyle \small \bullet$ The ions reached to xylem are further transported to all parts of the plant through the transpiration stream.  
$\displaystyle \small \bullet$ The chief sinks for the mineral elements are growing regions such as apical and lateral meristems, young leaves, developing flowers, fruits and seeds, Storage organs.
$\displaystyle \small \bullet$ Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
$\displaystyle \small \bullet$ Mineral ions are also frequently remobilized, particularly from older, senescing parts (e.g. older dying leaves) to younger leaves.  

PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINK
$\displaystyle \small \bullet$ Sucrose is transported by the vascular tissue phloem from a source to a sink.
$\displaystyle \small \bullet$ The glucose prepared at the source (by photosynthesis) is converted to sucrose (a disaccharide).  
$\displaystyle \small \bullet$ Sucrose is moved into the companion cells and then into the living phloem sieve tube by active transport (loading).
$\displaystyle \small \bullet$ Food (sucrose) is transported by phloem from source to sink. The part of plant that synthesize the food is called source and part where food is used or stored is called sink.
$\displaystyle \small \bullet$ The source and sink can be reversed by the plants depending upon the season or plant’s need. So, the direction of movement in the phloem is bi-directional.