transpiration

pathway of water

entry:

• sap in RHC has a lower W.P. than soil solution due to presence of sugars and mineral salts actively pumped in. Water enters via osmosis

• Entry of water dilutes RHC’s cell sap. RHC cell sap. now has a higher W.P than its adjacent cell. Water enters the inner cells via osmosis until it enter the xylem vessels.

entry of mineral ions

diffusion:

• when amt. in soil > amt. in RHC

active transport:

• when amt in RHC> amt. in soil

root pressure

• pressure resulting from constant entry of water into the roots

• gives rise to guttation

• causes sap to ooze from freshly cut stems

  mechanism:

• living cells around the xylem use active transport to pump ions into the xylem

• W.P. of xylem is lowered

• water flows into xylem from living cells by osmosis and flows upwards

capillary action

• tendency for water to move up inside capillary tube due to forces of cohesion between water molecules and adhesion between water molecules and xylem walls

transpiration definition

• loss of water as water vapours from the aerial parts of a plant , especially through the stomata of a leaf

transpiration pull

• the suction force caused by transportation which results in water moving up the xylem.

• MAJOR force in water transport

mechanism:

• transpiration occurs when water vapour diffuses out of intercellular air spaces in the leaf to the environment

• movement of water out of cells to replace thin film of moisture that has evaporated decreases W.P of cell sap

• Mesophyll cells absorb water via osmosis from cells deeper in the leaf, which in turn absorb water from xylem vessels

• produces a suction force that pulls the column of water in the xylem vessels up

effect of air humidity

INCREASES:

• conc. of water vapour outside the leaf increases in external environment

• conc. gradient of water vapour between intercellular air spaces and external environment becomes less steep

• rate of diffusion of water vapour out of stomata decreases

• rate of transpiration decrease

DECREASES:

• conc. of water vapour outside leaf decreases in external environment

• conc. gradient of water vapour between intercellular air spaces and external environment becomes more steep

• rate of diffusion of water vapour out of stomata increases

• rate of transpiration increases

effect of wind speed/air movement

INCREASES:

• conc. of water vapour outside leaf in external environment decreases

• conc. gradient between intercellular air spaces and external environment becomes more steep

• rate of diffusion of water vapour out of stomata increases

• rate of transpiration increases

DECREASES:

• conc. of water vapour outside leaf in external environment increases

• conc. gradient between intercellular air spaces and external environment becomes less steep

• rate of diffusion of water vapour out of stomata decreases

• rate of transpiration decreases

effect of temperature

INCREASES:

• conc. of water vapour inside leaf in intercellular air spaces increases, increased rate of evaporation of thin film of moisture

• conc. gradient between intercellular air spaces and external environment becomes more steep

• rate of diffusion out of stomata increases

• rate of transpiration increases

DECREASES:

• conc. of water vapour inside leaf in intercellular air spaces decreases, decreased rate of evaporation of thin film of moisture

• conc. gradient between intercellular air spaces and external environment becomes less steep

• rate of diffusion out of stomata decreases

• rate of transpiration decreases

effect of light intensity

INCREASES:

• size of stomata increases

• rate of diffusion of water vapour out of stomata increases

• rate of transpiration increases

DECREASES:

• size of stomata decreases

• rate of diffusion of water vapour out of stomata decreases

• rate of transpiration decreases

importance

• cools plants by removing latent heat of vaporisation

• maintains cell turgidity by bringing water to various plant parts

• delivers dissolved mineral salts along with water in transpiration stream

• delivers water up plant for photosynthesis and for use as a solvent/medium for biochemical reactions

wilting

ADVANTAGES:

• reduced transpiration rate

• prevents excessive water loss

DISADVANTAGES:

• reduced rate of photosynthesis

  • reduced light absorption and intake of carbon dioxide