2. General introduction
• Chemical potential- quantitative expression of
free energy.
• Water flows from region of higher potential to
lower potential
• Water potential- chemical potential/ partial
molal volume.
• Water potential denote effects of solute,
pressure and gravity.
3. • Water potential measured by psychrometers.
Sometimes by liquid filled microcapillary.
• Psychrometer take advantage of latent heat of
vaporisation.
• Water moves from less negative water potential to
more negative water potential.
• Water moves into and out of the cell in response to
water gradient. The direction of flow is determined by
direction of water potential.
• Hydraulic conductivity describes how readily water
move into . and out of the cell across cell membrane.
• Short half time means fast equilibration. Cell with high
surface to volume ratios, high membrane hydraulic
conductivity and stiff cells will rapidly come to
equillibration with their surroundings.
4. • Plants grown under saline environment have
lower value of osmotic potential. Low water
potential allow root cells to extract water from
saline water.
• Plant may also exhibit quite negative water
potential under draught conditions. Water stress
typically lead to accumulation of solutes in
cytoplasm and vacuoles of plant cells, thus
allowing cells to maintain turgor pressure despite
low water potential.
• In very dry soils, water potential may fall to
permanent wilting point at which plant cannot
maintain turgor pressure.
5. ROOT PRESSURE
• Roots can generate root pressure. (0.05-0.5 M
Pa).
• Positive root pressure assosciated with
guttation. It helps to dissolve xylem bubbles.
• Water movement in xylem is by bulk flow
along pressure gradient. Only root pressure is
not sufficient but pressure tension that
develops in leafs are also required.
6. • Cohesion and adhesion makes the activation energy of
transition to vapor phase very high and xylem structure
minimises the existence of nucleating sites for bubble
formation.
• In soil and xylem, liquid water moves by bulk flow in
response to pressure gradient.
• When liquid water is transported across membrane
driving force is water potential difference across
membrane. Such osmotic flow occurs when cells absorb
water and when roots transport water from soil to xylem.
• In vapor phase water moves primarily by diffusion until it
reaches outside air where convection becomes dominant.
