The document discusses osmotic potential, which is the potential of water molecules to move from a hypotonic solution to a hypertonic solution across a semi-permeable membrane due to differences in solute concentration. It describes how osmotic potential is always negative due to the presence of solutes, and how it is measured using a vapor pressure osmometer (VPO) which can measure samples as small as 2 microliters. The VPO provides accurate measurements of osmotic potential but has limitations for direct measurement of multiple tissue samples due to lengthy equilibration times.

1. Measurment of osmotic
potential

2. Osmotic potential
• The potential of water molecules to move
from a hypotonic solution (more water,
less solutes) to a hypertonic
solution (less water, more solutes) across a
semi permeable membrane.

3. Cont..
• A pure water contains no solutes, thus, it
should have zero (0) water potential. And also
for this reason, the value of osmotic potential
of a solution is always negative since the
presence of solutes will always make
a solution have less water than the
same volume of pure water.

4. Cont..
• when two solutions are isotonic the osmotic
potentials will be equal, and there will be no
net movement of water molecules.
• When different, the solution that
is hypotonic (diluted solution,
less solutes more water) will have higher
osmotic potential (less negative ψπ ).

5. Cont..
• whereas the solution that
is hypertonic (concentrated solution,
more solutes less water) will have lower
osmotic potential (more negative ψπ).
Difference in osmotic potentials will
cause water molecules to move from
a hypotonic solution to a hypertonic solution.

6. Cont..
• The pressure probe, which is routinely used
to measure the turgor potential (Ψp) of
individual epidermal cells in Tradescantia
virginiana (L.), has also been used to sample
small volumes of vacuolar fluid from these
same cells (as low as 0.02 nl)
formeasurement of
cellular solute (osmotic) potential (Ψs) in a
micro .

7. Measurment of osmotic potential
• The vapor-pressure osmometer (VPO) is used
to measure osmotic potential (ψπ) in plant
water relations.
• The VPO was designed for 10 μL solution
samples but its performance can be extended
to suit smaller volumes and actual tissue
samples.

8. Cont..
• Measurement of ψπ by the VPO was
compared to end-window (EW) psychrometry
on known saline solutions and tissue samples
varying from 10 to 2 μL.
• The VPO was a more sensitive instrument
compared to the small EW psychrometer
when using 10 μL of solution in the standard
dish.

9. Cont..
• Volumes varying from 3 to 10 μL could be
used with reasonable accuracy and precision if
a thinner disk (to ensure saturation) or smaller
dish was used.
• Small samples such as root tips required a
suitably small volume chamber to reduce
effects of lowering ψπ.

10. Cont..
• For leaves from a field study, variability in
plant replicates was greater than VPO
sensitivity, so precision was comparable to
using psychrometers.
• For direct measurement of water potential,
lengthy equilibration times made the VPO an
unsuitable method for measuring multiple
samples.

11. Cont..
• The VPO could be used for both small solution
volumes and direct measurement of ψπ in
tissue.

12. Thank
you