SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

48
l/J =
o
J1.w - J1.w
Vw
l/J is of ten expressed in MPa (1 MPa = 10 bar). An alternative expression
relates the ratio of water vapour pressure in the system (p) to that of
saturated vapour (po) as follows:
RT P
l/J =- ln-
Vw Po
where R is the gas constant 8.3143 J mol- 1 K-l and T is the absolute
temperature (K).
As for soil, plant water potential can be divided into various components:
Le. osmotic potential (l/Jrr)' turgor pressure (l/J p ), matric potential (l/J m ) and
gravitational potential. As the gravitational component is only 0.01 MPa
m-l it can, except in high trees, be neglected. Hence:
l/J = l/Jrr + l/J p + l/J m
For plant tissue in equilibrium with its surroundings the total water
potential (l/J) is the same throughout the system; Le. in xylem, wall,
cytoplasm and vacuole. However, the components of the total water
potential may be quite different. In the vacuole the total water potential
arises largely from osmotic and turgor forces, whereas in wall and xylem
matric forces are predominant. The osmotic potential of the apoplastic
water (water in wall and xylem) is high, less than -O.OS MPa.
Plant cells (cytoplasm and vacuole) behave as osmometers caused by the
semi -permeable plasma membrane, and l/J within cells equilibrates with
the surroundings within seconds. The main components relevant to plant
cells are turgor potential (l/J p ) and osmotic potential (l/J rr ). Hereby:
The turgor pressure (l/J p ) is the pressure difference between that inside and
outside the cell wall.
An useful approximation of l/J rr is the van't Hoff relation:
l/J = -RTC
rr s (S)
where C s is the concentration of solutes (mol m -3 solvent). R and T are
as defined above. Typical cell sap from many plants has an osmotic
potential (l/J rr ) of about -1 MPa.
The water relations of plant cells (tissues) are of ten described by the
Höfler diagram (Fig. 1) which shows the interdependence of cell volume
l/J, l/J rr and l/J p as the tissue loses water (decrease of RWC). In a fully turgid
(1)
(2)
(3)
(4)