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