Determination of the Parameters of Lichen Water Relations
Lichens are poikilohydric, i.e. can lose most of the water from their thalli, and then recover metabolic activity when re-wetted. They share this feature with most bryophytes and many algae, but poikilohydry is absent from the vast majority of higher plan
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Determination of the Parameters of lichen Water Relations RICHARD
P.
BECKETT
Introduction Lichens are poikilohydric, i.e. can lose most of the water from their thalli, and then recover metabolic activity when re-wetted. They share this feature with most bryophytes and many algae, but poikilohydry is absent from the vast majority of higher plants that are termed homoiohydric. Surprisingly little information is available on the basic water relations oflichens; only a few measurements of water potential and its components exist in the literature. Water potential (\ji) is an expression of the chemical potential or free energy status of water (Kramer and Boyer 1995). It is a measure of the driving force that causes water to move into a system, or from one part of a system to another. It is only possible to measure the chemical potential of water in a given system relative to the chemical potential of pure, free water, i.e., of water containing no solutes and bound by no forces. We take the \ji of pure water to be zero under standard conditions of temperature and pressure. The water potential of a plant cell under isothermal conditions has three components: Equation 1 where \jin is osmotic potential, \jip is turgor potential, and \jiM is matric potential. Osmotic potential is the contribution made by dissolved solids and is always negative in sign. Turgor potential is the contribution made by pressure exerted by the cell wall and is positive, or zero in plasmolysed cells. Matric potential is the contribution made by water-binding colloids and surfaces in the cell. It is negative in sign, and in living homoiohydric plant cells is usually sufficiently small to be ignored, although in lichen tissues at low water contents it is likely to be important. Richard P. Beckett, University of Natal, School of Botany and Zoology, Scottsville, Pietermaritzburg, 3209, Republic of South Africa (phone +27-33-260-5141;fax +27-33260-5105; e-mail [email protected])
I. C. Kranner et al. (eds .), Protocols in Lichenology © Springer-Verlag Berlin Heidelberg 2002
15 Determination of the Parameters of Lichen Water Relations
c
o
OSMOTIC
INCREASED
INCREASED BOUND
ADJUSTMENT
ELASTICITY
WMER FRACTION
A
B
NO ADAPTATION
f
100
90
100
80
100
80
100
80
RELATIVE Willl'ER CONTENT ('fb)
Fig.!. Representations of the relationships among relative water content and the components of water potential in a tissue. A: no adaptation to drought. B, C, and D: turgor maintenance by osmotic adjustment, increase in elasticity, and increase in bound-water fraction, respectively. The horizontal dashed line indicates the IjI that in A is associated with zero turgor. In B, C, and D the IjI for zero turgor is considerably lower. The vertical dashed lines within each diagram indicate the relative water content and turgor associated with the constant 1jI. Note that for constant IjI the relative water content is unchanged in B, decreased in C, and increased in D. Taken from Radin (1983).
Measurement of water potential enables the metabolic
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