The Physiology of Flowering Plants - KHAM PHA MOI

64 WATER RELATIONSshrinking cell contents press less strongly on the wall, and as aconsequence of the solutes being concentrated into a smaller volumeand lowering the C p component. When the stage is reached wherethe protoplast no longer presses against the wall, the cell is said to beflaccid; nowC p ¼ 0; and C ¼ C p (3:4)Box 3.1There is some variation in notationregarding the components ofwater potential. Osmotic potential,which here is represented by Cp,has by various workers been calledCs (for solute), Co (for osmotic),or just plain p. It is also quite commonto find subscripts such asCplant or Csoil used to denote thewater potentials of varioussystems.If still more water is removed, the effect depends on the mode ofremoval (Fig. 3.1). If the water is removed by evaporation, drying outin air, the cell shrivels in size and the wall caves in or folds as theshrinking protoplast pulls on it. The degree of flexibility of the wallmay determine how much water can be removed. The tissue becomeswilted. If, however, the water is removed by immersion of the cell in asolution of low C p , plasmolysis results: the protoplast shrinks awayfrom the wall and the external solution fills the space between theplasma membrane and the wall; there is no further decrease in overallcell size. The relationships between C, C p and C p are showngraphically in Fig. 3.2. Under field conditions, wilting is more usualthan plasmolysis. Loss of water beyond a limit, which varies with thetissue, is fatal; this is discussed in Chapter 13 under desiccationstress.Actual values of cell C of plants growing in the field in a temperateclimate and with a fairly adequate supply of water fall mostly inthe range of –0.1 to –2.0 MPa, but may fall well below this in times ofwater shortage, and in extreme climates or habitats such as deserts orsalt marshes; in the latter the apparently generous external watersupply is at a low C owing to the osmotic effect of the salt. Plant Cvalues below –10 MPa have been recorded.Quantitative consideration of plant water potentials needs clearthinking. Because values of C of plants and in the natural environmentare negative, one must be careful to keep in mind thate.g. –2 MPa is lower than –1 MPa. Sometimes the terms ‘higher’ and‘lower’ are avoided by referring to ‘less negative’ and ‘more negative’values. One must also accustom oneself to thinking of zero as a highvalue, the highest that the C of plant or soil in most cases can attain.(Plant tissues under high pressure, such as squirting glands or squirtingfruits, which can eject their contents to considerable distances,may have positive C values. But these play no significant part inoverall plant water relations.)3.3.2 Measurement of water potential and its components inplant cells and tissuesThe overall C of a cell or tissue can be measured by exposing replicatesamples of the tissue to a graded series of C, either by immersing thesamples in solutions of known C, or by enclosing them in atmospheresof known C (vapour pressure). Changes in water content ofthe samples are detected by weighing the samples before and after