Can There Be a Positive Water Potential in Plants?

Abstract

= P=DPD u L Vw where the reference state is pure, free water at atmospheric pressure, and V, is the partial molal volume of water. The units of ?p are bars or atmospheres. is acknowledged in discussions of plant water relations that the water potential of pure, free water, thus, is zero, and the water potential of water in the plant may be zero (when ? = P) but usually has a negative value (Slatyer, 1967; Kramer, 1969). This latter point suggests that there is a possibility of the plant water potential being greater than the reference state, i.e., that plant water potential could be positive. In fact, after looking through the latest general discussions of plant water relations, I find that several authors do indeed allow for such a possibility. is felt by these authors that positive water potentials could arise in the xylem of plants when the xylem contents are subjected to pressure, i.e., when there is a positive root Barrs (1968) is quite cautious in admitting the possibility of a positive water potential when he says, Total water potential is also normally zero or some negative value, but it could conceivably become positive under the influence of root pressure, although this would be a very unusual condition. Slatyer (1967) is slightly less cautious when he states that, It is probably only positive during the process of guttation or similar phenomena. Crafts (1968), however, allows for a positive water potential in all plants apparently when in discussing seed germination he states, As the embryo expands and grows into a seedling, water potential in the young xylem elements passes the zero value and becomes positive; the seedling exhibits guttation from the leaf tips and the excised root produces xylem exudate.' Such statements make one wonder if an earlier statement of Meyer's (1956) may simply have been perpetuated without much thought given to the plausibility of it. Meyer (1956) said, Occurrence of negative diffusion pressure deficits in plants is rare: the only common example is when the sap in xylem cells or vessels is subjected to a root pressure. seems that great importance is attached to the fact that under such condi-