Crafts et al. — 104— Water in Plants 



concentrations and temperature is held constant. The difference in liquid level before 

 and after immersion of the cylinder is recorded as the initial volume of each cylinder. 

 The cylinders are next transferred to larger volumes of the same solutions and are 

 permitted to come to equilibrium, the time being related to the diameter. For beetroot 

 cylinders 9 mm. in diameter and 28 mm. long, 8 hours were found to be sufficient (Cur- 

 rier, 1943). The cylinders are then freed of excess moisture, and their volumes again 

 determined. Absence of any change in volume is assumed to mean that the cylinder is 

 immersed in a sucrose solution equivalent to the DPD of water in the tissue. If such 

 a solution is not included, a graph may be prepared from which a fairly accurate esti- 

 mate of the DPD may be made. Errors due to non-uniform swelling of the tissue are 

 eliminated by this immersion method. 



Change in volume of a tissue piece may be affected by intercellular air. Increase 

 in turgor reduces the size of the intercellular spaces, as evidenced by the escape of gas 

 bubbles. Thus, the actual change in cell volume would be greater than that indicated 

 by tissue measurements. Similarly, the spaces might increase in size on shrinkage of 

 the tissue, giving low values. There should be no error in DPD measurements where 

 no, or only slight, volume change occurs. 



While the simplified or strip method involves fewer errors than the cell method, 

 there remain the difficulties due to removal of intercellular pressure at the cut surfaces, 

 and the low order of volume change of some tissues due to inelastic walls. The suit- 

 ability of each tissue must be determined in advance. 



The Weight Method: — This method involves the same principle, that is a sugar 

 concentration is sought which produces neither gain nor loss in weight of water by the 

 tissue. A major source of error is in reproducing comparable conditions of surface 

 moisture at the time of weighing the tissue pieces. Also, any surface active material 

 released from cut or injured cells that reduces the surface tension of the sugar solu- 

 tions to a point where wetting of the intercellular capillary spaces takes place will in- 

 troduce an error. This infiltration can be quite significant under some conditions 

 (AsHBY and Wolf, 1947). 



Meyer and Wallace (1941) found that the volume method (based on change in 

 length) and the weight method gave practically the same results when applied to 

 potato tuber tissue (Figure 34). Also that permeation of sugar into or loss of solutes 

 from the tissues at concentrations near the DPD were insignificant, and even at other 

 concentrations were not sufficient to change the trend of results. 



Other Methods for DPD: — Stocking (1945) devised a procedure 

 for studying the DPD of water in tissues of intact plants growing in 

 the field. Using squash plants, he injected sugar solutions of known con- 

 centrations into the hollow petioles of leaves of comparable size and age. 

 At periodic intervals thereafter he withdrew small samples of the solutions 

 and determined their concentrations using an Abbe hand refractometer. 

 Not only could he measure the DPD of water in the tissues at a given time, 

 but he could follow changes throughout a considerable period, dependent 

 mainly upon the volume of solution that could be held by the hollow petiole. 

 From his data he determined the diurnal trend in water deficit in the leaves 

 and calculated tensions in the xylem that corresponded to these. Values 

 varying between 0.2 and 9.1 atm. were calculated. 



A method for examining the DPD or water absorbing power of rela- 

 tively intact root tissue is credited to Sabinin (1925). When plants grow- 

 ing in water culture are excised above the root they usually exhibit exuda- 

 tion as a result of the phenomenon termed "root pressure." If sucrose or 

 mannitol is dissolved in the solution surrounding the roots, exudation 

 ceases and if a potometer is affixed to the bleeding stem it will be found 

 that the movement of water through the root may be reversed. By care- 

 fully adjusting the concentration of the bathing solution until movement 

 ceases entirely, a value will be obtained that might be considered in equilib- 

 rium with the DPD of the root system. However, van Overbeek (1942) 

 has shown by cryoscopic determination of the osmotic concentration of the 

 exudate and of the bathing solution that an active mechanism may be in- 



