Chapter VIII _129— Active Relations 



These authors could not escape the conclusion that water absorption 

 under the conditions of their experiments was in some manner linked to 

 vital processes. Among these processes, respiration and protein synthesis 

 appeared to proceed proportionately to water intake, even though the con- 

 centration of respirable reserves (sugars, amino acids) were depleted to a 

 greater extent than in other cultures where protein synthesis and water 

 absorption were less. The suggestion was offered "that actively metaboliz- 

 ing cells which can grow may absorb water in a manner which has but little 

 relation to any conventional osmotic or suction pressure theory but may 

 be more directly linked with metabolic processes (respiration and protein 

 synthesis) ; processes which are determined by oxygen and affected by the 

 nature of the salts present in the external solution." 



In the view of Reinders (1942), increased hydration of the protoplasm 

 may possibly result from the formation of osmotically active solutes in the 

 "so-called free water" of the protoplasm. 



The problem was further investigated by Commoner, Fogel and Mul- 

 LER (1943), who reported an "active absorption of water against an osmotic 

 gradient." A decrease in wet weight by tissue immersed in aerated 0.2 M 

 (hypertonic) sucrose solution could be prevented by the addition of auxin 

 (indole-3-acetic acid). Where KCl or fumarate were also present, the 

 tissue increased in wet weight. In the absence of auxin, water was lost. 

 The authors postulated an active process, one closely associated with solutes, 

 especially the four-carbon dicarboxylic acids, to be responsible. The sug- 

 gestion that auxin controls absorption or accumulation of salt, thus affect- 

 ing the intake of water, does not concur with the statement that water enters 

 actively against an osmotic gradient. Figure 38 graphically represents 

 some of the data of this investigation. 



Van Overbeek (1944) criticized the work of Commoner, et al., for 

 two reasons. The lack of aseptic conditions during the tests prolonged for 

 a week or more might introduce complicating factors. And since Reinders 

 observed marked absorption in distilled water, the suggestion that the auxin 

 effect is on salt metabolism must be erroneous. The experiments were re- 

 peated under both aseptic and non-aseptic conditions, with the result that 

 the effect of auxin on water absorption of potato was confirmed. Greater 

 regularity, however, obtained under aseptic conditions (Figure 39). Van 

 Overbeek was unable to demonstrate an accelerating effect due to the addi- 

 tion of KCl or fumarate to distilled water, 0.2 M sucrose or 0.2 M mannitol 

 solutions containing auxin. Naphthalene acetic acid acted similarly to in- 

 doleacetic acid. By means of cryoscopic determinations, sap expressed from 

 the auxin treated discs was found to have a lower osmotic pressure than that 

 from the control discs, so that on this basis the effect of auxin is not one of 

 increasing the amount of osmotically active solutes. The decrease in OP 

 was in line with the increased volume of the tissue so that probably there 

 was no change in total solute. 



We must conclude with van Overbeek that the effect of auxin must be 

 either (a) one of decreasing the wall pressure or (^) an effect of "active" 

 pressure. While the first is a definite possibility, there is also reason to be- 

 lieve that water absorption may be in some way associated with the increase 

 in respiratory activity produced by auxin. 



Active forces leading to retention of water against an apparent osmotic 

 gradient would, on first consideration, be sought in rapidly growing tissue. 

 The potato tuber is a dormant structure, yet potentially it is capable of 



