1 84 Fundamentals of Auxin Action 



ceased. The application of respiratory inhibitors also prevented the 

 water uptake. These studies and the subsequent ones of Hackett and 

 Thimann (1952) establish that water uptake in response to auxin is 

 dependent upon oxidative metabolism. Bonner et al (1953) have 

 shown in addition that dinitiophenol could block water uptake with- 

 out inhibiting respiratory rate, and suggest that phosphorylation re- 

 actions provide the energy for water uptake phenomena. These char- 

 acteristics of water uptake are precisely similar to the characteristics 

 of growth. 



There seems to be no doubt that water uptake is an integral part 

 of growth, and yet there is some difficulty in determining whether 

 water uptake is the cause or only the consequence of growth. Burstrom 

 (1953) has attempted to distinguish between reversible and irreversible 

 water uptake (growth) by a comparison of the behavior of roots in 

 solutions of various osmotic values. Water uptake which could be 

 reversed by plasmolysis was subtracted from that which was essentially 

 irreversible. The irreversible uptake in hypotonic solutions was found 

 to be independent of the osmotic value of the external medium, which 

 leads Burstrom to conclude that water uptake is a consequence and 

 not the cause of growth. 



Levitt (1947) has leveled another criticism at the concept that the 

 driving force of growth is an active water uptake. He calculates that 

 the energy required for the assimilation of water by some secretion 

 mechanism against an osmotic gradient would require energy utiliza- 

 tion far beyond the capacity of the cells to supply. Thus, for beet cells 

 to maintain a turgor of 5 atmospheres, Levitt calculated that the en- 

 tire dry matter of the beet would be utilized in three months, assum- 

 ing that all of the respiratory energy was applied to the w^ater uptake 

 mechanism. 



It seems difficult to expect, then, that plant cells may grow by 

 means of an active uptake of water against an osmotic gradient. And 

 if the theory of Commoner et al (1943) that water is taken up by 

 an osmotic mechanism driven by respiration is not accepted, it be- 

 comes difficult too to expect that growth may be driven by an osmotic 

 water uptake. If water uptake forces other than simple osmotic ones 

 may be found, then the concept of growth as a function of such forces 

 may be reopened. 



THEORIES OF CELL WALL EFFECTS 



Heyn (1940) has observed that auxin applications cause a pro- 

 nounced increase in flexibility and extensibility of cell walls. He pro- 

 posed that such an increased extensibility would result in a drop of 



