AUXIN-INDUCED WATER UPTAKE! 



J. Bonner, L. Ordin, and R. Cleland 

 Kerckhoff Biological Laboratories, California Institute of Technology, Pasadena, California 



The increase in cell size which is induced in plant tissue by applied auxin is 

 an expression of and intimately related to the net uptake of water by the 

 tissue under the influence of the hormone. In order to find out how auxin 

 increases cell size it is therefore necessary to find out how auxin 

 brings about net water uptake. Three general kinds of suggestions have been 

 made concerning mechanisms which might be involved in such auxin-induced 

 net water uptake. One proposal is that auxin in some manner softens or 

 plasticizes the cell wall. A second suggestion is that auxin causes deposition 

 of cell-wall material. A third notion has been that auxin brings about non- 

 osmotic transport of water into the plant cell. The first two suggestions 



Figure 1. Elongation of Avena 

 coleoptile sections in the presence or 

 absence of 0-5 M mannitol. Indole- 

 acetic acid {lAA) 5-0 mgIL added at 

 arrow. After Ordin et al. (1955). 



envisage an osmotic entry of water into the cell in response to lowered wall 

 pressure. The third suggestion envisages an actual active transport of water 

 against an osmotic gradient. We have investigated the extent to which the 

 auxin-induced net water uptake of Avena coleoptile sections is controlled by 

 purely osmotic principles (Ordin, Applewhite, and Bonner, 1955). For this 

 work, elongation of sections has been used as a measure of net water uptake. 

 This is justifiable since diameter changes during auxin-induced elongation of 

 coleoptile sections are negligible and since increases in length are closely 

 correlated with increases in wet weight of this tissue. 



Data on the time course of elongation of sections in media of two different 

 osmotic concentrations are given in Figure 1. It may be noted that the osmotic 

 concentration of the initial tissue is approximately 0-4 M. In the experiment 

 oi Figure 1 the media in both cases contained potassium maleate buffer in low 



■f" This paper was read at the Conference by J. Bonner. 



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