SALT ACCUMULATION AND MODE OF ACTION 



OF AUXIN. 



A PRELIMINARY HYPOTHESIS 



T. A. Bennet-Clark 



Botany Department, King's College, University of London 



INTRODUCTION 



Hypotheses on the mechanism of auxin action should, of course, provide 

 explanations of the very numerous different effects which it seems to evoke : 

 stem extension, bud inhibition, abscission delay, and the like. If, at the 

 beginning, we consider stem elongation only, it may be said that three types 

 of hypothesis have been advanced : (a) that the cell wall is rendered more 

 plastic and is thus subjected to turgor stetching, {h) that active water and 

 possibly solute uptake is promoted, (c) that polysaccharide synthesis is 

 stimulated and the wall 'grows'. 



The supporters of each of these types of hypothesis do not appear to have 

 suggested or thought of any molecular mechanisms which would carry out 

 the process involved. This may be explained or excused on the grounds that 

 little or nothing is known of the mechanism of synthesis of the polysaccharide 

 material of cell walls. This is true also of active water and solute uptake, 

 apart from the Lundegardh anion-respiration-cytochrome hypothesis and 

 the contractile protein hypothesis of Goldacre; in neither case, however, are 

 any precise molecular details specified, nor is there any reason to think that 

 3-indolylacetic acid (lAA) would be involved in either of these processes. 



SUMMARY OF EXPERIMENTAL FINDINGS 



It will be convenient before enunciating a working hypothesis, the aim of 

 which is primarily to stimulate and direct research, to present the major 

 series of experimental results suggesting this possibly novel view of the 

 mechanism. 



(i) Wightman (1955) has shown that considerable extension growth of 

 wheat coleoptiles occurs under influence of lAA without corresponding- 

 production of cellulose. Our own results indicate also that in the absence of 

 external supplies of sugar, wall extension occurs without appreciable gain of 

 cellulose, 'hemicelluloses' or 'pectins' during a 10-hour period. 



(ii) This extension is markedly influenced by the osmotic pressure of the 

 medium and by the nature of the ions present in it. Results of typical experi- 

 ments are given in Figure 1, which shows the extensions of 10-mm segments of 

 oat coleoptiles plotted against time. The growth of segments treated with a 

 series of solutions of different osmotic pressure to which lAA had been added 

 at a concentration of 1 mg/1. are given together with that of corresponding 

 controls without lAA. 



The initial extension rates are negative (shrinkage) in the higher osmotic 

 pressures. That o.p. which just balances the suction pressure (s.p.) of the 



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