Metallic Ions, Auxin Action, and Chelating Agents 379 



the kinetic constants for this binding. We can determine what con- 

 centration of calcium is needed to get half maximal binding of the 

 exchangeable bound calcium, and we can measure the time constant 

 for the binding. Now, the happy coincidence is that both the con- 

 centration of calcium needed for half-maximal effect and the time 

 constant for the binding is about the same for the exchangeable 

 binding of calcium as it is for the binding of the calcium which 

 causes inhibition of growth rate or inhibition of bendability. There- 

 fore, I conclude that it really is exchangeably-bound calcium that 

 goes into the tissue and gets bound somewhere that exerts these 

 effects. The only remaining question then is to discover what is in 

 the tissue that binds the calcium. This is not so hard to do as it 

 might seem. If we drop our tissue in boiling water and kill it, it still 

 retains its cation binding capacity. If we take tissue and make a 

 cell wall preparation from it, we find that this portion has essentially 

 all of the cation-binding capacity of the intact tissue. So, I think the 

 calcium ions that inhibit the growth rate and are exchangeably bound 

 are bound in the cell wall. 



The final question is, What is it in the cell wall that binds the 

 calcium? There are free carboxyl groups in the cell wall and, as 

 has been shown by Peter Ray and by our own group during the past 

 two years, there are free pectic carboxyl groups in the cell wall and 

 these account quantitatively for the cation-binding capacity of the 

 wall. It seems to us that it is these free pectic carboxyl groups that 

 bind the calcium in the cell wall which are then the calcium ions 

 that cause inhibition of growth rate and inhibition of cell wall 

 bendability. 



Just one last comment. In these same experiments it has been 

 shown that if we grow Avena seedlings in distilled water and then 

 harvest the coleoptile sections and make cell wall preparations from 

 them, then these cell wall preparations contain calcium to such an 

 extent that there is enough calcium to occupy about one-fifth of all 

 the free caiboxyl groups in the cell wall. We propose, therefore, that 

 perhaps one way that EDTA works in causing increases in growth 

 rate is to pry out some of this calcium which is endogenously present 

 in the cell wall, calcium which is contributed to the cell wall from 

 calcium which the seed contains. Perhaps here again we're dealing 

 with rigidity of cell wall induced by calcium. 



Dr. Fawcett: The discovery by Cohen, Ginzburg, and Heitner- 

 Wirguin (Nature. 181: 686. 1958) that the ultraviolet absorption spec- 

 tra of lAA and of NAA are profoundly altered by the presence of 

 cupric, but not calcium or magnesium ions, led them to postulate 

 that the cupric ion reacted with these acids to form chelate complexes. 



