Metabolism and mode of action 



very rapid change over from negative to positive extensions in the first 60 to 

 120 minutes of exposure to water or solutions makes detailed interpretations 

 difficult; one can, however, be reasonably confident that initial suction 

 pressures are of the order of 7 to 10 atm and it is consequently of interest to 

 note the effects of very small external osmotic pressures of salts of bivalent 

 or trivalent cations. 



Figure 2 shows the extension against time of coleoptile sections placed 

 either in solutions containing only lAA at 1 mg/1., or in solutions of the same 

 lAA concentration with additions of calcium and praseodymium chlorides 

 of various o.p. (the trivalent ion used was the commercially available 

 mixture of praseodymium and neodymium chloride; hydrolysis of alu- 

 minium chloride was thought to offer a possible complication). Solutions of 



0-Vaim.Y, 



8-1 atm.K 



0-55 atm.Ca 

 ^5atm.Pr 



Figure 2. Extension growth o/Avena coleoptile plotted against time with 1 mgjl. lAA and additions of 

 KCl.CaCla and PrClg of various osmotic pressures indicated at the right-hand side of the curves. 



PrClg with an o.p. of 2 atm or over completely inhibit extension even though 

 the internal s.p. has almost certainly a minimum value of the order of 7 to 

 10 atm. The effect is therefore almost certainly not osmotic. 



The converse of these multivalent ion effects is also found. Substances 

 which chelate such ions, like ethylenediaminetetraacetic acid (EDTA), not 

 only counteract the effect of artificially added Ca++, but act for a limited 

 period of time as 'growth substances'. The initial growth-promoting effect is 

 of rather short duration in the case of wheat coleoptile tissue, and thus bears 

 some resemblance to the promoting effects observed with high concentrations 

 of auxin. The falling off in growth rate and activity after some 6 to 8 hours 

 may perhaps be ascribed to chelation and removal of essential ions of the 

 respiration complex such as Mg++ and perhaps others. The falling off in 

 growth rate with time is not very noticeable in oat coleoptile sections with 

 EDTA concentrations of 10~^ M or lower. 



Figure 3 shows the extension of oat coleoptiles in water (as control) and in 

 EDTA. 10~^ or 10"^ M ammonium oxalate acts similarly as a 'growth 

 substance' whereas the same concentrations of ammonium chloride give the 

 same extension as water. These results, which will be published in extenso in 



286 



