Studies With C^''-labeled Ethylene 111 



In the same manner, the association of ethylene-C^^ with a trypto- 

 phan-Hke compound suggests an explanation for ethylene in auxin- 

 controlled responses, especially apical dominance and abscission. By 

 altering or blocking indole-3-acetic acid synthesis, the cause for loss 

 of apical dominance and the stimulation of abscission in plants ex- 

 posed to ethylene becomes apparent. It has also been found that ethyl- 

 ene inhibits catalase but stimulates peroxidase activity (16). This may 

 be important in explaining the effects of ethylene upon auxin- 

 controlled responses. True assessment of the significance of possible 

 ethylene-auxin interactions, however, awaits the results of experiments 

 now in progress. 



SUMMARY 



Experiments with Coleus and cotton plants exposed to various 

 levels of ethylene-Ci^ have provided quantitative evidence supporting 

 the hormonal nature of ethylene. Both radioautography and direct 

 counting of the radioactivity have confirmed that exogenous ethylene 

 is absorbed and transported readily by the vegetative organs of the 

 two species. The amount of ethylene absorbed in general increases 

 with the dosage administered and is greatly influenced by the physio- 

 logical activity of the plant or tissue and according to whether "fresh" 

 or "aged" ethylene is used. The causes for the changes occurring dur- 

 ing the "aging" of ethylene, as described in this paper, and the reasons 

 for the increase in its biological reactivity, are presently unknown. 



Considerable of the ethylene-C^^ absorbed is unreacted or in loose 

 combination in the plant since two-thirds or more of the absorbed 

 radioactivity can be removed by oven-drying or other techniques and 

 when introduced through the roots it is partially lost via the foliage. 

 The transport form of ethylene is unknown, but the evidence sug- 

 gests that it is translocated both as a gas and as water-soluble 

 metabolites. 



The "fixed" ethylene apparently does not enter readily into normal 

 metabolic pathways that result through respiration in carbon dioxide. 

 More ethylene is fixed by rapidly growing flowering plants than by 

 younger plants, but not necessarily by the older, mature leaves. In- 

 tact leaves absorbed and fixed higher amounts of ethylene-Ci^ than 

 detached leaves. Although more fixation occurred in the dark, more 

 metabolites were translocated under conditions permitting photosyn- 

 thesis and transpiration. 



At least 18 radioactive metabolites were formed in leaves of plants 

 after exposure to ethylene-Ci^ for 15 hrs. From 5 to 9 radioactive, 

 water-soluble metabolites have been isolated in different experiments; 

 two of these having acidic properties and giving a positive phosphate 

 reaction contained the majority of the water-soluble radioactivity. 



