GEORGE S. AVERY, JR. IO7 



growth processes, and which is in some way catalyzed by auxin. It 

 represents a small but variable fraction of the total respiration." Here 

 for the first time was promising evidence of hormone stimulation of 

 respiration and stimulation of growth. At this point growth studies seem 

 to have fallen by the wayside for a number of years. Respiration studies, 

 as influenced by hormones, were no longer carried out with parallel 

 fact-finding on growth. 



Evidence for the stimulation of respiration with the hormone indole- 

 acetic acid comes from the work of Berger and Avery (1,2,3,4). They 

 found both malic and alcohol dehydrogenase enzyme systems isolated 

 from coleoptiles to be stimulated by pretreatment of the tissue with 

 indoleacetic acid, but not by the addition of indoleacetic acid to the 

 in vitro preparations. The activity of alcohol and malic dehydrogenases 

 in Avena coleoptile tissue was increased respectively 200 and 150 per 

 cent when segments of the coleoptile were treated with relatively high 

 concentrations of indoleacetic acid (10 mg. per hter — the same concen- 

 tration found most effective by Commoner and Thimann). Although 

 not linked experimentally with studies on growth this evidence supports 

 the view that both these dehydrogenases are closely concerned with 

 growth, and that the hormone indoleacetic acid controls growth, at 

 least in part, by activating them. 



Employing the same high concentration of indoleacetic acid, Berger, 

 Smith, and Avery (5) reported increases of 35 per cent or more in 

 oxygen uptake in Avena coleoptile segments in sucrose solutions. But 

 they found malate, and therefore presumably fumarate, to function as a 

 substrate rather than as a catalyst in this reaction. Skoog (15) points out 

 that the role of substrate rather than catalyst agrees with Lundegardh's 

 conclusion that fumaric and malic acids may be intermediaries but not 

 catalysts in respiration of wheat roots. 



The most extensive recent study on Avena coleoptile tissue (8) includes 

 a single experiment that should be mentioned here. It concerns hormone 

 stimulation of respiration and growth. Bonner now reports growth and 

 respiration, as determined at the end of a 24-hour growth period, to be 

 stimulated 40 and 38 per cent respectively in the presence of indoleacetic 

 acid at a concentration of 10 mg. per liter. 



Coleoptile tissue has also been investigated for its respiratory response 

 to the synthetic hormone, 2,4-dichlorophenoxyacetic acid (2,4-D), and 

 increases of 20 per cent or more in oxygen uptake in coleoptile segments 



